It is critical to monitor the state of health (SOH) of the Li-ion batteries to ensure a safe operation and to extend the service life of the batteries in electric vehicles. In this work, we demonstrated that the equivalent capacitance (Cp) and resistance (Rp) of the electrode interface derived using a first-order RC equivalent circuit under a large galvanostatic pulse (LGPM) condition can be correlated with SOH. For both the cathode and the anode, the interfacial kinetics of Li-ions were analyzed to study the electrochemical properties of active particles. The RC parameters of the equivalent circuit were correlated with the diffusion kinetics of Li-ions near the interface between the electrolyte and the active nano/micro-particles during fast charging/discharging. For fresh LiFePO4 (LFP)/Li half-cells, the values and the change of Cp and Rp were explained using the hypothesis of interparticle ion transport under a non-equilibrium condition. For graphite/Li half-cells, the buffering of Li-ions by the solid-electrolyte interphase (SEI) layer was speculated to affect Cp and Rp under a non-equilibrium condition. In commercial LFP/graphite batteries, the Cp values of unhealthy batteries were found to be higher than those of healthy batteries. In further tests, the Cp values of the half cells with the graphite anode recovered from the unhealthy batteries were found to be higher than those of the half cells with graphite from the healthy batteries. The half cells with LFP from the unhealthy batteries behaved similarly to those with LFP from the healthy batteries. With additional analysis on the microstructure, we proposed that the deterioration of the LFP/graphite batteries was mostly due to the formation of a thicker SEI on the graphite anode. The method developed in this work can be integrated in EVs at a low calculation cost. More importantly, we gained a better understanding of the interfacial kinetics of Li-ions during a non-equilibrium process.

•The NiCo2O4/C composite is successfully prepared by a facile solution route.•The NiCo2O4/C is applied as counter electrode for the reduction of triiodide in the dye-sensitized solar cells.•The NiCo2O4/C composite possesses a good electronic conductivity and high electrochemical activity.•The NiCo2O4/C CE shows the photovoltaic performance of 6.27% and the fill factor of 0.60.•Due to the synergistic effect between C and NiCo2O4, the NiCo2O4/C CE possesses both high efficiency and stability.NiCo2O4/carbon black (NiCo2O4/C) composite is successfully synthesized by a facile solution route, and used as counter electrode (CE) for dye-sensitized solar cells (DSCs). The DSCs based on NiCo2O4/C composite CE achieves a power conversion efficiency of 6.27%, which is much higher than that of NiO/C (5.07%), Co3O4/C (4.82%) or pristine C (4.34%). Also, the fill factors of DSCs devices with the NiCo2O4/C CE are better than that of other CEs. Compared to pristine C, the NiCo2O4/C composite has a marked improvement on electrocatalytic performance for the reduction of triiodide. Due to the synergistic effect between C and NiCo2O4, the NiCo2O4/C CE possesses both high efficiency and stability.

•Surface NH2-rich nanoparticle (A-SiO2) with a tightening network is synthesized.•A-SiO2 could solidify ionic-liquid electrolyte in much less amount (5 wt%).•The best efficiency of DSCs based on ionic-liquid gel electrolyte is up to 8.1%.The surface properties of nanoparticles have a significant influence on the properties of the gel electrolytes. Herein, the surface NH2-rich nanoparticle (A-SiO2), with a tightening network, is synthesized by silanizing SiO2 nanoparticles with pre-polymerized aminopropyltriethoxysilane, which is further employed to prepare ionic-liquid gel electrolytes for dye-sensitized solar cells. The addition of a small amount of A-SiO2 can effectively solidify the ionic-liquid, whereas a large number of NH2 groups on the SiO2 surface leads to a large negative shift of the TiO2 conduction band edge, and can react with I3− in the form of a Lewis complex, resulting in an increase in the concentration of I− and a decrease in the concentration of I3− in the electrolyte. In addition, the ionic-liquid gel electrolyte possesses thixotropic behavior, which allows it to easily penetrate into the inner part of the TiO2 mesoporous film. As a result, large improvements of the photovoltage from 695 mV to 785 mV and of the photocurrent from 13.3 mA cm−2 to 14.9 mA cm−2 are achieved. This leads to significant enhancement of the power conversion efficiency, from 6.2% to 8.1%, for the cell with A-SiO2 compared to that of the pristine ionic-liquid electrolyte.Download high-res image (402KB)Download full-size image

SnO2 is an important alternative to TiO2 for use as a semiconductor in dye-sensitized solar cell (DSSC) photoanodes. In this work, we prepared SnO2 and Al-doped SnO2 nanocrystals via a simple hydrothermal method, and found for the first time that the tuning of the conduction band and suppression of charge recombination are simultaneously improved in the Al-doped samples. The electron lifetime is significantly improved and the conduction band edge is shifted negatively by doping the SnO2 photoanode with Al. Compared to the undoped SnO2 DSSCs (AM 1.5, 100 mW cm−2), the power conversion efficiency (η) of the optimized Al-doped SnO2 DSSCs is enhanced by 75%. After being treated with TiCl4, the highest η of DSSCs based on Al-doped SnO2 nanocrystals is approximately 6.91%, which is a high overall photoconversion efficiency for SnO2-based DSSCs.

In this work, the interfacial properties of a series of metal-free organic naphthodithienothiophene (NDTT)-based photosensitizers adsorbed on TiO2 surfaces were investigated by a combination of ab initio calculations and experimental measurements. The calculations and experiments reveal that because of the efficient charge transfer from the adsorbed dyes to TiO2 nanocrystal surface there is an upward shift for the energy levels of dyes and a downward shift for the conduction band of surface TiO2 and that the band gaps for both of them are also reduced. Such electronic level alignments at the interface would lead to increased light absorption range by adsorbed dyes and increased driving force for charge injection but reduced open-circuit potential (Voc). More interestingly, we found that molecule engineering of the donor group and introducing additional electron-withdrawing unit have little effect on the electronic level alignments at the interface (because band gaps of the dyes adsorbed on TiO2 surfaces become approximately identical when compared with those of the dyes measured in solution) but that they can affect the steric effect and the charge separation at the interface to tune Voc and the short-circuit current density (Jsc) effectively. All these findings suggest that optimizing the interfacial properties of dyes adsorbed on TiO2 surfaces by synchronously modifying steric effects of dye molecules anchored on TiO2 and charge-transfer and separation properties at the interfaces is important to construct efficient dye-sensitized solar cells.Keywords: ab initio calculations; DSSCs; electronic structure; interface; metal-free photosensitizer; steric effect

A series of cyclometalated ruthenium complexes with bis(benzimidazolyl)benzene ligands were prepared and their applications in dye-sensitized solar cells are presented. The Ru(III/II) redox process of these complexes occurs at +0.71 V vs. Ag/AgCl. All complexes show broad absorptions extending into the near-infrared (NIR) region. The length of alkyl chains on the benzimidazole rings were found critical to the device performance. Sensitizer 4b with octyl substituents exhibits the best cell performance under the standard air mass 1.5 sunlight (η = 3.7%, Jsc = 9.85 mA cm−2, Voc = 555 mV, FF = 0.67). The device exhibits appreciable action in the NIR region between 700 and 850 nm.

