Approaching the radiative limits for wide bandgap perovskite solar cells using fullerene blend electron transport interlayers

Abstract

Performance losses in positive–intrinsic–negative architecture perovskite solar cells are dominated by nonradiative recombination at the perovskite/organic electron transport layer interface, which is particularly problematic for wider bandgap perovskites. Large endeavours have been dedicated to the replacement of fullerenes, which are the most commonly used class of electron transport layers, with limited success thus far. In this work, we demonstrate blending the fullerene derivatives [6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM) and indene-C₆₀ bis-adduct (ICBA) as a thin interlayer between 1.77 eV bandgap perovskite and an evaporated C₆₀ layer. By tuning the fullerene blend to a trace 2% by mass of PCBM in ICBA, we remarkably form an interlayer which features improved energetic alignment with the perovskite and the PCBM : ICBA fullerene mixture, together with a stronger molecular ordering and an order of magnitude higher electron mobility than either neat PCBM or ICBA. Additional molecular surface passivation approaches are found to be beneficial in conjunction with this approach, resulting in devices with 19.5% steady state efficiency, a fill factor of 0.85 and an open-circuit voltage of 1.33 V, which is within 10% of the radiative limit of the latter two device parameters for this bandgap. This work highlights the complex nonlinear energetic behaviour with fullerene mixing, and how control of the energetics and crystallinity of these materials is crucial in overcoming the detrimental recombination losses that have historically limited perovskite solar cells.