Role of PC60BM in defect passivation and improving degradation behaviour in planar perovskite solar cells

Titanium oxide (TiO2) is the most commonly employed electron transport layer (ETL) in planar perovskite solar cells (PSCs) structure (FTO/c-TiO2/CH3NH3PbI3/Spiro-OMeTAD/Ag). However, it suffers from defects such as oxygen vacancies and structural defects due to the formation of Ti3+ ions. While defects in TiO2 could play a role in determining the device performance, perovskite layer in PSCs also exhibits traps at interfaces and grain boundaries causing increased recombination losses lowering the power conversion efficiencies (PCE) of the devices. In this manuscript, we demonstrate the use of a thin interlayer of [6, 6]-phenyl-C60-butyric acid methyl ester (PC60BM) as a passivation layer between TiO2 and perovskite layer in planar PSC structure, as analysed using aging and stability studies. The study reveals that the quality of the interface between the constituent layers improves after introduction of PC60BM as passivation layer thus causing an enhancement in the performance of devices. The performance of the devices showed a peculiar behaviour when measured at different times with ~40% enhancement in PCE after 50 h followed by a drop when measured after 150 h. The initial improvement in the device performance was studied using photoluminescence (PL) spectroscopy which showed a blue shift in the emission while impedance spectroscopy measurements showed a significant reduction in charge transfer resistance after 50 h, aided by an improved band alignment as suggested by Kelvin probe force microscopy (KPFM). Further, we show that initiation of degradation in perovskite starts from interface instead of degradation in the bulk of perovskite layer.