Critical Evaluation of the Photovoltaic Performance of (AgI) x (BiI 3 ) y Thin Films from the Viewpoint of Ultrafast Spectroscopy and Photocurrent Experiments

Silver iodobismuthate thin films obtained from mixtures of AgI and BiI 3 have been frequently suggested as promising alternatives for lead-based perovskite materials in photovoltaic applications. Here, we investigated (AgI) x (BiI 3 ) y thin films with stoichiometric ratios ( x / y ) of 3:1, 2:1, 1:1, and 1:2 produced via a low-temperature (<100 °C) spin coating process from AgI/BiI 3 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 × 10 17 cm –3 and well approximated by a monoexponential decay with a rate constant k rec in the range of 2.0–4.3 × 10 8 s –1 , which is consistent with trap-mediated charge-carrier recombination. Moreover, pronounced coherent phonon dynamics was observed for all of these (AgI) x (BiI 3 ) 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 AgBi 2 I 7 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.