應用於多接面高效率矽薄膜太陽能電池之穿隧複合接面及中間反射層的開發

Abstract

此研究矽基薄膜太陽能電池由27.12 MHz電漿輔助化學氣相沉積系統所製作。氫化非晶矽雙接面電池相較於微晶矽雙接面電池有成本較低的優勢，同時相對於相同吸收層厚度的單接面非晶矽電池也有較高的穩定度。本論文研究穿隧複合接面來提升在氫化非晶矽雙接面電池，利用不同的材料和結構做為穿隧複合接面幫助載子傳輸與複合進而提高電池的效率。首先，有摻雜的微晶矽當作穿隧複合接面可提供缺陷能階提高載子複合，利用二氧化碳電漿形成很薄一層晶種層提升微晶矽薄膜的結晶率以達更好複合效果，同時也藉由提高氫流量比率使該層更易結晶進而減少厚度。此實驗穿隧複合接面的優化厚度為23奈米，結晶率為53%，應用此接面電池的開路電壓、短路電流密度、填充系數、轉換效率分別提升至1.78 V、7.6 mA/cm2、72.8%及9.86%。 另一方面，氫化非晶矽鍺之雙接面太陽能電池具有能吸收長波長光譜的優勢，本研究也著力於應用穿隧接面於非晶矽鍺之雙接面電池的研究。我們應用一層氫化微晶氧化矽當成中間反射層，以增加反射到頂部的光而達到較好的電流匹配使短路電流提升。此實驗再兼顧導電率下提高氫化微晶氧化矽的氧含量，使其達到較小的折射率以增加反射量。此外，高氧含量的氫化微晶氧化矽也可當成種晶層去增加穿隧複合接面的結晶率。最後應用穿隧接面及中介反射層之矽鍺雙接面電池，其開路電壓、短路電流密度、填充系數、轉換效率分別提升為1.61 V、8.23 mA/cm2、68.2%及9.03%。

In this thesis, the Si-based thin-film tandem solar cells were prepared by a 27.12 MHz radio-frequency plasma-enhanced chemical vapor deposition (PECVD) system. In order to improve the performance of the a-Si:H / a-Si:H tandem solar cell, we introduced a microcrystalline silicon (μc-Si:H(n)) tunneling recombination junction (TRJ) layer between the top and the bottom cells to assist carrier recombination as well as the cell performance. The conductivity of μc-Si:H(n) film was the critical factor to affect cell performance. A thicker μc-Si:H(n) layers can have better conductivity but would increase the absorption loss in the cells. The TRJ was optimized to be thin and highly crystallized. In our results, the optimized thickness of the μc-Si:H(n) was 23 nm and the crystalline volume fraction (XC) was approximately 53%. In addition, TRJ was also employed in a-Si1-XGeX:H tandem cell due to its higher optical absorption in long wavelength region. In this structure, in order to increase the optical reflection for the top cell, we used the n-type hydrogenated microcrystalline silicon oxide (μc-SiOX:H(n)) as an intermediate reflecting layer (IRL) between a-Si:H top cell and a-Si1-XGeX:H bottom cell. The μc-SiOX:H(n) reduced the optical loss in a-Si1-XGeX:H bottom cells due to a wider bandgap. Moreover, the enhancement of the current gain in a-Si:H top cell was obtained due to the stronger reflection. The employment of μc-SiOX:H(n) reflective layer can also act as a seed layer for the TRJ afterwards to further increase the crystalline fraction. As a result, we used μc-Si:H(n) and/or μc-SiOX:H(n) layers as TRJ or IRL to obtain optimum cell efficiencies. The open circuit voltage (VOC), short circuit current density (JSC) fill factor (F.F.) and conversion efficiency (η) of a-Si:H / a-Si:H cell with the optimized μc-Si:H(n) TRJ were improved to 1.78 V, 7.6 mA/cm2, 72.8% and 9.86%, respectively. For a-Si:H / a-Si1-XGeX:H solar cell with μc-SiOX:H(n)/μc-Si:H(n) TRJ structure, the VOC, JSC, F.F. and η were improved to 1.61 V, 8.23 mA/cm2, 68.2% and 9.03%, respectively.