Quadruple-Junction Thin-Film Silicon-Based Solar Cells

The direct utilization of sunlight is a critical energy source in a sustainable future. One of the options is to convert the solar energy into electricity using thin-film silicon-based solar cells (TFSSCs). Solar cells in a triple-junction configuration have exhibited the highest energy conversion efficiencies within the thin-film silicon photovoltaic technology. Going further from the state-of-the-art device structures, this thesis works on the concept of quadruple-junction TFSSCs, and explores the potential and feasibility of such configuration.The initial experimental realization of quadruple-junction TFSSCs is demonstrated in Chapter 2. The fabricated thin-film a-SiOx:H/a-Si:H/nc-Si:H/nc-Si:H solar cells showed favorable fill factors (FF) and exceptionally high open-circuit voltages (VOC) up to 2.91 V, suggesting a high quality of the material depositions and of the process control. Optical simulations were used in the design of the device structure, to precisely control the thickness and optical absorption in the layers. This preliminary experiment indicated how improvements can be made by better light management.The spectral response of the component subcells is important information for the study of multi-junction solar cells, and the accurate measurement of such properties turns out to be challenging. Chapter 3 analyzes the mechanism of the spectral response measurement of multi-junction solar cells, by means of modeling the optoelectrical response of the subcells and their internal interactions. The formation of measurement artifacts, and their dependence on cell properties and measurement conditions, are elucidated. The analyses lead to comprehensive guidelines on how to conduct a trustworthy measurement and sensible data interpretation.Absorbing semiconductor materials with different bandgaps are desirable for multi-junction solar cells. Thin-film a-SiGex:H cells have been developed to accommodate an absorber material with an intermediate bandgap between that of a-Si:H and nc-Si:H. Chapter 4 reports the development of a-SiGex:H cells using mixed-phase SiOx:H materials in the doped layers. Bearing the band alignment in mind, the optimization of p- and n-type SiOx:H layers resulted in satisfying device performance. The use of SiOx:H p- and n-layers offers great flexibility when integrating the cell in a multi-junction solar cell. Chapter 5 describes the development of quadruple-junction TFSSCs using four different absorber materials. The thin-film wide-gap a-Si:H/narrow-gap a-Si:H/a-SiGex:H/nc-Si:H solar cells promotes reasonable spectral utilization because of the descending bandgap along the direction of light incidence. The tunnel recombination junctions between the subcells have been optimized to ensure effective interconnections thus the proper functioning of the multi-junction device. Advanced light management, which involved the use of modulated surface textured front electrode, was arranged for enhancing the optical performance. These investigations reveal the potential of quadruple-junction TFSSCs.Chapter 6 evaluates the benefit of multi-junction solar cells with different number of subcells. The gains and losses inherent in adding more subcells have been critically assessed from the optical and electrical points of view. The effects of optical reflection, parasitic absorption, tunnel recombination junctions, and filtered illumination in multi-junction cells on the performance were investigated. In general, all types of losses increase with the number of subcells. Among them, the filtered illumination in the subcells can play a significant role in case of a large number of subcells. These results show that such comprehensive analysis helps to judge whether it is reasonable to develop a multi-junction solar cell with a certain structure.Photovoltaic Materials and Device

Thin-film amorphous silicon germanium solar cells with p-and n-type hydrogenated silicon oxide layers

Mixed-phase hydrogenated silicon oxide (SiOx:H) is applied to thin-film hydrogenated amorphous silicon germanium (a-SiGe:H) solar cells serving as both p-doped and n-doped layers. The bandgap of p-SiOx:H is adjusted to achieve a highly-transparent window layer while also providing a strong electric field. Bandgap grading of n-SiOx:H is designed to obtain a smooth transition of the energy band edge from the intrinsic to n-doped layer, without the need of an amorphous buffer layer. With the optimized optical and electrical structure, a high conversion efficiency of 9.41% has been achieved. Having eliminated other doped materials without sacrificing performance, the sole use of SiOx:H in the doped layers of a-SiGe:H cells opens up great flexibility in the design of high-efficiency multi-junction thin-film silicon-based solar cells.Photovoltaic Materials and DevicesElectrical Sustainable Energ

