Spontaneous open-circuit voltage gain of fully fabricated organic solar cells caused by elimination of interfacial energy disorder
The realization of high open-circuit voltage (Voc) 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 Voc gain in efficient OSCs without sacrificing the short-circuit current (Jsc) 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 Voc 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 Voc 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.