In this paper, the pre-synthesized monodisperse PbS quantum dots were assembled on TiO2 films through direct adsorption. In consideration of the easy reaction between Pb-rich surface of PbS QDs and dissociative S2− of polysulfide electrolyte, a relatively mild post-treatment was selected to produce a CdS capping layer by chemical bath deposition route using thiourea as the sources of sulfur. Vis-NIR absorption spectra was adopted to confirm the change of the size and size distribution of the absorbed PbS QDs. Finally, the optimized treatment led to a stable panchromatic solar cell with 2.67% conversion efficiency and short circuit photocurrent density of 15.14 mA cm−2.

The TiO2, Sb-doped and Mg-doped TiO2 nanoparticles were prepared by the hydrothermal method and characterized by scanning electron microscopy (SEM), Energy Dispersive X-ray (EDX), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS). The photoelectric conversion efficiency of the dye-sensitized solar cells (DSSCs) with photoanodes based on the optimized structure of Sb-doped TiO2 layer/TiO2 layer/Mg-doped TiO2 layer achieved 8.84%, which is noticeably higher than that of the undoped (7.34%) and single doped DSSCs (8.49% for Sb-dopant and 8.15% for Mg-dopant respectively). The flat band potential of the optimized-structure films shifts from −0.49 V (vs.SCE) to −0.59 V (vs.SCE), which is beneficial to the increase of Voc. The higher transfer rate of electrons in the optimized-structure films than in the undoped TiO2 and single doped TiO2 films is confirmed by IMPS and IMVS measurements, which is favorable to the higher Jsc. This work shows that tuning the film structure through different dopants is a new method to enhance the performance of DSSCs and has good potential for application in photoenergy conversion devices.

•Oriented bamboo-type TiO2 NT arrays were prepared by AV anodization of Ti for DSSCs.•Increased dye loading in inner & outer walls due to bamboo structure were estimated.•Electron transfer time of TiO2 NT arrays was unaffected by the bamboo structure.•Cell efficiency of 7.36% was achieved due to higher dye loading.Highly ordered, bamboo type TiO2 nanotube (NT) arrays were prepared by anodization of Ti foils in an electrolyte of 0.5 wt% NH4F in ethylene glycol using square-wave voltage. The morphology of obtained arrays consisting of closely packed TiO2 NTs with inner diameter of 90 nm, wall thickness of 20 nm and length of 26 μm was characterized by scanning electron microscopy (SEM). Poly-crystalline anatase structure with crystallites of 28 nm in average size was determined by X-ray diffraction (XRD). Intensity-modulated photocurrent spectroscopy (IMPS) was employed to study the electron transport properties of TiO2 NT arrays used in dye-sensitized solar cells (DSSCs). Dye molecules were found to cover both the inner and outer surface of the bamboo type TiO2 NT arrays. Electrochemical impedance spectroscopy (EIS) showed reduced interfacial resistance and increased interfacial capacitance for the bamboo type TiO2 NT arrays compared with the smooth type arrays, indicating an increase in surface area of the bamboo type TiO2 NT arrays, which resulted in the higher dye loading. An increased conversion efficiency of 7.36% was achieved for the DSSCs based on the bamboo type TiO2 NT arrays attributing to the significant increase of dye loading resulted from the bamboo structure.

Mn3O4-anchored reduced graphene oxide (Mn3O4/RGO) nanocomposites have been successfully synthesized via a facile, effective, energy-saving technique. The surface morphology and structure of the Mn3O4/RGO composites were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR), high resolution transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), Brunauer–Emmett–Teller (BET) surface area and Barrett–Joyner–Halenda porosity (BJH). The composite material was applied as the counter electrode (CE) for dye-sensitized solar cells (DSSCs). Electrochemical impedance spectra (EIS), cyclic voltammetry (CV) and Tafel polarization measurements revealed a better electrocatalytic performance for the reduction of triiodide in Mn3O4/RGO based DSSCs, compared with the devices with reduced graphene oxide (RGO) CEs. The Mn3O4/RGO improves the fill factor by 56% and the power conversion efficiency of this kind of DSSC reaches 5.90%, which is much higher than the DSSC assembled from RGO CEs. A Pt counter electrode as a reference was also tested.

In order to increase the electron-donating ability of the donor part of the organic dye, the dithiafulvenyl (DTF) group as an additional electron donor is introduced into a triphenylamine (TPA) unit to form a donor–donor (2D) structure in this paper. Two new organic dyes WD9 (one 2-cyanoacetic acid as an electron acceptor) and WD11 (two 2-cyanoacetic acids as electron acceptors) containing this DTF–TPA 2D structure are designed, synthesized and applied in dye-sensitized solar cells (DSSCs). It is found that the double donor structures (DTF–TPA) not only contribute to enhancement of the electron-donating ability, but also inhibit aggregation between dye molecules and prevent I3− in the electrolyte from recombining with injected electrons in TiO2. Under AM 1.5G irradiation (100 mW cm−2), a maximum power conversion efficiency (η) of 4.09% is obtained for the WD11-based DSSC, higher than that of DTF-free dye L0 (η = 2.47%). These results have demonstrated that the incorporation of the DTF group into the organic dye will be an effective approach to develop high-performance metal-free organic dyes.

Vertically aligned TiO2 nanotube arrays (~8 μm long, ~110 nm wide) have been fabricated through anodic oxidation of Ti-metal sheet in fluoride-containing electrolyte. By changing the volume ratio of ethylene glycol and diethylene glycol in the electrolyte, TiO2 nanotube arrays with different tube-to-tube lateral spacing, i.e., closely packed, just separated, and fully separated, have been synthesized and applied as photoanodes for dye-sensitized solar cells (DSSCs). Photovoltaic efficiency of 2.99 %, 3.34 %, and 3.44 % has been obtained for DSSCs based on the closely packed, just separated, and fully separated TiO2 nanotube arrays, respectively, illustrating the effect of tube-to-tube lateral spacing of TiO2 nanotube arrays on the performances of DSSCs. It is suggested that fully separated TiO2 nanotube arrays are beneficial to the conversion efficiency of DSSCs due to higher dye loading and faster electron transfer.

