Investigation Of Amorphous Silicon Germanium Solar Cells And Amorphous Silicon/Amorphous Silicon Germanium/Microcrystalline Silicon Triple Junction Solar Cellswith Electro-optically Modified Structure

A potential to achieve high efficiency and stability simultaneously for silicon base triple junction solar cell makes it an important way to improve silicon base thin film solar cells’ performance further. Amorphous silicon germanium(a-SiGe:H) materials become the preferred choice of its middle sub-cell’s intrinsic material owing to their excellent opticalcharacteristics of high absorption coefficient and adaptive band gaps. However, the deteriorated material quality with germanium incorporation has imposed restrictions on the further improvement of a-SiGe:H cells and corresponding triple junction solar cells. New techniques are desperately needed to break the bottleneck. In this thesis, with RF-PECVD process, based on a-SiGe:H materials with device-quality level, aiming to the issues emerging in their application to cell structures, for the purpose of achieving high efficiency a-SiGe:H single junction solar cells with a broadband light response, excellent electrical performance and applicable middle sub-cells in triple junction solar cells, we systematically studied the modification of a-SiGe:H cells’electrical and optical structures from the device design point of view. Meantime, with the aim of interconnecting other sub-cells with low optical and electrical losses, we applied the modified a-SiGe:H cells to amorphous silicon/amorphous silicon germanium/microcrystalline silicon(a-Si:H/a-SiGe:H/μc-Si:H) type triple junction solar cells and systematically investigated the modification of top/middle and middle/bottom tunnel recombination junctions in triple junction solar cells. Detailed description of the research was listed as following:Firstly, aiming to the bulk recombination on account of the deterioration of a-SiGe:H intrinsic layer’s quality and enhanced electric field screening effect with germanium incorporation and the interface recombination owing to band gap discontinuity and enhanced defect density at P/I or I/N interfaces,(Dthe research on the amorphous silicon(a-Si:H) buffer layer’sdifferent band gaps at the P/I interface shows an a-Si:H buffer layer with high quality and a proper band gap can efficiently modify the band gap discontinuity, lower the interface defect density and electric field screening effect, then improve the electrical performance of a-SiGe:H cells.② Based on the optimized interfaces, the modulation effect of band gap grading structures in intrinsic layers on the cells’performance were investigated. The experimental results show different charge and recharge processes at charged defect layers under forward biased voltages are the reason why a difference in fill factors exists between cells with different grading slope signs. A-SiGe:H cells with positive grading structures can efficiently lower the electric field screening effect on the hole transport and collection, then improve the fill factor.③By studying the optimized electrical structures for a-SiGe:H cells with high and low germanium content(42%and30%), optimized electrical structures that are applicable to a-SiGe:H cells with different germanium content were firstly proposed. The a-SiGe:H cell with high germanium content fabricated with the proposed structure shows nearly the same electrical performance as that of the a-SiGe:H cell with low germanium content, and this will serve as a potent guidance for the choice of high electrical performance middle sub-cells with various band gaps to fabricate triple junction solar cells with current-matching and high electrical performance.④Aiming at the stronger electric field screening effect exists in a-SiGe:H cells with high Ge content compared with cells with low Ge content, we firstly proposed the finely tuned band gap grading structure based on the linear band gap grading. The experimental results illustrate that the hole transport in bulk regions can be efficiently improved by reducing the germanium incorporation in the bulk region near the P/I interface and a-SiGe:H cells’ fill factor can be elevated further.⑤The function of modifying a-SiGe:H cells’ electrical performance by means of reducing the leakage current for n type μe-SiOx:H layers with proper oxygen content was firstly proposed. This method can efficiently improve the fill factor of PIN type a-SiGe:H cells and a fill factor as high as70.05%which is the highest value among the reported PIN type a-SiGe:H cells was achieved.Secondly, based on the electrical structure modulation of a-SiGe:H cells, the modulation investigation of the optical structure was carried out.