Centimetre-scale perovskite solar cells with fill factors of more than 86 per cent

Owing to rapid development in their efficiency¹ and stability², perovskite solar cells are at the forefront of emerging photovoltaic technologies. State-of-the-art cells exhibit voltage losses^3-8 approaching the theoretical minimum and near-unity internal quantum efficiency^9-13, but conversion efficiencies are limited by the fill factor (<83%, below the Shockley-Queisser limit of approximately 90%). This limitation results from non-ideal charge transport between the perovskite absorber and the cell's electrodes^5,8,13-16. Reducing the electrical series resistance of charge transport layers is therefore crucial for improving efficiency. Here we introduce a reverse-doping process to fabricate nitrogen-doped titanium oxide electron transport layers with outstanding charge transport performance. By incorporating this charge transport material into perovskite solar cells, we demonstrate 1-cm² cells with fill factors of >86%, and an average fill factor of 85.3%. We also report a certified steady-state efficiency of 22.6% for a 1-cm² cell (23.33% ± 0.58% from a reverse current-voltage scan).