Multifunctional films can have important applications. Transparent and flexible films with high conductivity and magnetic properties can be used in many areas, such as electromagnetic interference (EMI) shielding, magnetic switching, microwave absorption, and also biotechnology. Herein, novel highly conductive and superparamagnetic thin films with excellent transparency and flexibility have been demonstrated. The films were formed from a poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS; Clevios PH1000) aqueous solution added with iron oxide (Fe3O4) nanoparticles that have a size of ∼20 nm by spin-coating. The PEDOT:PSS/Fe3O4 films have a high conductivity of 1080 S/cm through treatment with methylammonium iodide in an organic solvent. The high-conductivity PEDOT:PSS/Fe3O4 films can also have a saturation magnetization of 25.5 emu/g and an EMI shielding effectiveness of more than 40 dB in the 8–12.5 GHz (X band) frequency range. The PEDOT:PSS/Fe3O4 films have additional advantages, like excellent transparency, good mechanical flexibility, low cost, and light weight. In addition, we fabricate flexible PEDOT:PSS/Fe3O4 silk threads with a high magnetism and conductivity.Keywords: Fe3O4 nanoparticle; highly conductive; mechanical flexibility; PEDOT:PSS; superparamagnetic;

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is a promising candidate as the next-generation thermoelectric (TE) material. Its TE properties are strongly dependent on its chemical and electronic structures. In this paper, we investigated the effect of PEDOT molecular weight on the TE properties of PEDOT:PSS films by a comparative study on two commercial grades of PEDOT:PSS, Clevios P, and Clevios PH1000. Dynamic light scattering (DLS) and Raman spectra imply that the PEDOT of Clevios PH1000 possesses longer conjugated chains than that of Clevios P. The TE properties of both the Clevios P and Clevios PH1000 films can be significantly enhanced through various post treatments, including solvent treatment, germinal diol treatment, organic solution treatment, and acid treatment. After these treatments, the treated Clevios PH1000 films constantly show both superior Seebeck coefficients and electrical conductivities over the treated Clevios P films. It is attributed to the higher molecular weight of PEDOT for the former than the latter. For the treated Clevios PH1000, longer PEDOT chains result in large PEDOT domains, facilitating the charge conduction a semimetallic behavior. Tuning the oxidation level of PEDOT:PSS is a facile way to enhance their TE property. A base treatment with sodium hydroxide was subsequently performed on both the treated Clevios P and Clevios PH1000 films. The power factors of both grades of PEDOT:PSS films were remarkably increased by a factor of 1.2–3.6. Still, both the conductivity and the Seebeck coefficient of a based-treated Clevios PH1000 film are superior over those of a control Clevios P film. The highest power factor the former is 334 μW/(m K2) for the former while only 11.4 μW/(m K2) for the latter. They are different by a factor of about 30 times.Keywords: electrical conductivity; molecular weight; PEDOT:PSS; post treatment; Seebeck coefficient; thermoelectric property;

•Water-soluble vitamins are used to treat PEDOT:PSS.•The conductivity enhancement depends on the chemical structure of vitamins.•The highest conductivity enhancement was observed for PEDOT:PSS treated with vitamin B3.•The mechanism for conductivity enhancement is proposed.Intrinsically conducting polymers can have important application in biology because they can be conductive and have good biological compatibility. Poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) has been the most popular conductive polymer in biological application due to its solution processability in water. PEDOT:PSS can be used as electrode materials or active materials of biological devices or circuits. It is important to study the effect of biomaterials on the structure and properties of PEDOT:PSS films. In this work, water-soluble vitamins that are biomaterials needed for organisms are used to treat PEDOT:PSS. They can significantly enhance the conductivity of PEDOT:PSS from 0.3 S cm−1 up to higher than 1000 S cm−1. The conductivity enhancement depends on the structure of vitamins. The highest conductivity enhancement was observed for PEDOT:PSS treated with vitamin B3. The vitamin-induced changes in the structure and properties of PEDOT:PSS were studied by UV–Vis absorption spectroscopy, temperature-dependence of resistance measurements, atomic force microscopy and cyclic voltammetry. The characterizations indicate that vitamins can induce phase segregation between PEDOT and PSS and the conformational change of the PEDOT chains. These discoveries are important to understand the application of PEDOT:PSS in biology and the development of new biological application of PEDOT:PSS.Download high-res image (120KB)Download full-size image

AbstractIt is very important to study the crystallization of hybrid organic–inorganic perovskites because their thin films are usually prepared from solution. The investigation on the growth of perovskite films is however limited by their polycrystallinity. In this work, methylammonium lead triiodide single crystals grown from solutions with different methylammonium iodide (MAI):lead iodide (PbI2) ratios were investigated. We observed a V-shaped dependence of the crystallization onset temperature on the MAI:PbI2 ratio. This is attributed to the MAI effects on the supersaturation of precursors and the interfacial energy of the crystal growth. At low MAI:PbI2 ratio (<1.7), more MAI leads to the supersaturation of the precursors at lower temperature. At high MAI:PbI2 ratio, the crystal growing plans change from (100)-plane dominated to (001)-plane dominated. The latter have higher interfacial energy than the former, leading to a higher crystallization onset temperature.

Thermoelectric (TE) materials are important for the sustainable development because they enable the direct harvesting of low-quality heat into electricity. Among them, conducting polymers have attracted great attention arising from their advantages, such as flexibility, nontoxicity, easy availability, and intrinsically low thermal conductivity. In this work, a novel and facile method is reported to significantly enhance the TE property of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) films through sequential post-treatments with common acids and bases. Compared with the as-prepared PEDOT:PSS, both the Seebeck coefficients and electrical conductivities can be remarkably enhanced after the treatments. The oxidation level, which significantly impacts the TE property of the PEDOT:PSS films, can also be well tuned by controlling the experimental conditions during the base treatment. The optimal PEDOT:PSS films can have a Seebeck coefficient of 39.2 µV K−1 and a conductivity of 2170 S cm−1 at room temperature, and the corresponding power factor is 334 µW (m−1 K−2). The enhancement in the TE properties is attributed to the synergetic effect of high charge mobility by the acid treatment and the optimal oxidation level tuned by the base treatment.

Readily obtained highly conductive, transparent, and flexible poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) films is urgently needed in printing of flexible transparent electrodes. A simple and facile method to enhance the electrical conductivity of PEDOT:PSS films is reported. The conductivity is increased by four orders of magnitude after adding solid chloroplatinic acid (H2PtCl6) into the pristine PEDOT:PSS solution. The H2PtCl6-doped PEDOT:PSS film exhibits a sheet resistance of 44 ± 5 Ω □-1 and a transmittance of 84 ± 1% at 550 nm, corresponding to a figure of merit of 47 ± 4. Comparative study shows addition of solid acid like H2PtCl6 is more effective in conductivity enhancement than addition of polar organic solvents, such as dimethyl sulfoxide or ethylene glycol. The mechanism for the conductivity enhancement is attributed to both in situ doping and phase separation of PEDOT:PSS. PEDOT is oxidized and doped by Pt4+ of H2PtCl6, which is reduced simultaneously to Pt2+. Proton transfer from H2PtCl6 to PSS− of PEDOT:PSS causes formation of neutral PSSH, leading to phase separation between insulating PSS and conducting PEDOT. Such a phase separation results in conformational changes of PEDOT chains and reduction in energy barrier for charge hopping.

Perovskite solar cells (PSCs) have received great attention due to their high power conversion efficiency and low fabrication cost. The perovskite layer is usually prepared from a solution of precursors. We found that the PbI2 purity has a significant effect on the crystallinity, charge carrier dynamics, and photovoltaic properties of the perovskite films. Planar heterojunction PSCs using highly pure PbI2 showed a high power conversion efficiency (PCE) of 16.4%, which was higher than that of control PSCs with low purity PbI2 by 30–40%. Steady-state photoluminescence (PL), time-resolved PL (TR-PL) and femtosecond transient absorption measurements (FS-TA) revealed that impurities can lower the electron lifetime and increase the non-radiative recombination. This study implies that the PCEs of the perovskite solar cell devices could be further boosted by controlling the precursor purity.

Wearable electronic devices are becoming increasingly popular. They can bring a tremendous impact to human life. Wearable sensors, a class of wearable electronic devices, have attracted considerable attention because of their importance in healthcare. In this study, graphene-based wearable sensors, which can be directly integrated into clothes or textile products, are fabricated by a simple and cost effective method. Non-woven fabric (NWF) is considered as a green and low cost textile material, and its properties can be engineered for specific functions, such as medical systems, clothing, filter and packaging. A piece of NWF was dipped into graphene oxide (GO) solution and then reduced in HI acid. The GO reduction was confirmed by both X-ray photoelectron spectra and scanning electron microscopy. In the final product, reduced GO sheets were evenly coated on the NWF surface. The as-prepared graphene-NWF (GNWF) sensors exhibited a negative gauge factor at small strain. The signal has good reproducibility in response to stretching, bending and pressure. The highest gauge factor is −7.1 at 1% strain, and the highest sensitivity is 0.057 kPa−1. The GNWF sensors are able to respond to a series of human motions with the differentiation of various degrees of motions, and it can monitor small scale motions such as pulse and respiration.

Fast-growing flexible and stretchable electronics, such as robots, portable electronics and wearable devices, are regarded as the next-generation electronic devices. Flexible or even stretchable electromagnetic interference (EMI) shielding materials with high performance are needed to avoid the adverse effects of electromagnetic radiation produced by these devices. In this work, highly conductive and stretchable polymer films were prepared by blending a conductive polymer, poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS), with highly stretchable waterborne polyurethane (WPU). The two polymers have good miscibility at a wide range of blending ratios. The conductivity of the composite films increases while the stretchability decreases with the increase of PEDOT:PSS loading. At a 20 wt% PEDOT:PSS loading, the composite films show a conductivity of 77 S cm−1 and an elongation at break of about 32.5%. More interestingly, they exhibit a high EMI shielding effectiveness (SE) of about 62 dB over the X-band frequency range at a film thickness of only 0.15 mm.

Hybrid organic–inorganic perovskite solar cells (PSCs) have attracted great interest owing to their low fabrication costs and high power conversion efficiency. Most studies have focused on the devices with methylammonium lead trihalide perovskites. Here, we explore a new perovskite with mixed organic cations and mixed halides, MA1−xFAxPbI3−yCly. MA1−xFAxPbI3−yCly films can be fabricated by annealing at a temperature of 80–110 °C. Planar heterojunction PSCs using this perovskite as the active material can exhibit a high power conversion efficiency (PCE) of up to 18.14% with short-circuit photocurrent density (Jsc) of 21.55 ± 0.55 mA cm−2, open-circuit voltage (Voc) of 1.100 ± 0.010 V, and fill factor (FF) of 0.75 ± 0.02. The PCE is much higher than those of the control devices with other commonly employed perovskites including MAPbI3, MAPbI3−yCly, MAPbI3−yBry, and MA1−xFAxPbI3. The superior performance is mainly attributed to the enhancement of Jsc, which is a result of long charge diffusion lengths due to the presence of mixed organic cations and mixed halides. In addition, there is no obvious hysteresis in the J–V curves along the forward and reverse scan directions. The formation of undesirable δ-phase perovskite that has a band gap of 2.8 eV is not observed in the MA1−xFAxPbI3−yCly films. These findings pave the way for the design of new hybrid perovskites with stronger light absorption over a wide range, lower charge recombination, and improved charge transport properties through compositional engineering.

