Flame-made ultra-porous TiO₂ layers for perovskite solar cells

We report methyl ammonium lead iodide (MAPbI₃) solar cells with an ultra-porous TiO₂ electron transport layer fabricated using sequential flame aerosol and atomic layer depositions of porous and compact TiO₂ layers. Flame aerosol pyrolysis allows rapid deposition of nanostructured and ultra-porous TiO₂ layers that could be easily scaled-up for high-throughput low-cost industrial solar cell production. An efficiency of 13.7% was achieved with a flame-made nanostructured and ultra-porous TiO₂ electrode that was coated with a compact 2 nm TiO₂ layer. This demonstrates that MAPbI₃ solar cells with a flame-made porous TiO₂ layer can have a comparable efficiency to that of the control MAPbI₃ solar cell with the well-established spin-coated porous TiO₂ layer. The combination of flame aerosol and atomic layer deposition provides precise control of the TiO₂ porosity. Notably, the porosity of the as-deposited flame-made TiO₂ layers was 97% which was then fine-tuned down to 87%, 56% and 35% by varying the thickness of the subsequent compact TiO₂ coating step. The effects of the decrease in porosity on the device performance are discussed. It is also shown that MAPbI₃ easily infiltrates into the flame-made porous TiO₂ nanostructure thanks to their high porosity and large pore size.