Non-conjugated electrolytes as thickness-insensitive interfacial layers for high-performance organic solar cells

Non-conjugated electrolytes, PMDETA-DBO and PEDETA-DBO, are developed through the quaternization of diethylenetriamine derivatives with 1,8-dibromooctane, which can interestingly be deployed as low-cost, thickness-insensitive and low-temperature processable electron transporting layers (ETLs) for organic solar cells (OSCs). Compared with PMDETA-DBO with methyl groups, PEDETA-DBO with hydroxyethyl groups leads to better photovoltaic performance in OSCs, as PEDETA-DBO not only has stronger capability to tune the work function of cathodes, but also allows better accommodation of the penetrated electron acceptors into ETLs for efficient electron extraction. Fullerene-based OSCs with PEDETA-DBO achieve PCEs of 10.41% and 9.63% under 10 and 50 nm layer thicknesses, respectively. And the OSCs based on the PM6:Y6 blend achieve PCEs of 16.57% and 15.33% under the thicknesses of 5 and 30 nm, respectively. We reveal that the electron acceptors spun atop could penetrate into the network of non-conjugated electrolytes, wherein the contact doping between the electrolytes and acceptors results in efficient electron extraction with a desirable thickness-insensitive property in inverted OSCs. Moreover, the conversion of tertiary amines into ammonium helps suppress the degradation of non-fullerene acceptors in inverted OSCs. Overall, this work provides a strategy to exploit low-cost and thickness-insensitive non-conjugated ETLs for OSCs.