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In this work, direct observation of ambipolar self-doping in hybrid lead-iodide perovskites has been reported through scanning tunneling spectroscopy (STS) and, thereby, density of states (DOS). Self-doping phenomenon in CH3NH3PbI3 (MAPbI3) and CH(NH2)2PbI3 (FAPbI3) through precursor stoichiometry has led to an alteration in the Fermi energy, and hence a change in the type of electronic conductivity, without affecting the inherent band gap of the materials. From STS and the respective DOS spectra, the band energies of the perovskites with respect to the Fermi energy for a range of precursor ratios have been estimated. The “direct” measurement of band edges with respect to Fermi energy inferred a gradual change in the electronic conductivity from p-type to n-type as both the perovskites were reacted from PbI2-deficient to PbI2-rich precursors. The results have been correlated with point defects generated due to the growth environment (stoichiometry of precursors) of perovskites, providing a new dimension to probe the Fermi energy of hybrid perovskites.
Paul et al. (Thu,) studied this question.