Materials and Light Management for High-Efficiency Thin-Film Silicon Solar Cells

Abstract

Direct conversion of sunlight into electricity is one of the most promising approaches to provide sufficient renewable energy for humankind. Solar cells are such devices which can efficiently generate electricity from sunlight through the photovoltaic effect. Thin-film silicon solar cells, a type of photovoltaic (PV) devices which deploy the chemical-vapor-deposited hydrogenated amorphous silicon (a-Si:H) and nanocrystalline silicon (nc-Si:H) and their alloys as the absorber layers and doped layers, are one of the promising PV technologies. Compared to other PV technologies, thin-film silicon solar cells have several important advantages such as the use of abundant and non-toxic materials, low processing temperature, short energy payback time and mature large-area manufacturing techniques. Despite the many advantages, thin-film silicon (TF-Si) technology is suffering from the drop in the PV market share due to the relatively low efficiency compared to c-Si, CIGS, and CdTe solar cells. This thesis is devoted to the development of advanced materials and novel light-trapping structures to increase the power conversion efficiency of thin-film silicon solar cells. To achieve maximal light absorption in the absorber layers, implementation of light-trapping structures is crucial for thin-film silicon solar cells. The status of light-trapping techniques is briefly summarized in chapter 1, together with the background knowledge for thin-film silicon solar cells. To design an effective light-trapping scheme for solar cells, both the optical performance and the influence on the electrical performance of solar cells have to be considered. The light-trapping structure itself should not give additional parasitic absorption losses, or this loss should be minimized. The morphology of the light-trapping substrate should be suitable for the growth of high-quality materials. Meanwhile, absorption losses in the supporting layers such as front electrodes, doped layers, and back reflectors should be minimized. In chapter 2, the fabrication of plasmonic back reflectors (BRs) based on self-assembled Ag nanoparticles and their application in a-Si:H solar cells are presented. It has been experimentally demonstrated that the optimized plasmonic back reflector can provide light trapping performance comparable to state-of-the-art random textures, without obvious deterioration of open-circuit voltage (Voc) and/or fill factor (FF). This conclusion is based on the fair comparison with high performance n-i-p solar cells and state-of-the-art p-i-n solar cells deposited on Asahi-VU substrates. The combined optical and electrical design of plasmonic back reflectors follows in chapter 3. The design rules of plasmonic back reflectors based on self-assembled Ag nanoparticles are discussed in detail. The shape of Ag NPs, the thickness of ZnO:Al spacer layers, materials on top of Ag NPs, and nanoparticle size are crucial for the performance of plasmonic BRs. By following the design ruless, an 8.4% efficiency plasmonic a-Si:H solar cell has been achieved. The application of the plasmonic back reflector in low bandgap nc-Si:H solar cells is discussed in chapter 4. The light trapping performance in nc-Si:H solar cells is improved by using the plasmonic BRs with a broad angular scattering and low parasitic absorption loss through tuning the size of silver nanoparticles. The nc-Si:H solar cells deposited on the improved plasmonic BRs demonstrate a high photocurrent comparable to the one achieved by the state-of-the-art textured Ag/ZnO BR. The commonly observed deterioration of fill factor is avoided by using nc-SiOx:H as the n-layer for solar cells deposited on plasmonic BRs. In chapter 5, micro-textures on glass with large opening angles and smooth U-shape morphology are proposed and applied to nc-Si:H solar cells for the first time. The micro-textures can provide both efficient light trapping and suitable morphology for the growth of high-quality nc-Si:H materials under a high deposition rate. A higher Voc and FF can be achieved in reference to the cells using nano-textured substrates. For thick solar cells (i-layer thicker than 2 µm), high Voc and FF values are maintained. Particularly, the Voc only drops from 564 to 541 mV as solar cell thickness increases from 1 to 5 ?m. The use of micro-textures paves the road to develop multijunction solar cells with a higher efficiency as will be shown in chapter 7. High-efficiency multijunction thin-film silicon solar cells require both high Voc and high blue spectral response in the top a-Si:H cell. In chapter 6, the mixed-phase p-SiOx films are investigated and used as window layer in high Voc a-Si:H p-i-n solar cells. The use of p-SiOx as window layer results in a higher Voc and a better spectral response than the standard p-SiC based window layer. Consequently, a-Si:H solar cells with Voc >1 V and FF >70% have been obtained. A high initial efficiency of 14.4% has been achieved in a-Si:H/nc-Si:H tandem solar cells deposited on the Asahi-VU substrates. Chapter 7 presents the implementation of highly transparent modulated-surface-textured (MST) front electrodes as light-trapping structures in multijunction TF-Si solar cells. The MST substrates comprise a micro-textured glass as developed in chapter 5, a thin layer of hydrogenated indium oxide (IOH), and a sub-micron nano-textured ZnO layer grown by low-pressure chemical vapor deposition (LPCVD ZnO). The MST front electrode has a good transparency and conductance, can provide efficient light trapping for each subcell and a suitable morphology for the growth of high-quality silicon layers. Efficiencies of 14.8% (initial) and 12.5% (stable) have been achieved for a-Si:H/nc-Si:H tandem solar cells with the MST front electrode and the high-performance a-Si:H top cells as developed in chapter 6, surpassing efficiencies obtained on state-of-the-art LPCVD ZnO. A short summary of this thesis is given in chapter 8. Perspectives to further improve the performance of thin-film silicon solar cells are suggested and discussed. The light-trapping performance of modulated-surface-textured front electrodes can be further improved by replacing the wet-etched glass with honeycomb textures, without sacrifice in electrical performance of solar cells. The honeycomb textures can be easily applied to superstrate configuration by mature UV-NIL technique. In the end, the hybrid a-Si:H/organic multijunction device configuration is proposed to avoid the use of thick nc-Si:H solar cells. A high efficiency of 11.6% has been achieved in the hybrid tandem configuration with a total absorber layer thickness less than 500 nm. By deploying the triple-junction structure, a high efficiency of 13.2% has been obtained while the thickness of absorber layers stack is below 1µm. With further efforts on this concept, performance comparable to the traditional devices based on a-Si:H and nc-Si:H can be expected while the total processing time is much shorter and the cost for manufacturing and materials is lower.