Numerical study of Cu(In,Ga)Se₂ solar cell performance toward 23% conversion efficiency

Abstract

The effects of conduction band grading in a Cu(In,Ga)Se2 (CIGS) thin film with an average bandgap of 1.4 eV on solar cell performance were investigated by changing the minimum bandgap (E gmin) and its position, employing the software wxAMPS. The calculation was carried out, taking CdS/CIGS heterointerface recombination into account, by incorporating a thin defective layer into the interface. For CIGS with a flat conduction band profile, i.e., without conduction band grading, the effects of the valence band offset (ΔE V) between a CdS layer and a CIGS layer with bandgaps from 1.05 to 1.6 eV were investigated. It was found that efficiency was increased by up to 3% by changing the conduction band profile from flat to double-graded, with a deep notch located in the vicinity of the CdS/CIGS interface. On the other hand, efficiency was increased by over 6% and reached 22% by increasing ΔE V up to 0.3 eV in the case of CIGS with a bandgap of 1.35 eV. Finally, an efficiency of 23.4% was achieved by combining a single-graded conduction band profile with a ΔE V of 0.3 eV. This result shows that a single-graded conduction band profile is required for high-efficiency wide-bandgap CIGS solar cells if the recombination at the CdS/CIGS heterointerface can be suppressed.

Fabrication of Cu(In,Ga)Se2 solar cells with a single graded band profile

“…The conduction band profile of typical CIGS solar cells fabricated by the three stage method has double graded (DG) structure which is useful for the improvement of carrier collection efficiency by an internal electric field, and the improvement of open circuit voltage ( V OC ) by bandgap‐widening in a depletion region. On the other hand, single graded (SG) structure has been reported lower V OC than DG structure due to no bandgap‐widening in a depletion region . However, in this study from experimental results we found that SG structure has higher efficiency than DG structure.…”

“…To apply the SG structure to CIGS solar cells with high Ga contents, we fabricated SG‐CIGS solar cells with E g = 1.2 eV at the surface side and E g = 1.6 eV at the back side ( E g(avg) = 1.4 eV) which was structure optimized by Hirai et al and investigated the influence of the OVC layer on solar cell performance by compared to DG‐CIGS with E g(avg) = 1.4 eV. Figures and show J – V characteristics, external quantum efficiency (EQE) and GGI depth profile, respectively, in DG‐ and SG‐CIGS solar cells with E g(avg) = 1.4 eV.…”

Theoretical and experimental investigation of the recombination reduction at surface and grain boundaries in Cu(In,Ga)Se2 solar cells by valence band control

“…Moreover, it could be confirmed that the high η of about 18% was also obtained by insertion of a thinner Cu(In,Ga) 3 Se 5 layer like a few nanometers even though the layer had high N AG of 10 18 cm -3 . So far in our group, it has been revealed that the open circuit voltage is improved due to suppression of interfacial recombination by repelling holes at the ∆E V if wide gap layer with ∆E V is inserted [11,12]. From these calculation results, it was shown that the ∆E V control at CdS/CIGS and GBs is very important for realization of high-efficiency CIGS solar cells and that the insertion of thin Cu(In,Ga) 3 Se 5 layer is a promising technique even though the layer has high defect density.…”

“…Cu(In,Ga) 3 Se 5 is one of the materials which has lower Cu contents more than CIGS and has valence band offset (ΔE V ) of about 0.3 eV [8,10]. Our group has revealed that suppression effect of interfacial recombination is obtained due to repelling holes by the ΔE V , if a wide gap layer with the ΔE V is inserted at the CdS/CIGS interface for CIGS solar cell [11,12]. In this study we disclosed a correlation between formation of a Cu-poor layer and solar cell characteristics, and comprehensively investigated influence by variation of Cu component in surface, GB and grain interior (GI) of CIGS thin films by theoretical and experimental.…”

Электрические и оптические свойства пленок Cu-=SUB=-2-=/SUB=-Zn(Fe,Mn)SnS-=SUB=-4-=/SUB=-, изготовленных спрей-пиролизом

“…Успехи фотовольтаического преобразования энергии излучения с помощью тонкопленочных солнечных эле-ментов связаны с фотопреобразователями на основе пле-нок CdTe (эффективность ∼ 21%) и CuInGaSe (CIGSe) (> 20%) [1][2][3]. Содержание в данных материалах ток-сического Cd, редких и дорогих металлов In и Ga существенно препятствует их широкому коммерческому использованию.…”