•The S-MoOx xerogel is synthesized via sol-gel derived method.•S-MoOx solution and film feature superior solubility, morphology and electronic properties.•Performance comparable to device with PEDOT:PSS HTL achieved using sol-gel derived S-MoOx.•Wider temperature tolerant from room temperature to 250 °C for this S-MoOx layer.•Dramatically enhance the stability of the OSCs using S-MoOx replacement of PEDOT:PSS.While solution processable MoOx films (S-MoOx) are typically sensitive to thermal treatment temperature and oxygen, we report an approach with the features of a wide temperature tolerance, water-free and solution-processed S-MoOx film for organic solar cells (OSCs), The structural, morphological, electronic properties of synthesized S-MoOx film were studied in detail by using field emission scanning electron microscopy (SEM), Contact angle meter, UV–visible Spectrophotometer (UV–vis), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and scanning kelvin probe force microscopy (SKPFM). These analysis shows that the S-MoOx thin films possess the suitable morphology and electronic properties for application in OSCs. The S-MoOx film shows wide temperature tolerance from room temperature to 250 °C. The S-MoOx was introduced into OSCs in normal and inverted configuration by acting as anode interface layer. The devices shows relatively high performance (ca 7.40%) based on varying temperature treated S-MoOx. The stability of the OSCs is dramatically enhanced via using the S-MoOx layers replace the conventional PEDOT:PSS layer both in air and in nitrogen atmosphere.

•The s-MoOx xerogel is synthesized via sol-gel derived method.•The s-MoOx xerogel has good solubility in alcohol solvent.•s-MoOx solution and film feature superior solubility, morphology and electronic properties.•Performance comparable to device with PEDOT:PSS HTL achieved using sol-gel derived s-MoOx.•Dramatically enhance the stability of the OSCs using s-MoOx replacement of PEDOT:PSS.Water-free solvent soluble, low-temperature processed metal oxides are important for preparing efficient and stable electronic devices, as well as the convenience in simplifying the device production process. Here we reported a facile approach with the features of low-temperature and solution-based process for the formation of a MoOx (s-MoOx) film as interface layer in polymer solar cells (PSCs). The absorbability, elementary composition, electronic property and surface microstructure of the s-MoOx are investigated in detail by ultraviolet–visible spectrophotometer (UV–vis), X-ray photoelectron spectrometry (XPS), ultraviolet photo-electron spectrometer (UPS) and atomic force microscopy (AFM). These investigations confirmed that such MoOx xerogel has high solubility in the organic alcohol solvents, such as ethanol and methanol. Meanwhile, this s-MoOx can be applied as the interfacial layer in organic solar cells via a low-temperature treatment (about 100 °C) due to its proper physical properties, and a power conversion efficiency (PCE) over 3% was achieved. In addition, the devices with s-MoOx shows excellent air-stability, and the PCE efficiency can maintain about 84% of its initial value after 100 h exposure in air, which is dramatically enhanced comparing with the common devices with PEDOT:PSS layer.

•A novel, low-temperature, solution-processable MoOx is facile synthesized.•MoOx features superior solubility, film morphology and electronic properties.•Highly efficient OLED is demonstrated with solution-processed MoOx.•The mechanism of MoOx promoting OLED performance is systematically clarified.Low cost, high throughout and large scale production techniques, such as roll-to-roll, printing and doctor blading, boost the favorite of electronic devices with all solution process. While MoOx are conventionally formed via high temperature and vacuum deposition, we develop a novel, lower-temperature, solution-processable MoOx hole injection layer (HIL) and cast successful application in organic light-emitting diodes (OLEDs). The characterization of MoOx is presented in detail using X-ray diffraction, atomic force microscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy and impedance spectroscopy measurements. The results show MoOx features amorphous phase structure, superior film morphology and exceptional electronic properties. With solution-processed MoOx as HIL, highly efficient OLED is demonstrated. The luminous efficiency has been enhanced by 56% in comparison with that of the counterpart using evaporated MoOx. The main reasons for the substantially improved performance are the tailored surface work function and appropriate hole injection capacity correspondingly result in optimizing carrier balance in OLED. Our results pave a way for advancing MoOx-based organic electronic devices with solution-processable techniques.

