Strong self-trapping by deformation potential limits photovoltaic performance in bismuth double perovskite

Abstract

Bismuth-based double perovskite Cs2AgBiBr6 is regarded as a potential candidate for low-toxicity, high-stability perovskite solar cells. However, its performance is far from satisfactory. Albeit being an indirect bandgap semiconductor, we observe bright emission with large bimolecular recombination coefficient (reaching 4.5 ± 0.1 × 10−11 cm3 s−1) and low charge carrier mobility (around 0.05 cm2 s−1 V−1). Besides intermediate Fröhlich couplings present in both Pb-based perovskites and Cs2AgBiBr6, we uncover evidence of strong deformation potential by acoustic phonons in the latter through transient reflection, time-resolved terahertz measurements, and density functional theory calculations. The Fröhlich and deformation potentials synergistically lead to ultrafast self-trapping of free carriers forming polarons highly localized on a few units of the lattice within a few picoseconds, which also breaks down the electronic band picture, leading to efficient radiative recombination. The strong self-trapping in Cs2AgBiBr6 could impose intrinsic limitations for its application in photovoltaics.

Highly mobile hot holes in Cs 2 AgBiBr 6 double perovskite

“…Figure 1B illustrates the x-ray diffraction (XRD) pattern of the prepared Cs 2 AgBiBr 6 double perovskite thin film (synthesis details in the Supplementary Materials). All peaks are in good agreement with theoretical calculations and previous experimental results for Cs 2 AgBiBr 6 , ruling out the presence of other phases and unreacted precursors in the synthesized sample ( 23 , 24 ).…”

“…In principle, the thermalization process in perovskites is governed by efficient carrier-carrier scattering, and its scattering rate is expected to scale with 1/ε 2 (with ε as the optical dielectric constant). As ε for both conventional Pb-based organic-inorganic perovskites and Cs 2 AgBiBr 6 falls into a similar range (~5 to 7) ( 3 , 23 ), no substantial difference in thermalization time for both systems is expected. Therefore, one might expect a photoconductivity peak following the thermalization process - (100-200 fs) after optical excitations.…”

Coherent Acoustic Interferometry During the Photo-Driven Oxygen Evolution Reaction Associates Strain Fields with the Reactive Oxygen Intermediate

“…It is also worth noting that such a surface piezoelectric effect has been associated with perovskite octahedra rotation, 67 which could reduce the effects of strain on OER activity. 21 Other than deformations directly associated with the hole polaron, 44 changes in free carrier concentrations can also contribute to interfacial strain. Under CC conditions, VB holes are the dominant free carrier near the surface because of the maintained Schottky barrier and resulting electron drift away from the surface (fig.…”

“…where is the probe vacuum wavelength, ( ) is the real part of the index of refraction in 0.1% Nb:STO (fig. S9), is the acoustic velocity in STO (calculated in Table 1), 42 CAWs as detected by picosecond interferometry (as opposed to a resonantly enhanced mechanism, 43,44 which does not follow the SBS phase matching conditions). For STO, the physical picture that follows is cartooned in fig.…”

Coherent Acoustic Interferometry During the Photo-Driven Oxygen Evolution Reaction Associates Strain Fields with the Reactive Oxygen Intermediate

“…CAWs as detected by picosecond interferometry (as opposed to a resonantly enhanced mechanism, 43,44 which does not follow the SBS phase matching conditions). For STO, the physical picture that follows is cartooned in fig.…”