Perovskite solar cells (PSCs) have attracted great attention due to their high power conversion efficiencies (PCEs) and low fabrication cost. The composition of the precursor solution determines the compositions of perovskite films. Excess precursor(s) may be used in the solution for the fabrication of perovskite films. However, it is still unclear how an excess precursor like PbI2 affects the structure and properties of the perovskite layer and the photovoltaic performance of PSCs. In this work, we investigated the effect of excess PbI2 that has a large bandgap on the electronic structure and properties of perovskite films and the photophysics and photovoltaic performance of PSCs. The presence of slightly excess PbI2 can affect the crystal structure and thus shift the Fermi level of perovskites. It can increase the open-circuit voltage (Voc) and thus the PCE of PSCs. However, the presence of a large amount of excess PbI2 is detrimental to the photovoltaic performance of PSCs. It can shorten the carrier lifetime, increase the resistance of the perovskite films, and decrease the fill factor (FF) and PCE of PSCs.

Conducting polymers have promising thermoelectric application because they have many advantages including abundant elements, mechanical flexibility, and nontoxicity. The thermoelectric properties of conducting polymers strongly depend on their chemical structure and microstructure. Here, we report a novel and facile method to significantly enhance the thermoelectric properties of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) films through a treatment with organic solutions of inorganic salts. N,N-Dimethylformamide (DMF) and a common inorganic salt like zinc chloride (ZnCl2) are used as the solvent and solute of the solutions, respectively. The treatments can significantly increase both the Seebeck coefficient and electrical conductivity of the PEDOT:PSS films. The thermoelectric properties of the PEDOT:PSS films are sensitive to the experimental conditions, such as the salt concentration, treatment temperature, and the cation of the salts. After treatment at the optimal experimental conditions, the PEDOT:PSS films can exhibit a Seebeck coefficient of 26.1 μV/K and an electrical conductivity of over 1400 S/cm at room temperature. The corresponding power factor is 98.2 μW/(m·K2). The mechanism for the enhancement in the thermoelectric properties is attributed to the segregation of some PSSH chains from PEDOT:PSS and the conformation change of PEDOT chains as a result of the synergetic effects of inorganic salts and DMF.Keywords: electrical conductivity enhancement; inorganic salt; organic solution; PEDOT:PSS; Seebeck coefficient; thermoelectric property

A transparent electrode is an indispensable component of optoelectronic devices, and there as been a search for substitutes of indium tin oxide (ITO) as the transparent electrode. Poly(3,4-ethylene dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is a conducting polymer that is very promising as the next generation of materials for the transparent electrode if it can obtain conductivity as high as that of ITO. Here, we report the treatment of PEDOT:PSS with organic solutions to significantly enhance its conductivity. Common organic solvents like dimethylformamide and γ-butyrolactone and common organic salts like methylammonium iodide and methylammonium bromide are used for the organic solutions. The conductivity of pristine PEDOT:PSS films is only ∼0.2 S/cm, and it can be increased to higher than 2100 S/cm. The conductivity enhancement is much more significant than control treatments of PEDOT:PSS films with neat organic solvents or aqueous solutions of the organic salts. The mechanism for the conductivity enhancement is the synergetic effects of both the organic salts and organic solvents on the microstructure and composition of PEDOT:PSS. They induce the segregation of some PSSH chains from PEDOT:PSS. Highly conductive PEDOT:PSS films were studied as the transparent electrode of polymer solar cells. The photovoltaic efficiency is comparable to that with an ITO transparent electrode.Keywords: conductivity enhancement; organic salt; organic solution; PEDOT:PSS; polymer solar cells

•Ultrasoniation lowers the conductivity of PEDOT:PSS.•Ultrasonication lowers the acidity of PEDOT:PSS.•PEDOT:PSS with low conductivity is used in polymer solar cells.•PEDOT:PSS with low conductivity is used in perovskite solar cells.Poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) has been widely used as the hole transport material in optoelectronic devices. To avoid the cross talk among different crossbars, PEDOT:PSS with low conductivity is required. It thus has a high loading of the non-conductive PSSH. The PSSH-to-PEDOT weight ratio is 6 for Clevios P VP Al 4083 that is the most popular polymer as the hole transport layer. However, the acidic PSSH brings severe problems to the device stability and performance. Here, PEDOT:PSS solutions with low acidity can be prepared through a facile treatment of PEDOT:PSS solution by probe ultrasonication. Two grades of PEDOT:PSS, Clevios PH1000 and Clevios P, with a PSSH-to-PEDOT weight ratio of 2.5 were treated by probe ultrasonication. The ultrasonication can lower the viscosity and the colloidal sizes of PEDOT:PSS solutions and conductivity of PEDOT:PSS films. The pH value of probe-ultrasonicated Clevios P was 2.12, higher than that (1.77) of pristine Clevios P VP Al 4083. The ultrasonication-treated PEDOT:PSS solutions were used as hole transport layer in polymer solar cells and perovskite solar cells. The photovoltaic performances of these solar cells are comparable to that of control devices employing Clevios P VP Al 4083 PEDOT:PSS as the hole transport layer.

This article presents a facile one-pot synthetic method to prepare ternary NiAuPt nanoparticles on reduced graphene oxide (rGO) nanosheets (NiAuPt-NGs) through the simultaneous chemical reduction of metal precursors and GO in solution and an investigation of NiAuPt-NGs as electrocatalysts toward ethanol oxidation reaction (EOR). The NiAuPt nanoparticles grow on the rGO sheets after the chemical reduction of their precursors. They consist of tightly coupled nanostructures of Ni, Au, and Pt, which have neither an alloy nor a core–shell structure, as revealed by X-ray diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy. As indicated by the Raman spectra, GO is reduced to rGO more completely in the presence of the metal precursors than in the absence of the metal precursors. The electrocatalysis of NiAuPt-NGs toward EOR in alkaline medium was investigated by cyclic voltammetry, chronoamperometry, and impedance spectroscopy. NiAuPt-NGs can effectively catalyze EOR. The ternary NiAuPt-NGs give rise to a high peak current density for EOR, which is more than 8 times higher than that on the monometallic Pt-NGs, 4 times higher than that on the bimetallic NiPt-NGs, and almost 2 times higher than that on the bimetallic AuPt-NGs. In addition, NiAuPt-NGs substantially lower the onset potential for EOR. It is −803 mV vs SHE, which suggests the excellent tolerance of NiAuPt-NGs against the residues of EOR. The high electrocatalytic activity of NiAuPt-NGs is attributed to the synergetic effect of the three nanostructured metals for EOR.Keywords: durability; electrocatalysis; ethanol oxidation reaction; NiAuPt; ternary nanoparticles

Perovskite solar cells (PSCs) have attracted considerable attention because of their low fabrication cost and impressive energy conversion efficiency. The perovskite layer of planar PSCs is usually prepared by coating a solution of perovskite precursors, that is, PbI2, PbCl2 and methylammonium iodide (MAI), on poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS, Clevios P VP Al 4083). Here, the deposition of the perovskite layer from its precursor solution saliently affects the electronic structure and properties of PEDOT:PSS films and thus the photovoltaic performance of planar PSCs. The conductivity of PEDOT:PSS is significantly enhanced from 10−3 to 101 S cm−1. The conductivity enhancement is not due to the solvent but mainly MAI. Even more significant conductivity enhancement occurs for PEDOT:PSS films after being coated with a dimethylformamide (DMF) solution of MAI, while pure DMF only slightly increases the conductivity of PEDOT:PSS by a factor of 2–3. PEDOT:PSS films become rougher after the deposition of a perovskite or MAI layer. The conductivity enhancements are attributed to the phase segregation of PSSH chains from PEDOT:PSS and the conformational change of PEDOT chains. The treatment of PEDOT:PSS with the organic solutions of MAI and solvents of perovskite precursor solutions also affects the photovoltaic performance of the planar PSCs.

Double interlayers consisting of a zwitterionic small molecule layer and a LiF layer were introduced between the electron transport layer and the cathode of perovskite solar cells. The double interlayers improve the photovoltaic efficiency to 13.2%, which is higher than that of control devices without the double interlayer (9.2%) or with LiF (11.0%) or rhodamine 101 zwitterion (12.1%) alone.

Perovskite solar cells (PSCs) have been attracting considerable attention because of their low fabrication cost and impressive energy conversion efficiency. Most PSCs are built on transparent conductive oxides (TCOs) such as fluorine-doped tin oxide (FTO) or indium tin oxide (ITO), which are costly and rigid. Therefore, it is significant to explore alternative materials as the transparent electrode of PSCs. In this study, highly conductive and highly transparent poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) films were investigated as the transparent electrode of both rigid and flexible PSCs. The conductivity of PEDOT:PSS films on rigid glass or flexible poly(ethylene terephthalate) (PET) substrate is significantly enhanced through a treatment with methanesulfonic acid (MSA). The optimal power conversion efficiency (PCE) is close to 11% for the rigid PSCs with an MSA-treated PEDOT:PSS film as the transparent electrode on glass, and it is more than 8% for the flexible PSCs with a MSA-treated PEDOT:PSS film as the transparent electrode on PET. The flexible PSCs exhibit excellent mechanical flexibility in the bending test.Keywords: flexible solar cell; interface engineering; PEDOT:PSS; perovskite solar cell; transparent electrode;

Graphene is considered to be one of the most interesting materials because of its unique two-dimensional structure and properties. However, commercialization and large-scale production of graphene still face great challenges at the moment. Thermal reduction of graphene oxide (GO) can be an effective method to fabricate graphene in large scale, but the need for inert gas protection and high reaction temperature leads to high cost of production, thus limiting the production capacity of graphene. In this paper, for the first time we report a facile, safe, and scalable method to achieve simultaneous thermal reduction and nitrogen doping of GO in air at much lower reaction temperature while upholding a high-quality end product. The reduction and nitrogen doping of GO are evidenced by ultraviolet–visible absorption spectroscopy, X-ray diffraction, Raman spectroscopy, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. The nitrogen-doped reduced GO (NrGO) fabricated via this method has a high carbon/oxygen ratio of 15 and a nitrogen content of 11.87 atom %. The NrGO is also investigated by applying it as an electrocatalyst for the oxygen reduction reaction. As a result, the catalytic activity has presented itself as much higher than that of the undoped rGO.Keywords: air; graphene; nitrogen-doped; reduced graphene oxide; thermal reduction

Stretchable and conductive materials can have important application in many areas, such as wearable electronics and healthcare devices. Conducting polymers have very limited elasticity because of their rigid conjugated backbone. In this work, highly stretchable and conductive polymer films are prepared by coating or casting aqueous solution of poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) and a soft polymer, including poly(ethylene glycol), poly(ethylene oxide), or poly(vinyl alcohol). The soft polymers can greatly improve the stretchability and the conductivity of PEDOT:PSS. The elongation at break can be increased from 2% up to 55%. The soft polymers can also enhance the conductivity of PEDOT:PSS from 0.2 up to 75 S cm–1. The conductivity is further enhanced by adding dimethyl sulfoxide (DMSO) or ethylene glycol (EG) into the aqueous solutions of the polymer blends. Polymer blends with an elongation at break of close to 50% and a conductivity of 172 S cm–1 are attained.Keywords: conductive polymer; elastic; PEDOT:PSS; polymer blend; stretchable conductor

•Contact between two electronic materials significant affects the electrical behavior.•Contact between Au nanoparticles and Al is studied by XPS.•Al significantly affects the XPS spectrum of Au nanoparticles.•Electron transfer occurs from Al into Au nanoparticles capped with a conjugated thiol.The electronic contact between a bulk metal and metal nanoparticles can be significantly different from that between two bulk metals due to the unique electronic structure in the nanometer size. In this work, the electronic contact between Au nanoparticles and Al is studied by X-ray photoelectron spectroscopy. Al is deposited on a layer of Au nanoparticles capped with conjugated 2-naphthalenethiol (Au-2NT NPs) in high vacuum by e-beam deposition at room temperature. The Au 4f X-ray photoelectron spectrum (XPS) significantly changes after the Al deposition. New XPS bands with higher binding energy appear. The angle dependence of the Au 4f XPS bands indicates that the electron transfer takes place at the contact between Al and Au-2NT NPs. In contrast, the Al deposition hardly changes the Au 4f XPS spectrum for Au nanoparticles capped with saturated 1-dodecanethiol. The effect of the Al deposition on the Au 4f XPS spectrum of Au nanoparticles capped with 2-naphthalenethiol is attributed to the electron transfer from Al through the conjugated 2-naphthalenehiol into the core of Au nanoparticles, as the Fermi energy of Al is higher than Au. This understanding on the contact between metal and metal nanoparticles provides guidance for the development of novel electronic devices.