The interlayer inserted between the active layer and ITO has been demonstrated to be crucial for the performance of inverted polymer solar cells (i-PSCs). In this work, we find that ionic liquids (ILs) can significantly enhance the efficiency of i-PSCs. With the ZnO/IL interfacial layer, PTB7-Th:PC71BM i-PSCs can exhibit a champion power conversion efficiency (PCE) of 10.15%, which is among the highest PCEs reported thus far for single-junction bulk heterojunction solar cells through the solution process. The IL layer and ZnO/IL combination layer with low work function, good optical transmittance, improved electron extraction and reduced resistance at the cathode interface have been demonstrated to be excellent and general interfacial layers for i-PSCs.

•Thickness controllable graphene oxide (GO) film was prepared via layer-by-layer assembly technique.•The performance of the device depends on the deposition layers of polyelectrolyte and GO.•PCE of 6.04% was achieved, which is comparable to that of PEDOT:PSS device.Graphene oxide (GO) is widely used as an interfacial material in applications such as organic light emitting diodes and photovoltaic devices. Herein we report a layer-by-layer (LbL) assembled GO thin film as an anode interfacial layer (AIL) for efficient polymer solar cells (PSCs). The GO thin film is fabricated by alternately depositing cationic polyelectrolyte poly(diallyldimethylammonium chloride) (PDDA) and GO on ITO/glass substrate, which possesses controllable thickness by adjusting LbL deposition frequency. The presence of ultrathin GO films improves the work function of ITO, leading to a better contact between the active layer and ITO anode. With the optimized number of deposition times, the efficiency of 6.04% for the PSC with PDDA-GO bilayer (GO-2) as the AIL was achieved.

We assembled a ternary blend bulk heterojunction polymer solar cell (PSCs) containing P3HT (donor) and PC61BM (acceptor) incorporated with a dihexyl-quaterthiophene (DH4T) small molecule oligomer as a third component. By optimizing the contents of DH4T, we increased the power conversion efficiency of ternary P3HT:DH4T:PC61BM PSCs to 4.17% from 3.44% of binary P3HT:PC61BM PSCs under AM 1.5 G of 100 mW/cm2 intensity. The major improvement is from the increase of the short circuit current and fill factor that is due to the increased light absorption at short wavelength, the balanced charge carrier transportation and the enhanced hole evacuation by a DH4T-enriched layer at the anode interface. In this work, we demonstrate that the efficiency of the PSCs can be enhanced by using low-bandgap conjugated polymer and its oligomer as donors and fullerene derivatives as acceptors.

•The inserted of BenMeIm-Cl ionic liquids as cathode interface layer for inverted polymer solar cells.•The ionic liquids own the advantages of low temperature fabrication, variety category, simple synthesis and environment-friendly nature.•The nanoscale thickness of ionic liquids can be controlled by a spontaneous vertical phase separation self-assembled structure.•This one-step self-assembled procedure greatly simplifies the fabrication of IPSCs.The interlayer inserted between active layer and ITO has been a key issue for improving electron extraction in inverted polymer solar cells (IPSCs), while the ideal interlayer for IPSCs has not been well developed. In this work, we presented a spontaneous vertical phase separation (SVPS) self-assembled bilayers structure with BenMeIm-Cl ionic liquid (IL) interfacial bottom layer and a photoactive top layer via a single spin-coated step of BenMeIm-Cl IL and organic donor–acceptor composite and achieved a PCE as high as 8% based on IPSCs with PTB7 as the donor. The presence of BenMeIm-Cl IL reduces the work function of ITO and leads to a better energy-level matching for efficient charge-transfer. The driving force of SVPS self-assembled structure is from the relative surface energy difference between organic materials and BenMeIm-Cl ILs, together with their interactions with the substrates. This self-assembled process procedure pave the way to simplify the manufacturing of low-cost and large-area organic electronic devices.

A novel non-fullerene small molecule electron acceptor TTzBT-DCAO, which contains all electron-withdrawing units of 2,1,3-benzothiadiazole, oligothiazole and alkyl cyanoacetate, has been synthesized and characterized. Its photophysical, electrochemical, and photovoltaic properties have been investigated. The material has favorable HOMO and LUMO levels of −5.88 and −3.60 eV, and shows strong absorption in the visible spectrum up to 650 nm. The small molecule:non-fullerene bulk-heterojunction organic photovoltaics (OPVs) were constructed based on two small molecules SF8TBT and TTzBT-DCAO. The influence of the donor:acceptor composition on device performance was investigated. The open-circuit voltages of the devices are over 1.20 V, which is among the highest values reported for single-junction OPVs. The results indicate that small molecules with all electron-withdrawing units could provide a novel route to efficient solution-processed OPVs with high open-circuit voltages.

