Mitsutaro Umehara

Cu2Sn1-xGexS3 (x = 0.17) Thin-Film Solar Cells with High Conversion Efficiency of 6.0%

We have fabricated Cu2Sn1-xGexS3 thin-film solar cells by cosputtering deposition of Cu and Sn followed by sulfurization in S and GeS2 vapors. The conversion efficiency was significantly improved to be as high as 6.0% compared with the values of Cu2SnS3 solar cells similarly fabricated. Scanning electron microscopy observation revealed that alloying with Ge accelerated the grain growth during the sulfurization process, contributing to the improvement in the conversion efficiency. The bandgap energy of Cu2Sn0.83Ge0.17S3 was about 1.0 eV, which is suitable for bottom cells used in double-junction solar cells.

Cu2Sn1− x Ge x S3 solar cells fabricated with a graded bandgap structure

We fabricated Cu2Sn1− x Ge x S3 (CTGS) solar cells with a graded bandgap structure in order to improve their photovoltaic performance. Bandgap gradation was formed by changing the Ge/Sn ratio in the depth direction of the CTGS layers. The composition profile of each sample was measured by secondary ion mass spectrometry, and we confirmed that the Ge/Sn ratio near the buffer layer was lower than that near the back electrode. This means that the bandgap increases with depth from the surface. The performance of the cells was improved to over 6.7% in conversion efficiency.

Laser Annealing to Form High-Temperature Phase of FeS2

We fabricated single-phase pyrite thin films of FeS2 by laser annealing of multi-phase FeS2 films. Sputter-deposited FeS films followed by sulfurization in sulfur vapor at high temperatures were mainly composed of the high-temperature phase (pyrite) but contained a small amount of the low-temperature phase (marcasite) that likely grew when the samples were naturally cooled after the sulfurization. We applied the rapid cooling feature of laser annealing to preventing the marcasite phase formation. No trace of marcasite phase was observed in Raman spectra and X-ray diffraction patterns of the laser-annealed samples. We analyzed temporal evolution of the sample temperature during the laser-annealing processes to confirm that the laser heating induced phase change of the small amount of marcasite to pyrite and the rapid cooling prevented marcasite regrowth.

Photovoltaic properties of Cu2ZnSnS4 cells fabricated using ZnSnO and ZnSnO/CdS buffer layers

To improve the photovoltaic properties of Cu2ZnSnS4 (CZTS) photovoltaic cells, we investigated the use of novel buffer layer materials. We found that Zn1− x Sn x O y fabricated by atomic layer deposition functioned as an effective buffer layer. The short-circuit current density increased by 10% because of a decrease in the absorption loss in the short-wavelength region. With Zn0.70Sn0.30O y layers, the conversion efficiency was 5.7%. To reduce interface recombination, a thin CdS layer was inserted between the ZnSnO and CZTS layers. The CZTS cells fabricated using ZnSnO/CdS double buffer layers showed a high open-circuit voltage of 0.81 V.

Improvement of red light response of Cu2Sn1−xGexS3 solar cells by optimization of CdS buffer layers

A cross-over anomaly has been observed in Cu2Sn1−xGexS3 (CTGS) solar cells, which degrades the solar cell performance under red light illumination. This is a critical drawback for bottom cells because it should work under red light illumination, filtered through top cells. We suppressed this cross-over anomaly by optimizing the deposition conditions of CdS buffer layers, which improved the performance of the cells under red light illumination. The mechanism of this phenomenon was discussed, and we concluded the origin of the anomalies was attributed to accepter-like defects in CdS buffer layers. Furthermore, we also inferred a model for the conduction band offset of the CTGS/CdS interface, which will aid the design of high efficiency solar cells.

Band slope in CdS layer of ZnO:Ga/CdS/Cu2ZnSnS4 photovoltaic cells revealed by hard X-ray photoelectron spectroscopy

For compound semiconductor photovoltaic cells with a common structure of the window-layer (WL)/buffer-layer (BL)/absorbing-layer (AL), the band slope in BLs, affecting the conversion efficiency, was directly and non-destructively measured by hard X-ray photoelectron spectroscopy. We demonstrated that the band slope in CdS-BLs sandwiched between WLs and Cu2ZnSnS4 (CZTS)-ALs reflected the trend of the work functions of WLs ( ϕ WL). This result implies that the larger downward band slope to the WL can be achieved using a smaller ϕ WL. The relatively large downward band slope of ∼0.5 eV to the WL was estimated in our ZnO:Ga/CdS/CZTS sample with a higher conversion efficiency of 9.4%, which indicates that the conversion efficiency of CZTS cells can be improved by a larger downward band slope to the WL.