Le Quang Phuong

Dynamic Optical Properties of CH3NH3PbI3 Single Crystals As Revealed by One- and Two-Photon Excited Photoluminescence Measurements

The dynamic optical properties of perovskite CH3NH3PbI3 single crystals were studied by means of time-resolved photoluminescence (PL) spectroscopy at room temperature. The PL peak under one-photon excitation exhibits a red-shift with elapsing time, while two-photon PL is time-independent and appears at lower energy levels. The low-energy two-photon PL can be attributed to emissions from the localized states because of strong band-to-band absorption and photon re-absorption of the emitted light in the interior region. We revealed that the PL behaviors can be explained by the diffusion of photocarriers generated in the near-surface region to the interior region. The excitation fluence dependence of the one-photon PL dynamics is also discussed in terms of the electron-hole radiative recombination and carrier diffusion effects.

Free Carriers versus Excitons in CH3NH3PbI3 Perovskite Thin Films at Low Temperatures: Charge Transfer from the Orthorhombic Phase to the Tetragonal Phase

We have investigated the dynamic optical properties of CH3NH3PbI3 (MAPbI3) perovskite thin films at low temperatures using time-resolved photoluminescence, optical transient absorption (TA), and THz TA spectroscopy. Optical spectroscopic results indicate that the high-temperature tetragonal phase still remains in the MAPbI3 thin films at low temperatures in addition to the major orthorhombic phase. The fast charge transfer from the orthorhombic phase to the tetragonal phase is likely to suppress the formation of excitons in the orthorhombic phase. Consequently, the near-band-edge optical responses of the photocarriers in both the tetragonal and orthorhombic phases of the MAPbI3 thin films are more accurately described by a free-carrier model, rather than an excitonic model even at low temperatures.

Photocarrier localization and recombination dynamics in Cu2ZnSnS4 single crystals

We have studied the photocarrier localization and recombination dynamics in Cu2ZnSnS4 single crystals at room temperature. The band-gap energy and tail states below the band edge were evaluated by a combination of photoluminescence (PL), PL excitation, photocurrent, and femtosecond transient reflectivity spectroscopy. The photocarriers are rapidly localized to shallow tail states within a typical time constant of several picoseconds to a few tens of picoseconds. The sub-nanosecond PL decay dynamics indicate the importance of multiple carrier trapping processes in the shallow tail states. Therefore, it is concluded that the tail states dominate the optical responses of Cu2ZnSnS4 single crystals.

Temperature-dependent photocarrier recombination dynamics in Cu2ZnSnS4 single crystals

Time-resolved photoluminescence (PL) measurements have been used to study the temperature-dependent photocarrier recombination dynamics in Cu2ZnSnS4 (CZTS) single crystals. We found a significant change of nearly four orders of magnitude of the PL decay time, from microseconds at low temperatures to subnanoseconds at room temperature. The slow PL decay at low temperatures indicates localization of the photocarriers at the band tails. Due to the large band tail states, the PL decay time depends strongly on both the photon energy and excitation density. It is pointed out that the drastically enhanced nonradiative recombination at high temperatures is one of the main factors that determine the power conversion efficiency of CZTS-based solar cells.

Ultrafast free-carrier dynamics in Cu2ZnSnS4 single crystals studied using femtosecond time-resolved optical spectroscopy

We studied the dynamics of photogenerated carriers in Cu2ZnSnS4 (CZTS) single crystals using femtosecond transient reflectivity (TR) and optical pump-THz probe transient absorption (THz-TA) spectroscopy. The TR and THz-TA decay dynamics consistently showed that free carriers have long lifetimes of up to a few nanoseconds. The excitation-photon-energy-dependent TR measurements revealed a slow picosecond energy relaxation of free carriers to the band edge in CZTS. The relaxation and recombination dynamics of free carriers were affected by nonradiative recombinations at the surface. Our results revealed a global feature of energy relaxation and recombination processes of free carriers in CZTS single crystals.

