| AVS 57th International Symposium & Exhibition | |
| Energy Frontiers Topical Conference | Wednesday Sessions |
| Session EN+TF-WeM |
| Session: | CIGS, CZTS and Chalcopyrite Films & Solar Cells |
| Presenter: | R. Krishnan, University of Florida |
| Authors: | R. Krishnan, University of Florida T.B. Song, University of Florida V.U. Chaudhari, University of Florida E.A. Payzant, Oak Ridge National Laboratory R. Noufi, National Renewable Energy Laboratory T.J. Anderson, University of Florida |
| Correspondent: | Click to Email |
Chalcopyrite Cu(In,Ga)Se2 is one of the most promising absorber materials for high efficiency thin film solar cells with reported conversion efficiency exceeding 20%. The National Solar Technology Roadmap for CIGS PV specifically calls for deposition rates of “30–40 μm/h and <1 μm CIGS absorber thickness” by 2015. This is compared to the current estimate in the Roadmap of “5 μm/h and 1.25–3 μm CIGS absorber thickness.” The comparison translates into a reduction of the absorber synthesis time from ~15 to 36 min to ~1.5 to 2 min. The challenge is to get high throughput and yield with columnar grain growth while retaining the high efficiency. In this work in-situ high temperature X-ray diffraction with and without selenium overpressure is used to determine absorber synthesis mechanisms for various precursors structures. Qualitative analysis of the data gives information on the reaction pathway and quantitative analysis of the data yields rate constants.
This presentation summarizes studies on the selenization of elemental stacked layers of copper, indium, gallium and selenium in two different configurations. In the first configuration (sample A), copper was first deposited on glass/Mo substrates, followed by gallium, indium and selenium. In the other configuration (sample B), gallium was first deposited followed by indium, copper and selenium. ICP results showed that both the samples were copper poor. Reaction pathways were followed with and without selenium overpressure and isothermal soaking experiments were performed to obtain the kinetic parameters using the Avrami growth model to reduce the data. The reaction pathways were similar for both the configurations, showing formation In, Cu9Ga4, and Cu2Se initially, followed by Se crystallization and formation of the intermediates In4Se3, CuSe2,CuSe and CIS, and finally yielding product CuIn1-xGaxSe2. The value of x computed from Vegard’s law yielded 0.35 for sample A and 0.37 for sample B. The activation energy computed from the Avrami model yielded 88.4(±12) and 125.1((±9) kJ/mole for samples A and B, respectively. The decrease in the local Avrami exponents (0.21-0.26) suggests the existence of an inhomogeneous distribution of nuclei during growth or interdiffusion of gallium and indium with simultaneous grain growth. Additional characterization such as SEM and TEM were performed to provide physical and compositional support of the pathway.