Genki Yamashita

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.

Intrinsic carrier multiplication efficiency in bulk Si crystals evaluated by optical-pump/terahertz-probe spectroscopy

We estimated the carrier multiplication efficiency in the most common solar-cell material, Si, by using optical-pump/terahertz-probe spectroscopy. Through close analysis of time-resolved data, we extracted the exact number of photoexcited carriers from the sheet carrier density 10 ps after photoexcitation, excluding the influences of spatial diffusion and surface recombination in the time domain. For incident photon energies greater than 4.0 eV, we observed enhanced internal quantum efficiency due to carrier multiplication. The evaluated value of internal quantum efficiency agrees well with the results of photocurrent measurements. This optical method allows us to estimate the carrier multiplication and surface recombination of carriers quantitatively, which are crucial for the design of the solar cells.

Sensitive monitoring of photocarrier densities in the active layer of a photovoltaic device with time-resolved terahertz reflection spectroscopy

We demonstrate the sensitive measurement of photocarriers in an active layer of a GaAs-based photovoltaic device using time-resolved terahertz reflection spectroscopy. We found that the reflection dip caused by Fabry-Perot interference is strongly affected by the carrier profile in the active layer of the p-i-n structure. The experimental results show that this method is suitable for quantitative evaluation of carrier dynamics in active layers of solar cells under operating conditions.

Evaluation of complex conductivity in a circularly polarized field by terahertz time-domain reflection spectroscopy with a phase shifter

We propose a smart technique for directly evaluating complex conductivity in a circularly polarized field with no polarimeter by time-domain reflection spectroscopy using a terahertz (THz) phase shifter and a polarized beam splitter. We performed THz time-domain spectroscopy on a doped InSb wafer under a magnetic field with a δ-phase shifter based on parallel metal waveguides. Despite the π/2 phase shifter with low accuracy, we obtained the conductivity spectrum shifted by cyclotron frequency while maintaining a Drude-like spectral shape. This technique paves the way for new magneto-optical spectroscopy methods.

Quantitative study of energy-transfer mechanism in Eu,O-codoped GaN by time-resolved photoluminescence spectroscopy

In order to investigate the excitation processes in Eu,O-codoped GaN (GaN:Eu,O), the time-resolved photoluminescence signal including the rising part is analyzed. A rate equation is developed based upon a model for the excitation processes in GaN:Eu to fit the experimental data. The non-radiative recombination rate of the trap state in the GaN host, the energy transfer rate between the Eu3+ ions and the GaN host, the radiative transition probability of Eu3+ ion, as well as the ratio of the number of luminescent sites (OMVPE 4α and OMVPE 4β), are simultaneously determined. It is revealed and quantified that radiative transition probability of the Eu ion is the bottleneck for the enhancement of light output from GaN:Eu. We also evaluate the effect of the growth conditions on the luminescent efficiency of GaN:Eu quantitatively, and find the correlation between emission intensity of GaN:Eu and the fitting parameters introduced in our model.

Quantitative monitoring of the internal field in the depletion layer of a GaAs-based solar cell with terahertz radiation

We measured the time profiles of terahertz (THz) radiation emitted from a GaAs-based solar cell under weak excitation with ultrashort optical pulses. The time-domain THz waveform directly reflects the characteristic saturation and overshooting of the drift current under high fields in the thin depletion layer of the solar cell, from which we can evaluate the internal electric field quantitatively. We also measured the time profiles of THz radiation for simultaneous excitation with ultrashort pulses and continuous light and observed the reduction of the internal field in the depletion layer of the solar cell during operation. Since nonlinear responses of the drift current in the depletion layer can be observed, we conclude that THz measurements provide a key technology for characterizing the solar cell performance that can be expected under practical operating conditions.We measured the time profiles of terahertz (THz) radiation emitted from a GaAs-based solar cell under weak excitation with ultrashort optical pulses. The time-domain THz waveform directly reflects the characteristic saturation and overshooting of the drift current under high fields in the thin depletion layer of the solar cell, from which we can evaluate the internal electric field quantitatively. We also measured the time profiles of THz radiation for simultaneous excitation with ultrashort pulses and continuous light and observed the reduction of the internal field in the depletion layer of the solar cell during operation. Since nonlinear responses of the drift current in the depletion layer can be observed, we conclude that THz measurements provide a key technology for characterizing the solar cell performance that can be expected under practical operating conditions.

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.