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Recently, low temperature solution-processed tin oxide (SnO) as a versatile electron transport layer (ETL) for efficient and robust planar heterojunction (PH) perovskite solar cells (PSCs) has attracted particular attention due to its outstanding properties such as high optical transparency, high electron mobility, and suitable band alignment. However, for most of the reported works, an annealing temperature of 180 °C is generally required. This temperature is reluctantly considered to be a low temperature, especially with respect to the flexible application where 180 °C is still too high for the polyethylene terephthalate flexible substrate to bear. In this contribution, low temperature (about 70 °C) UV/ozone treatment was applied to in situ synthesis of SnO films deposited on the fluorine-doped tin oxide substrate as ETL. This method is a facile photochemical treatment which is simple to operate and can easily eliminate the organic components. Accordingly, PH PSCs with UV-sintered SnO films as ETL were successfully fabricated for the first time. The device exhibited excellent photovoltaic performance as high as 16.21%, which is even higher than the value (11.49%) reported for a counterpart device with solution-processed and high temperature annealed SnO films as ETL. These low temperature solution-processed and UV-sintered SnO films are suitable for the low-cost, large yield solution process on a flexible substrate for optoelectronic devices.

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Currently, most efficient perovskite solar cells (PVKSCs) with a p-i-n structure require simultaneously electron transport layers (ETLs) and hole transport layers (HTLs) to help collecting photogenerated electrons and holes for obtaining high performance. ETL free planar PVKSC is a relatively new and simple structured solar cell that gets rid of the complex and high temperature required ETL (such as compact and mesoporous TiO2). Here, we demonstrate the critical role of high coverage of perovskite in efficient ETL free PVKSCs from an energy band and equivalent circuit model perspective. From an electrical point of view, we confirmed that the low coverage of perovskite does cause localized short circuit of the device. With coverage optimization, a planar p-i-n(++) device with a power conversion efficiency of over 11% was achieved, implying that the ETL layer may not be necessary for an efficient device as long as the perovskite coverage is approaching 100%.

The development of solution processable perovskite solar cells (PSCs) has progressed rapidly, and the their highest power conversion efficiency (PCE) has recently surpassed 22%. Further studies to promote market-oriented PSCs call for further reducing the manufacturing cost of the device and addressing the concerns about the possible outflow of toxic lead. To reduce the level of environmental pollution and prevent the health hazard caused by degraded devices (solid waste) and possible lead outflow and to conserve resources, we adopted low-temperature solution-processed, multirecycled glass/FTO/c-TiO2 (m-TiO2) substrates from the degraded devices to fabricate efficient planar heterojunction (PH) and mesoporous (M) PSCs in an environmentally friendly and energy-conserving manner. This is realized by simple and low-temperature processes, including organic solvent washing, ultrasonic cleaning, and UV–ozone treatment. After two rounds of substrate recycling, the PH PSC and M PSC still exhibited peak efficiencies of 11.87% and 11.03%, respectively, indicating the feasibility of recycling used substrates for sustainable, energy and resource conservation-oriented, and environmentally friendly energy production.