Very recently, lead-halide perovskites have received much attention as a promising class of low-cost and high-performance photovoltaics materials since the first report of all-solid-state perovskite solar cells (SCs) in 2012 [1]. Organic-inorganic hybrid CH₃NH₃PbI₃ is a model material of metal-halide perovskites, and its optical and electronic properties have been insensitively studied. It is known that perovskite CH₃NH₃PbI₃ has excellent optoelectronic properties for SC applications: large optical absorption coefficients, extremely low densities of traps and defects, and long diffusion lengths of carriers. Because the energy conversion efficiencies of SCs depend on the luminescence quantum efficiency of the target semiconductor material [2,3], photoluminescence (PL) spectroscopy is one of the most powerful techniques for characterizing and developing new materials and structures for SC devices. By taking advantage of high-efficiency PL of lead-halide perovskites, we can directly monitor their carrier generation, recombination, and transport dynamics of SCs using time-resolved PL spectroscopy [4]. Here, we summarize the optical properties of solution-processed CH₃NH₃PbI₃ and CH₃NH₃PbBr₃thin films and single crystals and discuss the physical origin of high conversion efficiencies in perovskite SCs.

Optical studies on high-quality thin films and single crystals are required for a deep understanding of the intrinsic optoelectronic properties of lead-halide perovskites. We studied photocarrier dynamics of high-quality CH₃NH₃PbI₃ and CH₃NH₃PbBr₃thin films and single crystals using one-photon excited PL (1PL) and two-photon excited PL (2PL) spectroscopy [5-7]. In thick single crystals, the photocarriers are generated at the near-surface region under one-photon excitation and in the interior region under two-photon excitation. The time-integrated 1PL peak energy is higher than the 2PL peak energy. The 1PL peak energy shifts to lower energy with delay time after laser excitation. The 1PL lifetime is shorter than the 2PL lifetime. These dynamical PL behaviors of thick single crystals are different from those of thin films. In thin films, the photocarriers are created homogeneously throughout the sample thickness under one- and two-photon excitation [8]. The spatial distribution of photocarriers determines the PL spectrum and dynamics: The PL dynamics of thick single crystals can be explained by fast carrier diffusion and photon recycling (photon emission and reabsorption) [5-7].

In SC device structures, the perovskite CH₃NH₃PbI₃ layer is sandwiched between the mesoporous TiO₂electron-transport layer and the spiro-OMeTAD hole-transport layer. Charge carrier dynamics in the perovskite solar cells were studied by means of PL and photocurrent (PC) imaging spectroscopy. Simultaneous PL and PC imaging measurements revealed a positive correlation between the PL intensity and PL lifetime, and a negative correlation between PL and PC intensities [8]. These correlations are a result of the competition between the escape of photocarriers from the perovskite layer and the recombination of photocarriers within the perovskite layer. Moreover, from the excitation-fluence dependence of the PL decay curves in thin films and SCs, we conclude that the carrier injection at the interface determines the device performance of perovskite SC [9,10]. Our systematic PL studies provide an essential information for developing high-performance devices based on metal-halide perovskites.

Part of this work was supported by JST-CREST (Grant No. JPMJCR16N3).

References

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