Energy Level Alignment of Formamidinium Tin Iodide in Contact with Organic Hole Transport Materials

Tin-based perovskites provide environmentally less problematic alternatives to established lead halide-based absorbers used in today's most efficient perovskite solar cells. However, tin-based cells still suffer from reduced power conversion efficiency and stability when compared to lead-based analogues. To facilitate the adoption of the very successful n-i-p cell geometry, efficient contact to hole transport materials (HTMs) is needed that can be deposited on top of the tin-based perovskite layer, ideally by physical vapor deposition to protect the underlying perovskite. As possible candidates for such HTMs, the growth and the interface formation of 4,4',4''-tris[phenyl(m-tolyl)amino]triphenylamine (m-MTDATA), pentacene, and copper phthalocyanine with formamidinium tin iodide by Kelvin probe force microscopy are systematically studied. For the contact to m-MTDATA, the formation of a Mott-Schottky junction is revealed, whereas an interfacial dipole is formed for the latter two. Doping densities within the organic semiconductor are determined and indicate ion migration from the perovskite.