Too Many Junctions?: A Case Study of Multijunction Thin-Film Silicon Solar Cells

The benefit of two-terminal multijunction solar cells in regard to the numberof junctions (subcells) is critically evaluated. The optical and electrical lossesinherent in the construction of multijunction cells are analyzed using informationfrom thin-film silicon photovoltaics as a representative case. Althoughthe multijunction approach generally reduces the thermalization and nonabsorption losses, several types of losses rise with the number of subcells.Optical reflection and parasitic absorption are slightly increased by addingsupporting layers and interfaces. The output voltages decline because of thetunnel recombination junctions, and more importantly of the illuminationfiltered and reduced by the top subcell(s). The loss mechanisms consumethe potential gains in efficiency of multijunction cells. For thin-film silicon,the triple-junction is confirmed to be the best performing structure. Moregenerally, only when each component subcell shows a high ratio between theoutput voltage and the bandgap of the absorber material, a multijunction cellwith a large number of subcells can be beneficial. Finally, the high voltageand low current density of multijunction cells with a large number of subcellsmake them difficult to optimize and manufacture, vulnerable to any changesin the solar spectrum, and thus less practical for the ordinary terrestrialapplications.Photovoltaic Materials and DevicesElectrical Sustainable Energ

Geometrical optimisation of core-shell nanowire arrays for enhanced absorption in thin crystalline silicon heterojunction solar cells

Background: Elongated nanostructures, such as nanowires, have attracted significant attention for application in silicon-based solar cells. The high aspect ratio and characteristic radial junction configuration can lead to higher device performance, by increasing light absorption and, at the same time, improving the collection efficiency of photo-generated charge carriers. This work investigates the performance of ultra-thin solar cells characterised by nanowire arrays on a crystalline silicon bulk. Results: Proof-of-concept devices on a p-type mono-crystalline silicon wafer were manufactured and compared to flat references, showing improved absorption of light, while the final 11.8% (best-device) efficiency was hindered by sub-optimal passivation of the nanowire array. A modelling analysis of the optical performance of the proposed solar cell architecture was also carried out. Results showed that nanowires act as resonators, amplifying interference resonances and exciting additional wave-guided modes. The optimisation of the array geometrical dimensions highlighted a strong dependence of absorption on the nanowire cross section, a weaker effect of the nanowire height and good resilience for angles of incidence of light up to 60°. Conclusion: The presence of a nanowire array increases the optical performance of ultra-thin crystalline silicon solar cells in a wide range of illumination conditions, by exciting resonances inside the absorber layer. However, passivation of nanowires is critical to further improve the efficiency of such devices.Photovoltaic Materials and DevicesElectrical Sustainable Energ

L10 FePt nanoparticles with distinct perpendicular magnetic anisotropy prepared on Au buffer layers by a micellar method

FePt nanoparticles were self-assembled on a MgO(001) substrate by a micellar method. We introduced an Au buffer layer to control the lattice orientation and the magnetic alignment of FePt nanoparticles. A distinct c-axis preferred orientation of the FePt nanoparticles was achieved during the thermal annealing treatment. The driving force of lattice reorientation is considered to be the result of the stress caused by the lattice misfit between Au and FePt. The degree of c-axis orientation is significantly enhanced with increasing Au thickness, which is attributed to the decrease of the in-plane lattice and the improved crystal quality of the Au layer. Perpendicular magnetic anisotropy was observed for the FePt samples with the Au buffer layer. The out-of-plane coercivity and remanence ratio are3.1 kOe and0.8, respectively, which far exceed the in-plane values.?2011 American Institute of Physics