①From the surface morphology, light scattering analysis of substrates, light absorption simulations and cell fabrication points of view, by studying the influence of the surface feature sizes of wet-etched sputtering ZnO:Al substrates with various etching times on a-SiGe cells’ optical performance, the results show that substrates’ surface feature sizes can be optimized with the aim of maximizing the optical absorption in a-SiGe:H intrinsic layers without sacrificing the electrical performance of cells. The optimal surface feature sizes which can maximize the light absorption in the intrinsic layers of a-SiGe:H single junction solar cells with1.48eV average band gap are (168nm,732nm).②By investigating the influence of n-μc-Si:H/n-μc-SiOx:H double-layer structure on the optical and electrical performance of a-SiGe:H cells, we firstly found that expect for the electrical gain in a-SiGe:H cells, the resulting refractive index grading can obviously elevating the long-wavelength response in a-SiGe:H cells. The n type μc-SiOx:H layers with a graded refractive index can improve the electrical performance of a-SiGe:H cells to some extent while keeping the long-wavelength response compared with n type μc-SiOx:H layers with a constant refractive index.③Based on the investigation of irrespective applications of p type nanocrystalline silicon layers and window layers with a reduced boron content in a-SiGe:H cells, a p type double-layer structure combining the two layers was proposed. The results show that by combining the high transparency properties of p type nanocrystalline silicon layers and the interface band gap match role of window layers with a reduced boron content, a-SiGe:H cells’ short wavelength response can be elevated obviously while keeping high electrical performance.④Based on the fabrication processes of a-SiGe:H cells with a electro-optical modulation, highest initial efficiencies of9.70%(Voc=775.90mV, FF=65.10%, Jsc=19.21mA/cm2) and10.59%(Voc=744.70mV, FF=66.79%,Jsc=21.29mA/cm2) were achieved for a-SiGe:H single junction cells without or with a back reflector. Our results achieved the domestic leading technology, the international advanced level.Thirdly, we studied the modulation of tunnel recombination junctions between a-SiGe:H middle sub-cell with a-Si:H top sub-cell or uc-Si:H bottom sub-cell when we applied a-SiGe:H cells with a modulated electro-optical structure in a-Si:H/a-SiGe:H/μc-Si:H triple junction solar cells.①Aiming to the issue that there is no way to measure the performance of tunnel recombination junctions directly, a fast diagnostic approach with high accuracy, namely contrastive analysis, compared with traditional n/p junction or pin/p diagnostic methods was proposed.②The investigation of the influence of n-μc-SiOx:H layers inserted in the middle/bottom tunnel recombination junction on the performance of triple junction solar cells was carried out. We proposed a middle/bottom tunnel recombination junction structure with low losses when a n-μc,c-SiOx:H layer is inserted. The results illustrate that this structure can realize both the interconnection between middle and bottom sub-cells with low losses and the improvement of short circuit current in middle sub-cells.③Based on high performance top, middle and bottom sub-cells with well designed structures, we successfully found the source of triple junction solar cells’ loss with the contrastive analysis. The results prove that middle sub-cells with p type microcrystalline silicon layers with a low activation energy can aid the transport of the optical generated holes at the tunnel junction, thereby elevating the performance of the tunnel junction and reducing the open circuit voltage losses. An initial efficiency of11.63%(Voc=1.75V, FF=67.97%, Jsc=9.77mA/cm2) was achieved for the resulting a-Si:H/a-SiGe:H tandem cells with a total thickness of290nm.④Aiming to the issue that a difference of the open circuit voltage and fill factor resulting from the different spectrum environments for single junction solar cells and middle sub-cells exists, we firstly proposed an approach which modifies the P/I buffer layer’s thickness to reduce the fill factor and compensate the open circuit voltage. This approach can obviously elevate the open circuit voltage of triple junction solar cells while keeping the fill factor, thereby improving the cell efficiency.⑤Based on the structure modulation of a-Si:H, a-SiGe, μc-Si:H sub-cells, top/middle and middle/bottom tunnel recombination junctions, and the finely tuned a-SiGe:H middle sub-cell’s structure, An optimal initial efficiency of15.06%(Voc=2.20V, Jsc=9.04mA/cm2, FF=75.93%) for a-Si:H/a-SiGe:H/μc-Si:H triple junction solar cells (with an active area of0.253cm2) was achieved. Our results achieved the domestic leading technology, the international advanced level.