No abstract

A method for direct inkjet printing of silver nanowire (Ag NW) to form transparent conductive network as the top electrode for inverted semi-transparent organic photovoltaic devices (OPV) was developed. The highest power conversion efficiency of the poly(3-hexylthiophene):phenyl-C61–butyric acid methyl ester (P3HT:PC61BM) based OPV was achieved to be 2.71% when the top electrode was formed by 7 times of printing. In general, devices with printed Ag NW top electrode had similar open-circuit voltage (VOC, around 0.60 V) but lower fill factor (FF, 0.33–0.54) than that of device with thermally deposited Ag opaque electrode (reference device). Both FF and short-circuit current density (JSC), however, were found to be increasing with the increase of printing times (3, 5, and 7), which could be partially attributed to the improved conductivity of Ag NW network electrodes. The solvent effect on device performances was studied carefully by comparing the current density-voltage (J-V) curves of different devices. The results revealed that solvent treatment on the anode buffer layer during printing led to a decrease of charge injection selectivity and an increase of charge recombination at the anode interface, which was considered to be the reason for the degrading of device performance.

For the inverted organic solar cells (OSCs), the interface contacts between the ZnO electron transporting layer and the organic active layer play an important role in the device performance and stability. Since the solution-processed ZnO surface always contains some base or zinc salt contaminants, we explored how the surface pH conditions influence the performance and stability of the nonfullerene acceptor (NFA) cells. A tight relationship between the surface pH condition and the device performance and stability was established. Specifically, device performance and stability were improved by treating the ZnO films with acid solutions but worsened after base treatment. The large number of hydroxyl groups on the surface of the solution-processed ZnO films was proved to be the main reason for the surface pH condition-related performance, which caused oxygen-deficient defects and unfavorable vertical phase separation in the blend films, hindered the photogenerated charge transfer and collection, and consequently resulted in low short-circuit current density and power conversion efficiency (PCE). The surface −OH groups also boosted the photocatalytic activity and led to fast degradation of the nonfullerene acceptor. Removal of the surface −OH groups can alleviate such problems. Different acid solutions, ZrAcac, 2-phenylethylmercaptan (PET), and glutamic acid (GC), were used to treat the ZnO films, and PET treatment was the most effective treatment for performance improvement. An efficiency of 16.46% was achieved for the PM6:Y6 cells and the long-term stability under continuous illumination conditions was significantly improved with a T 80 lifetime of over 4000 h (4410 h), showing the excellent long-term stability of this heterojunction solar cell. Our understanding of the surface pH condition-related device performance and stability would guide the development of a feasible method for solving the interface problems in OSCs. We also provide a practical strategy to modify ZnO with acid solutions for high-performance and stable NFA OSCs.

Organic solar cells (OSCs) have gained a rapid development in the past two decades and the power conversion efficiency (PCE) of single-junction OSC has recently approached 20%. The novel materials and device engineering are two key factors of this evolution. In this review, the device engineering, including morphology characterization and optimization, device physics, flexible and large-area OSCs, and stability of OSCs are systematically summarized. In addition, the current challenges, problems and future developments are also discussed.

With the aim of developing high‐performance flexible polymer solar cells, the preparation of flexible transparent electrodes (FTEs) via a high‐throughput gravure printing process is reported. By varying the blend ratio of the mixture solvent and the concentration of the silver nanowire (AgNW) inks, the surface tension, volatilization rate, and viscosity of the AgNW ink can be tuned to meet the requirements of gravure printing process. Following this method, uniformly printed AgNW films are prepared. Highly conductive FTEs with a sheet resistance of 10.8 Ω sq−1 and a high transparency of 95.4% (excluded substrate) are achieved, which are comparable to those of indium tin oxide electrode. In comparison with the spin‐coating process, the gravure printing process exhibits advantages of the ease of large‐area fabrication and improved uniformity, which are attributed to better ink droplet distribution over the substrate. 0.04 cm2 polymer solar cells based on gravure‐printed AgNW electrodes with PM6:Y6 as the photoactive layer show the highest power conversion efficiency (PCE) of 15.28% with an average PCE of 14.75 ± 0.35%. Owing to the good uniformity of the gravure‐printed AgNW electrode, the highest PCE of 13.61% is achieved for 1 cm2 polymer solar cells based on the gravure‐printed FTEs.

Zinc oxide (ZnO) nanoparticles are widely used as electron- transport layer (ETL) materials in organic solar cells and are considered to be the candidate with the most potential for ETLs in roll-to-roll (R2R)-printed photovoltaics. However, the tendency of the nanoparticles to aggregate reduces the stability of the metal oxide inks and creates many surface defects, which is a major barrier to its printing application. With the aim of improving the stability of metal oxide nanoparticle dispersions and suppressing the formation of surface defects, we prepared 3-aminopropyltrimethoxysilane (APTMS)-capped ZnO (ZnO@APTMS) nanoparticles through surface ligand exchange. The ZnO@APTMS nanoparticles exhibited excellent dispersibility in ethanol, an environmentally friendly solvent, and remained stable in air for at least one year without any aggregation. The capping of the ZnO nanoparticles with APTMS also reduced the number of surface-adsorbed oxygen defects, improved the charge transfer efficiency, and suppressed the light-soaking effect. The thickness of the ZnO@APTMS ETL could reach 100 nm without an obvious decrease in the performance. Large-area APTMS-modified ZnO films were successfully fabricated through roll-to-roll microgravure printing and exhibited good performance in flexible organic solar cells. This work demonstrated the distinct advantages of this ZnO@APTMS ETL as a potential buffer layer for printed organic electronics.

No abstract

Solution-processed organic-inorganic hybrids composing of MoO3 nanoparticles and PEDOT:PSS were developed for use in inverted organic solar cells as hole transporting layer (HTL). The hybrid MoO3:PEDOT:PSS inks were prepared by simply mixing PEDOT:PSS aqueous and MoO3 ethanol suspension together. A core-shell structure was proposed in the MoO3:PEDOT:PSS hybrid ink, where PEDOT chains act as the core and MoO3 nanoparticles connected with PSS chains act as the composite shell. The mixing with PEDOT:PSS suppressed the aggregation of MoO3 nanoparticles, which led to a smoother surface. In addition, since the hydrophilic PSS chains were passivated through preferentially connection with MoO3, the stronger adhesion between MoO3 nanoparticles and the photoactive layer improved the film forming ability of the MoO3:PEDOT:PSS hybrid ink. The MoO3:PEDOT:PSS hybrid HTL can therefore be feasibly deposited onto the hydrophobic photoactive polymer layer without any surface treatment. The use of the MoO3:PEDOT:PSS hybrid HTL resulted in the optimized P3HT:PC61BM- and PTB7:PC61BM-based inverted organic solar cells reaching highest power conversion efficiencies of 3.29% and 5.92%, respectively, which were comparable with that of the control devices using thermally evaporated MoO3 HTL (3.05% and 6.01%, respectively). Furthermore, less HTL thickness dependence of device performance was found for the hybrid HTL-based devices, which makes it more compatible with roll-to-roll printing process. In the end, influence of the blend ratio of MoO3 to PEDOT:PSS on photovoltaic performance and device stability was studied carefully, results indicated that the device performance would decrease with the increase of MoO3 blended ratio, whereas the long-term stability was improved.