Large grain Cu(In,Ga)Se2 thin film growth using a Se-radical beam source
Introduction
Cu(In,Ga)Se2 (CIGS) thin films can be prepared by a variety of methods. Among these, multi-source evaporation has proven to date to be the most promising method to obtain high-quality photovoltaic-grade CIGS films [1]. Multi-source evaporation, however, has been seen to date as a laboratory-scale production technique. One of the disadvantages of multi-source evaporation holding back scaling up of this method to industrial application has been the large amount of Se source material consumed during growth. This leads to increased production costs, more frequent maintenance of the growth chamber, and increased levels of industrial waste. To solve this issue, we have developed a CIGS growth technique that utilizes a radio frequency (rf)-plasma cracked Se-radical beam source in the multi-source evaporation method [2], [3]. A significant reduction in the amount of Se source material used by more than a factor of 10 over that used by a conventional Se evaporation has been demonstrated. In addition to this merit, CIGS films grown with a Se-radical source exhibited highly dense, smooth surfaces, and large grain size. This is attributed to the modification of growth kinetics due to the high reactivity of the active Se-radical species produced and the resulting enhanced migration during growth. Se radicals are expected to enhance surface migration, which may increase the tendency toward two-dimensional growth similar to the case of ZnSe growth using a thermally cracked Se source [4].
In this study, we have applied the Se-radical source for film growth of CuGaSe2 (CGS), low-temperature growth of CuInSe2 (CIS), and CIGS films. The In–Ga composition ratio of the CIGS and the growth temperature have been known as critical parameters controlling variations in film properties and solar cell performance. In general, CGS (x=1) films and low-temperature (here we note that ‘x’ is the composition ratio of [Ga]/[In+Ga] in CIGS in at% and ‘low-temperature’ is 400 °C where CIGS growth on polyimide films is possible)-grown CIGS films exhibit small grain size compared with CIGS films grown at x<1 or at high temperature (∼550 °C). In view of the previous results seen for CIGS films grown at 550 °C, Se-radical source grown films can be expected to exhibit large grain size and modified surface morphologies. Here, the film properties of CGS grown at 550 °C, low-temperature-grown CIS, and CIGS films using a Se-radical source have been studied. Photovoltaic performance of solar cells fabricated using Se-radical source grown films has been also examined.
Section snippets
Thin film growth
CIGS thin films were grown on Mo-coated soda-lime glass substrates by the three-stage process using a molecular beam epitaxy apparatus equipped with elemental Cu, In, and Ga Knudsen-cell sources and an rf-cracking unit equipped Se-radical source. The growth chamber was also equipped with an elemental Se Knudsen-cell source for conventional CIGS film growth. Detailed growth conditions of CIGS films using a conventional Se-evaporative source and an Se-radical source have been described elsewhere
CGS thin films
Cross-sectional and surface SEM images of CGS thin films grown using a conventional Se-evaporative source and an Se-radical source are shown in Fig. 1(a) and (b), respectively. These films were grown under nominally identical conditions except for difference in Se source. The Se-radical source grown CGS film exhibited large grain size compared with the Se-evaporative source grown CGS film as shown in Fig. 1(b); similar results have been seen for CIGS films grown at a maximum substrate
Summary
We have grown CGS, CIS, and CIGS thin films using an rf-plasma cracked Se-radical source alternative to the conventional Se-evaporative source. All films grown with an Se-radical source exhibited highly dense surfaces and large grain size with strong (1 1 2) texture in comparison with Se-evaporative source grown films. Although further investigation and development is necessary concerning Se-radical source grown CGS films and corresponding devices, solar cells made from low-temperature-grown CIS
Acknowledgements
This work was supported in part by the Incorporated Administrative Agency, New Energy and Industrial Technology Development Organization (NEDO) under the Ministry of Economy, Trade and Industry (METI).
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