HYSTERESIS IN METHYLAMMONIUM LEAD IODIDE PEROVSKITE SOLAR CELLS: THE EFFECT OF CHANGING ELECTRON TRANSPORT LAYERS ON OBSERVED HYSTERESIS

Abstract

Perovskite solar cells (PSCs) have developed to the point where, currently, power conversion efficiencies (PCEs) in excess of 20% have been reported. This in contrast to the first PSC which was reported in 2009 with an efficiency of 3.8 %. This increase in PCE has helped to position PSCs as a photovoltaic technology which could potentially be commercialized. However, hysteretic current-density – voltage (J-V) behaviour has posed a challenge in the development of PSCs. When determining the efficiency of a PSC, their J-V characteristics are measured. However, their J-V response is sensitive to the voltage scan direction used when carrying out J-V tests. During these tests, measurement starts at 0V and ends at a suitable positive value. The direction in which the voltage was initially applied is then reversed during a second voltage scan. The resulting PCEs from each scan differs. This calls into question which value should be taken as correct. This variation in J-V response could also pose power quality issues when used in real world situations. This work explores the origins of hysteresis in PSCs which use Titanium Dioxide (TiO2) as an electron transport layer (ETL) and PSCs which use [6,6]-Phenyl C61 butyric acid methyl ester (PCBM). Devices illustrated in this work which were fabricated with a compact TiO2 ETL, demonstrated more pronounced hysteresis when compared to devices which were fabricated using a mesoporous TiO2 ETL. Hysteresis was minimal in the PSC which made use of the PCBM ETL. This difference in hysteretic behaviour is attributed to differences in the built-in electric fields present in devices using compact, and/or mesoporous TiO2 as an ETL.