Asynchronous Photoexcited Electronic and Structural Relaxation in Lead-Free Perovskites

Abstract

Vacancy-ordered lead-free perovskites with more-stable crystalline structures have been intensively explored as the alternatives for resolving the toxic and long-term stability issues of lead halide perovskites (LHPs). The dispersive energy bands produced by the closely packed halide octahedral sublattice in these perovskites are meanwhile anticipated to facility the mobility of charge carriers. However, these perovskites suffer from unexpectedly poor charge carrier transport. To tackle this issue, we have employed the ultrafast, elementalspecific X-ray transient absorption (XTA) spectroscopy to directly probe the photoexcited electronic and structural dynamics of a prototypical vacancy-ordered lead-free perovskite (Cs3Bi2Br9). We have discovered that the photogenerated holes quickly self-trapped at Br centers, simultaneously distorting the local lattice structure, likely forming small polarons in the configuration of Vk center (Br2dimer). More significantly, we have found a surprisingly long-lived, structural distorted state with a lifetime of ∼59 μs, which is ∼3 orders of magnitude slower than that of the charge carrier recombination. Such long-lived structural distortion may produce a transient "background" under continuous light illumination, influencing the charge carrier transport along the lattice framework.

Polarons and Charge Localization in Metal‐Halide Semiconductors for Photovoltaic and Light‐Emitting Devices

“…[ 109 ] Alternative spectroscopic approaches have included optical Kerr effect spectroscopy and 2D spectroscopy, [ 84,110,111 ] both of which are also able to provide sub‐picosecond time resolution, as well as slower techniques such as X‐ray transient absorption, which can give insights into structural distortions in a material. [ 112–114 ] We would caution that, although these techniques can provide valuable information regarding ultrafast processes in metal‐halide semiconductors, in general there can be a variety of contributing signals at early times and great care needs to be applied when interpreting specific transient experimental signals.…”

“…[ 10 ] Another study combined optical and x‐ray transient absorption with DFT calculations to argue for the formation of small polarons in Cs 3 □Bi 2 Br 9 , through the formation of V k centers (or, Br 2 − dimers). [ 112 ] Doping of Cs 2 □SnCl 6 nanocrystals with antimony leads to an additional low‐energy emission band, ascribed to triplet‐state self‐trapped excitons, [ 173 ] and a combination of Raman spectroscopy and fitting of PL linewidths found intermediate electron–phonon coupling strengths in Cs 2 □SnI 6 . [ 174 ] The inclusion of larger cations at the A‐site in A 2 □SnI 6 (A = Cs, MA, FA) leads to increased lattice anharmonicity and octahedral tilting, caused by changes in the interoctahedral I–I distances, leading to more tightly bound polarons and lower polaron mobilities.…”

Ultrafast Excited-State Localization in Cs2AgBiBr6 Double Perovskite

“…The phenomenon of self-trapping occurs when the local lattice distortion caused by a photoexcited charge carrier is sufficiently strong that the charge carrier rapidly relaxes into the energetic state associated with this local deformation, 27 such that its localization length may approach the length of a single unit cell of the lattice. 28 Self-trapping of charge carriers has been reported in related materials, whether for electrons in CsPbI 3 29 or for holes in CsPbBr 3 , 30 in other bismuth-based materials such as Rb 4 Ag 2 BiBr 9 31 and Cs 3 Bi 2 Br 9 , 32 and layered metal halide perovskites. 33 For Cs 2 AgBiBr 6 , the proposed self-trapping 34 has also been synonymously 35 described as the formation of small polarons 24 or color centers, 25 with broad photoluminescence (PL) emission 25 , 34 and low charge-carrier mobility 24 attributed to its occurrence.…”

Recent Advances and Opportunities of Lead-Free Perovskite Nanocrystal for Optoelectronic Application

“…However, the PLQY of such QDs is still relatively low, inferior to Pb-based perovskite counterparts, which is mainly due to the residual surface defects caused by dangling bonds on the QD surface [131]. To improve the PL performance, Clanions were employed as the passivating ligands to suppress the surface defect and enhance the radiative recombination of the MA 3 [133,134]. This modified synthesis method with appropriate ligands and Br-rich surface is quite useful for synthesizing highly efficient Bi 3+ -based perovskite QDs.…”