Exciton Dynamics and Electron–Phonon Coupling Affect the Photovoltaic Performance of the Cs₂AgBiBr₆ Double Perovskite

Abstract

Lead-free double perovskites have been proposed as promising nontoxic photovoltaic materials for the replacement of lead perovskites. While the latter ones reach remarkably high power conversion efficiencies (PCEs) above 23% in small lab devices, the lead-free double perovskites so far have severely underperformed, with PCEs below 3% for the prototypical system Cs 2 AgBiBr 6 , in spite of considerable optimization efforts by several groups. Here, we present a detailed study of Cs 2 AgBiBr 6 thin films deposited on poly(methyl methacrylate) and mesoporous TiO 2 . Femtosecond UV−vis−NIR transient absorption experiments clearly identify the presence of excitons. In addition, strong electron−phonon coupling via Froḧlich interactions is observed in terms of pronounced coherent oscillation of a strong A 1g optical phonon mode of the double perovskite at 177 cm −1 . Similar behavior is also found for the related vacancy-ordered perovskite Cs 3 Bi 2 Br 9 and the parent compound BiBr 3 . Excitonic effects and electron−phonon coupling are known to induce unwanted electron−hole recombination and hamper carrier transport. New strategies will thus be required for efficient carrier extraction at the interfaces of the double perovskite with electron and hole transport layers.

Post‐Treatment Engineering of Vacuum‐Deposited Cs2NaBiI6 Double Perovskite Film for Enhanced Photovoltaic Performance

“…www.advancedsciencenews.com www.pss-a.com with smooth surfaces. The X-ray diffraction (XRD) pattern in Figure 1d demonstrates the diffraction peaks of Cs 45.78 , which are in agreement with the literature. [30] The diffraction peaks at 2θ of 9.72 and 39.46 are due to the presence of trace amount of Cs 3 Bi 2 I 9 side product, which is due to the decomposition of Cs 2 NaBiI 6 during the cooling period of the chemical synthesis process.…”

“…However, the exciton PL emission at 550 nm significantly increases when the c ‐TiO 2 layer is applied, revealing the higher exciton recombination rate at the interface. For double perovskite materials, it has been found that excitonic effects and electron–phonon coupling are their intrinsic characteristics, which could induce unwanted electron–hole recombination and hamper carrier transport . Also, the presence of the c ‐TiO 2 layer could somehow enhance this excitonic effects and electron–phonon coupling phenomenon.…”

“…For double perovskite materials, it has been found that excitonic effects and electron-phonon coupling are their intrinsic characteristics, which could induce unwanted electron-hole recombination and hamper carrier transport. [45] Also, the presence of the c-TiO 2 layer could somehow enhance this excitonic effects and electron-phonon coupling phenomenon. Therefore, such effects might be the reason that leads to the poor FF of Cs 2 NaBiI 6 -based PSCs.…”

Towards predictive band gaps for halide perovskites: Lessons from one-shot and eigenvalue self-consistent GW

“…We note that all experimental band gaps cited here are optical gaps. MAPbX 3 (X=I, Br), CsSnBr 3 , and Cs 2 TlAgX 6 (X=Br, Cl) possess exciton binding energies <50 meV 7,16 , while the exciton binding energy of Cs 2 AgBiBr 6 has recently been reported to be ∼270 meV 35 .…”

Hot carrier relaxation in Cs2TiIyBr6−y (y = 0, 2 and 6) by a time-domain ab initio study

“…To understand the relaxation process better, the averaged carrier-phonon NAC in Cs 2 TiI y Br 6Ày was calculated. 22,24,45 The formula for the average carrier-phonon NAC is…”