Development of flexible perovskite solar cells by the low-temperature fabrication of TiO2 electron transport layers

Flexible and light-weight perovskite solar cells (PSCs) have been considered due to the expectation of low production cost and wide application ranges. Low temperature processes are required in the fabrication of flexible perovskite solar cells because flexible substrates cannot resist process temperatures over 150 ºC. An electron transport layer (ETL) is commonly titanium-oxide (TiO 2 ) formed by high temperature sintering (˃400ºC). The formation of ETL is crucial for flexible PSCs in order to complete the entire low-temperature process. Here, we have employed our original single crystalline TiO 2 nanoparticles with high purity of brookite phase (BK TiO 2 NPs), and could form TiO 2 ETLs below 150 ºC. We succeeded to fabricate flexible PSCs on polyethylene naphthalate (PEN) substrates with the power conversion efficiency of 13.08% (in reverse scan). The present work provides us a good finding to realize the low-cost flexible PSCs in the future.