Ultrathin Flexible Transparent Composite Electrode via Semi-embedding Silver Nanowires in a Colorless Polyimide for High-Performance Ultraflexible Organic Solar Cells

Ultraflexible organic solar cells (OSCs) with both high power conversion efficiency (PCE) and good mechanical robustness are still challenging, in which flexible transparent composite electrodes (FTCEs, substrate-cum-electrodes) play critical roles. Here, an ultrathin FTCE (∼9 μm) via semi-embedding a silver nanowire electrode in a colorless polyimide (CPI) substrate was developed, which simultaneously possessed outstanding performance such as low square resistance (Rsq ∼ 12.7 Ω sq-1), high optical transmittance (T550 ∼ 86.3%), smooth surface (root-mean-square ∼ 0.32 nm), and excellent thermal, mechanical, and solution producing stability. Prior to the FTCE fabrication, four CPI samples with the number-average molecular weight ranging from 35.9 to 177.5 kDa were prepared and their optical, mechanical, and thermal properties were studied in detail. Moreover, the effect of the molecular weight on the minimum thickness that can withstand the following solution production of ultraflexible OSCs was investigated, which revealed that the molecular weight of CPI here should be above 81.4 kDa. Based on the FTCE, an ultraflexible OSC with a high PCE value of 14.37% and outstanding mechanical robustness was constructed, in which the PCE could still maintain above 96% of its initial value after 1000 bending cycles at a bending radius of 0.5 mm.