Fundamental Carrier Lifetime Exceeding 1 µs in Cs₂AgBiBr₆ Double Perovskite

Abstract

air-stability of some compositions has motivated researchers to find lead-free alternatives. [7,8] A wide range of materials classes have recently been explored computationally and experimentally. [8,9] In evaluating the potential of these new materials for photovoltaics, much of the focus has been on the bandgap, stability, absorption coefficient, and phase. [8,[10][11][12][13] The minority-carrier lifetime has received less attention, yet has historically limited the development of new photovoltaic materials. [14,15] However, the reported lifetimes of lead-free alternatives to the perovskites have typically ranged from <0.1 to ≈10 ns. [15][16][17] Silver-bismuth double perovskites (e.g., Cs 2 AgBiBr 6 and Cs 2 AgBiCl 6 ) have recently been found to be an exception. Time-resolved photoluminescence (TRPL) measurements of these materials show an initial drop in photo luminescence (PL) over a nanosecond timescale by 0.5-2 orders of magnitude, followed by a slow tail in PL decay. These PL decay traces are fit with a bi-or triexponential model and the longest time constant is attributed to the fundamental lifetime, which has been found to be ≥100 ns. [18][19][20] Given that these materials have also been found to be more stable than methylammonium lead iodide, [18,20] they have attracted significant interest, with many recent investigations of new families of double perovskite compounds with novel properties.There is current interest in finding nontoxic alternatives to lead-halide perovskites for optoelectronic applications. Silver-bismuth double perovskites have recently gained attention, but evaluating their carrier lifetime and recombination mechanisms from photoluminescence measurements is challenging due to their indirect bandgap. In this work, transient absorption spectroscopy is used to directly track the photocarrier population in Cs 2 AgBiBr 6 by measuring the ground state bleach dynamics. A small initial drop is resolved in the ground state bleach on a picosecond timescale, after which the remaining photocarriers decay monoexponentially with a lifetime of 1.4 µs. The majority of the early-time decay is attributed to hot-carrier thermalization from the direct transition to the indirect bandgap, and the 1.4 µs lifetime represents the recombination of most photocarriers. From this lifetime, a steady-state excess carrier density of 2.2 × 10 16 cm −3 under 1 sun is calculated, which is an order of magnitude larger than that for methylammonium lead iodide, suggesting that charge transport and extraction can be efficient in Cs 2 AgBiBr 6 solar cells. Double PerovskiteLead-halide perovskites display remarkable optoelectronic properties, with long diffusion lengths >1 µm, [1,2] strong optical absorption on the order of 10 5 cm −1 , [3] and high photoluminescence quantum efficiencies >80%. [4] These properties have led to rapid increases in the efficiency of perovskite solar cells (up to a certified power conversion efficiency of 22.7%) [5] and light emitting diodes (>11% external quantum efficiency) [6] over a sho...

From Lead Halide Perovskites to Lead‐Free Metal Halide Perovskites and Perovskite Derivatives

“…[183][184][185] The VBM is mainly derived from Ag 4d-Br 5p antibonding states, whereas the CBM is mainly derived from Bi 6p-Br 4p antibonding states. [183] Interestingly, for high-quality Cs 2 AgBiBr 6 single crystals, the carrier lifetimes were reported to as high as 668 ns by Slavney et al [155] (Figure 15c) and even >1 µs by Hoye et al [191] These long carrier lifetimes may be related to the indirect bandgap nature of Cs 2 AgBiBr 6 . [185] Also, because of the orbital mismatch, the electronic dimensionality of Cs 2 AgBiBr 6 is lower than 3D, which qualitatively explains the calculated larger (and anisotropic) carrier effective masses of Cs 2 AgBiBr 6 , [183] as compared to its Pb counterpart.…”

Progress of Lead‐Free Halide Double Perovskites

“…[159] However, Cs 2 AgBiBr 6 shows a fascinating property, as the initial rapid decay is followed by a long PL lifetime ≈660 ns, which is much higher than that of MAPbBr 3 (≈170 ns) and is close to that of MAPbI 3 (736 ns-1 µs). [160] In the absence of other limiting photophysical properties exhibited by this compound, such excellent charge transport properties could make it endure higher film thickness with enhanced light absorption and higher PV performance. For this reason, thicker absorber layer may be required for Cs 2 AgBiBr 6 .…”

“…[165] For Cs 2 AgInBr 6 to exhibit decent optoelctronic properties with shallow defects, it must be grown in an Ag-rich and Br-poor condition. [160] Copyright 2018, Wiley-VCH. Conversely, Li et al proposed an In 3+ poor and Cl − rich growth condition to realize suppressed and shallow level defects in the Cs 2 AgInCl 6 analog.…”

Research Update: Bismuth-based perovskite-inspired photovoltaic materials

“…This was two orders of magnitude slower than the photoluminescence decay. 111 Whilst the bandgap is too large for single-junction solar cells, it is suitable for a top-cell in 4-terminal tandem devices on silicon, but the low absorption coefficient may limit the achievable efficiencies. 82 AgBiS 2 thin films have also been investigated for photovoltaic applications.…”

“…Optical, transient absorption and photoemission spectroscopy measurements strongly suggested that the photoluminescence decay related to recombination to defect states in the band-gap, rather than across the indirect bandgap. 111 While no standard iii . However, sub-bandgap peaks dominated the photoluminescence spectrum due to deep defect states introduced by the dopant.…”