DLC has been attractive as semiconductor materials for solar cell due to its biological friendliness, flexible microstructures, and especially its tunable band gap. In order to fabricate high-efficiency multiband gap solar cell, it is important to control the sp3/ sp2 bonds ratio of DLC film corresponding to optical band gap (Eg). There are many references reporting the relations among the fabrication conditions, Eg, sp3/ sp2, and ID/ IG. However, a more comprehensive database is needed for controllable fabrication. Especially, the quantitative relationship of sp3/ sp2 ratio to Eg of DLC film by PECVD is unclear. In this paper, 36 sets of DLC films were fabricated by RF-PECVD. Characterization methods of XPS, Raman spectroscopy, and IR absorption have been used to determine the sp3/ sp2 ratio of DLC films. UV/visible light absorption method has applied to evaluate Eg. The Eg obtained is in the range 1.45–3.0 eV. Our results agree well with the references. The XPS spectra gives a linear relationship as Eg = − 0.161 (± 0.136) + 26.095 (± 1.704) · {sp3 (XPS)/sp2}, the Raman spectra shows a linear function that Eg = 1.327 (± 0.046) + 0.428 (± 0.036) · (ID/IG), as well as the FTIR analysis demonstrates that Eg = − 0.492 (± 0.093) + 0.464 (± 0.044) · {sp3 (FTIR)/sp2}.

    Abstract
    1 Introduction
    2 Experimental
        2.1 Fabrication of DLC films by PECVD
        2.2 Characterization of DLC films
    3 Result and discussion
        3.1 Morphologies characterization
        3.2 Ultraviolet visible spectrophotometer analysis
        3.3 X-ray photoelectron spectroscopy
        3.4 Raman spectroscopy
        3.5 Fourier transform infrared spectrum
    4 Conclusions
    Acknowledgements 
    References

• Hongxun Sheng(College of Energy, Xiamen University)
• Wenwen Xiong(College of Energy, Xiamen University)
• Songsheng Zheng(College of Energy, Xiamen University)
• Chao Chen(College of Energy, Xiamen University)
• Song He(College of Energy, Xiamen University)
• Qijin Cheng(College of Energy, Xiamen University)