Solution processed and self-assembled polymerizable fullerenes/metal oxide as an interlayer for high efficient inverted polymer solar cells

Abstract

A novel polymerizable fullerene derivative, 2-(2-(2-methoxyethoxy)ethoxy)ethyl undec-10-enyl malonate C₆₀ (EEMC), functionalized with ethenyl and tri-(ethylene oxide) (TEO), has been rationally designed and synthesized. The functionalized fullerene can easily self-assemble on the surface of ZnO to form an immobilized and robust fullerene/ZnO electron-transporting layer (ETL) for the inverted solar cells. Additionally, the solution processed and assembled fullerene/ZnO ETL can be further stabilized by a polymerized network resulting from ethenyl groups undergoing subsequent thermal annealing. The oxygen groups of TEO anchoring on ZnO effectively passivate the surface traps of ZnO and induce a smoother and more hydrophobic surface. Moreover, the polymerized EEMC (PEEMC) also promotes a better matched energy level and more intimate interfacial contact and improves active layer morphology compared to the bare ZnO. As a result, the performance of the inverted solar cell device is improved by incorporation of such novel ETL. Furthermore, methanol treatment on the PEEMC (MT-PEEMC) is found to better optimize the device performance. The device based on a blend of a low band-gap polymer thieno[3,4-b]thiophene/benzodithiophene (PTB7) with [6,6]-phenyl C₇₁-butyric acid methyl ester (PC₇₁BM) delivers a high efficiency of 7.5%. Note that a solution-processed solar cell with MT-PEEMC as ETL and a sol–gel derived VO_x as holes transfer layer (HTL) achieves a PCE of 7.3%. In addition, the self-assembled and polymerized interlayer also improves the durability of device lifetime.