Chemical Composition Dependence of Cu 2 ZnSnS 4 Absorbers Fabricated by Sulfurization of Thermal Evaporated Metal Precur
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Chemical Composition Dependence of Cu2ZnSnS4 Absorbers Fabricated by Sulfurization of Thermal Evaporated Metal Precursors and Solar Cell Performance Liyuan Zhang, Sreejith Karthikeyan, Mandip J. Sibakoti, Stephen A. Campbell Department of Electrical and Computer Engineering, University of Minnesota, 200 Union Street SE, Minneapolis, MN 55455, U.S.A. ABSTRACT We investigate the synthesis of kesterite Cu2ZnSnS4 (CZTS) thin films using thermal evaporation from copper, zinc and tin pellets and post-annealing in a sulfur atmosphere. The effects of chemical composition were studied both on the absorber layer properties and on the final solar cell performance. It is confirmed that CZTS thin film chemical composition affects the carrier concentration profile, which then influences the solar cell properties. Solar cells using a CZTS thin film with composition ratio Cu/(Zn+Sn) = 0.87, and Zn/Sn = 1.24 exhibited an open-circuit voltage of 483 mV, a short-circuit current of 14.54 mA/cm2, a fill factor of 37.66 % and a conversion efficiency of 2.64 %. Only a small deviation from the optimal chemical composition can drop device performance to a lower level, which confirms that the CZTS solar cells with high conversion efficiency existed in a relatively narrow composition region. INTRODUCTION The need for sustainable energy sources is always current, and solar cells are generally viewed as one of the main solutions. Thin film solar cells have an especially high potential since only a small amount of active material is needed. Cu(In,Ga)Se2 (CIGS) is one of the highestperforming commercial alternatives, but due to the high price and limited availability of the metal indium, it has been increasingly interesting to search for replacements for this material. One of the most promising alternatives is Cu2ZnSnS4 (CZTS), which is p-type with a band gap of 1.4 to 1.5 eV, close to the optimum value for solar energy conversion [1]. The optimal CZTS composition for solar cells has been empirically found to be Cu-poor and Zn-rich, specifically Cu/(Zn+Sn) = 0.8-0.95, and Zn/Sn = 1.1-1.25 [2]. Some theoretical research shows that reducing Cu and increasing Zn reduces the excess carrier concentration since CuZn antisite defect is the dominant acceptor in near-stoichiometric CZTS. This also enhances the formation of Vcu defect, which is the primary acceptor in Cu deficient CZTS thin film. The reduction of deep states associated with ZnSn and (Cum)Zn, where m can be one to three, is another reason that Cu-deficient and Zn-rich films show better performance [3]. In this report, CZTS thin films with different chemical compositions were formed by thermal evaporation and a post-deposition sulfurization process. Completed solar cells were fabricated based on those CZTS thin films and device measurements were also conducted to study the relationship between film composition, carrier concentration and solar cell performance from an experimental point of view. EXPERIMENTAL DETAILS
The metal precursor layers (Cu, Zn, and Sn) were deposited in a stack on Mo
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