A dual post-treatment was carried out by introducing CdS sandwiched between quantum dots (QDs) and ZnS, which leads to a large increase in photocurrent for CdS/CdSe sensitized solar cells, resulting from the improved electron injection from QDs to TiO2. As a result, a high solar-to-energy conversion efficiency of 5.47% was achieved.

A high-performance Pt-free counter electrode (CE) based on poly(3,4-ethylenedioxythiophene) (PEDOT) film for plastic dye-sensitized solar cells (DSCs) has been developed via a facile solid-state polymerization (SSP) approach. The polymerization was simply initiated by sintering the monomer, 2,5-dibromo-3,4-ethylenedioxythiophene (DBEDOT), at the temperature of 80 °C, which can be applied on the plastic substrate. The cyclic voltammetry measurements revealed that the catalytic activity of the SSP-PEDOT CE for triiodide reduction is comparable with that of the Pt CE. Under optimized conditions, the power conversion efficiency of a DSC with a N719-sensitized TiO2 photoanode and the SSP-PEDOT CE is 7.04% measured under standard 1 sun illumination (100 mW cm–2, AM 1.5), which is very close to that of the device fabricated under the same conditions with a conventional thermally deposited Pt CE (7.35%). Furthermore, taking advantage of the compatibility of the SSP-PEDOT with the plastic substrates, a full plastic N719-sensitized TiO2 solar cell was demonstrated, and an efficiency of 4.65% was achieved, which is comparable with the performance of a plastic DSC with a sputter-deposited Pt CE (5.38%). These results demonstrated that solid-state polymerization initiated at low temperature is a facile and low-cost method of fabricating the high-performance Pt-free CEs for plastic DSCs.Keywords: 2,5-dibromo-3,4-ethylenedioxythiophene; electrochemical impedance spectra; flexible dye-sensitized solar cells; plastic substrate; poly(3,4-ethylenedioxythiophene); solid-state polymerization;

Introducing oligo-organosiloxane grafting oligo(ethylene oxide-co-propylene oxide) dimethylamine (OEA) into PEO polymer electrolyte, at low degree loading (OEA:PEO = 0.05:0.95, w/w), improved remarkably the performance of solid state dye sensitized solar cell (SDSCs), resulting in an overall conversion efficiency of 3.06% under AM 1.5 illumination of 100 mW cm−2. The addition of OEA into PEO electrolyte not only significantly increased ionic conductivity and apparent diffusion coefficients of I3− in the electrolyte, but also promoted interfacial contacts of electrolyte/TiO2 and interfacial property of electrolyte/counter electrode. The quaternized OEA (OEI) was also introduced to the PEO electrolyte. XRD patterns revealed that the addition of small amount of OEA or OEI did not reduce the crystallinity of PEO much while its crystal orientation was preferentially directed. Besides, IR measurements illustrated the existence of hydrogen bond between the dimethylamine groups on OEA and the hydroxyl groups on PEO. The crystal orientation and also the hydrogen bond were deduced benefited the electrolytic properties and further the device photovoltaic performances.

•Sn-doped TiO2 were fabricated by the hydrothermal method with six Sn sources.•We study the effect of Sn sources on photoelectric properties of DSSCs.•Organic tin dopant source is a suitable tin source.•We also study the performance of DSSCs based on Ta-, Nb-, and Sb-doped TiO2.Six different Sn sources were applied to prepare Sn-doped TiO2 at low concentration (0.5 mol%) via hydrothermal method and its performance as the photoanode of dye-sensitized solar cells (DSSCs) was investigated. The Sn-doped TiO2 was characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). Compared with the pure TiO2 based DSSCs (7.61%), higher conversion efficiencies of 8.55% and 8.66% were achieved by the DSSCs based on the tetramethyltin doped and the tetra-n-butyltin doped TiO2, respectively. And they were found to improve the open-circuit voltage due to the negative shift of Vfb of TiO2 and enhance the short circuit current density due to the faster electron transport in the Sn-doped TiO2 films. Chloride ion, containing in Sn source, has a negative effect on DSSCs based on Sn-doped TiO2. IMVS and EIS measurements indicate that the charge recombination increases with increasing of Cl− doping content, and this is the main reason for the decline in efficiency. Based on these results, we further studied the performance of DSSCs based on Ta-, Nb-, and Sb-doped TiO2, and the results indicate that the metallo-organic compound is an appropriate doping source for the high performance of DSSCs.Sn-doped TiO2 was synthesized by hydrothermal method using six different Sn sources. High conversion efficiencies of 8.55% and 8.66% were achieved by the DSSCs based on the tetramethyltin doped and tetra-n-butyltin doped TiO2, where efficiencies improved by 12.4% and 13.8% compared with that of the cells based on pure TiO2. Metallo-organic compound could be an appropriate doping source for the DSSCs based on Sn-doped TiO2.

•TiO2 microspheres were synthesized by a simple and low cost method.•The microspheres size could be controlled by varying the adding amount of CTAB.•A possible formation mechanism of the microspheres was proposed.•A high photon to electricity conversion of 9.33% has been achieved.This report describes a facile method for preparing mesoporous TiO2 microspheres (TMs) for scattering layer in DSSCs. In this method, TMs can be synthesized by direct hydrolysis of TiCl4 in ethanol aqueous solution at 40 °C, and the microspheres size could be easily controlled by varying the adding amount of CTAB. The TMs were characterized by scanning electron microcopy (SEM), X-ray diffraction (XRD), and nitrogen sorption analysis. A possible formation mechanism of the TMs was proposed on the basis of time-dependent measurements by TEM. Photoanodes consisting of the TMs as scattering layers were then fabricated. The use of TMs largely enhances the dye loading and gives highly effective light scattering. Using TMs with a diameter of 400 nm (TM400), a high photon to electricity conversion of 9.33% has been achieved, indicating a 24% increase in the conversion efficiency compared to the photoanode without scattering layer (7.54%). The charge transport behavior of the cells was investigated by intensity-modulated photocurrent and photovoltage spectroscopic analysis (IMPS and IMVS), and the results indicates that the DSSCs with TM400 as a scattering layer has a higher ηcc (80.2%) value than the DSSCs without a scattering layer (71%). This method provides a feasible way for fabricating low-cost scattering layers in DSSCs.This report describes a simple and low-cost method for preparing mesoporous TiO2 microspheres (TMs) for scattering layer in DSSCs. The maximum photoelectric conversion efficiency (η) of 9.33% has been achieved by the photoanodes made from TMs with a diameter of 400 nm, indicating a 24% increase in the η compared to the photoanode without scattering layer (7.45%).

