The Fabrication of CZTS/CZTSSe Solar Cells by Sputtering and Sulfidation(Selenization)

Thin film solar cells will consume a little of material and have potentials in low cost, mass production and flexible substrate applications. The most interesting substrates are steel foils, since they can be coated in a roll-to-roll process at high temperatures of up to 600°C. In the last few years, thin film solar cells draw people’s attention due to their great potential application in the future, especially for CIGS and CdTe solar cells. First solar is a production company of CdTe solar cells. Their annual production will be beyond 2GW in the year of 2011. Solar frontier is the other solar company for producing CIGS solar cells. Their annual production will be beyond 1GW in the year of 2012. Despite the promise of these technologies, restrictions on heavy metal usage for Cd and limitations in supply for In and Te are projected to restrict the production capacity of the existing chalcogen-based technologies to ＜100GWp per year, a small fraction of our growing energy needs, which are expected to double to 27TW by 2050. Earth-abundant copper-zinc-tin-chalcogenide kesterites, Cu2ZnSnS4 and Cu2ZnSn(S,Se)4, have been examined as potential alternatives for the two leading technologies, reaching promising but not yet marketable efficiencies of 6.7% and 10.1%, respectively. Therefore, Cu2ZnSnS4 and Cu2ZnSn(S,Se)4 will be investigated in this project. However, fabricating high-efficiency Cu2ZnSnS4 solar cells face two main problems. One is low fill factor due to high series resistance. The non-uniformity of absorber quality, the backside ZnS segregations (small grains) and a presence of blocking back contact are responsible for high series resistance. The other is low open circuit voltage due to high recombination rate at the interface between CZTS and buffer layer. The possible reasons are SnS existing on the surface of CZTS and large amount of recombination centers at the interface. One can achieve a high efficiency CZTS solar cell if they can solve these problems. In literatures, it is known that the metal composition ratios in higher efficiency CZTS solar cells should be in the range of Cu/(Zn+Sn)=0.85 、Zn/Sn=1.2 、Cu/Sn=1.8~1.9 and Zn/(Cu+Zn+Sn)= 0.25~0.31. First, the stacked layer thickness of metals is calculated to achieve the metal composition ratios. Multiple periods of very thin Cu/SnS/Cu/ZnS/Cu stack sequence are sputtered on SLG substrates. Multiple periods of very thin layer will enhance inter-diffusion in precursor. This selecting stack sequence will avoid the SnS existing on the surface of CZTS and the backside ZnS segregations. Optimizing the sputtering parameters and annealing processing will be examined to enhance the inter-diffusion. Secondly, a very clean environment is used to sulfurize the precursor. Various temperature profiles of the sulfurization are examined to find a good quality absorber layer. Thirdly, the surface treatment of CZTS thin film by chemical solutions is carried out. Various Cd solutions in chemical bath deposition will be tested to improve a good interface between CdS and CZTS. Finally, a re-selenizing treatment to CZTS thin film is performed to obtain a CZTSSe absorber. It is known that CZTSSe solar cells have higher efficiency than CZTS solar cells. Doping selenium to CZTS will change the bandgap and the material property. The effects of the ratio of sulfur to selenium on film quality are also investigated in this project.

Project ID:PB10108-2821
External Project ID:NSC101-2221-E182-068