2, Centre Suisse d'Electronique et de Microtechnique (CSEM), Neuchâtel, , Switzerland
3, University of Oxford, Clarendon Laboratory, Parks Road, Oxford, , United Kingdom
Organometallic halide perovskite solar cells have gained considerable interest in recent times. Despite the devices’ low cost, reported power conversion efficiencies have risen rapidly and now exceed 22 %.  However, most reported efficiencies have been obtained on an active area markedly smaller than 1 cm2. Demonstrating similar efficiencies on upscaled devices for industry has been a principal challenge faced by the technology.
While TiO2 is the most commonly used electron transport layer (ETL) in the field, it has been shown to produce pronounced hysteresis in the current-voltage curve for planar devices. TiO2 also reduces the long term stability of the cell namely due to its photocatalytic action under UV illumination. Band gap engineering studies in recent years suggest SnO2 as a viable replacement due to its enhanced conduction band alignment which may allow for improved electron extraction when compared to TiO2 based devices. 
Here we demonstrate the deposition of nominally undoped TiO2 and SnO2 ETLs by thermal atomic layer deposition (ALD) on fluorine doped tin oxide (FTO) and indium doped tin oxide (ITO) coated glass substrates. These are then fabricated into planar perovskite devices with an active area up to 1 cm2. ALD growth of metal oxides is carried out below 200 °C to accommodate silicon-perovskite heterojunction cells. 
The resulting cell performances of 18.3 % power conversion efficiency (PCE) on 0.09 cm2 and 15.3 % PCE on 1.04 cm2 active areas are discussed in conjunction with the significance of growth parameters and ETL composition.
 J. P. Correa Baena et al., Energy Environ. Sci., 8, 2928
 J. P. Mailoa et al., Appl. Phys. Lett. 106, 121105