•Printable TiO2 paste was prepared by using an inorganic (Sb, In) doped SnO2 sol (SITO-sol) as crosslinking agent for the first.•The SITO-sol can also act as a kind of binder in the paste.•Porous and robust hybrid TiO2 film adheres strongly onto the plastic substrate.•The prepared photoanode shows enhanced electron transport property.•Plastic DSCs fabricated by the presented process show high efficiency up to 6.75%.The presented work demonstrates a novel technique to prepare printable TiO2 paste, employing a (Sb, In) doped SnO2 sol (SITO-sol) as a crosslinking agent and binder, to construct plastic DSCs. Porous and robust hybrid TiO2 films with enhanced conductivity and electrochemical properties were obtained using the newly developed TiO2 paste. Compared with DSCs prepared from a pristine TiO2 colloid, plastic DSCs fabricated by the presented process show a significant improvement (49%) in power conversion efficiency (6.75% vs. 4.54%). The mechanism for the enhanced performance was investigated.

Ca salts [CaSO4]-doped TiO2 electrodes prepared with well-optimized condition by the hydrothermal method show an increase in short-circuit (JSC), resulting in a power conversion efficiency (PCE) of 8.35%, which is higher than that of the undoped TiO2 thin film (7.33%). The doping of Ca shifts the flat band potential of TiO2 photoanode positively and increases the electron density indicated by the Mott–Schottky plot. The driving force of injecting electrons from the LUMO of the dye to the conduct band of TiO2 improves apparently because of the positive shift of the flat band potential. The intensity-modulated photocurrent spectroscopy measurement confirms that increased electron density accelerates the electrons transfer rate in the Ca-doped TiO2 thin films by comparison to undoped films.Highlights► Ca-doped TiO2 electrode showed a power conversion efficiency of 8.35%. ► The doping of Ca shifts the flat band potential of TiO2 photoanode positively. ► The driving force of injecting electrons is improved. ► We confirmed that increased electron density by measurement.

In order to increase the electron-donating ability of the donor part of the organic dye, two phenothiazine groups, as additional electron donors, were introduced into a triphenylamine unit to form a starburst donor–donor (2D) structure in this paper. Three new organic dyes (WD-6, WD-7 and WD-8) containing this starburst 2D structure and a 2-cyanoacetic acid acceptor linked by various conjugated linkers (benzene, thiophene, and furan) have been designed, synthesized and applied in dye-sensitized solar cells (DSSCs). The introduction of a phenothiazine group with a butterfly conformation in the triphenylamine donor parts has a good influence on preventing the molecular π–π aggregation due to the starburst 2D structure of the organic dye. The conjugated linker effects on the photophysical, electrochemical and photovoltaic properties of these organic dyes were investigated in detail. The DSSCs made with these organic dyes displayed remarkable overall conversion efficiencies, ranging from 4.90–6.79% under an AM 1.5 solar condition (100 mW cm−2). The best performance was found for organic dye WD-8, in which a furan group was the conjugated linker. It displayed a short-circuit current (Jsc) of 14.43 mA cm−2, an open-circuit voltage (Voc) of 682 mV, and a fill factor (ff) of 0.69, corresponding to an overall conversion efficiency of 6.79%. The different photovoltaic behaviors of the solar cells based on these organic dyes were further elucidated by the electrochemical impedance spectroscopy.

Two new triphenylamine-based starburst dyes (WD-2 and WD-3) are designed, in which the carbazole/phenothiazine groups are used as secondary electron donor and the rhodanine-3-acetic acid moiety as the acceptor. We report the synthesis, photophysical and electrochemical properties of the dyes as well as their applications in dye-sensitized solar cells (DSSCs). Under standard global AM 1.5 solar condition, the WD-2 sensitized cell gives a short circuit photocurrent density Jsc = 6.4 mA cm−2, an open circuit voltage Voc = 600 mV, a fill factor ff = 0.8, corresponding to an overall conversion efficiency of 3.1%. Under the same conditions, the WD-3 sensitized cell gives Jsc = 4.7 mA cm−2, Voc = 570 mV, and ff = 0.78, corresponding to an overall conversion efficiency of 2.1%. The conversion efficiency is increased about 47% from WD-2 sensitized cell to WD-3 sensitized cell. The research results show that the inefficient electron injection from the excited dyes into the conduction band of TiO2 results in the low efficiencies of DSSCs based on the two dyes.Graphical abstractHighlights► The phenothiazine group as secondary electron donor was introduced into organic dyes. ► The dyes with the starburst conformations can reduce aggregation between molecules and avoid the charge recombination processes of injected electrons with triiodide in the electrolyte. ► Power conversion efficiency of 3.1% was obtained for the DSSCs based on WD-2 without deoxycholic acid.

A novel thixotropic composite electrolyte was fabricated by using the amine group-functionalized silica nanoparticles as the gelator of an ionic liquid-based electrolyte for dye-sensitized solar cells. The SEM image of electrode deposited with electrolyte confirms that the high viscous gel electrolyte has a good contact with TiO2 electrode ascribed to the thixotropic property of the electrolyte. Based on the thixotropic and ionic liquid based-electrolyte containing the amine group-functionalized silica particles, the cells show better performance owing to the increased open-circuit voltage than that of the liquid device, yield a high energy conversion efficiency of 6.69% under AM 1.5 illumination at 100 mV cm−2 when the content of the amine group-functionalized silica is 15 wt.% and exhibit a long-term stability without sealing.Highlights► A novel thixotropic composite electrolyte was prepared by using MA-NH2 for DSSC. ► The value of Voc was increased steadily after adding the modified particles. ► Jsc first reached a maximum value, and then was almost identical to the reference. ► The efficiency of DSSC with this thixotropic gel electrolyte was up to 6.69%.

