Flame-made ultra-porous TiO layers for perovskite solar cells.

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