Optimization of solution-processed Cu(In,Ga)S 2 by tuning series and shunt resistance
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Jingbiao Cuia) Department of Physics, University of Memphis, Memphis, Tennessee 38152 (Received 14 February 2014; accepted 9 May 2014)
Solution-processed CuInGaS2 (CIGS) thin-film solar cells are promising for large-scale commercialization due to their economic process although the efficiency still needs to be improved to compete with vacuum-based materials. Systematic studies were performed to optimize the series and shunt resistance of hydrazine-based CIGS solar cells. Optimization was achieved through compositional adjustment of copper (Cu) near the p–n junction and gallium (Ga) near the back contact. Cu adjustments optimized the shunt resistance between 4000 and 5000 X cm2. Ga adjustments optimized the series resistance to 2 X cm2. Shunt and series resistance play vital roles in the fill factor. Fill factor was hence improved upward of 0.80 with the optimization of Cu and Ga. Chemical etching was also conducted to investigate the durability of the materials and to remove small crystals near the interface. Device conversion efficiencies were improved up to 12.4%. This study provides the implications for improving the device performance of chalcogenide solar cell materials.
I. INTRODUCTION
Solar energy conversion is a highly attractive process for clean and renewable power for the future. Cu(In,Ga) (Se,S)2/CdS/ZnO has proven itself to be a worthwhile photovoltaic combination achieving record efficiencies upward of 20%.1 These record efficiencies have been obtained on the layered thin-film structures through vacuum-based approaches.1,2 Along with high performance, the cost of making such devices is also high, which has become a barrier to the affordability of vacuumbased solar cells as a commercial energy source. As a result, solution-based approaches such as electrodeposition,3,4 nanoparticle precursors,5,6 and organic solutions7 have attracted significant attention in the last few years. While the solution-based approaches have lower fabrication costs, there are a few disadvantages. These disadvantages include, for example, the difficulty in obtaining the desired grain structure characteristics, eliminating impurities (e.g., C, O, Cl), and the chemical complexity inherent in a compound semiconductor consisting of several different elements.2 So far, the most effective solution-based approach is the hydrazine-based precursor process pioneered by researchers at IBM.8–13 Recorded efficiencies have reached up to 15%,9 on par with that of the vacuum-based approaches. The process is a completely a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2014.114 J. Mater. Res., Vol. 29, No. 11, Jun 14, 2014
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solution-based approach and involves dissolving precursor materials in anhydrous hydrazine. The hydrazine-based approach possesses advantages such as good grain size, less impurity incorporation, precise control of material composition through individual precursors, and large-scale thin-film deposition with desired homogeneity.11 Direct
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