Investigation of Cu(In,Al)Se2 thin film solar cell fabricated by using electrodeposition technique

• Main Authors: Kuo-ChanHuang, 黃國展
• Other Authors: Mau-Phon Houng
• Format: Others
• Language: en_US
• Published: 2015
• Online Access: http://ndltd.ncl.edu.tw/handle/y5vw25

博士 === 國立成功大學 === 微電子工程研究所 === 103 === In this dissertation, we exploit the low cost electrodepositon technique to investigate the fabrication of Cu(In,Al)Se2 thin film solar cell. The research content is divided into four segments. In the first segment, the cyclic voltammetric studies are used to realize the element’s and compounds reduction potentials and identify a suitable potential as co-electrodeposition. In combination with the XRD analysis, chemical reaction mechanism for presuming the formation routes of quaternary Cu(In,Al)Se2 films are defined. Furthermore, It is found that the SDS additive promotes the deposited potential of each element closing to each other for a better co-elecorodeposition environment, and simultaneously change the nucleation mechanism of Cu(In,Al)Se2 films from instantaneous nucleation to progressive nucleation. This feature is helpful to obtain a smooth precursor Cu(In,Al)Se2film and round-like structure. In addition, we find the stoichiometry of Cu(In,Al)Se2 film changes from Cu-rich to Cu-poor type and the morphology of Cu(In,Al)Se2 film transfers from round-like structure to cauliflower-like structure by increasing deposited potential. In the second segment, we focus on the adjustment of stoichiometry and optical energy band gap of Cu(In,Al)Se2 films. By adjusting the Al and In concentration is solutions, the ratio of Al to (Al+In) in Cu(In,Al)Se2 films can be successfully controlled from 0.21 to 0.42, and the corresponding optical energy band gap of Cu(In,Al)Se2 films can be varied from1.17 eV to 1.48 eV to match with optimum band gap value for the solar spectrum. Furthermore, X-ray diffraction (XRD) patterns reveal three preferred growth orientations along the (112), (204/220), and (116/312) planes for all species. In the third segment, we focus on the surface morphology of Cu(In,Al)Se2 films and the relationship between precursor Cu(In,Al)Se2 films and post-annealed Cu(In,Al)Se2 films. The nucleation mechanism of electrodeposited Cu(In,Al)Se2 films change from instantaneous nucleation to progressive nucleation is observed by increasing the copper concentration. The research results exhibit that precursor Cu(In,Al)Se2 films had roughly cauliflower-like and triangular structures with Cu-poor composition at instantaneous nucleation mechanism, whereas smooth and round structures with Cu-rich composition at progressive nucleation mechanism. After post-annealing treatment, the surface morphology of Cu-rich Cu(In,Al)Se2 films shows high quality with compact structures and large grains, that is more beneficial to be the absorber layer of solar cell. A 1.96% efficient Cu(In,Al)Se2 thin film solar cell fabricated by electrodeposition technique is first time achieved and publish in international journal. The corresponding values of open-circuit voltage (Voc), short-circuit current (Jsc), fill factor (FF), Rsh and Rs are 0.189 V, 29.21 mA/cm2, 35.4%, 125Ω and 2.82Ω, respectively. In the fourth segment, the binary structure precursor Cu(In,Al)Se2 films are utilized to improve the surface morphology and of Cu(In,Al)Se2 films and investigate the characteristics of CdS and Cu(In,Al)Se2 interface. It is found that the upper Cu–Se compounds in binary structure can form a liquid phases during the post-annealing process, which enhances elemental migration and promotion of large grains and smooth surface formation and reduction of RMS roughness less than 100 nm. The subsequently deposition of CdS film on binary structure Cu(In,Al)Se2 films exhibit good spreadability and smoothness, leading to efficiently diminish the distribution of leakage current paths. The dark current–voltage characteristics of the CdS/CIAS heterojuncions shows that the reverse dark current density is decreased by approximately one order of magnitude from 4.02 x 10-4 A/cm2 (single structure) to 4.26 x 10-5 A/cm2 (binary structure). Furthermore, the conversion efficiency of CIAS solar cells is enhanced from 0.52 % (single structure) to 1.44 % (binary structure) with increase in Voc and Jsc.