Dissertation
Dissertation > Industrial Technology > Electrotechnical > Independent power supply technology (direct power) > Photocell > Solar cells > Thin film solar cells

Investigation of Key Materials for Cu-based Compound Thin Film Solar Cells

Author LiuJun
Tutor WeiAiXiang
School Guangdong University of Technology
Course Microelectronics and Solid State Electronics
Keywords Thin film solar cells CIGS thin films Selenization of metallic precursors Chemical bath deposition ZnS thin films ZnSxSe1-xthin films
CLC TM914.42
Type Master's thesis
Year 2013
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The CuIn1-xGaxSe2(CIGS) thin film solar cell is always touted as the most promising solar cells due to its near-optimum band gap, high conversion efficiency, low cost and prominent stability. Typical CIGS solar cells structure utilizes the soda lime glass (SLG) substrate, continuously prepared with a Mo film as the back contact, a CIGS thin film as the absorb layer, a CdS film as the buffer layer, a high-resistance ZnO film and a doped high-conductivity ZnO film as the window layer and Ni/Al current collection grid. The study of the CIGS absorb layer is the key part for the fabrication of high quality CIGS thin film solar cells. Nowadays, the most popular preparation methods for the deposition of CIGS films are co-evaporation and seleniazation of metallic precurcors. The record efficiency of20.3%CIGS films solar cells were prepared by co-evaporation. But the equipment requirements of the co-evaporation are strict because there are many parameters need to be controlled, and the uniformity of large-area deposition is poor, so it is hard to achieve the lager-area deposition for this method. Comparing with co-evaporation, selenization of metallic precursors is easier to achieve the lager-area solar cell modules production. The H2Se is hypertoxic. expensive, inflammable and explosive, but it is a popular choice of selenium resource in traditional selenization process. In addition, CdS buffer layer causes serious environment problem which seriously hindered the development of CIGS thin films solar cells.Solid Se powder was used as the selenium source to prepare CIS or CIGS films by selenization of metallic precursors in this paper. In order to obtain high-performance CIGS absorb layer, the selenization process and sputtering of metallic precursors were investigated. The Cd-free buffer layers were deposited by chemical bath deposition (CBD), and the influence of the reaction process and deposition mechanism also have been analyzed. All the works are based on the intent to fabricate pro-environmental, high-efficiency and low-cost CIGS thin films solar cells. The research content of this thesis can be summarized into the following several parts:The preparation of CIS films by selenization of metallic precursors:CIS thin films were prepared by vacuum sputtering precursors of Cu-In and subsequently selenizied by "three-step heating method" used solid Se powder as selenium resource. The morphology, chemical composition and structure of absorb layer thin films were characterized by SEM, EDS. XRD and Hall Effect measurement. The research is mainly focus on the influence of heating mode, selenization atmosphere and selenization method for selenization process on the properties for CIS thin films. The results show that the optimal parameters of selenization process is the using of "three-step heating method" and "selenium vapor method" under the Ar atmosphere for the preparation of CIS films. According to this technique, the films prepared by selenizing the Cu-In precursors of CuIn/In/CuIn structure on Mo layer show a more reasonable stoichiometric proportion, and the Cu, In, Se atomic ratio of CIS films was1.06:1:1.76which close to1:1:2.The preparation of CIGS films by selenization of metallic precursors:In order to prepared the CIGS absorber layer with gradient band gap in the thickness direction, the different stacking types of Cu-In-Ga precursor layer with CuGa/In/CuGa, CuGa/Culn/CuGa and CuInGa/CuIn/CuInGa structures were prepared by alternately sputtering the Culn target and CuGa target. The results show that:The films prepared by selenization of metallic precursors are chalcopyrite structure and (112) crystal plane preferred orientation. T is much less under the protection of argon in selenization process. The loss of In element for CIS films selenizied by "selenium film method" is also lesser than by "selenium vapor method", and the stoichiometric proportion of the films is more reasonable. The CIGS thin films prepared by the precursors with stacking type of CuInGa/CuIn/CuInGa can effectively reduce the loss of In element, further restrain the content of Cu2-xSe compounds. The CIGS films also show smoother surface morphology, higher crystallinity and favorable electrical properties with the hall coefficient of+22.88cm3C-1the carrier concentration of+2.51×1017cm-3, the carrier mobility of19.8cm2V1s-1and the resistivity of3.11×104Ω·cm.The preparation of ZnS thin films chemical bath deposition (CBD):ZnS thin films respectively deposited in Na-citrate-ammonia system and hydrazine-ammonia system by using thiourea (SC(NH2)2) as source of S2-, zinc sulfate (ZnSO4) as Zn2+. This paper studied the impact of process parameters for ZnS thin film morphology, structure and optical properties and explored the optimal parameters. We explored the optimal parameters of chemical bath deposited ZnS thin films:0.1M zinc sulfate,0.82M hydrazine,0.7M ammonia,0.75M thiourea and the deposition temperature is70℃. According to the analysis of experimental results, we discussed the growth mechanism of chemical bath deposition ZnS thin films.The preparation of ZnSxSe1-x thin films chemical bath deposition (CBD):ZnSxSe1-x thin films were prepared by using the selenomethionine sodium sulfate (Na2SeSO3) as a source of Se2+, thiourea (SC(NH2)2) as source of S2+zinc sulfate as Zn2+and hydrazine-ammonia as the complexing agent on ordinary glass substrates by chemical bath deposition. The composition’x’was varied from0to1by changing the concentration of thiourea and sodium selenosulphate in the precursors. The difference on morphology, structural and optical properties of the ZnSxSe1-x thin films were investigated。The band gap energy of ZnSxSe1-x thin films can be controlled from2.80to3.77eV which is significant for optimizing the structure and improving the photoelectric conversion efficiency of the CIGS thin film solar cells.

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