Highly efficient electron transport based on double-layered PC₆₁BM in inverted perovskite solar cells

Electron transport layer (ETL) plays a major role in determining performance of perovskite solar cells (PSCs). [6, 6]-phenyl C₆₁butyric acid methyl ester (PC₆₁BM) has been the preferred choice as the ETL in inverted PSCs. However, it has some drawbacks including current leakage, low mobility, and difficulty in full coverage of the perovskite layer with rough surface, resulting in charge recombination losses, charge leakage pathways and inefficient charge transfer at the perovskite/ETL, thus contributing to lower device efficiency and stability. For the first time, we introduce a simple strategy of implementing double-layered (DL) PC₆₁BM as ETL to address the aforementioned challenges. The use of DL PC₆₁BM particularly resulted in reinforced morphology of the ETL with reduced pin-holes and surface roughness compared to the single-layer (SL) PC₆₁BM. This yielded increased electrical conductivity of the PC₆₁BM, suppressed charge recombination, and facilitated higher number of charge generation, transfer/collection at the perovskite/PC₆₁BM interface. Consequently, the DL PC₆₁BM-based PSC delivered a considerable increase of power conversion efficiency (PCE) to 18.06%, which is much higher than the SL-based pristine PSC with 14.20%. Furthermore, the transition to DL from SL PC₆₁BM exhibited longer ambient stability of the PSCs. This work can be easily adopted for other perovskite compositions.