The TiO2 and Sb-doped TiO2 nanoparticles were prepared by the hydrothermal method and were characterized by high resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), UV–vis diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The photoelectric conversion efficiency of the dye-sensitized solar cells (DSSCs) based on 1 mol% Sb-doped TiO2 electrode achieved 8.13%, which is noticeably higher than that of the undoped DSSCs (7.36%). The improvement of the performance of Sb-doped DSSCs was attribute to the positive shift of the conduction band, which is beneficial to electron injection efficiency from the LUMO of the dye to the TiO2 conduction band. Intensity-modulated photocurrent and photovolatage spectroscopic analysis reveals that the electron transport and recombination have influence on the performance of the DSSCs.

A new kind of carboxyl group-functionalized silica nanoparticles were synthesized and employed as framework to form “soggy sand” electrolyte for dye-sensitized solar cells. The property of this electrolyte has been evaluated by studying the effect of the carboxyl group-functionalized silica nanoparticles content on the performance of dye-sensitized solar cells. The cells based on electrolyte containing the carboxyl group-functionalized silica nanoparticles show better performance owing to the increased short-circuit photocurrent than that of the liquid device and yield the energy conversion efficiency of 7.1% under AM 1.5 illumination at 100 mV cm− 2 when the content of the carboxyl group-functionalized silica is 15 wt.%.Highlights► The COOH-modified SiO2 particles (MA-COOH) were synthesized. ► The MA-COOH particles were employed to solidify ionic liquids for DSSCS. ► The value of JSC and Dapp of I3− were enhanced after adding the modified particles. ► The efficiency of DSSC with this composite gel electrolyte was up to 7.10%.

In dye-sensitized solar cells (DSSCs), as the excited electrons from dye molecules are injected to the conduction band of semiconductor film through the acceptor moieties, the acceptor groups have significant influences on the photovoltaic properties of the dyes. In this paper, the effects of different acceptor groups (cyanoacetic acid and rhodanine-3-acetic acid) in two phenothiazine-triphenylamine dyes (PTZ-1 and PTZ-2) on the optical, electrochemical properties and photovoltaic performances were studied. In comparison with PTZ-2, the photovoltaic performance of PTZ-1 is significantly improved by replacing rhodanine-3-acetic acid to cyanoacetic acid. The conversion efficiency of solar cell based on the PTZ-1 is increased about 110%. The lower efficiency of solar cell based on PTZ-2 is mainly because the delocalization of the excited state is broken between the 4-oxo-2-thioxothiazolidine ring and the acetic acid, which affects the electron injection from PTZ-2 to the conduction band of TiO2.Highlights► The phenothiazine-triphenylamine organic dyes with different acceptors were studied. ► The absorption of PTZ-2 with rhodanine-3-acetic acid shows an obvious red shift. ► PTZ-1 gives effective and faster electron injection from the LUMO to TiO2 electrode. ► PTZ-1 with cyanoacetic acid as acceptor shows the better photovoltaic performance.

A photoplatinization technique was proposed to deposit Pt on a thin TiO2 layer modified indium tin oxide-coated polyethylene naphthalate (ITO/PEN) substrate at low temperature (about 50 °C after 1 h of UV irradiation) for the first time. The fabrication process includes coating and hydrolyzing the tetra-n-butyl titanate to form a TiO2-modified layer and the photoplatinization of the modified substrate in H2PtCl6/2-propanol precursor solution under UV irradiation. The obtained platinized electrodes were used as counter electrodes (CE) for flexible dye-sensitized solar cells (FDSCs). The well-optimized platinized electrode showed high optical transmittance, up to 76.5% between 400 and 800 nm (Tav), and the charge transfer resistance (Rct) was as low as 0.66 Ω cm2. A series of characterizations also demonstrated the outstanding chemical/electrochemical durability and mechanical stability of the platinized electrode. The FDSCs with TiO2/Ti photoanodes and the obtained CEs achieved a power conversion efficiency (PCE) up to 8.12% under rear-side irradiation (AM 1.5 illumination, 100 mW cm–2). The obtained CEs were also employed in all-plastic bifacial DSCs. When irradiated from the rear side, the bifacial FDSC yielded a PCE of 6.26%, which approached 90% that of front-side irradiation (6.97%). Our study revealed that, apart from serving as a functional layer for deposition of Pt, the thin TiO2 layer modification on ITO/PEN substrates also played an important role in improving the transparency and the mechanical properties of the CE. The effect of the thickness of the TiO2 layer for Pt coating on the performance of the CE was also investigated.Keywords: electrocatalytic activity; flexible dye-sensitized solar cell; optical transparency; photoplatinization; platinized electrodes

A series of Sn-doped TiO2 with Sn content ranging from 0.25 to 1 mol % were successfully synthesized by the hydrothermal method, and its performance as the photoanode of dye-sensitized solar cells (DSSCs) was investigated. TEM and XRD results indicate that the doping has no effect on the morphology and the crystal form of TiO2. The shift of XRD peaks observed at higher angle and the XPS results indicate Sn4+ ions incorporation into the TiO2 lattice. The flatband potential of Sn-TiO2 films shifts from −0.505 V (vs SCE) to −0.55 V with increasing Sn content from 0 to 1 mol at. %, which is beneficial to the increase of Voc. The higher transfer rate of electrons in the Sn-doped TiO2 films than in the undoped TiO2 films is confirmed by IMPS measurements, which is favorable to the higher Jsc. IMVS and EIS measurements indicate that the charge recombination increases with increasing Sn doping content. Taking these factors, the optimum efficiency of 8.31% was found at 0.5 mol % Sn-doped TiO2 based DSSCs, which gave an efficiency improved by 12.1% compared with that of the cells based on pure TiO2 (7.45%). This work shows that Sn-doped TiO2 is a most interesting material and has good potential for application in photoenergy conversion devices.

We prepared highly-ordered titanium dioxide nanotube arrays (TNAs) by anodizing Ti foils in F−-containing electrolytes. The crystalline nature and morphology of the TNAs were studied using X-ray diffraction patterns and scanning electron microscopy. We found the morphology of TNAs affects the light-to-electricity conversion efficiency (η) of dye-sensitized solar cells (DSSCs). The efficiency of DSSCs reached 5.95% under the condition of light illuminated from the counter electrode. The high efficiency of TNA-based DSSCs was attributed to the neat top surface of TNAs, which allows more dye molecule loading on the surface of the TiO2 nanotubes, and fewer electron recombination centers and a low interface resistance of integrated TNAs.