Photocarrier dynamics in undoped and Na-doped Cu2ZnSnS4 single crystals revealed by ultrafast time-resolved terahertz spectroscopy

We investigated the effects of sodium doping on the photocarrier dynamics in Cu2ZnSnS4 (CZTS) single crystals using optical pump-THz probe transient reflectivity (THz-TR) and time-resolved photoluminescence (PL) spectroscopy. The THz-TR and PL decay dynamics are influenced by sodium doping, and their sodium-induced changes are consistent with each other. These time-resolved measurements revealed that the lifetime of photocarriers increases with sodium doping. This result indicates that a part of defects is suppressed by doping sodium into CZTS and implies that sodium doping improves the charge transport properties of CZTS, leading to an improvement in the performance of CZTS-based solar cells.

Free excitons and exciton-phonon coupling in CH 3 NH 3 PbI 3 perovskite single crystals at low temperatures

Hybrid lead halide perovskites, such as methylammonium lead halides CH3NH3PbX3 (MAPbX3, X = I, Br, Cl), are emerging as an excellent class of materials for various optoelectronic applications including solar cells, light-emitting diodes, lasers, and photodetectors [1,2]. As for these perovskite-based devices, deep understanding of transport properties of excitons and charge carriers in perovskite layer is essentially needed in order to achieve a better performance. Although the recombination dynamics in MAPbX3 materials have been investigated extensively [3,4], there are limited works devoted to elucidating the scattering processes of excitons and charge carriers with phonons and impurities. Thus far, the spectral width extracted from conventional photoluminescence (PL) spectroscopy has been used to examine the electron- and exciton-phonon interactions in perovskite polycrystalline thin films [5]. However, polycrystalline thin films consist of grains with various sizes and compositions; these lead to a broad inhomogeneous PL spectral width, and therefore, prevent an appropriate evaluation of the scattering processes. In addition, PL spectrum at low temperatures is composed of multiple emission bands [4,5] and usually dominated by the emission band from the bound excitons due to their giant oscillator strength. A precise extraction of the spectral width of the free-exciton emission band could not be obtained qualitatively based on only PL measurement at low temperatures. Here, we report a study of the free-exciton properties and the exciton-phonon interaction in orthorhombic-phase MAPbI3 single crystals by means of simultaneous temperature-dependent photocurrent (PC) and PL measurements.

Nonlinear photocarrier recombination dynamics in mixed-halide CH3NH3Pb(I1− x Br x )3 perovskite thin films

Mixed-halide perovskites, whose bandgap energies can be widely controlled through choice of composition, are promising for various optoelectronic applications. Herein, we report the photocarrier recombination dynamics in mixed-halide CH3NH3Pb(I1− x Br x )3 perovskite films with different Br contents. We observed small changes in the single-carrier trapping rate with respect to the Br content. In contrast, the two-carrier radiative and three-carrier Auger recombination rates increased significantly with the Br content. These increases in the multicarrier recombination rates likely originated from the enhancement of the Coulomb interactions between electrons and holes caused by incorporating Br. Our findings are useful for designing mixed-halide perovskite-based optoelectronic devices.

Impact of alkali doping on carrier transport in Cu 2 ZnSn(S, Se) 4 thin films for solar cell applications

The effects of alkali doping (Na, Li, K) on the carrier transport in Cu2ZnSn(S, Se)4 (CZTSSe) thin films have been studied systematically using time-resolved THz spectroscopy. Na and Li doping seem to have no influence on the carrier mobility, whereas K doping leads to a noticeable decrease. In addition, we found that Na doping increases the lifetime of photocarriers, but Li and K doping do not. Consequently, we are able to evaluate the diffusion length of photocarriers as a function of the alkali dopants. This result provides a helpful orientation for future efforts to improve the performance of CZTSSe-based solar cells.

Photoelectrical properties of undoped and Na-doped Cu2ZnSnS4 single crystals measured by optical time-resolved spectroscopy

We have studied photocarrier recombination and relaxation dynamics in Cu2ZnSnS4 (CZTS) single crystals using a combination of various optical time-resolved spectroscopy. The band tails and deep defects govern primarily the photocarrier dynamics in CZTS. We pointed out the physical origins behind low power conversion efficiencies of CZTS-based solar cells. In addition, we found that the photocarrier lifetime increases as Na is doped in CZTS; this result implies that Na doping suppresses a part of nonradiative recombination centers in CZTS. Therefore, Na dopant is important to improve the photovoltaic performance of CZTS-based solar cells.