PTB7:PC71BM inverted polymer solar cells with a room-temperature TiOx/PEI electron transport layer achieve an average power conversion efficiency of 8.72% (the champion power conversion efficiency is 9.08%), which is much better than that of the control devices based on PEI (7.00%) or TiOx (7.38%). The room-temperature TiOx/PEI layer exhibits outstanding capacities, including increased electron mobility, reduced series resistance and improved electron extraction at the cathode interface.

The formation of interconnected phase-separated domains on sub-20 nm length scale is a key requirement for all-polymer solar cells (all-PSCs) with high efficiency. Herein, we report the application of crystalline poly(3-hexylthiophene) (P3HT) nanowires via an O-dichlorobenzene/hexane mixed solution blended with poly{(9,9-dioctylfluorenyl-2,7-diyl)-alt-[4,7-bis(3-hexylthiophen-5-yl)-2,1,3-benzothiadiazole]-2′,2″-diyl} (F8TBT) for the first time. The nanomorphology of P3HT:F8TBT all-PSCs can be controlled by P3HT nanowires. The improved film morphology leads to enhanced light absorption, exciton dissociation, and charge transport in all-PSCs, as confirmed by ultraviolet–visible absorption spectra, X-ray diffraction, transmission electron microscopy, atomic force microscopy, and time-resolved photoluminescence spectra. The P3HT nanowire:F8TBT all-PSCs could achieve a power conversion efficiency of 1.87% and a Voc of 1.35 V, both of which are the highest values for P3HT:F8TBT all-PSCs. This work demonstrates that the semiconductor nanowires fabricated by the mixed solvents method is an efficient solution process approach to controlling the nanomorphology of all-PSCs.Keywords: all-polymer solar cell; mixed solvents; nanoaggregation; nanomorphology; P3HT:F8TBT;

The power conversion efficiency of PTB7:PC71BM polymer solar cells was improved to 9.1% by treatment with methanol followed by a water- and alcohol-soluble conjugated polyelectrolyte cathode interface layer. This improvement in efficiency is a result of a combination of an enriched PC71BM ratio on the top surface in the active layer and the presence of the preferred dipole at the cathode interface.

•The design of low bandgap polymers of benzo[1,2-b:4,5-b′]dithiophene with fused thiadazole quinoxalines•Polymers with an extremely low bandgap of ~ 1.2 eV, extending the absorption to 1100 nm for films•Polymer solar cells with a 0.52% power conversion efficiencyTwo copolymers comprised of different fused thiadiazole electron withdrawing units and benzo[1,2-b:4,5-b′]dithiophene electron donating unit have been synthesized by Stille reaction. The structural, optical, electrochemical and photovoltaic properties of the copolymers were investigated. Both copolymers in the film state exhibited abroad absorption spectra with an extremely low bandgap of ~ 1.2 eV. Bulk heterojunction solar cells were further fabricated and the best device delivered a power conversion efficiency of 0.52% when thermal annealed at 90 °C.

Five 4,7-dithien-2-yl-2,1,3-benzothiadiazole (DTBT)-based conjugated copolymers with controlled molecular weight were synthesized to explore their optical, energy level and photovoltaic properties. By tuning the positions of hexyl side chains on DTBT unit, the DTBT-fluorene copolymers exhibited very different aggregation properties, leading to 60 nm bathochromic shift in their absorptions and the corresponding power conversion efficiencies (PCEs) value of photovoltaic cells varied from 0.38%, 0.69% to 2.47%. Different copolymerization units, fluorene, carbazole and phenothiazine were also investigated. The polymer based on phenothiazine exhibited lower PCE value due to much lower molecular weight owing to its poor solubility, although phenothiazine units were expected to be a better electron donor. Compared with the fluorene-based polymer, the carbazole-DTBT copolymer showed higher short circuit current density (Jsc) and PCE value due to its better intermolecular stacking.

TiO2 thin films deposited by magnetron sputtering possess excellent optical transmittance, high refractive index, good adhesion and chemical stability. In this manuscript, TiO2 thin films deposited by magnetron sputtering was used for the first time as an electron extraction layer in inverted polymer solar cells (IPSCs), and the effect of the TiO2 thickness on the photovoltaic performance of P3HT:PC61BM IPSCs was investigated. The highest PCE value of 3.75% was obtained when the thickness of TiO2 thin films was in the range between 42 nm and 73 nm. The absorption properties, morphology and structure of the TiO2 films were characterized by UV-Vis spectroscopy, SEM and Raman spectroscopy, and were related to the device performance of P3HT:PC61BM IPSCs. The results indicate that TiO2 films deposited by magnetron sputtering are an excellent electron extraction layer for IPSCs.

