Interface energetics in organo-metal halide perovskite-based photovoltaic cells

Abstract

Direct and inverse photoemission spectroscopies are used to determine materials electronic structure and energy level alignment in hybrid organic-inorganic perovskite layers grown on TiO 2 . The results provide a quantitative basis for the analysis of perovskite-based solar cell performance and choice of an optimal hole-extraction layer.

Perovskite-Inspired Photovoltaic Materials: Toward Best Practices in Materials Characterization and Calculations

“…In this method, the intersection of the leading edge with the background level determines the position of the band offset. 19,22,27,30,163 However, this approximation can lead to erroneous fits in the case of band edges that are not abrupt. In particular, inaccuracies may arise from this method if the density of states at the leading edge is low and there is no distinct tangent to take, as occurs for many PIMs reported.…”

“…19,27,162 Photoemission spectroscopy is a powerful tool to investigate the band structure of novel semiconductor materials and the position of the Fermi level in the bandgap. The techniques are applicable not only to thin films, 163 but also single crystals, for which band positions need to be precisely known for electrical characterization and device applications. 89,90 XPS, for instance, is not only useful for measuring the chemical composition and composition of materials by evaluating the core level area intensities, it can also be employed to determine the valence band density of states with respect to the Fermi level.…”

“…Similarly, an early investigation of hybrid organic-inorganic lead-halide perovskites showed that displaying the spectra in a semilogarithmic plot yields a better approximation of the valence maximum, as depicted in Figure 6b. 163 It should be noted that the background fitted by this method can be highly susceptible to the measurement conditions and more accurate measurements are only possible if monochromatic UV sources are used. Best practice in this case is to evaluate and compare both the linear and semilogarithmic representations of the measured spectra.…”

“…In particular, inaccuracies may arise from this method if the density of states at the leading edge is low and there is no distinct tangent to take, as occurs for many PIMs reported. 19,27,30,163 A method to visualize gap states is to plot the leading edge on a semilogarithmic scale, since band tails can be described by exponential functions. 172,173 This is illustrated in Figure 6a for organic small molecules.…”

Degradation by Exposure of Coevaporated CH3NH3PbI3 Thin Films

“…The ionization potential of CH 3 NH 3 PbI 3 can be deduced to be 5.71 eV, substantially lower than those obtained from spin cast films whose stoichiometry are known to deviate from ideal. [19,28] The band gap of CH 3 NH 3 PbI 3 was reported to be around 1.55 eV, [28,29] so the prepared film was a slightly n-type film. The XPS core level spectra of the CH 3 NH 3 PbI 3 film before and after exposure to oxygen are shown in Figure 2.…”