Tail state formation in solar cell absorbers leads to a detrimental effect on solar cell performance. Nevertheless, the characterization of the band tailing in experimental semiconductor crystals is generally difficult. In this article, to determine the tail state generation in various solar cell materials, we have developed a quite general theoretical scheme in which the experimental Urbach energy is compared with the absorption edge energy derived from density functional theory (DFT) calculation. For this purpose, the absorption spectra of solar cell materials, including CdTe, CuInSe 2 (CISe), CuGaSe 2 (CGSe), Cu 2 ZnSnSe 4 (CZTSe), Cu 2 ZnSnS 4 (CZTS) and hybrid perovskites, have been calculated by DFT particularly using very-high-density k meshes. As a result, we find that the tail state formation is negligible in CdTe, CISe, CGSe and hybrid perovskite polycrystals. However, coevaporated CZTSe and CZTS layers exhibit very large Urbach energies, which are far larger than the theoretical counterparts. Based on DFT analysis results, we conclude that the quite large tail state formation observed in the CZTSe and CZTS originates from extensive cation disordering. In particular, even a slight cation substitution is found to generate unusual band fluctuation in CZTS(Se). In contrast, CH 3 NH 3 PbI 3 hybrid perovskite shows the sharpest absorption edge theoretically, which agrees with experiment.

We find that coevaporated Cu2ZnGeSe4 has an ideal bandgap for solar cells (1.39 ± 0.01 eV) and shows quite reduced tail state absorption with a very low Urbach energy of 28 meV, which is far smaller than those of more studied Cu2ZnSnSe4 and Cu2ZnSnS4. The small tail states in Cu2ZnGeSe4 are found to originate from almost perfect cation ordering, while unusual tail state generation occurs in the Sn-based quaternary compounds by extensive cation substitution. Quite remarkably, the crystal total energy derived from first-principles calculations reveals a unified rule for the cation disordering, confirming that the lighter group-IV element (i.e., Ge) is essential for eliminating the tail state generation induced by cation mixing.

To achieve low on-state and switching losses simultaneously in SiC bipolar devices, the depth distribution of the carrier lifetime within the voltage blocking layer and the techniques used for observing the carrier lifetime distribution are important considerations. We developed a measurement system of the time-resolved free carrier absorption with intersectional lights (IL-TRFCA) for the nondestructive measurements of the depth distribution of the carrier lifetime in 4H–SiC thick epilayers. To confirm the reliability of the measurement results, we also performed TRFCA measurements to the cross section of the samples. As a result, although the lifetimes are underestimated owing to an inevitable diffusion of the carriers from the measurement region, the system was able to observe a carrier lifetime distribution up to a depth of 250 μm. Our IL-TRFCA system demonstrated a depth resolution of ∼10 μm, which is the best resolution among previously reported nondestructive measurement techniques. We consider the proposed system to be useful for the development of SiC bipolar devices.

The carrier lifetime is an important parameter for high voltage SiC bipolar devices because its distribution in drift layers affects the device performance. Observation techniques for carrier lifetime, along with the development of carrier lifetime control processes, are important to control carrier lifetime distribution. In this study, we developed a microscopic time-resolved free carrier absorption system that has a variable spot size of excitation light and two different probe light wavelengths (405 and 637 nm). By selecting a relatively small spot size of excitation light and the probe light of shorter wavelength (405 nm), the distribution of carrier lifetime was observed with a high spatial resolution of ∼3 μm. Additionally, by using a relatively large spot size of excitation light and the probe light that leads to stronger free carrier absorption (637 nm), an accurate measurement of carrier lifetime was obtained. The developed system enables the design and development of bipolar SiC devices with carrier lifetime distribution control.

The on-resistance of bipolar devices depends on the carrier lifetime determined by Shockley-Read-Hall, surface, radiation and Auger recombination processes. Values for the Auger recombination coefficient have been previously reported, but the values were constant in each report. However, the Auger recombination coefficient should have dependence on the excited carrier concentration and presence of the traps. In this study, we observed excited carrier recombination in 4H-SiC under the high injection condition by time resolved free carrier absorption measurements. As a result, we found dependence on the excited carrier concentration of the Auger recombination coefficient and negligible effects of the traps on the coefficient.