Electron-Transport-Layer-Assisted Crystallization of Perovskite Films for High-Efficiency Planar Heterojunction Solar Cells

Crystal engineering of CH3NH3PbI3 perovskite materials through template-directed nucleation and growth on PbI2 nuclei dispersed in a polar fullerene (C-60 pyrrolidine tris-acid, CPTA) electron transport layer (ETL) (CPTA:PbI2) is proposed as a route for controlling crystallization kinetics and grain sizes. Chemical analysis of the CPTA:PbI2 template confirms that CPTA carboxylic acid groups can form a monodentate or bidentate chelate with Pb(II), resulting in a lower nucleation barrier that promotes rapid formation of the tetragonal perovskite phase. Moreover, it is demonstrated that a uniform CH3NH3PbI3 film with highly crystalline and large domain sizes can be realized by increasing the spacing between nuclei to retard perovskite crystal growth via careful control of the preferred nucleation site distribution in the CPTA:PbI2 layer. The improved perovskite morphology possesses a long photoluminescence lifetime and efficient photocarrier transport/separation properties to eliminate the hysteresis effect. The corresponding planar heterojunction photovoltaic yields a high power conversion efficiency (PCE) of 20.20%, with a high fill factor (FF) of 81.13%. The average PCE and FF values for 30 devices are 19.03% +/- 0.57% and 78.67% +/- 2.13%, respectively. The results indicate that this ETL template-assisted crystallization strategy can be applied to other organometal halide perovskite-based systems.