Graphene oxide (GO) has attracted considerable attention due to its interesting structure and properties. The photoluminescence (PL) of GO is much stronger than that of graphene owing to the opening of an energy band gap. However, the origin of the PL bands in the ultraviolet and visible ranges remains controversial. In this paper, we report the dependence of the PL spectrum of GO on the pH value and concentration of GO aqueous solutions. It was discovered that PL in the visible range becomes prominent when the pH value is low and/or the GO concentration is high. As revealed by the time-resolved photoluminescence, the lifetime of the PL in the visible range is longer than that in the UV range. These results evidence the formation of excimers and prove that the PL band at the long wavelength is caused by the GO excimers.

Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is the most successful conducting polymer in terms of practical application. It possesses many unique properties, such as good film forming ability by versatile fabrication techniques, superior optical transparency in visible light range, high electrical conductivity, intrinsically high work function and good physical and chemical stability in air. PEDOT:PSS has wide applications in energy conversion and storage devices. This review summarizes its applications in organic solar cells, dye-sensitized solar cells, supercapacitors, fuel cells, thermoelectric devices and stretchable devices. Approaches to enhance the material/device performances are highlighted.

The practical application of dye-sensitized solar cells (DSCs) requires high photovoltaic efficiency and good photovoltaic stability. This work reports nanocomposites of two-dimensional graphene oxide (GO) and one-dimensional multi-walled carbon nanotubes (MWCNTs) as the gelator of gel electrolytes for quasi-solid state DSCs. The composite gels are formed by immobilizing an organic solvent, 3-methoxypropionitrile (MPN), with GO and MWCNTs, and the gel electrolytes are prepared by adding iodine, 1-methyl-3-propylimidazolium iodide, guanidinium thiocynate and 4-tert butylpyridine into the GO–MWCNT–MPN composite gels. GO sheets can gel organic solvents because of their hydrophobic and hydrophilic domains. The MWCNTs can reinforce the solid networks formed by GO sheets and reduce the ionic diffusion length of the redox species within the electrolyte as MWCNTs are conductive and catalytic toward the electrochemical reduction of I3−. The presence of MWCNTs in the gel electrolyte increases both the open-circuit voltage (VOC) and the short-circuit current (JSC) of DSCs so as to increase the power conversion efficiency (PCE). The optimal PCE of the DSCs with GO–MWCNT–MPN gel electrolyte is 7.12% under AM 1.5G illumination (100 mW cm−2), which is significantly higher than that (6.54%) of the gel DSCs without MWCNTs.

Organic solar cells based on poly(3-hexylthiophene) and [6,6]-phenyl-C61-butyric acid methyl ester were fabricated with electrodeposited TiOx electron extraction layers 5–180 nm thick. Electrodeposition under ambient conditions is an attractive, facile and viable approach to prepare metal oxide interfacial layers. The TiOx films obtained displayed a linear relationship between thickness and deposition time when fabricated under ambient conditions using an aqueous air stable peroxotitanium precursor. The precursor solution was prepared from titanium isopropoxide using a chelate process, which allowed water to be used as solvent due to considerably decreased sensitivity of the precursor solution towards hydrolysis. Highly conformal TiOx films, typically observed with vacuum deposition techniques, were obtained on the indium tin oxide substrate upon electrogeneration of OH– ions using H2O2 additive. Conversely, significantly rougher films with spherical growths were obtained using NO3– additives. Low temperature annealing at 200 °C in air was found to greatly improve purity and O stoichiometry of the TiOx films, enabling efficient devices incorporating the electrodeposited TiOx to be made. Using MoOx as the hole extraction layer, the maximum power conversion efficiency obtained was 3.8% (Voc = 610 mV; Jsc = 10.6 mA/cm2; FF = 59%) under simulated 100 mW/cm2 (AM1.5G) solar irradiation, whereas an efficiency of 3.4% was achieved with fully solution processed interfacial layers comprising the electrodeposited TiOx films and a surfactant-modified PEDOT:PSS hole extraction layer.Keywords: Electrodeposition; inverted organic photovoltaic; morphology; solution processed; thin film; titanium oxide;

Improving device efficiency and stability of polymer solar cells (PSCs) is crucial for their practical application. Although graphene oxide (GO) could replace the poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as the hole-collection material and improve the photovoltaic stability of PSCs, the power conversion efficiency is moderate because of its insulating nature. In this article, nanocomposites of two-dimensional reduced graphene oxide (rGO) and GO are used to replace the acidic PEDOT:PSS as the hole-collection material of PSCs. The nanocomposites are formed by dispersing rGO into aqueous solution of GO. GO serves as a surfactant, and it can stabilize rGO. The presence of rGO can quench the photoluminescence of GO in water. The nanocomposite films exhibit higher conductivity than GO films without rGO. They are used as the hole-collection material of PSCs. The optimal PSCs with poly(3-hexylthiophene) and [6,6]-phenyl-C61-butyric acid methyl ester exhibit such photovoltaic performances: short-circuit current density of 10.37 mA cm–2, open-circuit voltage of 0.60 V, fill factor of 67.66%, and power conversion efficiency of 4.21%. The photovoltaic efficiency is much higher than that of the control devices with GO only (3.36%) as the hole-collection material. In addition, the presence of rGO in GO gives rise to better stability for the PSCs in air than that of the devices with GO only. The devices with rGO:GO composites as the hole-collection materials exhibit much better stability in power conversion efficiency than the control devices with PEDOT:PSS.Keywords: graphene oxide; hole collection; polymer solar cells; reduced graphene oxide

•Asymmetrical resistive switches on devices with Au nanoparticles capped with 2-naphthalenethiol.•The resistive switches are sensitive to temperature.•The resistive switches become insignificant at low temperature.•At T > 220 K, the charge transport is thermally assisted.•At T < 220 K, temperature-insensitive tunneling becomes important.The interface between two bulk electronic materials can significantly affect the electrical behavior of electronic devices. But the interface between a bulk metal and metal nanoparticles has been rarely explored. This paper reports significant temperature effect on the asymmetrical resistive switches of polymer:nanoparticle memory devices. The devices have architecture of a polystyrene layer admixed with gold nanoparticles capped with conjugated 2-naphthalenethiol sandwiched between Au and Al electrodes. The devices exhibit significant resistive switches at room temperature. However, the resistive switches become less significant at temperature below 200 K, and they are not noticeable at 103 K. The temperature effect suggests that the resistive switches are assisted by the thermal energy. The charge transport through the devices has different mechanisms at high and low temperatures. At temperature above 220 K, the Poole–Frenkel emission is an important mechanism for the charge transport. At temperature below 220 K, the temperature-independent Fowler–Nordheim tunneling becomes an important process.

Long term device stability and high power conversion efficiency (PCE) are important for practical application of dye-sensitized solar cells (DSCs). Here, we report the use of ethyl cellulose (EC) and acid functionalized multi-walled carbon nanotubes (oMWCNTs) as a co-gelator to gel organic solvents and the application of these gels for quasi-solid state DSCs. The gels are formed by blending EC and oMWCNTs with methoxypropionitrile (MPN) and acetonitrile (ACN). The loadings of EC and oMWCNTs can be as low as 4 wt% and 1.5 wt%, respectively, for the gel formation. The total minimum gelator loading of EC and oMWCNTs is lower than that with EC alone. In the absence of oMWCNTs, the EC loading must be more than 12 wt% for the gel formation. Gel electrolytes were prepared by adding iodine, 1-methyl-3-propylimidazolium iodide (PMII), guanidinium thiocyanate and 4-tert-butyl pyridine into the EC-oMWCNT/ACN-MPN gels, and they were used to fabricate quasi-solid state DSCs. The optimal PCE of the gel DSCs was 6.97% under AM1.5G illumination, quite close to that of control liquid DSCs, while the gel DSCs have better long term stability than their liquid-state counterparts. The former retains more than 98% of the initial PCE after 30 days, whereas the PCE of the latter decreases to about 80%.

To increase the open-circuit voltage (Voc) of dye-sensitized solar cells (DSCs), it is crucial to enhance the photovoltaic efficiency of DSCs. Here, we report an effective method to significantly improve the Voc and photovoltaic efficiency of DSCs by using gel-coated composites of reduced graphene oxide (rGO) and single-walled carbon nanotubes (SWCNTs) as the counter electrode. Gel-coated rGO-SWCNT composites outperform Pt, rGO and SWCNTs in catalyzing the reduction of I3– and functioning as the counter electrode of DSCs. The Voc and power conversion efficiency (PCE) are 0.86 V and 8.37% for fresh DSCs with the composite of 80 wt % rGO and 20 wt % SWCNTs, significantly higher than those (Voc = 0.77 V, PCE = 7.79%) of control DSCs with Pt fabricated by pyrolysis as the counter electrode. The Voc value of DSCs with rGO-SWCNT composites as the counter electrode further increases to 0.90 V after one week. The high Voc and PCE are ascribed to the synergetic effects of rGO and SWCNTs in reducing the overpotential of the I3– reduction. RGO with high specific surface area can have high electrocatalytic activity, whereas SWCNTs give rise to high conductivity for the composites and facilitate the penetration of the redox species into rGO sheets by preventing the agglomeration of the rGO sheets. To the best of our knowledge, this is the first time to report iodide/triiodide DSCs with both high Voc and PCE.Keywords: carbon nanotubes; dye-sensitized solar cells; electrocatalysis; reduced graphene oxide;

In this paper, we report a novel method to effectively fabricate the mesoporous TiO2 films of dye-sensitized solar cells (DSCs) by formulating new TiO2 pastes. Graphene oxide (GO) is added into TiO2 nanoparticles pastes as an auxiliary binder. Thick mesoporous TiO2 films free of crack can be prepared by only single printing. TiO2–GO pastes and films were characterized by dynamic mechanical analysis, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), X-ray diffraction (XRD), thermogravimetric analysis (TGA), and FTIR spectroscopy. TiO2 pastes added with GO exhibit gel behavior. GO helps bind TiO2 nanoparticles together through the interactions between functional groups on GO and the surface species of TiO2 nanoparticles. The presence of 0.8 wt.% GO in the TiO2 paste (GO weight percentage with respect to the weight of TiO2) is sufficient to fabricate thick and crack-free TiO2 films via single printing. These mesoporous TiO2 films fabricated from the TiO2–GO pastes are investigated as the anode of DSCs. They can give rise to a power conversion efficiency (PCE) of 7.70% for DSCs under AM1.5G illumination, which is almost the same as that of control devices with the TiO2 mesoporous electrode fabricated from the conventional TiO2 paste without GO via four-fold printings.Graphical abstractHighlights► Graphene oxide is used as an auxiliary binder in TiO2 paste. ► Graphene oxide can improve the binding among the TiO2 nanoparticles. ► Graphene oxide prevents the appearance of crack on TiO2 films. ► High-quality TiO2 films can be fabricated from TiO2–GO pastes via single printing. ► Graphene oxide effectively improve the fabrication of dye-sensitized solar cells.

