Growing Copper(Indium,Gallium)Selenium2 Thin Film Solar Cells with High Efficiency and Low Production Costs

Cu(In,Ga)Se2 (CIGS)-based thin film solar cell has been commercialized recently due to its high energy conversion efficiency. We have designed an integrated satellite deposition system for producing CIGS solar cell with substrate size of 10cm × 10cm. This work mainly contains two parts with first part focusing on growing and characterizing high quality baseline solar cells and solar modules and second part concentrating on further reducing the material costs by growing thinner absorber layer with high efficiency.;The most difficult part in growing high quality CIGS solar cells originate from the absorber layers which contain p-type chalcopyrite structures with four different elements: Cu, In, Ga and Se. The widely used three-stage process is employed to co-evaporate In, Ga and Se first, then Cu and Se are evaporated to form the chalcopyrite CIGS structure and additional In, Ga and Se are deposited in the end to ensure an overall Cu deficiency, which is important for getting p-type semiconductors. The substrate temperatures during these three stages are carefully adjusted to introduce proper gallium gradients which is important for collecting electrons efficiently. Together with optimizing other layers we are able to get cell efficiency (area around 0.5 cm2) over 17%. To produce CIGS mini-modules, laser scribing as well as mechanical scribing are employed for series interconnection of individual cells using monolithic integration. The power and speed of laser together with the condition of mechanical scriber are carefully adjusted to ensure a minimum dead area in the module. Module (area around 80 cm2) with efficiency over 12% is produced.;Solar cells were fabricated and tested under varied temperature and weak light conditions. Temperature coefficient is compared between CIGS solar cells and other types of solar cells. Temperature coefficient is improved a lot with higher gallium content in the absorber layer. Weak light performance is shown to be increased a lot when copper percentage is lowered down. In order to examine the origin of beneficial effects from Cu-poor absorber, solar cells are grown with comparable grain sizes using our technique and I-V performances are examined under STM in grain/atomic scale. Leakage current is found to be mainly originates from boundary area. CIGS solar cells with Cu-poor absorber benefit from the reduced leakage from boundary area.;CIGS solar cells with thinner absorber thickness are studied and compared with conventional CIGS solar cells. We have found that high conversion efficiency solar cells can be grown for absorber thickness as thin as 1.5 µm. Further reduction in absorber thickness deteriorates solar cell performances in both Voc and Jsc resulting in conversion efficiency as low as 11%.;Two major approaches are performed to improve solar cell performances. Light trapping by etching AZO top contact for creating pyramid-structures to enhance light scattering. Efficiency is increased by more than 1.5% for solar cells with etched AZO surfaces. Solar cells with efficiency larger than 13% can be grown by using AZO etching. Another approach is by using suitable Ga content in absorber layer. Solar cells with efficiency as high as 14.17% are grown which makes thinner CIGS solar cells very competitive.