Current highest efficiency single junction perovskite solar cell architectures up to 22.6% utilise a dual electron transport layer comprising of a compact TiO₂ hole blocking layer and a thicker mesoporous TiO₂ layer which is infiltrated by perovskite [1]. This requires high temperature processing, is considered to be incompatible with many scalable processing techniques and may lead to degradation of the perovskite layer under normal irradiation (continuous UV exposure).

SnO₂ has emerged as a forerunner for planar, UV-stable hole-blocking layers, with two methods leading to efficiencies over 20% in literature [2,3]. Here, we investigated these proposed routes in the device architecture ITO/SnO₂/CsI_0.05((FAPbI₃)_0.83(MAPbBr₃)_0.17)_0.95/spiro-OMeTAD/Au and find highly reproducible PCEs of up to 19% utilizing nanoparticles without the use of interlayers.

Solution processed methods offer certain advantages, but alternatives such as atomic layer deposition (ALD) and vacuum-based techniques allow greater uniformity over larger areas. Thermally evaporated C₆₀ is commonly used in vapour deposition architectures [4], but this is more expensive and likely less stable than SnO₂. ALD of SnO₂ has been previously shown to give cells with optimized band alignment for efficient, barrier-free charge extraction from various perovskite absorber layers [4]. However, ALD is time consuming, limited by the solubility of underlayers and unsuitable for roll-to-roll deposition.

Vacuum-based techniques are widely used in industry and can avoid these disadvantages whilst still giving good layer uniformity at low temperatures. We report our latest work in developing scalable electron-beam evaporation of SnO₂ with efficiencies comparable to those of ALD and nanoparticle references. We also present our latest characterization work to better understand the requirements for SnO₂ to act as an effective hole-blocking layer to achieve high performance by minimizing parasitic losses.

1. Yang, Woon Seok, et al. "Iodide management in formamidinium-lead-halide–based perovskite layers for efficient solar cells." Science 356.6345 (2017).

2. Anaraki, Elham Halvani, et al. "Highly efficient and stable planar perovskite solar cells by solution-processed tin oxide." Energy & Environmental Science 9.10 (2016).

3. Jiang, Qi, et al. "Enhanced electron extraction using SnO2 for high-efficiency planar-structure HC(NH₂)₂PbI₃-based perovskite solar cells." Nature Energy 2 (2016).

4. Borchert, Juliane, et al. "Large-Area, Highly Uniform Evaporated Formamidinium Lead Triiodide Thin Films for Solar Cells." ACS Energy Letters 2.12 (2017).

5. Baena, Juan Pablo Correa, et al. "Highly efficient planar perovskite solar cells through band alignment engineering." Energy & Environmental Science 8.10 (2015).