低温超薄高效Cu(In,Ga)Se_2太阳电池的实现

The high efficiency sub-micrometer Cu(In,Ga)Se_2 solar cell prepared on low temperature

衬底温度保持恒定,在Se气氛下按照一定的元素配比顺序蒸发Ga,In,Cu制备厚度约为0.7μm的Cu(In0.7Ga0.3)Se2(CIGS)薄膜.利用X射线衍射仪分析薄膜的晶体结构及物相组成,扫描电子显微镜表征薄膜形貌及结晶质量,二次离子质谱仪测试薄膜内部元素分布,拉曼散射谱分析薄膜表面构成,带积分球附件的分光光度计测量薄膜光学性能.研究发现在Ga-In-Se预制层内,In主要通过晶界扩散引起Ga/(Ga+In)分布均匀化.衬底温度高于450C时,薄膜呈现单一的Cu(In0.7Ga0.3)Se2相;低于400C,薄膜存在严重的Ga的两相分离现象,且高含Ga相主要存在于薄膜的上下表面;低于300C,薄膜结晶质量进一步恶化.薄膜表层的高含Ga相Cu(In0.5Ga0.5)Se2以小晶粒形式均匀分布于薄膜表面,增加了薄膜的粗糙度,在电池内形成陷光结构,提高了超薄电池对光的吸收.加上带隙值较小的低含Ga相的存在,使电池短路电流密度得到较大改善.衬底温度在550C—350C变化时,短路电流密度JSC是影响超薄电池转换效率的主要因素;而衬底温度Tsub低于300C时,开路电压VOC和填充因子FF降低已成为电池性能减退的主要原因.Tsub为350C时制备的0.7μm左右的超薄CIGS电池转换效率达到了10.3%.

In the presence of Se, Cu（In0.7Ga0.3）Se2 （CIGS） thin films are prepared by the sequential evaporation of Ga, In, Cu at a constant substrate temperature between 250 ℃ and 550 ℃ on the Mo/soda lime glass substrates. The thickness values of films are about 0.7 μm. The structural and phase properties of CIGS films are studied by an X-ray diffractometer, the morphology and crystalline quality are characterized by a scanning electron microscope, the depth profiles of elements are measured by a secondary ion mass spectroscopy, the surface compositions are analyzed by a Raman spectrometer, and the optical properties of CIGS films are measured by a spectrophotometer with an integrating sphere. It is found that the films prepared at substrate temperature above 450 ℃ each exhibite a single Cu（In0.7Ga0.3）Se2 phase, and the homogenization of Ga/（Ga＋In） distribution in the Ga-In-Se precursor is achieved by the diffusion of In atoms through grain boundaries. As the substrate temperature is less than 400 ℃, a serious Ga phase separation is observed and the high content of Ga phase mainly exists at the top and bottom of CIGS films. Below 300 ℃, a serious deterioration of crystalline quality is found, and Ga atoms cannot effectively enter into the CIS lattice position to form CIGS. The films prepared at the substrate temperature less than 400 ℃ are covered with lots of Cu（In0.5Ga0.5）Se2 small grains, which results in the enhancement of the surface roughness and the formation of a light trapping structure at the interface of Cd/CIGS. Thus, the light absorption of solar cell is improved. In addition, the smaller gap value of the low Ga content phase also facilitats the light absorption, then the short-circuit current density of thinned solar cell is greatly improved. The analysis shows that the short-circuit current density is the main factor affecting the conversion efficiency of thinned solar cell prepared between 550 ℃-350 ℃. However, when the substrate temperature is below 350 ℃, the reduction of VOC and FF has become the main reason for the deterioration of solar cell. In conclusion, the efficiency of solar cell with 0.7 μm CIGS absorber prepared at substrate temperature of 350 ℃ reaches 10.3% due to the improvement of short-circuit current density.