Effects of Growth Conditions on Secondary Phases in CZTSe Thin Films Deposited by Co-evaporation
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Effects of Growth Conditions on Secondary Phases in CZTSe Thin Films Deposited by Coevaporation Douglas M. Bishop, Brian E. McCandless, Thomas C. Mangan, Kevin Dobson, and Robert Birkmire Institute of Energy Conversion, University of Delaware, 451 Wyoming Rd, Newark, DE 19711, U.S.A. ABSTRACT High temperature multi-source co-evaporation has been the most successful approach to fabricate record efficiency Cu(InGa)Se2 devices, yet many groups have been unable to replicate this success when transferring these methods to the Cu2ZnSnSe4 system. The difficulties stem from the dramatic differences in the thermochemical properties which result in decomposition and loss of volatile species, such as Zn and SnSe, at temperatures needed for growth. In coevaporation, decomposition and element loss must be managed throughout the entire growth process, from the back contact interface to the final terminating surface of the film. The beginning and ending phases of deposition encompass different kinetic regimes suggesting a phased approach to growth may be helpful. A series of depositions with different effusion profiles were used to demonstrate the effects of decomposition during different stages of growth. Secondary phase detection can be challenging in CZTSe, but a combination of SEM imaging and thin cross-section depth profile by EDS were found to best identify and locate the secondary phases that occur during different phases of growth for co-evaporated Cu2ZnSnSe4 films. Deposition with a uniform incident flux followed by shuttered vacuum cool-down yielded films with a ZnSe phase at the absorber/Mo interface and Cu-rich composition at the surface of the exposed film. Devices from these absorber layers never exceeded conversion efficiencies of 1%. Decomposition at the surface could be prevented by continuing effusion of Se and Sn during the cool-down of the substrate. Resulting films demonstrated more faceted grains as well as significantly improved device performance. Secondary phases that traditionally form at the back contact during the beginning of growth were minimized by decreasing the substrate temperature to 300°C during the initial stages of deposition which reduced the ZnSe formed at the Mo interface. The thermochemical origin of the secondary phases will be discussed and the performance of representative devices will be presented. INTRODUCTION Cu2ZnSnS4 and Cu2ZnSnSe4 (CZTS and CZTSe, respectively) have generated significant interest as earth abundant absorber materials for photovoltaic devices in the last half decade. The kesterite crystal structure can be derived from chalcopyrite structure, however, the thermochemical properties are quite different from Cu(InGa)Se2, leading to difficulties when growth and annealing strategies are translated directly to CZTSe [1-2]. In stark contrast to CIGS, record device efficiencies have often been achieved by solution-growth methods [3]. Research teams attempting high-temperature multi-source co-evaporation have reported mixed results with only one team (NREL) reporting >5% devi
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