Research on Crystalline Compound Semiconductors and Transparent-Conducting Oxides

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Part A: Monocrystalline CuInSe2 -based Semiconductors for Solar Cells The first generation commercial solar cells in the market are predominantly manufactured using bulk polycrystalline Si or monocrystalline Si. Although the thermal stability of the current Si bulk solar cells is excellent, the energy pay back time is still long. The relatively long energy pay back time is mainly due to the indirect bandgap requiring a large substrate thickness of 200 µm or more for sufficient absorption and the high growth temperatures. The energy pay back time of the second generation thin film solar cells must be reduced for large scale terrestrial applications. The way to achieve the low energy pay back time is to adopt a semiconductor with a direct band gap and high optical absorption coefficients. There are two main material systems being developed for the second generation solar cell fabrication: CuInxGa1-xSe2 and CdTe. Using these semiconductors, the amount of material usage and energy consumption during the cell manufacturing can be reduced. In the applicant’s laboratory at McGill, research work has been performed on the Bridgman growth of monocystalline CuInSe2 (CIS) and CuInGaSe2 (CIGS). Both CIS and CIGS have very large optical absorption coefficients in the solar spectrum and a thin film with a thickness as small as 0.4 µm instead of 200 µm is capable of absorbing 98% of above bandgap photons in the solar spectrum. The performance of thin film cells containing trace of Na (due to inter diffusion from soda lime glass substrates) is much better than those without Na. Unfortunately, it is more difficult to study Na effects in CIS and CIGS thin films and the causes of performance improvement have not been conclusive. Therefore, research work will be performed in this project to study effects of Na on the microscopic and electronic properties of monocrystalline bulk CIS and CIGS. Research work will be performed to prepare ingots from melts containing different amounts of Na. In addition, research work will be made to prepare photovoltaic cells on the CIS and CIGS substrates containing Na. Special attention will be paid to the effect of Na on defect density for the CIGS surface and CIS-CIGS /CdS interface face. We plan to obtain Na results which can allow thin film solar cells to reach efficiencies more than 22%. Part B: Transparent and Conducting Oxides for Electronic Applications Transparent conductive oxides (TCO) are often microcrystalline thin films which have high transmission in the visible wavelength range and high conductivity. Known TCOs include F- or Sb-doped tin oxide, Sn- or Zn-doped indium oxide and Al-, B- or Ga-doped zinc oxide. The resistivity of the TCO films is limited to a value slightly above 10-4 ohm-cm due to a relatively low mobility (~10 cm2/V-sec) at high doping density of more than 1021 cm-3. The high doping concentration has caused a decrease in optical transmission at least in the long wavelength region. In order to improve the performance of devices involving the TCOs, it is desirable to develop thin films with high electron mobility so that the doping concentration can be reduced to obtain the same level of resistivity or to decrease further the resistivity. In the present project, modulation doping will be explored in the conductive oxides in order to increase the mobility to substantially greater than 100 cm2/V-sec while maintaining the high doping concentration. The improved TCOs will be deposited on the CIS and CIGS substrates developed in part A to form solar cells. We also plan to achieve thin film transistor devices with exceptional switching performance compared to the existing technology.