Research on Crystalline Compound Semiconductors and Transparent-Conducting Oxides

A部分：太阳能电池用单晶CuInSe2基半导体 市场上的第一代商用太阳能电池主要是使用块状多晶硅或单晶硅制造的。虽然目前的硅体太阳电池的热稳定性很好，但能量回收期仍然很长。较长的能量回馈时间主要是由于间接带隙，需要200微米或更大的衬底厚度才能充分吸收和较高的生长温度。对于大规模的地面应用，必须缩短第二代薄膜太阳能电池的能量回收期。实现低能量回收期的方法是采用具有直接带隙和高光吸收系数的半导体。目前正在开发的第二代太阳能电池主要有两种材料体系：CuInxGa1-xSe2和CdTe。使用这些半导体，可以减少电池制造过程中的材料使用量和能源消耗。在申请人在McGill的实验室中，已经对单晶CuInSe2(CIS)和CuInGaSe2(CIGS)的布里奇曼生长进行了研究。CIS和CIGS在太阳光谱中都有很大的光学吸收系数，厚度为0.4微米而不是200微米的薄膜能够吸收太阳光谱中98%的上述带隙光子。含微量钠的薄膜电池(由于钠钙玻璃衬底的相互扩散)的性能比不含钠的薄膜电池好得多。遗憾的是，研究CIS和CIGS薄膜中的Na效应更加困难，性能改善的原因也没有定论。因此，本项目将研究Na对单晶体CIS和CIGS微观和电子性质的影响。将开展研究工作，从含有不同量钠的熔体中制备钢锭。此外，还将研究在含Na的CIS和CIGS衬底上制备光伏电池。重点研究了Na对CIGS表面和CIS-CIGS/CDS界面缺陷密度的影响。我们计划获得钠的结果，使薄膜太阳能电池的效率达到22%以上。 B部分：电子应用用透明导电氧化物 透明导电氧化物(TCO)通常是在可见光波段具有高透过率和高电导率的微晶薄膜。已知的TCO包括氟或锑掺杂的锡氧化物、锡或锌的掺杂的氧化铟和铝、硼或镓的掺杂的氧化锌。由于在1021 cm-3以上的高掺杂浓度下，TCO薄膜的迁移率相对较低(~10cm2/V-sec)，因此TCO薄膜的电阻率被限制在略高于10-4欧姆-厘米的值。高掺杂浓度至少在长波长区引起了光透过率的下降。为了提高TCOS器件的性能，人们希望开发具有高电子迁移率的薄膜，以降低掺杂浓度以获得相同水平的电阻率或进一步降低电阻率。在本项目中，将探索在导电氧化物中进行调制掺杂，以便在保持高掺杂浓度的同时将迁移率增加到基本上大于100 cm2/V-sec。改进后的TCO将沉积在A部分开发的CIS和CIGS衬底上，形成太阳能电池。我们还计划实现与现有技术相比具有卓越开关性能的薄膜晶体管器件。

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.