We demonstrate O2 plasma treated graphene oxides with a work function of 5.2 eV as a high performance hole transport layer in organic solar cells. The high transparency and high work function simultaneously increase short circuit current, threshold voltage and fill factor, resulting in a 30% increase in cell efficiency.

Inverted polymer solar cells (IPSCs) were fabricated with cesium carbonate (Cs2CO3) modified indium tin oxide (ITO) substrates as the electrode and molybdenum trioxide (MoO3) modified Al as the anode. The Cs2CO3 dissolved in 2-ethoxyethanol was spin-coated on ITO substrates, showing snowflake-like morphology characterized by the scanning electron microscope (SEM). The absorption, X-ray diffraction as well as the morphology of the active layer were measured before and after annealing treatment. The IPSCs with annealing treatments on the active layers and MoO3 layers exhibited the maximum power conversion efficiency (PCE) approaching to 2%, with open circuit voltage (Voc) of 0.57 V, short circuit current density (Jsc) of 8.8 mA/cm2 and fill factor (FF) of 38.7%. The performance of IPSCs was dramatically decreased by annealing treatment after the deposition of Al cathode, which may be due to the diffusion of Al atom crossing the MoO3 layer forming new channels for charge carrier collection. However, the new channels are not beneficial to the charge carrier collection, which is demonstrated from that the Jsc of IPSCs was evidently decreased from 8.8 to 4.6 mA/cm2 by annealing treatment after deposition Al layer. The annealing treatment after deposition of MoO3 could improve the interfacial contact to aid in electron extraction.Highlights► PCE of inverted polymer solar cells (IPSc) with Cs2Co3 modified ITO as cathode and MoO3/Al as anode is approaching to 2%. ► The snowflake-like morphology of Cs2CO3 was investigated by SEM. ► The annealing treatment before deposition Al layer has positive effect on improvement of IPSCs performance. ► EDX results directly demonstrate the vertical phase separation of P3HT:PCBM induced by annealing treatment.

We demonstrate O2 plasma treated graphene oxides with a work function of 5.2 eV as a high performance hole transport layer in organic solar cells. The high transparency and high work function simultaneously increase short circuit current, threshold voltage and fill factor, resulting in a 30% increase in cell efficiency.

The interlayer inserted between the active layer and ITO has been demonstrated to be crucial for the performance of inverted polymer solar cells (i-PSCs). In this work, we find that ionic liquids (ILs) can significantly enhance the efficiency of i-PSCs. With the ZnO/IL interfacial layer, PTB7-Th:PC71BM i-PSCs can exhibit a champion power conversion efficiency (PCE) of 10.15%, which is among the highest PCEs reported thus far for single-junction bulk heterojunction solar cells through the solution process. The IL layer and ZnO/IL combination layer with low work function, good optical transmittance, improved electron extraction and reduced resistance at the cathode interface have been demonstrated to be excellent and general interfacial layers for i-PSCs.

PTB7:PC71BM inverted polymer solar cells with a room-temperature TiOx/PEI electron transport layer achieve an average power conversion efficiency of 8.72% (the champion power conversion efficiency is 9.08%), which is much better than that of the control devices based on PEI (7.00%) or TiOx (7.38%). The room-temperature TiOx/PEI layer exhibits outstanding capacities, including increased electron mobility, reduced series resistance and improved electron extraction at the cathode interface.

A novel non-fullerene small molecule electron acceptor TTzBT-DCAO, which contains all electron-withdrawing units of 2,1,3-benzothiadiazole, oligothiazole and alkyl cyanoacetate, has been synthesized and characterized. Its photophysical, electrochemical, and photovoltaic properties have been investigated. The material has favorable HOMO and LUMO levels of −5.88 and −3.60 eV, and shows strong absorption in the visible spectrum up to 650 nm. The small molecule:non-fullerene bulk-heterojunction organic photovoltaics (OPVs) were constructed based on two small molecules SF8TBT and TTzBT-DCAO. The influence of the donor:acceptor composition on device performance was investigated. The open-circuit voltages of the devices are over 1.20 V, which is among the highest values reported for single-junction OPVs. The results indicate that small molecules with all electron-withdrawing units could provide a novel route to efficient solution-processed OPVs with high open-circuit voltages.