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Cu(In, Ga)Se2 Thin Films on Flexible Polyimide Sheet: Structural and Electrical Properties versus Composition

The structural and electrical properties of Cu(In, Ga)Se2 (CIGS) films grown on polyimide (PI) sheet using the three-stage co-evaporation process are investigated by x-ray diffraction spectra (XRD), scanning electron microscopy (SEM), Raman spectra, and Hall effect measurements, respectively. The results show that the properties of CIGS films on PI sheet are strongly dependent on the compositional ratio of Cu/(In+Ga) (Cu/III). In contrast to the non-stoichiometric CIGS films, stoichiometric CIGS films show better structural and electrical properties, such as a relatively larger grain size, lower resistivity and higher carrier concentration. The flexible CIGS solar cells on PI sheet with the conversion efficiencies of 9.7% and 6.6% are demonstrated for the CIGS absorber layer with Cu/III of 0.96 and 0.76, respectively (active area, 0.20 cm2). The cell efficiency for Cu-poor CIGS films is limited by a relatively lower open circuit voltage and fill factor.