Thin film solar cell based on $Cu(In,Ga)Se_2$ (CIGS) film have been the most successful material among compound semiconductor solar cells. High quality absorber film deposited by 3-stage process provided advance in cell efficiency above 20%. In industry’s point of view, film growth process more suitable for mass production and higher module efficiency are required for competition in photovoltaic industry. In this study, alternative growth process using evaporation was investigated and surface treatment using $Na_2$S was applied on CIGS film to enhance device performance.
For growth of CIGS film, high temperature above $500^\circ C$ is required for large grain and reduction of defects in film. Increased temperature can also help diffusion of Na from glass substrate to CIGS film, enhancing the device properties. Reducing temperature requirement will be important topic for industrial application of thin film solar cells. We found that the growth of a large grain with a Cu-deficient composition was possible under Se deficient environment at growth temperature below $500^\circ C$. Effect of growth temperature in the range of 350 to $550^\circ C$, and annealing temperature of film in Se atmosphere at 400 to $550^\circ C$ were studied. The elemental distribution was investigated after Se annealing and the quality of film was investigated using LTPL to compare defect characteristics in the film. Our results showed that the device performance of the CIGS solar cells with the two-stage co-evaporation was comparable to that of the cell prepared with three-stage process.
To enhance solar cell performance CIGS films were brought to surface treatment using $Na_2$S, in both low temperature process and standard 3 stage process CIGS. Conditions of post-deposition treatment (PDT) were evaluated by solar cell performance. Observation of film after $Na_2$S PDT showed the changes in band structure and atomic distributions on surface. Suppression of deep level defect by $Na_2$S was also detected. Control of 3 stage process and $Na_2$S PDT condition resulted 19.2% conversion efficiency with antireflective coating. Further study on alkali element treatment on CIGS can help enhance solar cell efficiency over 20%.
$Cu(In,Ga)Se_2$ (CIGS) 태양전지의 산업적 활용을 위해 중요한 과제로는 대량 생산에 용이한 공정의 개발과 모듈 효율의 증대가 있다. 현재 소면적의 고효율 소자를 제조하는데 사용되는 3단계 진공증발법을 이용한 태양전지들에서 20% 이상의 높은 효율이 보고되고 있다. 그러나 이 방법은 대면적화가 어렵고 $500^\circ C$이상의 공정온도가 필요하다. 본 연구에서는 Cu 및 Se이 부족한 조성에서의 액상 보조 성장에 의해 $500^\circ C$ 이하의 온도에서 큰 결정립을 가지는 CIGS 박막의 성장 과정을 분석하였다. 박막 성장시의 온도에 의한 영향을 $350^\circ C$ 에서 $550^\circ C$의 범위에서 관찰하였다. $400^\circ C$에서 성장한 박막의 Se 분위기에서의 열처리시 온도를 $400^\circ C$에서 $550^\circ C$로 변화시켰을 때 나타나는 박막 내의 원소 분포와 저온 photoluminescence(PL) 특성 및 태양전지 특성을 비교하였다.
태양전지 성능의 향상을 위해 CIGS 박막에 $Na_2$S를 사용한 표면처리를 진행하였다. $Na_2$S 공급량, 열처리 온도 및 시간을 변화시켰을 때 나타나는 태양전지 특성 변화를 관찰하였다. 표면처리 이후 CIGS 박막에서 나타나는 특성 변화에 대한 분석을 통해 에너지 밴드 구조 변화와 deep level defect 생성 억제를 볼 수 있었다. 3단계 진공증발법으로 제조한 CIGS 박막에 $Na_2$S 표면처리와 반사방지막 사용을 통해 19.2%의 효율을 얻을 수 있었다.
