Tuning the Crystallinity and Electron Mobilities of a PCBM:ICBA Fullerene Blend Interlayer for Wide Band Gap Perovskite Solar Cells

Despite significant advances in wide band gap perovskite solar cells over the past several years, non-radiative recombination at the perovskite-electron transport interface continues to be a critical obstacle limiting device performance. This study presents a solution utilizing a thin interlayer of blended fullerenes [6], [6]-phenyl C61 butyric acid methyl ester (PCBM) and indene-C60 bis-adduct (ICBA) between the 1.77 eV perovskite and evaporated C60. Optimizing the blend to a trace 2% by mass PCBM in ICBA results in a hybrid electron transport layer (ETL) with improved energetic alignment, stronger molecular ordering, an order of magnitude higher electron mobility compared to neat PCBM or ICBA. When combined with surface passivation, this approach resulted in devices with 19.5% steady state efficiency, a fill factor of 0.85 and open-circuit voltage (VOC) of 1.33 V, which is within 10% of the radiative limit of VOC for this bandgap. Here we highlight the complex nonlinear behavior with fullerene mixing, and how controlling the energetics and crystallinity as well as the electronic properties of these materials is vital in mitigating nonradiative recombination and achieving high performing wide band gap perovskite solar cells.