KEYWORDS: Solar cells, Internal quantum efficiency, Perovskite, Luminescence, Terahertz spectroscopy, Quantum efficiency, Time resolved spectroscopy, Spectroscopy, Diffusion, Probability theory
Internal quantum efficiency (IQE) is a key parameter determining solar cell power conversion efficiency. While reported IQEs of metal-halide perovskite solar cells are often close to one, the contributions of photoluminescence reabsorption (PLr) and surface recombination (SR) to IQE has not been elucidated. In this work, both effects are examined by photoluminescence spectroscopies and time-resolved terahertz spectroscopy (TRTS). Then PLr rate and SR velocity are extracted from TRTS kinetics by diffusion theory. At last a model is proposed to calculate the carrier-collection probability and discuss contributions of PLr and SR on the IQE.
Mixed halide, mixed cation lead perovskite films have been demonstrated to benefit tremendously from the addition of Cs and Rb into the perovskite formulation, resulting in high performance, enhanced reproducibility and stability. However, the root cause of these effects in these complicated systems is not well understood. We address the above challenge by tracking in situ the solidification of perovskite precursors during solution-casting using time-resolved grazing incidence wide-angle X-ray scattering (GIWAXS). In doing so, we can directly link the formation or suppression of different crystalline phases to the presence of Cs and/or Rb. In the absence of these elements, the multi-component perovskite film is inherently unstable, phase segregating into a solvated MAI-rich phase and a FABr-rich phase. Adding even one of the two (Cs or Rb) is shown to alter the solidification quite dramatically, promoting different solidification pathways. Importantly, the addition of both components in the optimal ratio can drastically suppress phase segregation and promotes the spontaneous formation of the desired perovskite phase. This result is also confirmed by elemental mapping of organic cations (FA+, MA+) and halide anions (I-, Br-) via time-of-flight secondary ion mass spectroscopy (ToF-SIMS). Perovskite precursors with an optimal combination of additives (7% Cs, 3% Rb) result in solar cells with 20.1% power conversion efficiency (PCE), outperforming formulation excluding Cs and Rb (PCE=14.6%). We propose that the synergistic effect is due to the collective benefits of Cs and Rb on the formation kinetics of the perovskite phase, and on the halides redistribution throughout the film. Importantly, our study points to new design rules for tuning the crystallization pathway of multi-component hybrid perovskites.
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