We report the formation of organogels of 3-methoxypropionitrile (MPN) using graphene oxide (GO) as the gelator and the use of these gels as the quasi-solid state electrolyte of dye-sensitized solar cells (DSCs). GO–MPN gels are prepared by mechanically grinding or ultrasonicating GO in MPN. The GO sheets form 3-dimensional solid networks in the gels, which hold the MPN solvent. The GO loading can be as low as 2.5 wt.% for the gel formation. Gel electrolytes were prepared by dispersing I2, 1-methyl-3-propylimidazolium iodide, guanidine thiocynate and 4-tert butyl pyridine into GO–MPN gels, and these were used for DSCs. The GO sheets can be fragmented into small pieces by ultrasonication, and smaller GO sheets can lead to a higher diffusion constant of the triiodide and a higher photovoltaic efficiency for the DSCs. DSCs with a GO–MPN gel electrolyte exhibit a photovoltaic efficiency of 6.70% under AM 1.5 G illumination (100 mW cm−2), quite close to that (7.18%) of the control liquid DSCs.

•Resistive switches are observed on devices with Au nanoparticles capped with 2-naphthalenethiol.•The resistive switches are sensitive to the electrodes.•The resistive switches are sensitive to the capping ligand of the Au nanoparticles.•The resistive switches are insensitive to the polymer matrix.•These results confirm the charge-transfer model for the resistive switches.Electronic devices with an polystyrene (PS) layer blended with Au nanoparticles capped with conjugated 2-naphthalenethiol (Au–2NT NPs) sandwiched between Au and Al electrodes exhibit bipolar resistive switches sensitive to the electrodes. This paper reports the effects of materials, including electrode materials, capping ligands of Au nanoparticles and matrix polymers, on the electrical behavior of the polymer:nanoparticle memory devices. Although the devices using Cu to replace Au as the top electrode exhibit resistive switches similar to those with Au, the threshold voltage for the resistive switch is higher, and the current density for the devices in the low conductivity state is lower. However, the threshold voltage and the current density are almost the same as those with Au as the top electrode, when a semiconductor, MoO3, is used to replace Au as the top electrode of the devices. The effects of these electrodes are attributed to the charge transfer at the contacts between Au–2NT NPs and the electrodes. The resistive switches are also sensitive to the capping organic ligand of the Au nanoparticles. The threshold voltage decreases and the current density increases, when conjugated benzenethiol is used to replace 2-naphthalenethiol. However, the current density dramatically decreases and the threshold voltage increases, when 2-benzeneethanethiol, a partially conjugated molecule, is adopted as the capping ligand of the Au nanoparticles. The effect of the capping ligands is ascribed to their effect on the charge tunneling across the Au–2NT NPs in the active layer and the contacts between Au–2NT NPs and electrodes. The devices with poly(methyl methacrylate) (PMMA) replacing PS as the polymer matrix exhibit resistive switches almost the same as those with PS, which indicates that the Au–2NT NPs rather than the polymer is the active material responsible for the resistive switches.Graphical abstract

Graphene deposited with metal nanostructures can have important applications in various areas. This paper reports the in situ deposition of gold nanostructures with well-defined shapes, including triangular nanoplate and cuboid, on unfunctionalized reduced graphene oxide (rGO) by a two-step process. The first step is to coat a layer of HAuCl4 on an rGO film on glass, which is prepared by the reduction of a graphene oxide film with aluminum in an HCl solution. The second step is to reduce the dry HAuCl4 layer to metallic Au nanostructures with ethylene glycol vapor at 160 °C for 15 min, which gives rise to the deposition of Au nanostructures with well-defined shapes on rGO. No solvent or additives are added during the chemical reduction. The shape of the Au nanostructures depends on the Au loading. The majority of the Au nanostructures are triangular nanoplates, when the Au loading is low. The Au nanostructures with hexagonal nanoplate and pentahedron nanoplate shapes become remarkable at high Au loading. Moreover, the shape of the Au nanostructures can be changed to cuboid by introducing acids into the HAuCl4. The different shapes of the Au nanostructures deposited on rGO are interpreted in terms of the generation and diffusion rates of the Au atoms.

Flexible transparent electrode materials are strongly needed for optoelectronic devices. We report a novel method to significantly enhance the conductivity of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) films through treatment with a fluoro compound, hexafluoroacetone (HFA). HFA hydrolyzes with water into a geminal diol, 1,1,1,3,3,3-hexafluoropropane-2,2-diol (HFP2OH) that has two –OH groups connected to the middle carbon atom. The conductivity increased from 0.3 to 1164 and 1325 S cm−1 after the treatment with HFA once and four times, respectively. The highly conductive HFA-treated PEDOT:PSS films can have a sheet resistances of 46 Ω □−1 and a transparency of around 83% at 550 nm. These values are comparable to those of indium tin oxide (ITO) on polyethylene terephthalate (PET). The conductivity enhancement is attributed to the HFP2OH-induced phase segregation of some hydrophilic PSSH chains from PEDOT:PSS and the conformational change of the conductive PEDOT chains, driven by the interactions between amphiphilic HFP2OH and PEDOT:PSS. The hydrophobic –CF3groups of HFP2OH preferentially interact with the hydrophobic PEDOT chains of PEDOT:PSS, while the hydrophilic –OH groups preferentially interact with hydrophilic PSS chains. The highly conductive PEDOT:PSS films were used to replace ITO as the transparent anode of polymer solar cells. Polymer solar cells based on poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) exhibited a photovoltaic efficiency of 3.57% under simulated AM1.5G illumination, comparable to the control devices with ITO as the anode.

We report highly efficient polymer solar cells (PSCs) with rhodamine 101, a conjugated zwitterion with positive and negative charges on the same molecule, as the electron-collection interlayer. Rhodamine 101 can be processed by solution processing techniques or thermal deposition. The rhodamine 101 interlayer simultaneously improves the short-circuit current, open-circuit voltage, fill factor so as to improve the photovoltaic efficiency of the PSCs with poly[[9-(1-octylnonyl)-9H-carbazole-2,7-diyl]-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl] (PCDTBT) and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as the active materials in comparison with control PSCs with Al or Ca/Al as the cathode. The photovoltaic efficiency reaches 6.15% for the PSCs with rhodamine 101/Al as the cathode under AM1.5G illumination, whereas the efficiencies are only 3.81% and 4.57% for the control PSCs with Al and Ca/Al as the cathodes, respectively. On the other hand, the improvement on the photovoltaic performance by the rhodamine 101 interlayer is less remarkable for the PSCs with poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) or PC71BM as the active materials. The photovoltaic efficiency is 4.25% for the PSCs with rhodamine 101/Al as the cathode, almost the same as that (4.20%) of the control PSCs with Ca/Al as the cathode. The high efficiency of the PSCs with the rhodamine 101 interlayer is ascribed to the lowering of the work function of metals by rhodamine 101 that has a strong dipole moment. The different effects of the rhodamine 101 interlayer on the two PSCs with PCDTBT and P3HT as the donor materials are attributed to different reactivities of the two polymers with active metals like Ca and Al. PCDTBT consisting of both electron-donating and electron-withdrawing units is more reactive with the active metals. The reaction between PCDTBT and Ca leads to low photovoltaic efficiency of the PSCs with Ca/Al as the cathode.

This paper reports the preparation of reduced graphene oxide (rGO) films through the gel formation of rGO with liquid polyethylene glycol (PEG) of low molecular weight and the application of the rGO films as the platinum (Pt)-free counter electrode of highly efficient iodide/triiodide dye-sensitized solar cells (DSCs). rGO prepared through the reduction of graphene oxide can form physical gels with PEG by mechanical grinding. The rGO sheets form 3-dimensional solid networks in the gels. The rGO–PEG gels are processed into rGO films on substrates with good adhesion by doctor blade and subsequent removal of PEG through heating. The rGO films can effectively catalyze the electrochemical reduction of triiodide and are used as the counter electrode of iodide/triiodide DSCs. The optimal photovoltaic efficiency of DSCs with the rGO counter electrode is 7.19% under AM1.5G illumination (100 mW cm−2), which is only slightly lower than that (7.76%) of control DSCs with Pt prepared by pyrolysis as the counter electrode. This is the highest photovoltaic efficiency observed on the DSCs with iodide/triiodide as the redox couple and graphene as the counter electrode. The rGO films exhibit better stability in catalyzing the electrochemical reduction of triiodide and give rise to better aging stability of DSCs than Pt prepared by pyrolysis. The photovoltaic efficiency of DSCs is strongly dependent on the thickness of the rGO film, and the optimal rGO film thickness is 15 μm. The photovoltaic performance of DSCs is also affected by the method to prepare the rGO–PEG gels. The photovoltaic efficiency is only 6.15% or lower when the rGO–PEG gels are prepared by ultrasonication rather than mechanical grinding. This is attributed to the different morphologies of the rGO films achieved by different methods.

Graphene has been attracting strong attention due to its interesting structure and properties and important applications in many areas. The process of the oxidation of graphite into graphene oxide (GO) and the subsequent reduction of GO into graphene is regarded as an effective process to produce graphene on a large scale. The quality of the reduced GO is strongly dependent on the reduction method. This paper reports the reduction of GO with Zn powder in neutral and alkaline aqueous solutions at room temperature. The reducing capability of Zn powder can be significantly improved through the complex formation of Zn2+ with other species in solution, which greatly lowers the Zn2+ concentration. Ethylenediaminetetraacetic acid (EDTA) can form an Zn–EDTA2− complex with Zn2+ and it is used for the reduction of GO by Zn in a neutral solution. The complex formation gives rise to quite low Zn2+ concentrations in solution. This effectively lowers the reduction potential of Zn/Zn2+ and enables the reduction of GO in neutral solutions. GO can be effectively reduced by Zn powder in alkaline solutions without EDTA as well. This is attributed to the complex formation of Zn2+ with OH−, where Zn2+ + 4OH−→ Zn(OH)42−. The reduced GO produced by these methods has high quality. Their C/O ratios for products obtained through GO reductions in neutral and alkaline solutions are 33.0 and 31.2 and their conductivities are 142 and 135 S cm−1, respectively.

