Critical Evaluation of the Photovoltaic Performance of (AgI)x(BiI3)y Thin Films from the Viewpoint of Ultrafast Spectroscopy and Photocurrent Experiments

Silver iodobismuthate thin films obtained from mixtures of AgI and BiI3 have been frequently suggested as promising alternatives for lead-based perovskite materials in photovoltaic applications. Here, we investigated (AgI)x(BiI3)y thin films with stoichiometric ratios (x/y) of 3:1, 2:1, 1:1, and 1:2 produced via a low-temperature (<100 degrees C) spin coating process from AgI/BiI3 solutions in dimethyl sulfoxide. Several critical observations on the basis of ultrafast broadband UV-vis transient absorption spectroscopy and photocurrent spectroscopy were made for these Ag-Bi-I materials, which will have a considerable impact on their photovoltaic performance: Their carrier recombination kinetics were independent of the initial carrier number density over the range of 1.3-6.6 x 1017 cm-3 and well approximated by a monoexponential decay with a rate constant krec in the range of 2.0-4.3 x 108 s-1, which is consistent with trap-mediated charge-carrier recombination. Moreover, pronounced coherent phonon dynamics was observed for all of these (AgI)x(BiI3)y compounds, thereby suggesting strong electron-phonon coupling, which will favor charge-carrier localization and nonradiative carrier recombination, in agreement with the virtually absent photoluminescence of these materials. In addition, Fourier-transform step-scan photocurrent spectroscopy (FTPS) on AgBi2I7 provided an Urbach energy of 70 meV and found indications for deep defects (0.6 eV below the band gap), which was also consistent with a trap-mediated recombination mechanism. A combination of all of these effects is likely responsible for the still quite low light-harvesting performance of this class of materials, which has been reported to show photovoltaic conversion efficiencies (PCEs) below 5%. Finally, for "AgI-rich" compounds (3:1, 2:1), we found a substantial separate contribution of carrier loss processes through ultrafast relaxation in AgI domains with time constants of 0.73 and 2.4 ps.