Large open‐circuit voltage boosting of pure sulfide chalcopyrite Cu(In,Ga)S₂ prepared using Cu‐deficient metal precursors

Abstract

We performed a comparative study to find out the reasons why it is necessary to prepare the pure sulfide chalcopyrite, Cu(InGa)S2 (CIGS) under a Cu‐deficient condition to improve solar cell performance. It has been shown that CIGS that was grown under Cu‐deficient condition exhibits large open‐circuit voltage (VOC) boosting compared with that the one grown under Cu‐excess condition. Thus, CIGS were prepared from Cu‐excess and Cu‐deficient metal precursor (MP) and characterized concerning the origins of the different VOC. We observed that CIGS prepared using Cu‐excess MP suffered large recombination at buffer/CIGS interface and in bulk, which resulted in serious limitation of the VOC. On the other hand, CIGS prepared from Cu‐deficient MP exhibited largely improved VOC with reduced recombination. We conducted defect analysis using the photoluminescence (PL) method, and CIGS prepared using Cu‐excess MP exhibited strong deep emissions. However, CIGS prepared from Cu‐deficient MP exhibited PL emission characteristics without deep level transition. This implies that a large number of deep level defects exist in the CIGS prepared using Cu‐excess MP, and this might be the reason for the large recombination limiting VOC of the device. Finally, we also present the reason for deep levels in the CIGS prepared using Cu‐excess MP.

Elaboration of wide bandgap CIGS on silicon by electrodeposition of stacked metal precursors and sulfur annealing for tandem solar cell applications

“…In the previous study by Barreau et al on CIGSu films prepared by coevaporation [3], similar trends were observed, with a decrease of the deep level peak when the deposition temperature was increased. In our case we observe similar luminescence spectra and can attribute the high energy peak to band to band, or shallow level to band transitions [20] in the top layer with a low GGI, and the lower energy peak to similar deep levels.…”

“…It is characterized by a sharp peak centered at 1.52 eV and a smaller peak at a lower energy of about 1.3 eV. Several studies address the analysis of luminescence spectra of copper indium gallium sulfide and copper indium sulfide as recalled in a recent paper from Kim et al [20]. This paper deals with layers formed under copper rich and copper poor conditions.…”

Over 15% efficient wide-band-gap Cu(In,Ga)S2 solar cell: Suppressing bulk and interface recombination through composition engineering

“…6,[13][14][15][16] Moreover, rather low QFLS and short charge-carrier lifetimes ($hundreds of ps) are typically observed for Cu(In,Ga)S 2 . 7,14 This implies significant non-radiative recombination in Cu(In,Ga) S 2 . The origin of non-radiative recombination lies in both bulk and interface (frontand back-contact) defects.…”

“…They observed higher Voc for Cu-poor over Curich Cu(In,Ga)S 2 , which was attributed to lower bulk defects and reduced interface recombination in the former. 7 All approaches to increase efficiency relied on toxic H 2 S, Cd-and/or KCN-based process. Moreover, the physical insights are still missing.…”

“…4 Therefore, sulfide chalcopyrite Cu(In,Ga)S 2 , due to its variable band gap between 1.5 and 2.4 eV, is receiving considerable interest as the absorber in the top cell of a tandem device. [5][6][7][8][9] Cu(In,Ga)S 2 adopts the chalcopyrite structure similar to high-efficiency Cu(In,Ga) Se 2 . Despite the high photoconversion efficiency (PCE) of 23.35% achieved using Cu(In,Ga)(S,Se) 2 , 10,11 the certified record PCE of pure sulfide Cu(In,Ga)S 2 solar cells remained limited to 15.5% thus far.…”