| AVS 60th International Symposium and Exhibition | |
| Energy Frontiers Focus Topic | Monday Sessions |
| Session EN+PS+TF-MoM |
| Session: | Thin Film, Organic, and Chalcogenide Solar Cells |
| Presenter: | E.S. Aydil, University of Minnesota |
| Authors: | B.S. Tosun, University of Minnesota J.T. Abrahamson, University of Minnesota A.A. Gunawan, University of Minnesota K.A. Mkhoyan, University of Minnesota S.A. Campbell, University of Minnesota E.S. Aydil, University of Minnesota |
| Correspondent: | Click to Email |
Kesterite and chalcopyrite thin film solar cells have an n-type metal sulfide (e.g., CdS, ZnS or ZnxCd1-xS) buffer layer deposited on the p-type absorber. This buffer layer is typically grown using chemical bath deposition (CBD) from metal salts and thiourea in basic aqueous solutions. In a typical CBD system, the entire solution, including the substrate, is heated. This leads to homogeneous nucleation of the metal sulfide particles in addition to the heterogeneous reaction that deposits the desired buffer layer on the substrate. This homogeneous nucleation and growth of particles wastes chemicals and causes problems with solar cells when the particles stick on the absorber surface. To solve these problems we have developed a continuous chemical bath deposition (CF-CBD) system that allows the deposition of ZnxCd1-xS films on 4-inch diameter substrates at temperatures ranging from 50 to 85 oC without significant solution temperature rise and without homogeneous nucleation and growth. Only the substrate is heated to the deposition temperature while the CBD solution is rapidly circulated between the bath and a chilled reservoir. The key features of the design are easily scalable and adaptable to roll-to-roll deposition. We have demonstrated that the CF-CBD system can be used to deposit uniform thickness and composition ZnxCd1-xS films across 4-inch diameter substrates for a variety of x values by changing the concentrations of the Zn and Cd salts in the bath. Addition of ethylenediaminetetraacetic acid disodium to the chemical bath deposition is the key to preventing deposition on surfaces other than the heated substrate surface and key to obtaining films with uniform composition and structure across the film’s thickness. Films were characterized using x-ray diffraction, Auger depth profiling, spectroscopic ellipsometry, transmission electron microscopy and Kelvin probe force microscopy. The effect of deposition parameters such as temperature, bath stirring rate and bath composition on the structure and composition of the films will be discussed.