Polymer solar cells (PSCs) with inverted structure can greatly improve photovoltaic stability. This paper reports a novel method to lower the work function of indium tin oxide (ITO) through the modification with a thin layer of zwitterions which have both positive and negative charges in the same molecule. Zwitterions have a strong dipole moment due to the presence of the two types of charges and are immobile under electric field. Zwitterions with both conjugated and saturated structure were investigated. A zwitterion thin layer is formed on ITO by spin coating a methanol solution of the zwitterion. The zwitterion-modified ITO sheets can be used as the cathode for the electron collection of inverted PSCs. The inverted poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PC61BM) PSCs can exhibit photovoltaic efficiency as high as 3.98% under simulated AM1.5G illumination (100 mW cm–2), which is comparable to that of PSCs with normal architecture. The effective electron collection by the zwitterion-modified ITO sheets is attributed to the reduction of the work function of ITO as a result of the dipole moment by the zwitterions. The zwitterion modification can lower the work function of ITO by up to 0.97 eV. The photovoltaic performance of PSCs and the reduction in the work function of ITO strongly depend on the chemical structure of the zwitterions.Keywords: dipole; inverted polymer solar cell; ITO modification; work function; zwitterions;

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is promising to be the next-generation transparent electrode of optoelectronic devices. This paper reports the differences between two commercially available grades of PEDOT:PSS: Clevios P and Clevios PH1000. The as-prepared PEDOT:PSS films from Clevios P and Clevios PH1000 solutions have close conductivities of 0.2–0.35 S cm–1. Their conductivities can be enhanced to 171 and 1164 S cm–1, respectively, through a treatment with hydrofluoroacetone trihydrate (HFA). The differences between Clevios P and Clevios PH1000 were studied by various characterizations on PEDOT:PSS aqueous solutions and PEDOT:PSS films. The gel particles are larger in Clevios PH1000 solution than in Clevios P solution as revealed by dynamic light scattering and fluorescence spectroscopy of pyrene in these solutions. These results suggest that PEDOT of Clevios PH1000 has a higher average molecular weight than that of Clevios P. The difference in the molecular weight of PEDOT for the two grades of PEDOT:PSS is confirmed by the characterizations on their polymer films, including atomic force microscopy and temperature dependences of the resistances of as-prepared and HFA-treated PEDOT:PSS films. The different molecular weights of PEDOT also gives rise to significant differences in the electrochemical behaviors of the two grades of PEDOT:PSS, as revealed by the cyclic voltammetry, in situ UV–vis–NIR absorption spectroscopy and potentiostatic transient measurements.Keywords: conductivity; molecular weight; PEDOT:PSS; rigid; transparent electrode;

To deposit nanostructured platinum (Pt) on substrates has fundamental and practical significance as Pt is a noble metal and an effective electrocatalyst. In this paper, we report a facile and scalable method to deposit Pt nanoparticles on substrates through the solventless chemical reduction of H2PtCl6 with vapor of ethylene glycol (EG) below 200 °C. The Pt nanoparticles have a size below 6 nm, and they homogeneously distribute on the substrates. They have good adhesion to the substrates. They are highly catalytic for the reduction of triiodide that is the chemical reduction at the counter electrode of dye-sensitized solar cells (DSCs). Though DSCs with Pt deposited by the EG vapor reduction and pyrolysis exhibit comparable high photovoltaic efficiency of more than 8% at high Pt loading, the photovoltaic efficiency of DSCs with Pt by the EG vapor reduction is much higher than that by pyrolysis when the Pt loading is quite low. DSCs with a Pt loading of 0.19 μg cm−2 deposited by this method can have a power conversion efficiency of 6.61%, which is even higher than that (6.19%) of DSCs with a Pt loading of 0.78 μg cm−2 deposited by pyrolysis. Thus, this method can significantly reduce the Pt loading to ¼ or even lower in comparison with Pt by pyrolysis which has been regarded as the method to deposit Pt with the highest electrocatalytic activity as the counter electrode of DSCs. The excellent electrocatalytic activity of Pt is attributed to the small Pt particle size.Graphical abstractHighlights► EG vapor can convert H2PtCl6 into Pt nanoparticles smaller than 6 nm. ► Pt nanoparticles have good adhesion to substrate. ► Pt nanoparticles have high electrocatalytic activity. ► This method can lower the Pt loading as electrocatalyst.

Ionic liquids are nonvolatile solvents and have high ionic conductivity. They can be used as stable electrolyte in many electrochemical systems. But their application is impeded by their high viscosity. This paper reports a novel method to reduce the viscosity of an ionic liquid, 1-propyl-3-methylimidazolium iodide (PMII) that is extensively used in dye-sensitized solar cells (DSSCs), by the addition of carboxylic group-functionalized multi-walled carbon nanotubes (oMWCNTs). The viscosity of PMII can decrease from 1380 cP to 400 cP at 25 °C after the addition of 0.10 wt% oMWCNTs. The viscosity reduction depends on the oMWCNT loading. Unfunctionalized MWCNTs hardly reduce the viscosity of PMII. The viscosity decrease is attributed to the reduction in the Coulombic attractions between the PMI+ cations and I− anions as a result of the hydrogen bond formation between PMI+ and carboxylic group of oMWCNTs. The presence of oMWCNTs gives rise to the increase in the size disparity of cations to anions of PMII and induces disordered structure. The addition of oMWCNTs does not affect the thermal stability of PMII. PMII–oMWCNT mixtures with reduced viscosity are used as the electrolyte of DSSCs. The addition oMWCNTs in the ionic liquid can significantly improve the photovoltiac efficiency of DSSCs. This is the first time to observe the viscosity reduction of ionic liquid by carbon nanotubes.

Deposition of nanostructured metals on substrates is important for the fundamental study and practical application, such as in optics and catalysis. In this paper, we report the deposition of gold (Au) nanoplates and porous platinum (Pt) structures on substrates through solvent-free chemical reductions of chloroauric acid (HAuCl4) and chloroplatinic acid (H2PtCl6) with ethylene glycol (EG) vapor at temperatures below 200 °C. The process includes two steps. The first step is the formation of a thin layer of a metal precursor on substrates by coating solution of a metal precursor. The thin metal precursor layer is subsequently dried by annealing. The second step is the chemical reduction of the metal precursor with EG vapor at 160 or 180 °C in the absence of solvent. Both the Au and Pt nanostructures deposited by this method have good adhesion to substrates, but they have different morphologies. The Au nanostructures appear as separate two-dimensional islands on the substrates, and up to 70% of them can be triangular nanoplates with the (111) crystal plane as the basal plane. In contrast, the reduction of H2PtCl6 gives rise to a 3-dimensional porous Pt structure on substrates. The different morphologies of nanostructured Au and Pt are tentatively related to the different surface energies of Au and Pt.

Poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) films with high conductivity can have important application as the transparent electrode of optoelectronic devices. In this paper, we report the significant conductivity enhancement of PEDOT:PSS through a treatment with germinal diols which have two hydroxyl groups connected to one carbon atom or amphiphilic fluoro compounds which have hydrophobic fluorocarbon groups and hydrophilic hydroxyl or carboxylic groups. Several compounds, including hexafluoroacetone, cyclohexanehexone, formaldehyde, acetaldehyde, and perfluorobenzophenone, which could convert into geminal diols, were used to treat PEDOT:PSS films. The conductivity enhancements are generally consistent with the equilibrium constants for the conversion of these compounds into geminal diols. PEDOT:PSS films were also treated with several amphiphilic fluoro compounds. The conductivity was significantly enhanced when PEDOT:PSS films were treated with hexafluoroisopropanol, trifluoroacetic acid and heptafluorobutyric acid, while it hardly changed when they were treated with 2,2,2-trifluoroethanol. Conductivities of more than 1000 S cm−1 were observed on the treated PEDOT:PSS films. The mechanism for the conductivity enhancement of PEDOT:PSS through the treatment with geminal diols or amphiphilic fluoro compounds is attributed to the phase segregation of PSSH from PEDOT:PSS and conformational change of the PEDOT chains as the results of the compounds-induced reduction in the Coulombic attraction between the positively charged PEDOT and negatively charged PSS chains.Graphical abstractHighlights► Geminal diols can significantly improve the conductivity of PEDOT:PSS. ► Amphiphilic fluoro compounds can significantly improve the conductivity of PEDOT:PSS. ► Insulator-to-metal transition occurs for some treated PEDOT:PSS film. ► The conductivity enhancements are due to phase segregation and conformational change.

Diketo-pyrrolo-pyrrole (DPP) is an important electron-deficient unit, and the alkylation of DPP can gives rise to good solubility of organic and polymer semiconductors in solvent. In this paper, we report our observations on the isomers of dialkyl DPP. The alkylation of DPP can take place on both the nitrogen and oxygen atoms, leading to the formation of three isomers. The presence of the isomers is evidenced by the nuclear magnetic resonance (NMR) and mass spectra. The yields of the isomers are affected by the experimental conditions, including the reaction temperature, reaction time and solvent. The isomers have different electronic structures and optical properties as revealed by the UV–Vis absorption spectroscopy, fluorescent spectroscopy, cyclic voltammetry and theoretical simulation using Gaussian 03 program with B3LYP/6-31G(d). Small molecules based on the DPP isomers with dialkylation at different sites were synthesized and investigated as donor materials of organic solar cells (OSCs). The photovoltaic performances of OSCs are consistent with the electronic structures of the DPP isomers.Graphical abstractHighlights► Alkylation of DPP can occur on both O and N atoms. ► Three isomers of dialkyl DPP are obtained. ► Dialkyl DPP isomers have different electronic structures. ► Molecules based on dialkyl DPP isomers are used as donor materials of OSCs. ► The performances of OSCs are consistent with the electronic structure of DPP isomers.

Polymer solar cells (PSCs) with inverted structure can greatly improve the photovoltaic stability. This paper reports surface modification of indium tin oxide (ITO) by spin coating a thin layer of various sodium compounds. ITOs with such a treatment were used as the cathode of the inverted PSCs with poly(3-hexylthiophene) (P3HT) as the donor, [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as the acceptor, and MoO3/Al as the anode. Among these sodium compounds, sodium hydroxide (NaOH) gives rise to high photovoltaic performance: an open-circuit voltage (Voc) of 0.58 V, short-circuit current density (Jsc) of 10.03 mA cm−2, fill factor of 0.67, and power conversion efficiency of 3.89% under AM 1.5G illumination (100 mW cm−2). The efficiency is significantly higher than that (0.35%) of the control devices with untreated ITO as the cathode and even slightly higher than that (3.61%) of PSCs with normal architecture. The high performance of the inverted PSCs is due to the reduction of the work function of ITO by almost 1 eV by NaOH. The reduction in the work function of ITO was also observed by other sodium compounds, and it is consistent with the association constants of the anions of the sodium compounds with proton. The mechanism for the reduction of the work function is attributed to the dipole formation and alignment of the sodium compounds on the ITO surface.

Transparent and conductive single-walled carbon nanotube (SWCNT) films were fabricated through gel coating. A gel was formed by directly dispersing a large amount of SWCNTs in a liquid nonionic surfactant like polyoxyethylene (12) tridecyl ether without a solvent by mechanical grinding and subsequent ultrasonication. SWCNT films were obtained by coating the gels on glass substrates and subsequently removing the surfactant through heating. The SWCNT films became very thin and transparent after heating at a temperature between 480 and 520 °C. They exhibited good uniformity and a high transmittance-to-sheet resistance ratio which is attributed to the long SWCNTs dispersed in the gels. Films with a sheet resistance of around 250 Ω □−1 at 80% transmittance were obtained. The films can be easily transferred from rigid substrates like glass to flexible polymer substrates like polyethylene terephthalate.

The conductivity of poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) films was significantly enhanced by preferential solvations of the hydrophobic PEDOT and hydrophilic PSS chains with cosolvents. When a PEDOT:PSS film prepared from the PEDOT:PSS aqueous solution was treated with water or a common organic solvent like ethanol, iso-propyl alcohol (IPA), acetonitrile (ACN), acetone, or tetrahydrofuran (THF), its conductivity did not change remarkably. But the conductivity was significantly enhanced when the PEDOT:PSS film was treated with a cosolvent of water and one of these common organic solvents. The conductivity enhancement was affected by several factors, including the ratio of the organic solvent to water, the dielectric constant of the organic solvent, and the temperature during the treatment. The conductivity enhancement from 0.2 S cm−1 to 103 S cm−1 was observed. The significant conductivity enhancement is attributed to the preferential solvation of PEDOT:PSS with a cosolvent. Water and the organic solvent of the cosolvent preferentially solvate the hydrophilic PSS and hydrophobic PEDOT chains, respectively. The preferential solvation of a PEDOT:PSS film with a cosolvent induces the phase separation of the insulator PSSH chains from the PEDOT:PSS film, aggregation of PSSH segments in the PEDOT:PSS film, and the conformational change of the PEDOT chains from coiled to linear. The cosolvent-treated PEDOT:PSS films were quite smooth and could be used to replace indium tin oxide (ITO) as the transparent electrode of electronic devices. Polymer photovoltaic cells (PVs) with the cosolvent-treated PEDOT:PSS films as the transparent electrode exhibited high photovoltaic performance.

