Efficiency Improvement of Narrow Band Gap Cu(In,Ga)(S,Se)2 Solar Cell with Alkali Treatment via Aqueous Spray Pyrolysis Deposition.

The chalcopyrite Cu(In,Ga)(S,Se)2 (CIGSSe) solar cell with a low band gap is a promising candidate for use as the bottom cell in high-efficiency tandem solar cells. In this study, we investigated narrow band gap CIGSSe solar cells, both with and without alkali treatment. The CIGSSe absorbers were fabricated using aqueous spray pyrolysis in an air environment, with the precursor solution prepared by dissolving constituent metal salts. We found that the power conversion efficiency (PCE) of the fabricated solar cell was significantly enhanced when rubidium postdeposition treatment (PDT) was applied to the CIGSSe absorber. The Rb-PDT facilitates defect passivation and a downshift of the valence band maximum of the CIGSSe absorber, thereby improving the power conversion efficiency and all device parameters. Due to these beneficial effects, a PCE of ∼15% was obtained with an energy band gap of less than 1.1 eV, making it suitable for use as the bottom cell in a highly efficient tandem solar cell.

Insights into High-Efficiency Ag-Alloyed CZTSSe Solar Cells Fabricated through Aqueous Spray Deposition.

Kesterite Cu2ZnSnSe4 (CZTSe), Cu2ZnSn(S,Se)4 (CZTSSe), and Cu2ZnSnS4 (CZTS) solar cells show considerably lower open-circuit voltages than their theoretical values. The large open-circuit voltage deficiency (Vocdef) hinders the improvement of the power conversion efficiency (PCE) and the development of the pathway to mass production of kesterite solar cells. The main reason behind the Vocdef is considered to be the low formation energy of Cu/Zn disorders and their highly distributed defect complexes. To diminish the Cu/Zn disorder, we substituted Ag with a relatively large atomic radius into the host CZTSSe as (AgxCu1-x)2ZnSn(S,Se)4 (ACZTSSe) and investigated its beneficial effect in a systematic way. The ACZTSSe absorbers were all fabricated using aqueous spray pyrolysis in ambient air. The device performance was found to increase up to the optimum Ag substitution and decrease after the optimum Ag substitution. Admittance spectroscopy revealed that the optimal substitution of Ag reduced the Cu-/Zn-related defects, that is, charge recombination centers, which further mitigates the band tailing issue and enhances the PCE of the solar cell, and higher Ag substitution induced the generation of deeper defects, which decreases the PCE back. At the optimum Ag content of Ag/(Ag + Cu) = ∼9%, the ACZTSSe solar cell with the highest PCE of 11.83% was obtained, where both the interface recombination and bulk recombination were found to be minimized.

Sulfur-Alloying Effects on Cu(In,Ga)(S,Se)2 Solar Cell Fabricated Using Aqueous Spray Pyrolysis.

We fabricated Cu(In,Ga)(S,Se)2 (CIGSSe) solar cells using aqueous spray based deposition, which is inexpensive and covers a large area. To apply the sprayed film to a photoabsorber of a solar cell, post-sulfo-selenization was carried out. Through the sulfo-selenization process, we were able to fabricate various S-alloyed CIGSSe films from S/(S + Se) = 0 (S-0.0) to S/(S + Se) = 0.4 (S-0.4). CIGSSe solar cells were made with the S-alloyed CIGSSe absorbers. Power conversion efficiency of CIGSSe solar cell was found to be increased with S-alloying up to S-0.3, and the best efficiency of 10.89% was obtained with the S-0.3 CIGSSe absorber. Comparison study of S-alloyed CIGSSe solar cells showed that enhanced efficiency in S-0.3 solar cell is due to the increased open-circuit voltage and an improved fill factor, which is induced by S-alloying. In addition, admittance spectroscopy revealed that the defect density of the deep level was developed in the S-alloyed S-0.3 CIGSSe absorber. However, the defect density was observed to be rather reduced. Details of characterization and analysis results are discussed in this paper.