The Effect of Sulfurization on the CuIn(Se,S)2 Photovoltaic Cell Synthesized By Electrodeposition

Photovoltaic is one of the most fascinating ways for direct solar energy conversion. The demand for photovoltaic is mainly covered by crystalline silicon; substantial cost reductions are necessary for large scale applications. A big challenge for thin film PV is the development of large area semiconductor thin films. The main candidate semiconductor materials for thin film solar cells at present are amorphous silicon, CdTe and Cu(In,Ga)(Se,S)₂.

Generally, even though physical vapor deposition (PVD) methods have been widely used for making the CIGS thin film ^1-3, its high process cost because of the vacuum system and difficulty on the large scale up still remained to overcome for mass production.

Non-vacuum methods, such as hydrazine-based solution process, paste coating, and electrodeposition have been studied to achieve the low cost process with high efficiency ^4-7. Electrodeposition could substitute for PVD methods because it has several advantages in cost competitiveness, easily scaled up, convenience control in room temperature and atmosphere pressure. Recently, the efficiency of CIGS solar cell manufactured by electrodeposition method exceeds 11% ⁸.

In this study, the effects of sulfurization temperature on CuIn(Se,S)₂ (CISeS) thin film solar cell have been investigated. Unlike well-known CIGS absorber, we adjusted the bandgap by controlling the ratio of Se and S, which could simplify the deposition process for obtaining absorber layer. 1㎛-CuInSe₂ layer was deposited on Mo/SLG (soda lime glass) at room temperature by electrodeposition method. To form CuIn(Se,S)₂ thin film, thin films were annealed in 5% H₂S-95% Ar atmosphere at 425-600℃. As the sulfurization temperature increased, the grain growth of CISeS films was improved and the ratio of Se/S decreased, then the optical bandgap was close to ideal value for solar cell (~1.4eV). However, MoS₂layers appeared at high temperature (≥500℃) and the film morphology was getting more porous with increasing temperature. As a result, the conversion efficiency decreased as the sulfurization temperature increased, and the maximum value of the conversion efficiency was 2.32% at 425℃.

 To improve the cell efficiency, the effects of sulfurization time on grain growth have been investigated. CuInSe₂ thin films were annealed in 5% H₂S-95% Ar atmosphere at 425℃ for 20-60min. When the sulfurization time increased, the crystallinity of CuIn(Se,S)₂ film improved. As a result, the conversion effieciency increased to 3.48% without anti-reflection coating layer.