探討搭配分佈式反射鏡與選擇性濾光膜的多層量子井太陽能電池之光子回收效應

Abstract

近年來，科學家注意到太陽能電池的理論極限效率受到內部光子耦合和光子回收效應的影響很大，因此，光性調控對III-V族太陽能電池等直接能隙的材料尤為重要。在本研究中，我們使用高自發輻射率的多層量子井太陽能電池搭配布拉格背反射鏡和選擇性濾光膜研究探討光子回收效應對太陽能電池的特性影響。 未加入濾光膜時，有背反射鏡的多層量子井太陽能電池在一個太陽光照射下相較無背反射鏡的元件，有較低的開路電壓與填充因子。然而在200個太陽光照射下，受到光子回收效應的影響，有背反射鏡的元件的開路電壓可增加約7.3mV，但是其填充因子會由於布拉格反射鏡造成串聯電阻增加，隨聚光率的提升而快速遞減。 接著，為了抑制太陽能電池正面的自發輻射損耗，我們蒸鍍由二氧化鈦與二氧化矽兩種高低折射率材料交互堆疊所形成的選擇性濾光膜作為表面結構，並且設計了880nm、910nm、與930nm 3種截止波長。從電致發光的量測結果確定選擇性濾光膜能抑制元件的發光，增加光子回收效應。元件加入選擇性濾光膜後，不管有無背反射鏡，在聚光下開路電壓與填充因子相較原本無濾光膜的元件皆有提升，由此證實抑制自發輻射的損耗，並增加光子回收效應能增加開路電壓與填充因子。其中多層量子井太陽能電池搭配布拉格反射鏡和截止波長910nm的選擇性濾光膜在200個太陽光照射下，有最大的開路電壓提升，約11mV。但是填充因子的增益卻較其他截止波長的元件小，主要原因可能是光子回收效應雖然能延長載子的生命週期，減少其非輻射複合損耗的機率，提升填充因子，但在高濃度載子的條件也會同時伴隨串聯電阻的增加，限制填充因子的增益。最後，雖然不論有無背反射鏡元件的填充因子都會隨著聚光率的提升而造成遞減，然而覆蓋選擇性濾光膜的元件，特別是有背反射鏡的元件，其填充因子遞減的程度會因為光子回收的效應而較為減緩。

In recent years, scientists have noted that the theoretical efficiency limit of solar cells can be largely affected by internal photon coupling and recycling effects. Therefore, light management has become an important issue for III-V and other solar cells made of direct bandgap materials. In this work, we employ multiple quantum well (MQW) solar cells with a high spontaneous emission rate to study the impact of photon recycling on the photovoltaic characteristics via the incorporation of Bragg mirrors and frontal selective filters. First, before the deposition of selective filters, the MQW solar cell with a Bragg mirror exhibits a lower open-circuit voltage (Voc) and fill factor (FF) than that without the mirror under one-sun illumination. However, the Voc is increased by 7.3 mV under 200 suns, compared to the reference, which is ascribed to the photon recycling effect via the back reflector. However, the fill factor is decreased significantly due to an increased series resistance, which arises from charge transport across the reflector composed of 10 dielectric pairs. Next, in order to suppress the spontaneous radiation losses toward the front surface, we deposit selective filters made of alternative titanium dioxide (TiO2) and silicon dioxide (SiO2) dielectric stacks, designed at cutoff wavelengths of 880nm, 910nm, and 930nm.The electroluminescence measurement confirms that light emission from the cell is inhibited by the filter, which in turn can lead to internal photon recycling. After incorporated with the selectivity filters, the MQW solar cells, with or without a back reflector, show enhanced Voc and FF compared to the reference counterpart without the filters, offering solid proof that suppress of spontaneous emission to increase photon recycling can effectively boost the photovoltaic characteristics. Moreover, the MQW solar cell with a Bragg reflector and selective filter at the 910nm cutoff wavelength exhibits a maximal Voc enhancement ~ 11mV. However, the FF enhancement is relatively small compared to other cutoff devices. We think that although photon recycling can improve the FF by reducing the probability of non-radiative recombination to increase carrier lifetime, the high carrier concentrations inside the cell is accompanied by the large series resistance; thus limit the FF improvement. Finally, although the FF is deteriorated by the increase of illumination concentration, the incorporation of selective filters can slow down the degradation due to photon recycling. This effect is particularly manifested for the MQW solar cells with both a Bragg reflector and selective filter.