A green method is reported to effectively and rapidly reduce graphene oxide to graphene with zinc powder at room temperature. The reduction is carried out by mixing graphene oxide and zinc powder in solution under ultrasonication. The reduction is complete within 1 min. The weight of the Zn powder should be at least as twice as that of graphene oxide to complete the reduction. The reduction of graphene oxide is confirmed by FTIR, UV–Vis absorption spectroscopy and Raman spectroscopy. The carbon/oxygen atomic ratio has increased from 2.58 to 33.5 after the reduction as determined by X-ray photoelectron spectroscopy. The reduced graphene oxide can have a conductivity of 15,000 S/m. It also has good thermal stability with the weight loss at 590 °C in air. The reduced graphene oxide can be readily re-dispersed into solutions of various surfactants.

This manuscript reports the modification of the interface between the mesoporous TiO2 work electrode and electrolyte of dye-sensitized solar cells (DSCs) with oxide layers deposited by the thermal evaporation of metals and subsequent oxidation with UV ozone. Both Al2O3 and MgO can be deposited on mesoporous TiO2 by this method, and their thickness can be precisely controlled. A thin layer of Al2O3 or MgO on the TiO2 work electrode can improve the photovoltaic efficiency. The optimal thicknesses are 14.1 and 4.9 Å for Al2O3 and MgO, respectively. The oxide effect has been investigated by the electrochemical impedance spectroscopy, cyclic voltammetry and UV–Vis–NIR absorption spectroscopy. The improvement in the photovoltaic efficiency by an oxide layer is attributed to the upward shift of the conduction band of TiO2, the passivation of the TiO2 surface, and the retardation of the charge recombination through the interface between TiO2 and electrolyte.Graphical abstractHighlights► Al2O3 and MgO were coated by thermal evaporation of metals and subsequent oxidation. ► The thickness of the oxide layers can be precisely controlled. ► Oxide layers were coated on TiO2 of DSCs to modify its interface with electrolyte. ► The oxide layers remarkably improve the photovoltaic performance of DSCs.

Copolymers with alternative electron-donating and electron-withdrawing units can exhibit a low-band gap and can be used as the donor materials in polymer photovoltaic cells (PVs). This paper reports the synthesis of copolymers of electron-donating ethynylfluorene and electron-withdrawing 3,6-dithiophen-2-yl-2,5-dihydropyrrolo[3,4-c]pyyrole-1,4-dione (DPP) via the Pd-catalyzed Sonogashira coupling and their application in polymer PVs. Applying microwave irradiation during the Sonogashira coupling can shorten the polymerization time from several days to a couple of minutes and improve the molecular weight. These polymers have band gaps of about 1.85 eV. Their energy levels indicate that they can be used as the donor materials for polymer PVs. Polymer PVs were fabricated using these polymers as donors and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as acceptor. Applying the microwave irradiation during the Sonogashira coupling can significantly improve the photovoltaic performance of the polymer PVs. Power conversion efficiency (PCE) of 2.25% was achieved on polymer PVs under AM 1.5G illumination (100 mW cm−2).Graphical abstractHighlights► We synthesized two low-bandgap donor-accepter copolymers. ► The polymerizations were carried out by the Sonogashira coupling. ► Microwave irradiation significantly shortens the polymerization time. ► Microwave irradiation significantly increases polymer molecular weights. ► Microwave irradiation improves the performance of polymer solar cells.

The photovoltaic stability of polymer solar cells (PSCs) can be greatly improved by adopting an inverted device structure. This paper reports high-performance inverted PSCs with lead monoxide (PbO)-modified indium tin oxide (ITO) as the cathodes. A thin PbO layer can effectively lower the work function of ITO from 4.5 to 3.8 eV. The optimal inverted PSCs with poly(3-hexylthiophene) (P3HT) as the donor and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as the acceptor exhibited high photovoltaic performance: open-circuit voltage of 0.59 V, short-circuit current density of 10.8 mA cm−2, fill factor of 0.632, and power conversion efficiency of 4.00% under simulated AM1.5G illumination (100 mW cm−2). The photovoltaic efficiency is significantly higher than that of the control inverted PSCs with unmodified ITO as the cathode. It is even better than that of the control PSCs with normal architecture, which have an optimal efficiency of 3.5%. The lowering in the work function by the PbO modification is attributed to the charge transfer between PbO and ITO, as evidenced by the X-ray photoelectron spectra.Graphical abstractHighlights► Inverted polymer solar cells with PbO-modified ITO exhibit high performance. ► A thin PbO layer can lower the work function of ITO. ► Lowering in the work function is due to the interactions between PbO and ITO.

Electronically and ionically conductive gels were fabricated by mixing and mechanically grinding neutral tetrathiafulvalene (TTF) and tetracyanoquinodimethane (TCNQ) in ionic liquids (ILs) like 3-ethyl-1-methylimidazolium dicyanoamide (EMIDCA), 1-ethyl-3-methylimidazolium thiocyanate (EMISCN), 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMITf2N), trihexyltetradecylphosphonium bis(trifluoromethylsulfonyl)imide (P14,6,6,6Tf2N), and methyl-trioctylammonium bis(trifluoromethylsulfonyl)imide (MOATf2N). Charge-transfer TTF–TCNQ crystallites were generated during the mechanical grinding as indicated by the UV–visibile–near-infrared (UV–vis–NIR) absorption spectroscopy, Fourier transform infrared (FTIR) spectroscopy, and X-ray diffraction. The charge-transfer TTF–TCNQ crystallites have a needle-like shape. They form solid networks to gelate the ILs. The gel behavior is confirmed by the dynamic mechanical measurements. It depends on both the anions and cations of the ILs. In addition, when 1-methyl-3-butylimidazolium tetrafluoroborate (BMIBF4) and 1-methyl-3-propylimidazolium iodide (PMII) were used, the TTF–TCNQ/IL mixtures did not behave as gels. The TTF–TCNQ/IL gels are both electronically and ionically conductive, because the solid phase formed by the charge-transfer TTF–TCNQ crystallites is electronically conductive, while the ILs are ionically conductive. The gel formation is related to needle-like charge-transfer TTF–TCNQ cyrstallites and the π–π and Coulombic interactions between TTF–TCNQ and ILs.

Metal nanoparticles with good adhesion to substrates are important in practical applications, such as in catalysis, but directly depositing metal nanoparticles on substrates with good adhesion from a solution of their precursors has been rarely explored. This work reports a two-step method to deposit catalytic platinum (Pt) nanoparticles with good adhesion to substrates by solution processing and chemical reduction of a Pt precursor at a relatively low temperature. The first step is to coat a layer of H2PtCl6 on a substrate, such as fluorine-doped tin oxide (FTO), indium tin oxide (ITO), or conductive poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). The second step is to cast ethylene glycol (EG) on the H2PtCl6 layer and convert H2PtCl6 into metallic Pt nanoparticles by heating at 160 °C for a few minutes. The Pt nanoparticles join together into a continuous Pt nanoparticle structure that is different from the dendritic structure and has good adhesion to substrates. The good adhesion of the Pt nanoparticles to the substrates is attributed to the deposition of the Pt nanoparticles with high surface energy and the disappearance of EG shortly after the nanoparticle formation. The Pt nanoparticles cannot be removed from the substrates by adhesive tape peeling or sonication. The continuous Pt nanoparticle strutures can be used as the catalyst for many energy-conversion electrochemical reactions, such as oxidation of methanol and reduction of triiodide. They were also used as the counter electrode of dye-sensitized solar cells (DSCs). The DSCs exhibited light-to-electricity conversion efficiency of 8.02% under AM1.5G illumination (100 mW cm−2) and good stability.

Polymer electrolytes are needed in many solid-state electronic and energy devices. But polymer electrolytes usually have a low ionic conductivity. This work reports the enhancement in the ionic conductivity of polyethylene oxide (PEO)-LiClO4 electrolyte by adding functionalized multiwalled carbon nanotubes (MWCNTs). MWCNTs are functionalized with carboxylic groups through oxidation with acids. They can be dispersed in acetonitrile solutions of PEO and LiClO4. The presence of functionalized MWCNTs can effectively enhance the ionic conductivity of the PEO-LiClO4 electrolyte, and the ionic conductivity enhancement depends on the loading of the functionalized MWCNTs. Enhancement by a factor of 3.3 was observed. The enhancement in the ionic conductivity is attributed to the functionalized MWCNT-induced decrease in the crystallinity of PEO and increase in the salt dissociation due to the Lewis acid–base interaction of the functionalized MWCNTs with PEO and LiClO4. The addition of functionalized MWCNTs can also effectively improve the mechanical properties of PEO films.

This paper reports the significant conductivity enhancement of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) films through a treatment with aqueous solutions of zwitterions for the first time. The conductivity enhancement was dependent on the structure of the zwitterions and the experimental conditions during the treatment, such as the concentration of the zwitterions and the temperature. Conductivity enhancement from 0.2 to 92.4 S cm−1 was observed on PEDOT:PSS films after a zwitterion treatment. The chemical and physical characterizations indicate the lowering of the energy barrier for charge hopping across the PEDOT chains, the loss of poly(styrene sulfonate) acid (PSSH) chains from the PEDOT:PSS film, and the conformational change of the PEDOT chains after the zwitterion treatment. These highly conductive PEDOT:PSS films could be used to replace indium tin oxide (ITO) as the transparent anode of polymer photovoltaic cells (PVs). Power conversion efficiency as high as 2.48% was observed on the polymer PVs with a zwitterion-treated PEDOT:PSS film as the transparent anode.

Significant conductivity enhancement was observed on transparent and conductive poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) films after a treatment with organic and inorganic acids, including acetic acid, propionic acid, butyric acid, oxalic acid, sulfurous acid, and hydrochloric acid. The conductivity could be enhanced from 0.2 to over 200 S cm−1, that is, by a factor of more than 1000. The conductivity enhancement was dependent on the structure of the acids and the experimental conditions during the treatment, such as the acid concentration and the temperature. The optimal temperature was in the range of 120 to 160 °C. The resistance dropped rapidly when a PEDOT:PSS film was treated with acid solution of high concentration, whereas it gradually increased and then decreased when it was treated with an acid solution of low concentration. The mechanism for this conductivity enhancement was studied by various chemical and physical characterizations. The temperature dependence of conductivity indicates that the energy barrier for charge hopping among the PEDOT chains become lower in the highly conductive PEDOT:PSS film after the acid treatment. The ultraviolet−visible−near-infrared (UV−vis−NIR) absorption spectroscopy, the X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) indicate the loss of polystyrene sulfonic acid (PSSH) chains from the PEDOT:PSS film after the acid treatment, and the atomic force microscopy (AFM) suggest conformational change of the polymer chains. Therefore, the conductivity enhancement is attributed to the loss of PSSH chains from the PEDOT:PSS film and the conformational change of the PEDOT chains, which are induced by the acids.Keywords: conducting polymer; conductivity; inorganic acid; organic acid; PEDOT:PSS; transparent

Mixtures of carbon nanotubes (CNTs), including multi-wall CNTs and single-wall CNTs, and polyoxyethylene(12) tridecyl ether (POETE), a nonionic surfactant and a fluid at room temperature, became gels after mechanical grinding. The heavily entangled multi-wall or single-wall CNTs debundled during the grinding and dispersed with fewer bundles in POETE. The mechanism for the gel formation was studied by the dynamic mechanical measurements and scanning electron microscopy. The results suggest that the formation of the CNT/POETE gel is the result of the physical-crosslinking CNT networks, mediated by the van der Waals interaction between CNTs and the nonionic surfactant. The gels were stable from room temperature up to 200 °C and did not shrivel even in vacuum. The CNT/POETE gels were electrically conductive and could be processed into conductive CNT films by coating the CNT/POETE gels on a substrate by a doctor blade and subsequent heating. POETE was removed during the heating, while the heating did not degrade the CNTs. The CNT films had a conductivity of about 13 S cm−1 and had good adhesion to the substrate.

