Thin-film photovoltaics are a promising alternative to their monocrystalline counterparts owing to their high-efficiency, comparable stability and potentially lower manufacturing cost. The most widely-studied thin-film materials currently under investigation for photovoltaic applications include the compound semiconductors CdTe [X. Wu, Solar Energy, vol. 77, p. 803, 2004], CuIn1−xGaxSe2 (CIGS) [Chirila et al., Nature Materials, vol. 10, p. 857, 2011], Cu2ZnSnS4 (CZTS) [D. Barkhouse et al., Progress in Photovoltaics, vol. 20, p. 6, 2012]; dye-sensitized solar cells [A. Yalla et al., Science, vol. 334, p. 629, 2011]; and organic semiconductor solar cells [Y. Liang et al., Advanced Energy Materials, vol. 22, p. E135, 2010]. Inorganic compound semiconductors which comprise high-efficiency solar cells are typically fabricated using expensive vacuum-based deposition although recent routes towards solution processing of CIGS and CZTS have exhibited high efficiency devices [M. Graetzel at al., Nature, vol. 488, p. 304, 2012]. Dye-sensitized and organic solar cells with lower record efficiencies are typically fabricated with solution-based deposition procedures but suffer from poor long-term stability. In addition, the relatively low production capacity of tellurium and indium makes CdTe and CIGS potentially commercially unattractive.
Perovskites [D. Mitzi et al., Science, vol. 267, p. 1473, 1995] are an alternative family of semiconductor materials which have been investigated for device applications [D. Mitzi at al., IBM Journal of Research and Development, vol. 45, p. 29, 2001]. For photovoltaics, perovskites have been used as the sensitizer in liquid electrolyte photoelectrochemical cells [J.-H. Im et al., Nanoscale, vol. 3, p. 4088, 2011; A. Kojima et al., Journal of the American Chemical Society, vol. 131, p. 6050, 2009], although in this previously reported electrolyte system, the perovskite absorbers decayed rapidly and the solar cells dropped in performance after only 10 minutes. Perovskites have also been used in solid-state photoelectrochemical cells [H.-S. Kim et al., Scientific Reports, doi:10.1038/srep00591; A. Kojima et al., ECS Meeting Abstracts, vol. MA2007-02, p. 352, 2007] and as the hole transporter in solid-state dye-sensitized solar cells [I. Chung, Nature, vol. 485, p. 486, 2012]. The main operating principle of sensitized solar cells, is that the role of light absorption, and charge transport are separated into different materials in the solar cell. This enables light absorbing materials, which would generate charge inefficiently if light was shone on a solid film of the material, to operate very efficiently in a sensitized solar cell. Hence, since there are examples of perovskites employed as sensitizers in meso-structured solar cells, or as hole-transporters in dye-sensitized solar cells, but no reports of solid films of perovskites operating efficiently in solar cells, it would be reasonable to assume that perovskites are not an ideal family of materials to employ as solid thin films in thin film photovoltaics.