Platinized electrodes were prepared by two-step constant current electrodeposition of Pt clusters on the Indium tin oxide-coated polyethylene naphthalate (ITO-PEN) substrates for use as counter electrodes in flexible dye-sensitized solar cells (DSSCs). The influence of the deposition conditions of two steps on the surface morphology of the deposited Pt was studied. Scanning electron microscopy and high-resolution transmission electron microscopy images showed the formation of well-separated and uniformly distributed Pt clusters on the substrates at optimum deposition conditions. The mean optical transmittance of the platinized electrodes was greater than 75% in the wavelength of 400–800 nm. The electrocatalytic activity for triiodide reduction was characterized by cyclic voltammetry and electrochemical impedance spectroscopy. Electrochemical reduction was used as an effective post treatment method for further improving the performance of electrocatalytic activity and optical transmittance. X-ray photoelectron spectroscopy analysis reveals an increase of the percentages of Pt(0) on the platinized electrode surface after post-treatment, leading to the improvement of the electrocatalytic activity. The energy conversion efficiency of 6.53 ± 0.45% for the flexible DSSC based on nanocrystalline TiO2/Ti photoelectrode and electrodeposited Pt/ITO-PEN counter electrode after electrochemical reduction was achieved under back illumination of 100 mW cm–2 (AM1.5).

We prepared highly ordered titanium dioxide nanotube arrays (TNAs) by anodizing Ti foils in F− containing electrolyte. The thickness and dye loading amount of TNAs were 26 μm and 1.06 × 10−7 mol cm−2, respectively. TiO2 nanoparticles (TNPs) were electrophoretically deposited on the inner wall of nanotube to produce coated nanotube arrays (TNAP). The dye loading was increased to 1.56 × 10−7 mol cm−2, and the electron transport rate improved. TNAs and TNAP were sensitized with ruthenium dye N3 to yield dye-sensitized TiO2 nanotube solar cells. The power conversion efficiency of TNA-based dye-sensitized solar cells (DSSCs) was 4.28%, whereas the efficiency of TNAP-based DSSCs increased to 6.28% when illuminated from the counter electrode. The increase of power conversion efficiency of TNAP-based DSSCs is ascribed to the increased surface area of TNAs and the faster electron transport rate.Highlights► TiO2 nanoparticles were electrophoretically deposited on TiO2 nanotube arrays. ► The charge transport rate increased after depositing TiO2 nanoparticles. ► The dye loading amount increased 47.2% after depositing TiO2 nanoparticles. ► The efficiency of dye sensitized solar cells increased from 4.28% to 6.28%.

1-Oligo(ethylene oxide)-3-methylimidazolium iodide (PEOMImI) was synthesized and applied to dye-sensitized solar cells (DSSCs) by blending it with different 1-alkyl-3-methylimidazolium iodides used as ionic liquid electrolytes. The 1-propyl-3-methylimidazolium iodide (PMII) blend enabled the DSSC to attain a higher solar energy conversion efficiency of 4.52% under a light intensity of 100 mW cm−2. The addition of N-methylimidazole (NMBI) to the electrolytes increased the conversion efficiency as compared to DSSCs based on NMBI-free electrolytes. The addition of both 1-allyl-3-methylimidazolium iodide (AMII) and NMBI enabled DSSCs to reach their highest solar energy conversion efficiency of 6.14% under a light intensity of 100 mW cm−2. The ionic conductivity and diffusion coefficient of the triiodide were found to be augmented dramatically after adding NMBI, which leads to an increase in the photocurrent density. The enhancement mechanism of NMBI in the electrolyte was investigated by Raman spectroscopy and differential scanning calorimetry, and it was mainly due to the enhancement of electron exchange in electrolytes.Highlights► Ionic liquid oligomer (PEOImI) is applied as electrolyte for DSSC to improve its efficiency. ► 1-Alkyl-3-methylimidazolium iodide is mixed with PEOImI to form electrolyte. ► The efficiency is further improved by adding N-methylimidazole (NMBI). ► NMBI enhances the Grotthus type diffusion of triiodide. ► A highest energy conversion efficiency of 6.14% has been attached at 100 mW cm−2.

The acidic ionic liquid polymer P [((3-(4-vinylpyridine) propanesulfonic acid) iodide)-co-(acrylonitrile)], which is named as P-HI for short, has been employed in ionic liquid electrolyte for dye-sensitized solar cells. The novel acidic ionic liquid polymer/ionic liquid composite polymer electrolyte does not contain iodine, and the performance of this electrolyte has been evaluated by studying the content of P-HI and the iodine concentration on the performance of dye-sensitized solar cells. The cell based on electrolyte containing 20 wt.% P-HI yields an overall energy conversion efficiency of 6.95% under AM 1.5 illumination at 100 mW cm−2.

A kind of polymer–metal complex gel electrolyte is successfully prepared and is used in dye-sensitized solar cells. Raman and X-ray photoelectron spectroscopy confirm the structure of this complex and is found that the metal ion reacts with nitrogen in the polymer. This novel electrolyte shows apparent diffusion coefficient of iodide of 8.37 × 10−7 cm2 s−1 and the energy conversion efficiency of 6.10% when the amount of ZnI2 is 0.04 M. By studying the dissociation active energy of the inorganic salt in electrolytes, we find that the metal salts can dissociate more easily after reacting with polymer and as a result can provide extra free iodide ion. The cell maintains ca. 93% of its initial efficiency after 20 d without further sealing, which shows good long-time stability.

Pt counter electrodes (CEs) with different platinum loading have been prepared using chemical reduced method on flexible indium-doped tin oxide coated polyethylene naphthalate (ITO-PEN) for dye-sensitized solar cells (DSSCs). H2PtCl6·6H2O terpineol solutions were screen printed on the transparent ITO-PEN substrates. After drying, H2PtCl6 was reduced by treating it in NaBH4 solution followed by the hydrothermal treatment at 100 °C. The obtained Pt CEs with different Pt-loading (2.4–7.7 μg/cm2) were characterized by SEM, XPS, electrochemical impedance and transmission spectrum measurement. The Pt CEs show high catalytic activity, low charge transfer resistance (0.26–1.38 Ω cm2) and good light transmittance (about 70% at 400–800 nm). The light-to-electricity conversion efficiency of the flexible DSSC fabricated with the prepared Pt CE and the TiO2 photoanode prepared on Ti substrate by screen printing technique attains 5.41% under the simulated AM 1.5 sunlight, which is almost same as that based on the thermal decomposited Pt CE on FTO-glass. Compared with other methods to prepare Pt CEs, chemical reduced method is simple and suitable for flexible polymer substrates and the large scale preparation of DSSCs.