The conductivity of conductive poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) films could be significantly enhanced through a treatment with aqueous solution of a certain salt, such as CuCl2 or InCl3. The anions of the salts had pronounced effect on the conductivity enhancement. Conductivity enhancement by a factor of more than 1000 was observed when CuBr2 or InI3 aqueous solution was used, while it became less significant and negligible when CuSO4 and Cu(CH3COO)2 aqueous solutions were used, respectively. The anion effect on the conductivity enhancement is consistent with the acid dissociation constants of the anions, which suggests that the conductivity enhancement is related to the dissociation of the salts and the association of the anions with PEDOT+. A salt with higher dissociation can benefit the association of the anions with PEDOT+, which leads to the loss of more PSS chains from the PEDOT:PSS film and more significant conductivity enhancement by the salt treatment.

Platinum precursors such as H2PtCl6 can be reduced to platinum (Pt) by polyols at temperatures below 200 °C. This method was developed to deposit nanostructured Pt films on various substrates through a solution process. High-quality Pt films with good adhesion to substrate were obtained on substrates like conducting polymer films, indium tin oxide (ITO), and polyimide by dropping a solution of H2PtCl6 in ethylene glycol (EG). Two types of Pt structures, that is, dense and porous Pt structures, were observed for the Pt deposited by EG reduction. The dense Pt structure is the result of Pt growth from the substrate immediately after the reduction. This Pt growth from the substrate can produce a continuous nanostructured Pt film with metallic luster and good adhesion to the substrate. The porous Pt structure is the result of the nucleation of Pt in solution. The Pt nucleation in solution gives rise to Pt particle formation. These Pt particles aggregate and finally precipitate as porous Pt on the dense Pt. The quality of the Pt film was affected by the experimental conditions, such as substrate, concentration, and pH value of H2PtCl6 solution in EG, and temperature during the reduction. Pt deposited by this method could be used as the counter electrode of high-performance dye-sensitized solar cells (DSCs). The DSCs exhibited a light-to-electricity conversion efficiency of 8.1%, quite close to that of the DSCs with Pt counter electrode prepared by the conventional pyrolysis. In addition, this low-temperature method enabled the Pt deposition on flexible substrate, which could be used as the flexible counter electrode for DSCs.

This article reports a novel method to significantly enhance the conductivity of conducting poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) films through a treatment with aqueous solutions of various salts, such as copper(II) chloride. Conductivity enhancement by a factor of about 700 was observed. Many salts were investigated, and the conductivity enhancement depended on the softness parameter of cations and the concentration of the salts in solution. A salt like copper(II) chloride or indium chloride, whose cation has positive softness parameter, could enhance the conductivity of the PEDOT:PSS film by 2 orders in magnitude, while other salt like sodium chloride or magnesium chloride, whose cation has negative softness parameter, gave rise to negligible effect on the conductivity. The mechanism for the conductivity enhancement was studied by various characterizations. It is attributed to PSS loss from the PEDOT:PSS film, and conformational change of PEDOT chains resulted from the salt-induced charge screening between PEDOT and PSS.

Perovskite solar cells (PSCs) have attracted much attention due to their impressive photovoltaic performance and low fabrication cost. The perovskite layer plays a critical role in light-to-electricity conversion. Here we study the effects of alkali metal ion dopants on the growth, structure and properties of perovskite films and the photovoltaic performance of PSCs. At a low doping level, the alkali metal ions can significantly affect the formation and properties of the perovskite films. They can enhance the short-circuit current density (Jsc) and power conversion efficiency (PCE) significantly. The Jsc increases significantly from 19.2 mA cm−2 to 21.0 mA cm−2 and 20.3 mA cm−2 for the PSCs with a methylammonium lead triode layer doped with 0.5 mol% K+ and 0.25 mol% Na+, respectively, and the fill factor (FF) increases from 0.74 to 0.79 for the devices with both doping ions. Correspondingly, the PCE increases from 13.7% to 15.3% and 14.6% for the PSCs, respectively. The efficiency improvement by K+ or Na+ doping is attributed to their effect in increasing the crystallinity and crystallite size of the perovskite films.

Perovskite solar cells (PSCs) have received great attention due to their high power conversion efficiency and low fabrication cost. The perovskite layer is usually prepared from a solution of precursors. We found that the PbI2 purity has a significant effect on the crystallinity, charge carrier dynamics, and photovoltaic properties of the perovskite films. Planar heterojunction PSCs using highly pure PbI2 showed a high power conversion efficiency (PCE) of 16.4%, which was higher than that of control PSCs with low purity PbI2 by 30–40%. Steady-state photoluminescence (PL), time-resolved PL (TR-PL) and femtosecond transient absorption measurements (FS-TA) revealed that impurities can lower the electron lifetime and increase the non-radiative recombination. This study implies that the PCEs of the perovskite solar cell devices could be further boosted by controlling the precursor purity.

Hybrid organic–inorganic perovskite solar cells (PSCs) have attracted great interest owing to their low fabrication costs and high power conversion efficiency. Most studies have focused on the devices with methylammonium lead trihalide perovskites. Here, we explore a new perovskite with mixed organic cations and mixed halides, MA1−xFAxPbI3−yCly. MA1−xFAxPbI3−yCly films can be fabricated by annealing at a temperature of 80–110 °C. Planar heterojunction PSCs using this perovskite as the active material can exhibit a high power conversion efficiency (PCE) of up to 18.14% with short-circuit photocurrent density (Jsc) of 21.55 ± 0.55 mA cm−2, open-circuit voltage (Voc) of 1.100 ± 0.010 V, and fill factor (FF) of 0.75 ± 0.02. The PCE is much higher than those of the control devices with other commonly employed perovskites including MAPbI3, MAPbI3−yCly, MAPbI3−yBry, and MA1−xFAxPbI3. The superior performance is mainly attributed to the enhancement of Jsc, which is a result of long charge diffusion lengths due to the presence of mixed organic cations and mixed halides. In addition, there is no obvious hysteresis in the J–V curves along the forward and reverse scan directions. The formation of undesirable δ-phase perovskite that has a band gap of 2.8 eV is not observed in the MA1−xFAxPbI3−yCly films. These findings pave the way for the design of new hybrid perovskites with stronger light absorption over a wide range, lower charge recombination, and improved charge transport properties through compositional engineering.

Perovskite solar cells (PSCs) have attracted considerable attention because of their low fabrication cost and impressive energy conversion efficiency. The perovskite layer of planar PSCs is usually prepared by coating a solution of perovskite precursors, that is, PbI2, PbCl2 and methylammonium iodide (MAI), on poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS, Clevios P VP Al 4083). Here, the deposition of the perovskite layer from its precursor solution saliently affects the electronic structure and properties of PEDOT:PSS films and thus the photovoltaic performance of planar PSCs. The conductivity of PEDOT:PSS is significantly enhanced from 10−3 to 101 S cm−1. The conductivity enhancement is not due to the solvent but mainly MAI. Even more significant conductivity enhancement occurs for PEDOT:PSS films after being coated with a dimethylformamide (DMF) solution of MAI, while pure DMF only slightly increases the conductivity of PEDOT:PSS by a factor of 2–3. PEDOT:PSS films become rougher after the deposition of a perovskite or MAI layer. The conductivity enhancements are attributed to the phase segregation of PSSH chains from PEDOT:PSS and the conformational change of PEDOT chains. The treatment of PEDOT:PSS with the organic solutions of MAI and solvents of perovskite precursor solutions also affects the photovoltaic performance of the planar PSCs.

Fast-growing flexible and stretchable electronics, such as robots, portable electronics and wearable devices, are regarded as the next-generation electronic devices. Flexible or even stretchable electromagnetic interference (EMI) shielding materials with high performance are needed to avoid the adverse effects of electromagnetic radiation produced by these devices. In this work, highly conductive and stretchable polymer films were prepared by blending a conductive polymer, poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS), with highly stretchable waterborne polyurethane (WPU). The two polymers have good miscibility at a wide range of blending ratios. The conductivity of the composite films increases while the stretchability decreases with the increase of PEDOT:PSS loading. At a 20 wt% PEDOT:PSS loading, the composite films show a conductivity of 77 S cm−1 and an elongation at break of about 32.5%. More interestingly, they exhibit a high EMI shielding effectiveness (SE) of about 62 dB over the X-band frequency range at a film thickness of only 0.15 mm.

Deposition of nanostructured metals on substrates is important for the fundamental study and practical application, such as in optics and catalysis. In this paper, we report the deposition of gold (Au) nanoplates and porous platinum (Pt) structures on substrates through solvent-free chemical reductions of chloroauric acid (HAuCl4) and chloroplatinic acid (H2PtCl6) with ethylene glycol (EG) vapor at temperatures below 200 °C. The process includes two steps. The first step is the formation of a thin layer of a metal precursor on substrates by coating solution of a metal precursor. The thin metal precursor layer is subsequently dried by annealing. The second step is the chemical reduction of the metal precursor with EG vapor at 160 or 180 °C in the absence of solvent. Both the Au and Pt nanostructures deposited by this method have good adhesion to substrates, but they have different morphologies. The Au nanostructures appear as separate two-dimensional islands on the substrates, and up to 70% of them can be triangular nanoplates with the (111) crystal plane as the basal plane. In contrast, the reduction of H2PtCl6 gives rise to a 3-dimensional porous Pt structure on substrates. The different morphologies of nanostructured Au and Pt are tentatively related to the different surface energies of Au and Pt.

Perovskite solar cells have attracted much attention due to their high power conversion efficiency and low fabrication cost. The efficiency of the devices depends on the charge transport and charge extraction at the interface between the perovskite and electron or hole transport material. In this study, we use Kelvin probe force microscopy (KPFM) to investigate the charge carrier generation, transport, and extraction mechanism of several different structures with/without the hole/electron interlayers. It was found that the perovskite films exhibited unbalanced charge-carrier transport and extraction in the p-i-n type structure. Meanwhile, the leakage current under bias conditions can be suppressed by using a [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) capping layer. The time-resolved photoluminescence (TR-PL) results reveal that the electrons have a longer lifetime and longer diffusion length than the holes in the perovskite layer. These results suggest that the electron extraction is more efficient than the hole extraction in the planar heterojunction devices of poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS)/perovskite/PCBM. These findings are beneficial for further optimizing the perovskite solar cells.

