Triazine-based Polyelectrolyte as an Efficient Cathode Interfacial Material for Polymer Solar Cells

Small-Molecule Electron Transport Layer with Siloxane-Functionalized Side Chains for Nonfullerene Organic Solar Cells

Active layer materials with silicone side chains have been broadly reported to have excellent long-term stability in harsh environments. However, the application of conjugated materials with silicone side chains in electron transport layers (ETLs) has rarely been reported. In this research, we synthesized for the first time a siloxane-modified perylene-diimide derivative (PDI-OSi) consisting of a side-chain substituent of siloxane and a conjugated group of perylene-diimide (PDI). The inserted siloxane functional groups not only can strengthen the light transmittance of PDI-OSi but also can remarkably expand its solubility and improve the film-forming ability and air stability of the material. Second, introducing siloxane-containing side chains can dramatically lower the work function and interfacial barrier of the electrode, thereby achieving a favorable ohmic contact. In addition, the moderate surface energy of siloxane functional groups makes PDI-OSi hydrophobic, which is conducive to forming excellent miscibility with hydrophobic active layers to promote charge transfer. When PDI-OSi is used as an ETL in organic solar cells (OSCs), operative exciton dissociation and more favorable surface morphology enable OSCs to realize a power conversion efficiency (PCE) of 13.99%. These results indicate that side-chain engineering with siloxane pendants is a facile strategy for constructing efficient OSCs.

Construction of Effective Polymer Solar Cell Using 1,7-Disubstituted Perylene Diimide Derivatives as Electron Transport Layer

The poor compatibility of an inorganic electron transport layer with the active layer and an ultrathin film organic material becomes a great obstacle in producing high-quality polymer solar cells with high-throughput roll-to-roll (R2R) method. Novel effective polymer solar cells had been fabricated by introducing 1, 7-disubstituted perylene diimide derivatives PDIH, PDIC, and PDIN as an electron transporting layer. It was noteworthy that PDIN could obviously improve the power conversion efficiency of solar cells that incorporated a photoactive layer composed of poly[(3-hexylthiophene)-2, 5-diyl] (P3HT) and the fullerene acceptor [6, 6-phenyl-C71-butyric acid methyl ester] (PC 71 BM). The power conversion efficiency varies from 1.5% for ZnO transparent cathode-based solar cells to 2.1% for PDIN-based electron transport layer-free solar cells. This improved performance could be attributed to the following reasons: the interaction between N atom in PDIN and O atom in indium tin oxide (ITO) reduced the work function of ITO, increased the built-in electric field, and thus lowered the electron transport barrier and improved the electron extraction ability of cathode, the appropriate roughness of the active layer increased the contact area with anode interfacial layer and enhanced the hole transport efficiency. These experimental results revealed that PDIN can be a promising novel effective material with a simplified synthesis process and lower cost as an electron transporting layer.

Engineering of Porphyrin Molecules for Use as Effective Cathode Interfacial Modifiers in Organic Solar Cells of Enhanced Efficiency and Stability

In the present work, we effectively modify the TiO₂ electron transport layer of organic solar cells with an inverted architecture using appropriately engineered porphyrin molecules. The results show that the optimized porphyrin modifier bearing two carboxylic acids as the anchoring groups and a triazine electron-withdrawing spacer significantly reduces the work function of TiO₂, thereby reducing the electron extraction barrier. Moreover, the lower surface energy of the porphyrin-modified substrate results in better physical compatibility between the latter and the photoactive blend. Upon employing porphyrin-modified TiO₂ electron transport layers in PTB7:PC₇₁BM-based organic solar cells we obtained an improved average power conversion efficiency up to 8.73%. Importantly, porphyrin modification significantly increased the lifetime of the devices, which retained 80% of their initial efficiency after 500 h of storage in the dark. Because of its simplicity and efficacy, this approach should give tantalizing glimpses and generate an impact into the potential of porphyrins to facilitate electron transfer in organic solar cells and related devices.