Abstract
Chalcopyrite compounds of Cu(In,Ga)Se2 and related alloys are among the most promising materials for photovoltaic applications. Sputtering of Cu-In-Ga precursors followed by selenization has been a preferred industrial process for Cu(In,Ga)Se2 solar cell manufacturing. In a sputtering process, many studies using co-sputtering or sequential sputtering from CuGa and In magnetron targets for preparation of the metallic precursors. In this study, the metallic precursors were deposited by sputtering a single Cu-In-Ga ternary target and compared with the In/CuGa stocked precursors and these samples were selenized using the Se vapor. It was observed that Ga tends to segregate near the Mo electrode after selenization thus reducing the band gap of the Cu(In,Ga)Se2 absorber near the surface. Since the open circuit voltage (Voc) depends on the band gap in the space charge region (SCR) near the surface of absorber. The device fabricated using this process, however, tends to have a relatively low Voc value due to the Ga element migration to near the Mo electrode. 一、Proposed and demonstrated a novel sandwiched precursor structure we demonstrated a novel sandwiched structure to improve the Ga distribution and grain growth in the absorption layer and thus increase the open circuit voltage Voc and Jsc. We discuss the employment of a novel precursor structure using a single CuInGa layer sandwiched between thin CuGa and In layers. This precursor structure was constructed by having a thin CuGa film on top of the CuInGa ternary layer and a thin In layer to the bottom of the CuGa/CuInGa stacked layer. It is observed that when a thin CuGa film was sputtered on top of the surface of CuInGa ternary precursor, it enhanced the grain growth of Cu(In,Ga)Se2 absorber and increased the Ga concentration in the space charge region, therefore improved the open circuit voltage (Voc). In addition, we observed when a thin In layer was added to the bottom of CuGa/CuInGa stacked layer, it reduced the minimum band gap of devices, and therefore increased the absorption of solar spectrum. By employing this novel structure, the open circuit voltage for the solar cell devices in our studies increased by 18.2% (from 390 mV to 460 mV), the short current density by 13.8% (from 29 mA/cm2 to 33 mA/cm2), and the conversion efficiency by 50 % (from 6.26 % to 9.52 %). 二、Investigation of selenization and sulferization process we discuss three kinds of selenization methods including (a) the RTP process, (b) H2Se selenization process and (c) sulferization after selenization process which were used in studying the Ga distribution and grain growth of Cu(In,Ga)Se2 absorbers under these three selenization processes. In an experiment study of selenization using RTP, our result shows by shortening the annealing time, CuGaSe2 and CuInSe2 would produce almost within the same time and therefore could reduce the segregation of Ga into the bottom of Cu(In,Ga)Se2 absorber. These experiments directly confirmed that the segregation of Ga element due to a difference in the formation temperature of the CuGaSe2 phase higher than that of the CuInSe2 phase. From the results of our study of the H2Se selenization process using XPS and SEM analyses, it further suggests a higher selenization temperature did not affect the Ga distribution in the absorber, however, it could enhance the grain growth near the bottom of Cu(In,Ga)Se2 absorber. As a result, it lead to an increase of the conversion efficiency of the solar cell devices from 9.5% to 12.8%; an enhancement of about 34%. From the experiment results of sulferization after the selenization process, the sulfur element incorporate into the Cu(In,Ga)Se2 absorber would form smaller grains. By comparing the results of GIXRD and SEM, it suggests that a lower selenization temperature would increases the S content in the surface area of Cu(In,Ga)(Se,S)2 film and form smaller grains of absorber and the energy band gap of the absorber. As a result, by using sulferization after the selenization process, the open circuit voltage (Voc ) of the device was further improved by 10%, and the overall conversion efficiency of the solar cell devices increased by about 10% from 12.8% to 14%. 三、Near infrared enhancement in Cu(In,Ga)Se2-based solar Near infrared enhancement in Cu(In,Ga)Se2-based solar cells utilizing a ZnO:H window layer were also investigated in this study. The hydrogen atoms incorporated into a ZnO film as a shallow donor could decrease the resistivity of ZnO film. The ZnO:H film has sa imilar resistivity to that of the ZnO:Al film of about 1.29×10-3 Ω-cm. The advantage of ZnO:H film is higher Hall mobility than ZnO:Al film and thus the carrier concentration of ZnO:H film is lower than that of ZnO:Al film which can decrease free carrier absorption in the NIR. It is found that the cell efficiency is enhanced by 4.8% for the ZnO:H device. This is attributed to the fact that the ZnO:H film has higher transmittance than the ZnO:Al film in the NIR which results in the improvement of short-circuit current (Jsc) from 34.5 to 35.6 mA/cm2.