We studied the dependence of the efficiency of Cu(In,Ga)Se2 (CIGS) solar cells on the fabrication process of CIGS films.
We grew the CIGS films at various substrate temperatures to investigate the effect of the substrate temperature on the efficiency of solar cells. We observed in the X-ray diffraction patterns that the CIGS films grown at substrate temperature of 535 °C had the largest texture coefficient for (220/204) plane and smallest FWHMs (full width at half maximum) for both the (112) and (220/204) planes. The solar cells (ITO/ZnO/CdS/CIGS/Mo) fabricated with CIGS films grown at 535 °C showed the highest efficiency. These results suggest that large texture coefficient of (220/204) in CIGS films is closely related to high efficiency of CIGS solar cells.
The amorphous CIGS films were crystallized by the rapid thermal process (RTP). We performed the RTP at four different conditions ; at the atmospheric pressure with argon flow, in the vacuum of 3×10-3 torr, and at the atmospheric pressure and in vacuum with additional constraint of CIGS film covered with a 0.7 mm thick glass cover. More crystallization was observed in the CIGS films heat-treated at atmospheric pressure than in the films heat-treated in vacuum. We found that the loss of selenium in the film heat-treated with a glass cover was less than that in the film heat-treated without a glass cover, but the crystallization in these films was not sufficient for the use in the CIGS solar cells. In this case, further crystallization process could not be proceeded due to the block of heat energy of halogen lamp by a glass cover. The solar cells (AZO/ZnO/CdS/CIGS/Mo) fabricated with CIGS films heat-treated at atmospheric pressure with argon flow showed the highest efficiency.
The thickness effect of ZnO layer in the solar cell with a structure of ITO/ZnO/CdS/CIGS/Mo was investigated by measuring the efficiency of solar cells with ZnO layer of two different thicknesses, that is, 50 and 90 nm. We found that the solar cell with a 50 nm-thick ZnO layer showed the larger efficiency. This result was consistent with the result of transmittance measurements, the 50 nm ZnO layer showed the highest transmittance.
Thesis Advisor: Prof. Ilsu Rhee