Impact of Cesium/Rubidium Incorporation on the Photophysics of Multiple‐Cation Lead Halide Perovskites

Abstract

Incorporating cesium (Cs) or rubidium (Rb) cations into multiple‐cation lead mixed halide perovskites (FA0.83MA0.17Pb(I0.83Br0.17)3) increases their photovoltaic performance. Herein, the fundamental photophysics of perovskites are investigated by steady‐state and transient optical spectroscopy and the reasons for the performance increase are revealed. Cs/Rb‐cation incorporation slightly increases the bandgap, whereas exciton binding energies remain in the range of a few meV. Urbach energies are reduced, suggesting improved perovskite microstructure upon Cs/Rb incorporation. Carrier density‐induced broadening of the photo‐bleaching following the Burstein–Moss model is observed, and the effective carrier masses are determined to be a few tenths of the electron rest mass. From fits of the high‐energy tail of the perovskite's photo bleach to Boltzmann's distribution, subpicosecond hot‐carrier cooling is revealed, implying strong carrier–phonon coupling. Importantly, the charge carrier recombination dynamics indicate that Cs/Rb‐incorporation reduces both the first‐order (trap‐assisted) and the second‐order (radiative) recombination, which appears to be the main reason for the observed performance increase upon Cs/Rb‐cation incorporation. Overall, this work presents a detailed study of the photophysics of multiple‐cation mixed halide lead perovskites and develops a concise picture of the impact of cesium/rubidium incorporation on the photophysics and device performance.

Photo-induced enhancement of lattice fluctuations in metal-halide perovskites

“…A blue shift of the photo-bleach peak induced by band filling is observed (inset in Fig. 1b ), implying that the photo-induced bandgap narrowing should be smaller than the Burstein-Moss shift Δ E BM , which can be estimated from the broadening of the TA photo-bleach peak (~38 meV for N = 3.5 × 10 18 cm −3 ) 44 . Hence, an upper limit of Δ R b / R b 0 ~ 10% can be determined from Eq.…”

Phase‐Pure Engineering for Efficient and Stable Formamidinium‐Based Perovskite Solar Cells

“…The optimized devices eventually realized higher PCEs, V oc, fill factor, and excellent thermal stability, which retained more than 95% of the initial PCE at 80 °C over 200 h. The role of Cs‐ and Rb‐incorporated in MA/FA mixed perovskite devices Cs 0.05 [FA 0.83 MA 0.17 ] 0.95 Pb(I 0.83 Br 0.17 ) 3 and Rb 0.05 [FA 0.83 MA 0.17 ] 0.95 Pb(I 0.83 Br 0.17 ) 3 on the photophysics and device performance proved that incorporation of Cs or Rb inevitably slightly increased the bandgaps of perovskites and exciton binding energies combined with the first‐principles calculations. [ 112 ] Interestingly, the short‐circuit currents were not affected, whereas the exciton binding energies are still lower than the thermal energy at room temperature. The charge carrier recombination dynamics show that incorporation of Cs or Rb may effectively reduce the trap‐assisted recombination and radiative recombination, which is also the main reason for the improvement of device performance.…”

Bandgap Engineering of Lead-Free Halide Double Perovskites

“…The excellent performance is mainly due to their superior optoelectronic properties, such as suitable and tunable direct band gap, high absorption coefficient, balanced and small carrier effective masses, long carrier lifetime and diffusion length, small exciton binding energy and high defect tolerance (Figure 1.2). 11,[13][14][15][16][17] Figure 1.2 Advantages and optoelectronic applications of lead halide perovskites.…”