What defines the perovskite solar cell efficiency and stability: fullerene-based ETL structure or film morphology?

In this contribution, we report the synthesis and structural characterization of a series of fullerene derivatives and their further systematic investigation as promising ETL (electron transport layer) materials in p-i-n perovskite solar cells (PSCs). The devices fabricated using a set of fullerene derivatives F1-F6 demonstrated high power conversion efficiencies (PCEs), up to 19.0%, compared to the 17.3% obtained with reference cells assembled using the benchmark ETL material, [6,6]-phenyl-C-61-butyric acid methyl ester (PC61BM). The improved photovoltaic performance of PSCs incorporating the fullerene derivatives originated from a decreased trap density at the perovskite/ETL interface and full coverage of the perovskite absorber layer, which was revealed by the photoluminescence (PL) spectra and infrared scattering scanning near field optical microscopy (IR s-SNOM). Also, we enhanced our experimental results with theoretical DFT and DFT-MD calculations which gave further insight on the dependence between the structure and electron mobility in these films. Significantly improved operational stability was achieved for non-encapsulated devices using fluorine-loaded fullerene derivative F5 as the ETL, which retained >60% of the initial efficiency after & SIM;1300 h of continuous illumination (1 sun), whereas the reference cells with PC61BM as the ETL degraded to & SIM;40% within 200 h under the same aging conditions. Therefore, the obtained results demonstrated that the molecular structure of the fullerene derivatives affects the performance of PSCs, whereas the film morphology plays a crucial role in defining the operational stability of the devices.