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

Abstract

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.

Ultrafast Carrier Dynamics and Electrical Properties of the Palladium-Based Vacancy-Ordered Perovskite Cs2PdBr6

“…Instead, the charge carrier dynamics in these Cs 2 PdBr 6 thin films appears to be similar to those of other systems investigated previously: For instance, fluence-dependent experiments on zero-dimensional (CH 3 NH 3 ) 3 Sb 2 I 9 found no changes of the time constants, which were explained by geminate recombination of locally trapped electrons and holes . Furthermore, silver iodobismuthates showed fluence-independent carrier dynamics, which was interpreted as a trap-mediated recombination . Geminate and trap-assisted recombination of the Cs 2 PdBr 6 films will be particularly problematic when it comes to photovoltaic applications.…”

“…As in our previous study of silver iodobismuthates, we describe the energy-dependent EQE norm by a simple model function, which consists of a sum of an exponential Urbach tail and a point defect with a Gaussian-type energy density of states: , EQE norm = EQE a exp true( prefix− E − E normalt E normalU true) + EQE b true[ 1 + normalerf ( E − E d σ d 2 ) true] Here, EQE a , E t , and E U are a scaling factor, an adjustable energy parameter (very close to the position of the lowest-energy direct transition), and the Urbach energy, respectively. The parameters EQE b , E d , and σ d 2 represent another scaling factor, the energy of the defect, and the variance of the Gaussian energy density of states of the point defect, respectively. , …”

“…Fourier transform step-scan photocurrent spectroscopy (FTPS) of Cs 2 PdBr 6 -coated interdigitated ITO substrates over the spectral range 633–1100 nm was performed using a setup that is based on a Bruker IFS 66v/s FTIR spectrometer. It was employed previously for the electrical characterization of hydrogenated amorphous silicon and silver iodobismuthate thin films . For experiments on Cs 2 PdBr 6 , the chopping frequency was 40 Hz.…”

Preparation of nano dumbbell-shaped silver bismuth iodide perovskite hybrids and their enhanced photocurrent properties

“…This new metal-perovskite nanostructure with high photocurrent paves the way to realize hotcarrier devices with a performance potentially exceeding that of conventional devices. 28,43 Heterostructures consisting of metal and semiconductor nanoparticles have been widely studied for plasmon-derived hot-electron devices (i.e., metal-semiconductor Schottky junction devices). The performances of these devices strongly depend on the efficiencies of plasmon-hot electron conversion at the metal-semiconductor interfaces.…”

“…Interestingly, rudorffites structure-type silver-incorporated bismuth halide perovskite with a direct band gap of about 1.7 eV to 1.83 eV is a suitable material as an efficient visible photon absorber. The silver bismuth heterostructure rudorffite-type perovskites, including Ag 3 BiI 6 , 21–23 Ag 2 BiI 5 , 24,25 AgBiI 4 26,27 and AgBi 2 I 7 , 28 possess comparatively higher stability under ambient atmospheric conditions. Accordingly, the rudorffite structure has become a good candidate for photovoltaic applications, resulting in a solar cell efficiency of up to 4% for Ag 3 BiI 6 .…”