Low Temperature Aqueous Solution-Processed ZnO and Polyethylenimine Ethoxylated Cathode Buffer Bilayer for High Performance Flexible Inverted Organic Solar Cells

High performance flexible inverted organic solar cells (OSCs) employing the low temperature cathode buffer bilayer combining the aqueous solution-processed ZnO and polyethylenimine ethoxylated (PEIE) are investigated based on Poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl-C-61-butryric acid methyl ester (P3HT:PC61BM) and Poly({4,8-bis[(2-ethylhexyl)oxy] benzo[1,2-b: 4,5-b']dithiophene-2,6-diyl}{3-fluoro-2-[(2-ethylhexy) carbonyl] thieno[3,4-b]thiophenediyl}):[6,6]-phenyl-C-71-butyric acid methyl ester (PTB-7: PC71BM) material systems. It is found that, compared with pure ZnO or PEIE cathode buffer layer (CBL), the proper combination of low-temperature processed ZnO and PEIE as the CBL enhanced the short circuit current density (J(SC)), resulting in better device performance. The increased J(SC) results from the enhanced electron collection ability from the active layer to the cathode. By using the ZnO/PEIE CBL, a power conversion efficiency (PCE) as high as 4.04% for the P3HT: PC61BM flexible device and a PCE as high as 8.12% for the PTB-7: PC71BM flexible device are achieved, which are higher than the control devices with the pure ZnO CBL or pure PEIE CBL. The flexible inverted OSC also shows a superior mechanical property and it can keep 92.9% of its initial performance after 1000 bending cycles with a radius of 0.8 cm. These results suggest that the combination of the low temperature aqueous solution processed ZnO and PEIE can be a promising cathode buffer bilayer for flexible inverted OSCs.