Chalcopyrite/Si Heterojunctions for Photovoltaic Applications

摘要

Films of CuInSe₂ (CIS) and CuGaSe₂ (CGS) were deposited on (100) Si by radiofrequency (RF) magnetron sputtering from stoichiometric CIS and CGS targets. Rutherford backscattering (RBS) analysis yielded a composition of Cu_0.8In_1.1Se_1.9 for CuInSe₂, which indicates that these films were Cu and Se poor. A composition of Cu_0.3Ga_1.5Se_2.0 for CuGaSe₂ shows Ga-rich and Cu-poor layers. Transmission electron microscopy (TEM) of cross-sectional samples established that the films were polycrystalline in nature and free of pinhole defects that normally short-circuit devices fabricated on glass with submicron absorber layers. From the electron and x-ray diffraction patterns, tetragonal chalcopyrite phases of the material were identified. Circular diodes, with a diameter between 100 μm and 400 μm, were fabricated on the grown films with a common Au back-contact. Diodes on both CIS and CGS films exhibited rectifying characteristics. From the polarity corresponding to the high and low currents, it was inferred that the grown films were p-type. These diodes exhibited photovoltaic response, and the forward-bias current increased by as much as two orders of magnitude when illuminated by a 75-W halogen lamp. The open-circuit voltages (V _OC) for these devices are expected to approach the turn-on voltage of the diodes, 0.5 V and 0.7 V, for the CGS/Si and the CIS/Si heterojunctions, respectively. Shunting caused by degenerate phases present in the CGS film is believed to have resulted in the observed lower turn-on voltage for the CGS-Si heterojunction diode.