Achieving High Efficiency in Solution-Processed Perovskite Solar Cells Using C-60/C-70 Mixed Fullerenes
ACS Appl. Mater. Interfaces 10(46):39590-39598 Nov 2018 

Lin, Hao-Sheng; Jeon, Il; Xiang, Rong; Seo, Seungju; Lee, Jin-Wook; Li, Chao; Pal, Amrita; Manzhos, Sergei; Goorsky, Mark S.; Yang, Yang; Maruyama, Shigeo; Matsuo, Yutaka


Fullerenes have attracted considerable interest as an electron-transporting layer in perovskite solar cells. Fullerene-based perovskite solar cells produce no hysteresis and do not require high-temperature annealing. However, high power conversion efficiency has been only achieved when the fullerene layer is thermally evaporated, which is an expensive process. In this work, the limitations of a solution-processed fullerene layer have been identified as high crystallinity and the presence of remnant solvents, in contrast to a thermally deposited C60film, which has low crystallinity and no remaining solvents. As a solution to these problems, a mixed C60and C70solution-processed film, which exhibits low crystallinity, is proposed as an electron-transporting layer. The mixed-fullerene-based devices produce power conversion efficiencies as high as that of the thermally evaporated C60-based device (16.7%) owing to improved fill factor and open-circuit voltage. In addition, by vacuum-drying the mixed fullerene film, the power conversion efficiency of the solution-processed perovskite solar cells is further improved to 18.0%. This improvement originates from the enhanced transmittance and charge transport by removing the solvent effect. This simple and low-cost method can be easily used in any type of solar cells with fullerene as the electron-transporting layer. 

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