N-Type Self-Doped Water/Alcohol-Soluble Conjugated Polymers with Tailored Energy Levels for High-Performance Polymer Solar Cells

Naphthalene diimide (NDI) and perylene diimide (PDI) based polymeric semiconductors with high mobility have shown great promise as electron transport materials (ETMs) in applications such as high-performance polymer solar cells (PSCs) and other optoelectronic devices. However, these NDI and PDI semiconductors usually have limited adjustment of the lowest unoccupied molecular orbital (LUMO) energy levels, which hinders their further use in the organic electronic device. Here, by using various degrees of esterification instead of imide groups, three perylenetetracarboxylic acid derivatives based, self-doped, n-type water/alcohol-soluble conjugated polymers (n-WSCPs) were developed, which can act as electron transport layers (ETLs) to produce high-performance PSCs. Owing to the distinct electron-deficient nature of the backbones, these nWSCPs exhibited different optoelectronic properties. The relationships between doping effects, charge-transporting capabilities, interfacial modifications, and structures of n-WSCPs were systematically investigated. When these n-WSCPs were used as ETLs, high-performance PSCs with the efficiencies of near 9% and over 10% were achieved in the combinations of poly[[2,6-4,8-di(5-ethylhexylthieny)benzo[1,2-b;3,3-Hdithiophene]-[3-fluoro-2[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7-Th)/[6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) and poly(2,5-thiophene-alt-5,5'-(5,10-bis(4-(2-octyldodecyl)thiophen-2yl)naphtho[1,2-c:5,6-c']bis([1,2,5]thiadiazole)) (PNTT)/PC71BM, respectively. The device of PNTT/PC71BM still exhibited a high PCE of 9.37% when the thickness of ETL was increased to 50 nm. Our results demonstrate the identical importance of energy level alignment and electron transporting in the design of n-WSCPs used for thickness-insensitive ETLs in PSCs for large area practical applications.