Flexible Cu(In, Ga)Se₂ Thin-Film Solar Cells on Polyimide Substrate by Low-Temperature Deposition Process

The electrical and structural properties of polycrystalline Cu(In,Ga)Se₂ films grown on polyimide (PI) substrates below 400° C via one-stage and three-stage co-evaporation process have been investigated by x-ray diffraction spectra (XRD), scanning electron microscopy (SEM) and Hall effect measurement. As shown by XRD spectra, the stoichiometric CIGS films obtained by one-stage process exhibit the characteristic diffraction peaks of the (In_0.68Ga_0.32)₂Se₃ and Cu(In_0.7Ga_0.3)₂Se. It is also found that the film structures indicate more columnar and compact than the three-stage process films from SEM images. The stoichiometric CIGS films obtained by three-stage process exhibit the coexistence of the secondary phase of (In_0.68Ga_0.32)₂Se₃, Cu_2-xSe and Cu(In_0.7Ga_0.3)₂Se. High net carrier concentration and sheet conductivity are also observed for this kind of film, related to the presence of Cu_2-xSe phase. As a result, when the CIGS film growth temperature is below 400° C, the three-stage process is inefficient for solar cells. By using the one-stage co-evaporation process, the flexible CIGS solar cell on a PI substrate with the best conversion efficiency of 6.38% is demonstrated (active area 0.16 cm²).