This paper reports the significant conductivity enhancement of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) films through a treatment with aqueous solutions of zwitterions for the first time. The conductivity enhancement was dependent on the structure of the zwitterions and the experimental conditions during the treatment, such as the concentration of the zwitterions and the temperature. Conductivity enhancement from 0.2 to 92.4 S cm−1 was observed on PEDOT:PSS films after a zwitterion treatment. The chemical and physical characterizations indicate the lowering of the energy barrier for charge hopping across the PEDOT chains, the loss of poly(styrene sulfonate) acid (PSSH) chains from the PEDOT:PSS film, and the conformational change of the PEDOT chains after the zwitterion treatment. These highly conductive PEDOT:PSS films could be used to replace indium tin oxide (ITO) as the transparent anode of polymer photovoltaic cells (PVs). Power conversion efficiency as high as 2.48% was observed on the polymer PVs with a zwitterion-treated PEDOT:PSS film as the transparent anode.

Transparent and conductive single-walled carbon nanotube (SWCNT) films were fabricated through gel coating. A gel was formed by directly dispersing a large amount of SWCNTs in a liquid nonionic surfactant like polyoxyethylene (12) tridecyl ether without a solvent by mechanical grinding and subsequent ultrasonication. SWCNT films were obtained by coating the gels on glass substrates and subsequently removing the surfactant through heating. The SWCNT films became very thin and transparent after heating at a temperature between 480 and 520 °C. They exhibited good uniformity and a high transmittance-to-sheet resistance ratio which is attributed to the long SWCNTs dispersed in the gels. Films with a sheet resistance of around 250 Ω □−1 at 80% transmittance were obtained. The films can be easily transferred from rigid substrates like glass to flexible polymer substrates like polyethylene terephthalate.

Polymer solar cells (PSCs) with inverted structure can greatly improve the photovoltaic stability. This paper reports surface modification of indium tin oxide (ITO) by spin coating a thin layer of various sodium compounds. ITOs with such a treatment were used as the cathode of the inverted PSCs with poly(3-hexylthiophene) (P3HT) as the donor, [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as the acceptor, and MoO3/Al as the anode. Among these sodium compounds, sodium hydroxide (NaOH) gives rise to high photovoltaic performance: an open-circuit voltage (Voc) of 0.58 V, short-circuit current density (Jsc) of 10.03 mA cm−2, fill factor of 0.67, and power conversion efficiency of 3.89% under AM 1.5G illumination (100 mW cm−2). The efficiency is significantly higher than that (0.35%) of the control devices with untreated ITO as the cathode and even slightly higher than that (3.61%) of PSCs with normal architecture. The high performance of the inverted PSCs is due to the reduction of the work function of ITO by almost 1 eV by NaOH. The reduction in the work function of ITO was also observed by other sodium compounds, and it is consistent with the association constants of the anions of the sodium compounds with proton. The mechanism for the reduction of the work function is attributed to the dipole formation and alignment of the sodium compounds on the ITO surface.

We report highly efficient polymer solar cells (PSCs) with rhodamine 101, a conjugated zwitterion with positive and negative charges on the same molecule, as the electron-collection interlayer. Rhodamine 101 can be processed by solution processing techniques or thermal deposition. The rhodamine 101 interlayer simultaneously improves the short-circuit current, open-circuit voltage, fill factor so as to improve the photovoltaic efficiency of the PSCs with poly[[9-(1-octylnonyl)-9H-carbazole-2,7-diyl]-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl] (PCDTBT) and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as the active materials in comparison with control PSCs with Al or Ca/Al as the cathode. The photovoltaic efficiency reaches 6.15% for the PSCs with rhodamine 101/Al as the cathode under AM1.5G illumination, whereas the efficiencies are only 3.81% and 4.57% for the control PSCs with Al and Ca/Al as the cathodes, respectively. On the other hand, the improvement on the photovoltaic performance by the rhodamine 101 interlayer is less remarkable for the PSCs with poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) or PC71BM as the active materials. The photovoltaic efficiency is 4.25% for the PSCs with rhodamine 101/Al as the cathode, almost the same as that (4.20%) of the control PSCs with Ca/Al as the cathode. The high efficiency of the PSCs with the rhodamine 101 interlayer is ascribed to the lowering of the work function of metals by rhodamine 101 that has a strong dipole moment. The different effects of the rhodamine 101 interlayer on the two PSCs with PCDTBT and P3HT as the donor materials are attributed to different reactivities of the two polymers with active metals like Ca and Al. PCDTBT consisting of both electron-donating and electron-withdrawing units is more reactive with the active metals. The reaction between PCDTBT and Ca leads to low photovoltaic efficiency of the PSCs with Ca/Al as the cathode.

The conductivity of poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) films was significantly enhanced by preferential solvations of the hydrophobic PEDOT and hydrophilic PSS chains with cosolvents. When a PEDOT:PSS film prepared from the PEDOT:PSS aqueous solution was treated with water or a common organic solvent like ethanol, iso-propyl alcohol (IPA), acetonitrile (ACN), acetone, or tetrahydrofuran (THF), its conductivity did not change remarkably. But the conductivity was significantly enhanced when the PEDOT:PSS film was treated with a cosolvent of water and one of these common organic solvents. The conductivity enhancement was affected by several factors, including the ratio of the organic solvent to water, the dielectric constant of the organic solvent, and the temperature during the treatment. The conductivity enhancement from 0.2 S cm−1 to 103 S cm−1 was observed. The significant conductivity enhancement is attributed to the preferential solvation of PEDOT:PSS with a cosolvent. Water and the organic solvent of the cosolvent preferentially solvate the hydrophilic PSS and hydrophobic PEDOT chains, respectively. The preferential solvation of a PEDOT:PSS film with a cosolvent induces the phase separation of the insulator PSSH chains from the PEDOT:PSS film, aggregation of PSSH segments in the PEDOT:PSS film, and the conformational change of the PEDOT chains from coiled to linear. The cosolvent-treated PEDOT:PSS films were quite smooth and could be used to replace indium tin oxide (ITO) as the transparent electrode of electronic devices. Polymer photovoltaic cells (PVs) with the cosolvent-treated PEDOT:PSS films as the transparent electrode exhibited high photovoltaic performance.

Graphene has been attracting strong attention due to its interesting structure and properties and important applications in many areas. The process of the oxidation of graphite into graphene oxide (GO) and the subsequent reduction of GO into graphene is regarded as an effective process to produce graphene on a large scale. The quality of the reduced GO is strongly dependent on the reduction method. This paper reports the reduction of GO with Zn powder in neutral and alkaline aqueous solutions at room temperature. The reducing capability of Zn powder can be significantly improved through the complex formation of Zn2+ with other species in solution, which greatly lowers the Zn2+ concentration. Ethylenediaminetetraacetic acid (EDTA) can form an Zn–EDTA2− complex with Zn2+ and it is used for the reduction of GO by Zn in a neutral solution. The complex formation gives rise to quite low Zn2+ concentrations in solution. This effectively lowers the reduction potential of Zn/Zn2+ and enables the reduction of GO in neutral solutions. GO can be effectively reduced by Zn powder in alkaline solutions without EDTA as well. This is attributed to the complex formation of Zn2+ with OH−, where Zn2+ + 4OH−→ Zn(OH)42−. The reduced GO produced by these methods has high quality. Their C/O ratios for products obtained through GO reductions in neutral and alkaline solutions are 33.0 and 31.2 and their conductivities are 142 and 135 S cm−1, respectively.

Wearable electronic devices are becoming increasingly popular. They can bring a tremendous impact to human life. Wearable sensors, a class of wearable electronic devices, have attracted considerable attention because of their importance in healthcare. In this study, graphene-based wearable sensors, which can be directly integrated into clothes or textile products, are fabricated by a simple and cost effective method. Non-woven fabric (NWF) is considered as a green and low cost textile material, and its properties can be engineered for specific functions, such as medical systems, clothing, filter and packaging. A piece of NWF was dipped into graphene oxide (GO) solution and then reduced in HI acid. The GO reduction was confirmed by both X-ray photoelectron spectra and scanning electron microscopy. In the final product, reduced GO sheets were evenly coated on the NWF surface. The as-prepared graphene-NWF (GNWF) sensors exhibited a negative gauge factor at small strain. The signal has good reproducibility in response to stretching, bending and pressure. The highest gauge factor is −7.1 at 1% strain, and the highest sensitivity is 0.057 kPa−1. The GNWF sensors are able to respond to a series of human motions with the differentiation of various degrees of motions, and it can monitor small scale motions such as pulse and respiration.

Long term device stability and high power conversion efficiency (PCE) are important for practical application of dye-sensitized solar cells (DSCs). Here, we report the use of ethyl cellulose (EC) and acid functionalized multi-walled carbon nanotubes (oMWCNTs) as a co-gelator to gel organic solvents and the application of these gels for quasi-solid state DSCs. The gels are formed by blending EC and oMWCNTs with methoxypropionitrile (MPN) and acetonitrile (ACN). The loadings of EC and oMWCNTs can be as low as 4 wt% and 1.5 wt%, respectively, for the gel formation. The total minimum gelator loading of EC and oMWCNTs is lower than that with EC alone. In the absence of oMWCNTs, the EC loading must be more than 12 wt% for the gel formation. Gel electrolytes were prepared by adding iodine, 1-methyl-3-propylimidazolium iodide (PMII), guanidinium thiocyanate and 4-tert-butyl pyridine into the EC-oMWCNT/ACN-MPN gels, and they were used to fabricate quasi-solid state DSCs. The optimal PCE of the gel DSCs was 6.97% under AM1.5G illumination, quite close to that of control liquid DSCs, while the gel DSCs have better long term stability than their liquid-state counterparts. The former retains more than 98% of the initial PCE after 30 days, whereas the PCE of the latter decreases to about 80%.

The practical application of dye-sensitized solar cells (DSCs) requires high photovoltaic efficiency and good photovoltaic stability. This work reports nanocomposites of two-dimensional graphene oxide (GO) and one-dimensional multi-walled carbon nanotubes (MWCNTs) as the gelator of gel electrolytes for quasi-solid state DSCs. The composite gels are formed by immobilizing an organic solvent, 3-methoxypropionitrile (MPN), with GO and MWCNTs, and the gel electrolytes are prepared by adding iodine, 1-methyl-3-propylimidazolium iodide, guanidinium thiocynate and 4-tert butylpyridine into the GO–MWCNT–MPN composite gels. GO sheets can gel organic solvents because of their hydrophobic and hydrophilic domains. The MWCNTs can reinforce the solid networks formed by GO sheets and reduce the ionic diffusion length of the redox species within the electrolyte as MWCNTs are conductive and catalytic toward the electrochemical reduction of I3−. The presence of MWCNTs in the gel electrolyte increases both the open-circuit voltage (VOC) and the short-circuit current (JSC) of DSCs so as to increase the power conversion efficiency (PCE). The optimal PCE of the DSCs with GO–MWCNT–MPN gel electrolyte is 7.12% under AM 1.5G illumination (100 mW cm−2), which is significantly higher than that (6.54%) of the gel DSCs without MWCNTs.

Perovskite solar cells (PSCs) have attracted great attention due to their high power conversion efficiencies (PCEs) and low fabrication cost. The composition of the precursor solution determines the compositions of perovskite films. Excess precursor(s) may be used in the solution for the fabrication of perovskite films. However, it is still unclear how an excess precursor like PbI2 affects the structure and properties of the perovskite layer and the photovoltaic performance of PSCs. In this work, we investigated the effect of excess PbI2 that has a large bandgap on the electronic structure and properties of perovskite films and the photophysics and photovoltaic performance of PSCs. The presence of slightly excess PbI2 can affect the crystal structure and thus shift the Fermi level of perovskites. It can increase the open-circuit voltage (Voc) and thus the PCE of PSCs. However, the presence of a large amount of excess PbI2 is detrimental to the photovoltaic performance of PSCs. It can shorten the carrier lifetime, increase the resistance of the perovskite films, and decrease the fill factor (FF) and PCE of PSCs.