具光線捕捉路徑太陽能電池內部量子效率模擬與分析

Abstract

本研究成功的建立了太陽能電池內部量子效率計算理論，以及模擬太陽能電池中基極區、射極區及空間電荷區對量子效率的貢獻，並分析了基極區厚度、射極區厚度、光線傳播數、光線傳播角度、載子擴散長度、載子表面復合速度、底部表面復合速度、電池表面抗反射率、電池底部反射率以及金字塔結構的使用等與量子效率之間的關係，並提供設計太陽能電池之理論參考。在電池表面抗反射部分，我們使用了氟化鎂(MgF)與硫化鋅(ZnS)的雙層抗反射結構以及金字塔結構太陽能電池，其較未使用抗反射膜的平板太陽能電池量子效率分別高約20%以及30%，而金字塔結構的角度若為54.75°時，其量子效率較角度為30°者高約21.7%。使用布拉格反射鏡為底部反射器結構時，當底部反射率達到100%時，光線在電池內的光線傳播數為l=2之量子效率則較光線傳播數為l=1時高約6.34%，以及具有光線傳播角度為θ=60°之量子效率較光線傳播角度為θ=0°時高約9.01%，若固定載子擴散長度，對整體量子效率影響較大的為基極區厚度。 另外在光線傳播數的影響上，根據理論，光線傳播數的極限值應為l=4n^2=51，其中n為矽的折射率約為3.5，光線若在電池中傳播51次後，其內部量子效率將較光線傳播數為l=1時高約26.98%，若具有光線傳播角度為 之量子效率值，當光線傳播數為l=25時,則量子效率值與光線傳播角度為 時相當，且研究中發現底部反射率的大小對量子效率的影響較表面穿透率的大小影響要大。另外若波長低於1000nm，具有光線傳播角度為θ=60°之量子效率值則較光線傳播角度為θ=0°時高約15%。我們也發現，載子擴散長度、光線傳播數以及光線傳播角度的增加可有效的使長波長光線(波長大於1000nm)提升位於電池內部的吸收率，但基極區的材料厚度加厚，隨著光線傳播數以及光線傳播角度的增加，內部量子效率則會隨之降低，因此若光線傳播數為l=51，載子的擴散長度為1000μm時，其內部量子效率較擴散長度為25μm者高約37%，且量子效率的峰值則出現在基極區厚度為50μm。相同的現象則出現於具光柵結構太陽能電池，由於光線的繞射角度為θm=0° 的條件下，光線傳播數若達l=51，則其量子效率峰值可達49%，並發生於基極區厚度為50μm。而其量子效率在基極區厚度wb小於150μm時，則較未具光柵太陽能電池高約9%，當基極區厚度大於150μm後，具有光柵結構者之內部量子效率則稍低於未具光柵結構者。上述提供了太陽能電池改變外部結構與內部結構所得到的內部量子效率與外部量子效率關係，因此當設計高效率太陽能電池時，可根據以上的物理現象與結果來提供足夠的元件設計方向與依據。

A theoretical analysis of the total internal quantum efficiency (IQE) of flat-band homo-junction silicon solar cell with back reflector using distributed Bragg reflectors and grating structure to improve the light trapping is presented and contributions of different regions of the structure to IQEs are simulated. An optical model for the determination of generation profile of the cell is adopted and multiple light passes are considered and compared to previous single light pass approach. It is found that the spatial widths of the cell, the surface recombination velocities, the front surface transmittance, the diffusion length, transmitted angle and the back reflector have significant impacts on the IQEs. With two light passes and normal incident light, the simulation result shows the IQEs can be increased over the one pass value by 6.34% and with a 60° light reflection angle, the IQEs can be further increased by 9.01% while assuming the reflectance at back structure closed to 100%. The effect on IQEs by back reflectance is more significant than that by front transmittance. Under multiple light passes simulation, up to l=4n^2=51 light trapping passes have been considered at wavelength range 900nm-1000nm, the cell can be enhanced by about 26.98%. With textured cell (facet angle α = 54.75°) and normal incident light, the simulation result shows the total IQE increases as the base thickness increases and is higher than that of the at band cell (α = 0°) by 21.7%. The total IQE becomes higher as the facet angle α varies from 0° to 54.75°. For α < 40°, the IQE of textured cell is similar to that of at band cell and for α > 40° the textured effect (increasing the number of light strikes) becomes more evident. Under fixed facet angle α = 54.75°, when the incident angle β is larger than 70°, the IQEs can be further increased by 8.9% as compared with that of normal incident case. As compared with at band cell with anti-reflection coating (which can reduce the Fresnel reflection loss), the total IQE of the textured cell is slightly lower for wavelength range 450nm-700nm and higher for wavelength range 700nm-1150nm. With l=4n^2H light trapping paths, the simulation result shows the best IQEs (~IQEb) with transmitted angle can reach to 73% which is 14% more than that with the normal incident. For the case of normal transmitted angle, the IQEb with diffusion length 1000μm is about 81% and is 37% higher than that with diffusion length 25μm. Under l=51 light trapping paths, the best achievable IQEb with grating structure is 49% at cell thickness wb=50μm, and the IQEb with diffraction angle θm=60° is larger than that with transmitted angle θl=0° and without light trapping by 9% and 39%, respectively. For cell thickness (H~wb>150μm), the IQEb with diffraction angle θm=60° is slightly smaller than that with transmitted angle θl=0°. It is observed that in present cell model the IQEs are affected more significantly by cell thickness, diffusion length and diffraction angle. The obtained results can provide essential information for designing a high-efficiency solar cell.