Efficient planar n-i-p type heterojunction flexible perovskite solar cells with sputtered TiO2 electron transporting layers

Abstract

The development of hybrid organo-lead trihalide perovskite solar cells (PSCs) comprising an electron transporting layer (ETL), a perovskite light absorber and a hole transporting layer (HTL) has received significant attention for their potential in efficient PSCs. However, the preparation of a compact and uniform ETL and the formation of a uniform light absorber layer suffer from a high temperature processing treatment and the formation of unwanted perovskite islands, respectively. A low temperature/room temperature processed ETL is one of the best options for the fabrication of flexible PSCs. In the present work, we report the implementation of a room temperature processed compact TiO₂ ETL and the synthesis of extremely uniform flexible planar PSCs based on methylammonium lead mixed halides MAPb(I_1−xBr_x)₃ (x = 0.1) via RF-magnetron sputtering and a toluene dripping treatment, respectively. The compact TiO₂ ETLs with different thicknesses (30 to 100 nm) were directly deposited on a flexible PET coated ITO substrate by varying the RF-sputtering time and used for the fabrication of flexible PSCs. The photovoltaic properties revealed that flexible PSC performance is strongly dependent on the TiO₂ ETL thickness. The open circuit voltage (V_OC) and fill factor (FF) are directly proportional to the TiO₂ ETL thickness while the 50 nm thick TiO₂ ETL shows the highest current density (J_SC) of 20.77 mA cm⁻². Our controlled results reveal that the room temperature RF-magnetron sputtered 50 nm-thick TiO₂ ETL photoelectrode exhibits a power conversion efficiency (PCE) in excess of 15%. The use of room temperature synthesis of the compact TiO₂ ETL by RF magnetron sputtering results in an enhancement of the device performance for cells prepared on flexible substrates. The champion flexible planar PSC based on this architecture exhibited a promising power conversion efficiency as high as 15.88%, featuring a high FF of 0.69 and V_OC of 1.108 V with a negligible hysteresis under AM 1.5 G illumination. Furthermore, the mechanical bending stability revealed that the fabricated devices show stable PCE up to 200 bending cycles. The interface properties revealed that the 50 nm thick TiO₂ ETL provides superior charge injection characteristics and low internal resistance. The present work provides a simplistic and reliable approach for the fabrication of highly efficient stable flexible perovskite solar cells.