Development of selenization/sulfurization-free CuIn(S,Se)2 for thin film solar cells applications
Date of Issue2015
School of Materials Science and Engineering
Energy Research Institute @ NTU
To meet the growing demand for energy and to reduce the depletion of fossil resources, technology for the harnessing of renewable energy is urgently needed. Among all the renewable energies, solar energy is one of the most clean, abundant and sustainable energy sources that can help to solve the world energy consumption problem. Harvesting and conversion of solar energy to electrical energy required the use of solar cells. Of all the different solar cell technology, copper-indium-gallium sulfide/selenide (CIGS) solar cell is one of the most promising thin film solar cells because of its high optical absorption coefficient, suitable band gap for harvesting a wide range of visible light from the sun as well as some unique electrical and optical properties. Compared to conventional vacuum-based synthesis, there is strong push towards the use of solution-based methods to prepare these CIGS films so that the processing cost can be reduced significantly. In this thesis, the solution-based synthesis of CuIn(S,Se)2 thin films as well as crystals and their application in the solar energy harvesting will be presented. Generally, for the solution-based synthesis routes, selenization/sulfurization process is usually needed to incorporate Se/S, to increase the crystallinity and to encourage grain growth. However, selenization/sulfurization is toxic and requires special processing system to cater for safety. Therefore this thesis sets out to investigate methods to synthesized absorber films without any selenization/sulfurization process and also ways to increase grains size. CuInS2, CuInSe2 and CuInSSe thin films were successfully synthesized without any selenization/sulfurization process. In this work, the thin films were prepared using a vulcanization process and then annealed. The vulcanization process occurred between the molecular precursors, when cross-links are formed between metal acetylacetonates and sulfur/selenium powder even before annealing. Using this method, CuInS2/CuInSe2/CuInSSe thin films could be formed at a relatively low temperature of about 350oC. It was found that crystallinity and morphologies of CuInS2/ CuInSe2/ CuInSSe could be improved by increasing the annealing temperature and annealing time. Raman characterization confirmed that no secondary phase was formed. CuInS2 was selected for the demonstration of solar cell because its film forming ability is better compared to CuInSe2 and CuInSSe This is because the poor solubility of Se in pyridine compared to S. However, since there is no high temperature selenization/sulfurization involved, the grain size of CuInS2 is still relatively small compared to those highly efficient Cu(InGa)(S,Se)2 solar cells. The solar cell efficiency for these CuInS2 is comparable to those Cu(InGa)(S,Se)2 solar cells without selenization/sulfurization and can be further improved by moderating the composition and also increasing the grain size. As mentioned earlier, large CIGS grains will result in films that showed better solar cell performance and one of the reasons is that they have fewer defect sites and recombination centers. As a route to larger grain size, nanoparticles is first synthesized and by encouraging growth in these individual CIGS nanoparticles, micron sized grains can be achieved. CuInSe2 nanoparticles were synthesized by solvothermal reaction at 180oC for 24hrs/48hrs. The effects of precursor concentration and type of solvents on the composition and morphology were investigated. Three different solvents were chosen: ethanol-DMF mixture, methanol-DMF mixture and water-DMF mixture. It was found that CuInSe2 nanoparticles could be obtained from both methanol-DMF mixture and water-DMF mixture but methanol-DMF mixture will result in higher purity CuInSe2 nanoparticles. Size and morphology of the nanoparticles could be tuned by precursor concentration. Raman spectroscopy revealed that although no secondary phases were formed, Cu-Au (CA) phase, which is believed to be harmful for device, existed in the synthesized CuInSe2. The detrimental CA phase can be reduced by increasing the reaction time and the precursor concentration. Photocurrent measurement indicated good photoresponse from the as-synthesized CuInSe2 and CuInSe2 with less CA phase showed improved electrical performance. As mentioned previously, larger grain size is preferred for good solar cells performance. Micron-sized CuIn(S,Se)2 grains were obtained by allowing the CuInSe2 nanoparticles to undergo a second solvothermal reaction step. TEM revealed that micron-sized plates are formed (more than 1μm) and the distribution of different elements is uniform in these plates. The mechanism for the formation of micrograin CuIn(S,Se)2 from CuInSe2 nanoparticles was investigated. In addition, the phases of final products after second-step solvothermal reaction with different reaction time were also studied by XRD and rietveld refinement. Photochemical studies on CuInSe2 and CuInSSe was carried out to investigate the electrical properties and photoresponse. CuInSSe is found to have a bandgap closer to the optimal value and larger carrier concentration. Solar cells fabricated using both CuInS2 thin film and CuInSSe microcrystals showed a better performance compared using just CuInS2 thin film. Our simple, environmental friendly synthesis and fabrication processes suggest a promising potential for the future solar harvesting applications.