Textured PbI2 photocathodes obtained by gas phase anion replacement

High quality PbI2 films are prepared by new route based on iodination of solution processed PbS films at room temperature. The PbI2 films are characterized by a highly texture morphology with flake-like particles sizing 500 and 800 nm across. Quantitative X-ray diffraction and resonance Raman spectroscopy demonstrate the crystallization of highly phase pure 2H-PbI2 with lattice parameters: a = 4.5505(9) Å, c = 6.9784(14) Å. Electronic structures
and vibrational modes are calculated employing DFT. Transmittance and reflectance measurements are characterized by a sharp absorption edge corresponding to a direct band gap transition of 2.41 eV. Photoelectrochemical measurements carried out in 0.1 M KI solution showed that the material behaves as a p-type semiconductor with an apparent donor concentration of the order of 1015 cm-3 as well as stable photoresponses associated with hydrogen evolution with nearly 50 % incident photon-to-current efficiency. Quantitative analysis of the spectral and potential dependence of the photocurrent responses shows that surface recombination is negligible even at potential close to the flat band value. We conclude that Ispecies stabilizes the semiconductor surface, generating an ideal semiconductor/electrolyte junction, while enabling the transfer of minority carrier from the conduction band edge dominated by lead antibonding p-orbitals.