Cu₂Sn_1-xGe_xS₃(x= 0.17) Thin-Film Solar Cells with High Conversion Efficiency of 6.0%

Abstract

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.

Influence of Ge composition in the Cu2Sn1−Ge S3 thin-film photovoltaic absorber prepared by sulfurization of laminated metallic precursor

“…However, the researches concerning with this CTGS alloy in the thin-film photovoltaic applications are still in the early initial phase and the study on CTGS alloy itself with various Ge contents is still premature compared to the study of individual Cu 2 SnS 3 (CTS) or Cu 2 GeS 3 (CGS) ternary compounds [13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32]. Up to now, only a few reports about the photovoltaic devices based on the CTGS with various Ge contents have been made [12,33,34]. Recently, Umehara et al achieved a conversion efficiency as high as 6% in the cell utilizing CTGS absorber with Ge/(Ge+Sn) ratio of x = 0.17 (E g = 1.02 eV) prepared by an open tube sulfurization technique showing that this material has a good potential for further improvement by adjusting to an optimum band gap by alloying with Ge [33,34].…”

“…Up to now, only a few reports about the photovoltaic devices based on the CTGS with various Ge contents have been made [12,33,34]. Recently, Umehara et al achieved a conversion efficiency as high as 6% in the cell utilizing CTGS absorber with Ge/(Ge+Sn) ratio of x = 0.17 (E g = 1.02 eV) prepared by an open tube sulfurization technique showing that this material has a good potential for further improvement by adjusting to an optimum band gap by alloying with Ge [33,34]. Hence, a detail study on CTGS alloy over the whole Ge composition range is an important issue in order to realize high efficiency photovoltaic devices utilizing this alloy material in the future.…”

Electrodeposition of germanium-containing precursors for Cu2(Sn,Ge)S3 thin film solar cells

“…An efficiency of 4.6% was recently achieved for solar cells based on this material [2]. Umehara et al demonstrated a higher device efficiency of 6.0% by replacing some of the tin with germanium, to form Cu 2 (Sn,Ge)S 3 (CTGS) [3]. The authors attributed the increased efficiency to the enrichment of grain boundaries with germanium, thus resulting in a local increase of the band gap [4].…”

“…These works showed that despite being possible to electrodeposit germanium from ionic liquids, there were several issues: the germanium sources are poorly soluble in the ionic liquids, non-standard experimental setups had to be used to cope with the volatility of the most common germanium compounds and deposition rates are low under standard conditions. In another approach, Bartlett et al electrodeposited amorphous germanium from liquid CH 3 [20,21]. Albeit successful, this approach presents some drawbacks: on the one hand, the need of a high pressure cell, which increases the complexity of the experiment and decreases output.…”

“…Various physical growth methods have been applied for the growth of compounds in the CTGS material system. Umehara and co-workers co-sputtered Cu-Sn precursors and thermally annealed them in the presence of sulphur and GeS 2 [3,5]. For CGS solar cells Araki et al deposited a Mo/Cu/Ge stack, with germanium being deposited via thermal evaporation and copper via e-beam evaporation [6].…”