Poly(vinylpyridine-co-ethylene glycol methyl ether methacrylate) (P(VP-co-MEOMA)) and α,ω-diiodo poly(ethylene oxide-co-propylene oxide) (I[(EO)0.8-co-(PO)0.2]yI) were synthesized and used as chemically cross-linked precursors of the electrolyte for dye-sensitized solar cells. Meanwhile, α-iodo poly(ethylene oxide-co-propylene oxide) methyl ether (CH3O[(EO)0.8-co-(PO)0.2]xI) was synthesized and added into the electrolyte as an internal plasticizer. Novel polymer electrolyte resulting from chemically cross-linked precursors was obtained by the quaterisation at 90 °C for 30 min. The characteristics for this kind of electrolyte were investigated by means of ionic conductivity, thermogravimetric and photocurrent–voltage. The ambient ionic conductivity was significantly enhanced to 2.3 × 10−4 S cm−1 after introducing plasticizer, modified-ionic liquid. The weight loss of the solid state electrolyte at 200 °C was 1.8%, and its decomposition temperature was 287 °C. Solid state dye-sensitized solar cell based on chemically cross-linked electrolyte presented an overall conversion efficiency of 2.35% under AM1.5 irradiation (100 mW cm−2). The as-fabricated device maintained 88% of its initial performance at room temperature even without sealing for 30 days, showing a good stability.

A novel low temperature method was used to prepare the mesoporous carbon (MC) counter electrode (CE) on indium-doped tin oxide coated polyethylene naphthalate (ITO-PEN) for flexible dye-sensitized solar cells (DSSCs). The obtained flexible MC CEs with carbon loading of 280 μg cm−2 were characterized by SEM, XRD and electrochemical impedance. The light-to-electricity conversion efficiency of the DSSC fabricated with the prepared flexible MC CE was 86% of that of DSSC based on the decomposited Pt CE.

Nanocomposite polymer electrolytes containing poly(ether urethane) (PEUR)/poly(ethylene oxide) (PEO)/modified SiO2 were prepared for all-solid-state dye-sensitized solar cells with a high efficiency of 4.86% and an active area of 0.25 cm2 under AM1.5 conditions at 100 mW cm−2 irradiation.

A poly(ether urethane) (PEUR)/poly(ethylene oxide) (PEO)/SiO2 based nanocomposite polymer is prepared and employed in the construction of high efficiency all-solid-state dye-sensitized nanocrystalline solar cells. The introduction of low-molecular weight PEUR prepolymer into PEO electrolyte has greatly enhance the electrolyte performance by both improving the interfacial contact properties of electrode/electrolyte and decreasing the PEO crystallization, which were confirmed by XRD and SEM characteristics. The effects of polymer composition, nano SiO2 content on the ionic conductivity and I3− ions diffusion of polymer-blend electrolyte are investigated. The optimized composition yields an energy conversion efficiency of 3.71% under irradiation by white light (100 mW cm−2).

Two latent chemical cross-linked precursors: (1) oligo-organophosphazene with propionitrile and iodo-(ethylene oxide-co-propylene oxide) and (2) oligo-organosiloxane grafting oligo-ethylene oxide and propylene oxide dimethylamine are synthesized to form all-solid-state polymer electrolyte for dye-sensitized solar cells employing in situ quaterization reaction. This novel electrolyte shows about 6.79 × 10−7 cm2 s−1 of the apparent diffusion coefficient of triiodide. The disintegrating temperature of 181 °C demonstrates good thermal stability of the solid-state electrolyte. Proper amount of 7 nm SiO2 modified with 3-triethoxysilylpropyl oligo(oxyethylene-co-oxypropylene) monomethyl urethane is added to the electrolyte and the photocurrent conversion efficiency reaches 2.66% compared with that of 1.81% with no additive.

Chemical fixing of xanthene dye (eosin Y) on the surface of TiO2 electrode was carried out by modifying the electrode with silane-coupling reagent to obtain stable dye-sensitized TiO2 electrode. Such silane modification can not only evidently enhance the stability of dye-sensitized TiO2 electrode but also improve the energy conversion efficiency of the assembled cells by increasing short-circuit photocurrent (JSC) and open-circuit photovoltage (VOC). It was found that the improvements of cell performances differ depending on the composition of the electrolyte. The optimum cell of the cell performance was achieved in the electrolyte with 0.5 mol/L TBAI/0.05 mol/L I2/EC:PC(3:1 w/w), yielding JSC of 4.69 mA·cm−2, VOC of 0.595 V, fill factor (FF) of 0.64 and η of 1.78%. Different spectroscopic techniques including UV-Vis spectra, fluorescence spectra, EIS and dark current measurements were employed to derive reasonable analysis and explanations.

The electrophoretic deposition combined with common pressure hydrothermal treatment was employed to prepare nanocrystalline TiO2 thin film from suspension of tetra-n-butyl titanate and P25 at low temperature. The tetra-n-butyl titanate was hydrolyzed and crystallized into anatase to interconnect nanocrystalline TiO2 particles and to stick them to a conductive substrate by common pressure hydrothermal treatment to improve the electron transport properties of the deposited thin film. A dye-sensitized solar cell based on TiO2 thin film prepared by the low temperature method yielded the conversion efficiency of 6.12%. Due to the relative slower electron transport rate in the deposited film, its conversion efficiency was slightly lower than that of the cell with TiO2 thin film prepared by the conventional high temperature sintering method. Since it is free of high temperature sintering step, this method can be used to prepare nanocrystalline TiO2 thin film on plastic polymer conductive substrate for fabrication of flexible dye-sensitized solar cell.

New kinds of additive, 4-alkyloxypyridne derivatives, were synthesized by introducing an alkyloxy group into the 4-position of 2-methylpyridine. The influence of these electrolyte additives on the short-circuit photocurrent (Jsc) of dye sensitized solar cells was investigated by combining electrochemical and spectral techniques. With the addition of pyridine derivatives to the electrolyte, a decrease in the rate of dye regeneration was observed by laser flash photolysis measurements and cyclic voltammetry, whereas, measurement of electrochemical impedence spectra showed an increase in the charge transfer resistance due to the formation of a complex between the pyridine derivatives and iodine, as identified by an absorption peak around 378 nm in the UV–Vis spectra. This leads to a decrease in Jsc of dye-sensitized solar cells. This adverse effect on the Jsc can be attributed to reaction or coordination between the dye cations and the iodine in the electrolyte.

