Polyamino acid interlayer facilitates electron extraction in narrow band gap fullerene-free organic solar cells with an outstanding short-circuit current

In this communication, we present a study to investigate interfacial characteristics modulated by electron extraction layers (EEL) comprising of a polyamino acid alpha-poly-L-lysine (alpha-PLL) in fullerene-free organic solar cells (OSC) based on narrow-band-gap PTB7-Th:IEICO-4 heterojunctions. Benefitting from effective interfacial modifications with the alpha-PLL EEL, PTB7-Th:IEICO-4F solar cells receive a considerable boost in the short-circuit current (26.6 mA/cm(2)) and ultimate power conversion efficiencies exceeding 12%, which is a result of promoted charge extraction. Aided by combinatorial analyses using electron spin resonance spectroscopy, angle-resolved X-ray photoelectron spectroscopy together with charge transport measurements, we found that the interfacial doping occurs at the alpha-PLL/IEICO-4F interface, possibly due to the low steric hindrance of the bared primary amines and non-conjugated backbones in alpha-PLL. The interface strategy with alpha-PLL displays a universal applicability, enabling to produce a series of high efficiency PCBM or non-fullerene OSCs with different device architectures. This work enriches fundamental insights into key interfacial processes and provides a promising pathway utilizing wide-accessible biomaterials to attain high performance OSCs with emerging fullerene-free photovoltaic materials.