Critical Evaluation
of the Photovoltaic Performance
of (AgI)x(BiI3)y Thin Films from the Viewpoint of Ultrafast Spectroscopy
and Photocurrent Experiments
posted on 2023-01-10, 21:05authored byAlexander Merker, Marius Morgenroth, Mirko Scholz, Thomas Lenzer, Kawon Oum
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 °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 × 1017 cm–3 and well approximated by a monoexponential decay
with a rate constant krec in the range
of 2.0–4.3 × 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.