A new kind of polythiophene derivative, Poly(3-{2-[4-(2-ethylhexyloxy)-phenyl]-vinyl}-2,2′-bithiophene) (PTh), was applied in dye-sensitized solar cell to extend the light response of nanocrystalline TiO2 electrode. UV-vis absorption and fluorescence spectra were employed to investigate the interaction of PTh with nanocrystalline TiO2. The absorption coefficient of the PTh was high in visible part of spectrum, and the fluorescence emission of the PTh can be efficiently quenched by TiO2 nanoparticles owing to charge injection from the excited singlet state of PTh to the conduction band of the TiO2 particles. Cyclic voltammetry measurements were performed to study the dye regeneration reaction at the nanocrystalline TiO2/electrolyte interface. The solar cell sensitized with PTh exhibited a short-circuit photocurrent (Isc) of 3.08 mA/cm2, an open circuit voltage (Voc) of 511 mV and an overall efficiency of 0.9% under the illumination of 100 mW/cm2 (AM 1.5).

Cauliflower-like TiO2 rough spheres, which are about 200 nm large, have greatly enhanced light harvesting efficiency and energy conversion efficiency of dye-sensitized solar cells (DSC), due to their high light scattering effect and large BET surface area (80.7 m2 g−1) even after calcinations at 450 °C for 30 min. The large size TiO2 rough and smooth spheres, produced at different initial temperatures by hydrolysis of Ti(OBu)4 with P105 (EO37PO56EO37) or F68 (EO78PO30EO78) tri-block copolymer as structural agents, have nearly the same diameter of ∼275 nm and strong light scattering effects in the wavelength of 400–750 nm. However, rough spheres have even higher light scattering effect and larger BET surface area than smooth spheres for the roughness of the surface. By adding 25 wt.% large TiO2 spheres into the over-layer of TiO2 film composed of ∼20 nm TiO2 particles as light scattering centers, the energy conversion efficiency of the film containing rough spheres reaches 7.36%, much larger than that of smooth spheres (6.25%). From another point of view, the TiO2 rough spheres may have the satisfying ability in other fields of application such as photo-catalysis, drug carriers and so on.

Quasi-solid-state electrolytes were prepared by employing the imidazole polymers to solidify the liquid electrolyte containing lithium iodide, iodine and ethylene carbonate (EC)/propylene carbonate (PC) mixed solvent. The ionic conductivity and diffusion behavior of triiodide in the quasi-solid-state electrolytes were examined in terms of the polymer content. Application of the quasi-solid-state electrolytes to the dye-sensitized solar cells, the maximum energy conversion efficiency of 7.6% (AM 1.5, 100 mW cm−2) was achieved. The dependence of the photovoltaic performance on the polymer content and on the different anions of the imidazole polymers was studied by electrochemical impedance spectroscopy and cyclic voltammetry. The results indicate the charge transfer behaviors occurred at nanocrystalline TiO2/electrolyte and Pt/electrolyte interface play an important role in influencing the photovoltaic performance of quasi-solid-state dye-sensitized solar cells.

4-Ethoxy-2-methylpyridine (EOP), as an electrolyte additive, was added into electrolyte of dye-sensitized solar cell, and its effect on the performance of solar cells and mechanism were studied by electrochemical and spectral measurements. With increasing in EOP concentrations, the flatband potential of TiO2 electrode shifted negatively and the rate of interfacial recombination at the TiO2 electrode/dye/electrolyte interfaces was improved. The net effect of EOP was to improve the open-circuit photovoltage, indicating that the negative shift of conduction band of TiO2 was the predominated factor on determining the open-circuit photovoltage. The decline of short-circuit current of solar cell was ascribed to the slow dye regeneration and the low catalytic activity of counter electrode on triiodide reduction due to addition of EOP.

Different paste has been used for preparing porous TiO2 thin film by screen-printing technique, the main component of it comes from commercial TiO2 P25 power. The dye-sensitized solar cell based on this TiO2 thin film without further chemical treatments exhibits high overall conversion efficiency of 5.81%–6.70%, even with low TiO2 content and thin film thickness. The experimental repeatability is nice and the properties of the films are uniform.

SnO2 is an important alternative to TiO2 for use as a semiconductor in dye-sensitized solar cell (DSSC) photoanodes. In this work, we prepared SnO2 and Al-doped SnO2 nanocrystals via a simple hydrothermal method, and found for the first time that the tuning of the conduction band and suppression of charge recombination are simultaneously improved in the Al-doped samples. The electron lifetime is significantly improved and the conduction band edge is shifted negatively by doping the SnO2 photoanode with Al. Compared to the undoped SnO2 DSSCs (AM 1.5, 100 mW cm−2), the power conversion efficiency (η) of the optimized Al-doped SnO2 DSSCs is enhanced by 75%. After being treated with TiCl4, the highest η of DSSCs based on Al-doped SnO2 nanocrystals is approximately 6.91%, which is a high overall photoconversion efficiency for SnO2-based DSSCs.

A dual post-treatment was carried out by introducing CdS sandwiched between quantum dots (QDs) and ZnS, which leads to a large increase in photocurrent for CdS/CdSe sensitized solar cells, resulting from the improved electron injection from QDs to TiO2. As a result, a high solar-to-energy conversion efficiency of 5.47% was achieved.

In order to increase the electron-donating ability of the donor part of the organic dye, two phenothiazine groups, as additional electron donors, were introduced into a triphenylamine unit to form a starburst donor–donor (2D) structure in this paper. Three new organic dyes (WD-6, WD-7 and WD-8) containing this starburst 2D structure and a 2-cyanoacetic acid acceptor linked by various conjugated linkers (benzene, thiophene, and furan) have been designed, synthesized and applied in dye-sensitized solar cells (DSSCs). The introduction of a phenothiazine group with a butterfly conformation in the triphenylamine donor parts has a good influence on preventing the molecular π–π aggregation due to the starburst 2D structure of the organic dye. The conjugated linker effects on the photophysical, electrochemical and photovoltaic properties of these organic dyes were investigated in detail. The DSSCs made with these organic dyes displayed remarkable overall conversion efficiencies, ranging from 4.90–6.79% under an AM 1.5 solar condition (100 mW cm−2). The best performance was found for organic dye WD-8, in which a furan group was the conjugated linker. It displayed a short-circuit current (Jsc) of 14.43 mA cm−2, an open-circuit voltage (Voc) of 682 mV, and a fill factor (ff) of 0.69, corresponding to an overall conversion efficiency of 6.79%. The different photovoltaic behaviors of the solar cells based on these organic dyes were further elucidated by the electrochemical impedance spectroscopy.