Strong performance enhancement in lead-halide perovskite solar cells through rapid, atmospheric deposition of n-type buffer layer oxides

Thin (approximately 10 nm) oxide buffer layers grown over lead-halide perovskite device stacks are critical for protecting the perovskite against mechanical and environmental damage. However, the limited perovskite stability restricts the processing methods and temperatures (≤110 °C) that can be used to deposit the oxide overlayers, with the latter limiting the electronic properties of the oxides achievable. In this work, we demonstrate an alternative to existing methods that can grow pinhole-free TiO_x (x ~ 2) films with the requisite thickness in <1 min without vacuum. This technique is atmospheric pressure chemical vapor deposition (AP-CVD). The rapid but soft deposition enables growth temperatures of ≥180 °C to be used to coat perovskites with or without a protective layer of PC₆₁BM. This is ≥ 70 °C higher than achievable by current methods and results in more conductive TiO_x films, boosting solar cell efficiencies by >2%. Likewise, when AP-CVD SnO_x (x ~ 2) is grown directly on the perovskite, there is also minimal structural damage to the underlying perovskite layer. The SnO_x layer is pinhole-free and conformal. When used to cover perovskite devices with a PC₆₁BM electron transport layer, shunting due to the pinholes in the spin-coated PC₆₁BM is reduced, resulting in increases in the steady-state efficiency from 16.5% (no SnO_x) to 19.4% (60 nm SnO_x), with fill factors reaching 84%. This work shows AP-CVD to be a versatile technique for growing oxides on thermally-sensitive materials.