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Cu2Sn1-xGexS3 (CTGS) is a compound composed of relatively abundant elements in the crust of the earth. The band gap of CTGS can be tuned by substituting elements at the Sn and Ge sites, making it an attractive material for low-environmental-impact solar cells. In this study, CTGS thin films were fabricated with a controlled [Ge]/([Ge] + [Sn]) composition ratio (x) by combining the co-evaporation method and sulfurization in an infrared furnace. Furthermore, the effect of Na on the CTGS and changes in the solar cell properties were investigated by stacking and sulfurizing NaF on the precursor fabricated using the co-evaporation method. As a result, CTGS with varying x was successfully fabricated by varying the deposition time of the Cu2GeS3 layer using co-evaporation. Additionally, CTGS prepared by doping with Na showed enlarged CTGS crystals compared to Na-free CTGS. The fabricated CTGS solar cells achieved a power conversion efficiency of more than 4.5% after doping with Na. [ABSTRACT FROM AUTHOR]

As a first step toward the realization of high-efficiency Cu2(Sn1-xGex)S3 (CTGS) solar cells, this study investigates the effects of the growth process, focusing on the crystalline structure and morphology of CTGS films formed via different growth processes. These processes include the sulfurization of the Ge/Cu/Sn-S precursor; sulfurization of Cu2SnS3 (CTS) with S and germanium sulfide (GeS) vapor; and co-evaporation of Cu, Sn, Ge, and S. The CTGS films obtained by sulfurization of the Ge/Cu/Sn-S precursor consistently exhibited the largest grain sizes. However, the surface roughness of CTGS films increased with increasing x ratio. Conversely, the CTGS films obtained by the re-sulfurization of CTS films previously formed in S and GeS mixed vapor exhibited both a large grain size and flat surface roughness, rendering them suitable for forming the p-n interface in a solar cell. Therefore, the re-sulfurization of CTS films previously formed in S and GeS mixed vapor is key for achieving high-efficiency solar cells. [ABSTRACT FROM AUTHOR]

In this study, as a novel approach to thin-film solar cells based on tin sulfide, an environmentally friendly material, we attempted to fabricate (Ge, Sn)S thin films for application in multi-junction solar cells. A (Ge0.42 Sn0.58)S thin film was prepared via co-evaporation. The (Ge0.42 Sn0.58)S thin film formed a (Ge, Sn)S solid solution, as confirmed by X-ray diffraction (XRD) and Raman spectroscopy analyses. The open circuit voltage (Voc), short circuit current density (Jsc), fill factor (FF), and power conversion efficiency (PCE) of (Ge0.42 Sn0.58)S thin-film solar cells were 0.29 V, 6.92 mA/cm2, 0.34, and 0.67%, respectively; moreover, the device showed a band gap of 1.42–1.52 eV. We showed that solar cells can be realized even in a composition range with a relatively higher Ge concentration than the (Ge, Sn)S solar cells reported to date. This result enhances the feasibility of multi-junction SnS-system thin-film solar cells. [ABSTRACT FROM AUTHOR]

Ag8SnS6 (ATS) has been reported to have a band gap of 1.33 eV and is expected to be a suitable material for the light-absorbing layers of compound thin-film solar cells. However, studies on solar cells that use ATS are currently lacking. The objective of this study is to obtain high-quality ATS thin films for the realization of compound thin-film solar cells using vacuum deposition and sulfide annealing. First, glass/SnS/Ag stacked precursors are prepared by vacuum deposition. Subsequently, they are converted to the ATS phase via sulfide annealing, and various process conditions, namely, annealing time, annealing temperature, and number of steps, are studied. By setting the heat treatment temperature at 550 °C and the heat treatment time at 60 min, a high-quality ATS thin film could be obtained. Multi-step heat treatment also produces thin films with nearly no segregation or voids. [ABSTRACT FROM AUTHOR]

The dependence of photoluminescence (PL) on sulfurization temperature of the Cu2SnS3 (CTS) thin films was investigated. CTS thin films were prepared at various sulfurization temperatures in the range of 500–600 ℃, and differences in the conversion efficiency of solar cells with the CTS thin films as absorption layer and the crystal structure of the CTS thin films were observed. In low-conversion-efficiency CTS films, which were a mix of monoclinic and cubic crystals, the PL spectrum only showed donor–acceptor pair (DAP) recombination luminescence due to deep defects. In high-conversion-efficiency CTS thin films, which contained only monoclinic crystals, the PL spectrum showed DAP recombination luminescence originating from the same deep defects. In addition band-edge luminescence was observed at room temperature. Thus, it was found that the conversion efficiency and crystal structure of CTS thin film can be easily estimated from PL measurements. [ABSTRACT FROM AUTHOR]

The effects of Ag in (Cu1–xAgx)2SnS3 (CATS) were investigated in terms of photoluminescence (PL) lifetime, carrier concentration, and solar cell performance with varying Ag/(Cu + Ag) ratios, x. The PL lifetime of CATS solar cells had a maximum value when x was within 7%. A similar tendency was observed for other parameters of the CATS thin films and solar cells. One of the reasons for this is that the number of non-radiative recombination centers in the CATS layer is suppressed by the sulfurization process with Ag. Therefore, even if the Ag content changes slightly in the 0%–11% range, the crystal quality is changed in CATS, resulting in a significant effect on the properties of solar cells. These investigations on the characterization and device physics can be applied to improve the efficiency of CATS solar cells. [ABSTRACT FROM AUTHOR]

Temperature‐dependent optical absorption spectra from transmittance and reflectance measurements of Cu2SnS3 (CTS) thin films which are promising material for solar cells were investigated. Thin film CTS samples with Cu‐poor, near‐stoichiometric, and Cu‐rich compositions were prepared on glass substrates by thermal co‐evaporation and sulfurization. At low temperature, three band‐to‐band (BB) transitions that are allowed between triple upper valence bands and a single lower conduction band were observed for all samples. The square of the product of the absorption coefficient and the photon energy was plotted to estimate the band gap energy for three bands of the Cu‐poor sample. On the other hand, excitonic (EX) transitions that corresponded to three bands were observed for the Cu‐rich and near‐stoichiometric samples. Temperature‐dependent optical absorption spectra with the triple BB and EX transitions were resolved using a simplified fitting equation with Lorentzian functions as the EX transitions. The band gap, EX transition, and exciton binding energies for the lowest energy band of the Cu‐rich sample were determined to be 0.945 eV, 0.936 eV, and 8.9 meV at 6 K, respectively. In the low‐temperature region, anomalous blue‐shifts of the estimated band gap energy with increasing temperature were obtained for three bands of all samples. [ABSTRACT FROM AUTHOR]