Spontaneous open-circuit voltage gain of fully fabricated organic solar cells caused by elimination of interfacial energy disorder

Abstract

The realization of high open-circuit voltage (V_oc) in organic solar cells (OSCs) mainly depends on the delicate donor (D) and acceptor (A) structures, meticulously optimized bulk heterojunction (BHJ) microstructure and functionalized interfacial materials. In this work, we demonstrate a spontaneous V_oc gain in efficient OSCs without sacrificing the short-circuit current (J_sc) and fill factor (FF). Using a combination of device studies and morphological analysis, we point out that the spontaneous phenomenon occurred at the bottom of the BHJ layer, which is caused by [6,6]-phenyl-C71-butyric acid methyl ester (PC₇₁BM) acceptors spontaneously migrating to the BHJ/ZnO interface under ambient conditions. Furthermore, physical characterization determines the relation between morphology evolution and spontaneous V_oc enhancement, which mainly results from the eliminated energetic disorder at the BHJ/ZnO interface. The investigation of surface energy parameters among selected components highlights the wetting coefficient as a dominant dynamic for this spontaneous phenomenon. Besides, a wide range of photovoltaic systems introduced for enabling parallel verification also confirm the effect of the interfacial surface energy on V_oc spontaneous enhancement in inverted solar cells. These findings exemplify the importance of surface energy modification as a tool for improved interfaces of layered morphology, and open new routes to device interfacial optimization using novel design strategies of photoactive materials and interfacial materials. Such a spontaneous phenomenon may open a novel field for making materials work in an intelligent way in organic electronics.