| Nano/micro-structured solar cells have attracted considerable attention due to their unique light-harvesting capabilities and the high photo-conversion efficiency. Detailed balance analysis is one of the most important methods to estimate the ultimate efficiency of solar cells under the ideal optical, electrical, material and other situations, which provides guidelines for improving the light-conversion efficiency of solar cells. In recent years, limiting efficiency research has obtained a distinct progress with the rapid progress of micro/nano-structured solar cells. However, for the newly-emerging field, much room still exists for further investigations, including the study of the radiative and Auger recombination mechanisms of electron-hole pairs and the design of cells with multiple semiconductor junctions.Based on crystalline silicon material system, this thesis focuses on the detailed balance analyses of nanowire/nanohole arrays and vertical/lying single nanowire solar cells. Some key mechanisms in the photoconversion process, including the carrier generation, carrier losses due to the radiative and Auger recombination, current-voltage(I-V) curve, photoelectric output, and photocurrent matching of double-junction solar cells, are studied extensively, in order to obtain the highly efficient photoconversion devices through the innovation on the design and operation principle of the solar cells.The core research achievements of this thesis are listed as follows:(1) Considering the silicon-based nanowire/nanohole array solar cells, the detailed balance analysis is performed to examine the short-circuit current density(Jsc), open-circuit voltage(Voc), and light-conversion efficiency(η), with considering the effects of Auger and radiative carrier recombination. A comprehensive optoelectronic simulation for the considered solar cells is conducted in multiphysics environment. The electrical parameters, such as the carrier generation, Jsc, Voc, η, and I-V curve are obtained; moreover, the radiative and Auger recombination rates are quantified. Result shows that the electrical parameters as well as the recombination rates calculated from detailed balance treatment and the comprehensive optoelectronic simulation are very close for the considered silicon-based nanowire/nanohole array solar cells.(2) Addressing the special single-nanowire configuration and the light-trapping mechanism, the detailed balance analyses are performed for the lying and vertical single-junction single-nanowire solar cells(S-SNSCs) based on the silicon material. Moreover, the Auger recombination is included for the first time for this kind of analysis. It shows that the lying S-SNSCs(LS-SNSCs) with radius of 300 nm can have the limiting efficiency of ~6.6%, exhibiting an enhancement ratio of 103% compared to the planar systems with an equivalent thickness. Besides, the vertical S-SNSCs(VS-SNSCs) with radius of 34 nm and nanowire length of 2000 nm can achieve a limiting efficiency of 203%(dependes on the definition of the incident cross-sectional area without breaking the energy conservation law), due to the dramatically enhanced optical antenna effect.(3) Based on the previous study on the LS-SNSCs, the a-Si:H/μc-Si:H lying tandem SNSCs(LT-SNSCs) are analyzed in order to examine the limiting efficiencies of these devices. It is found that tandem design leads to a higher(~2%) light-conversion capability than μc-Si:H LS-SNSCs with cell radius ranging from 0 to 500 nm. However, compared with the a-Si:H LS-SNSCs, the tandem design has a superior performance only for the cells with relatively large radii.(4) The preparation of vertical silicon microwire arrays by MaCE is proposed. With the etching model studied, we investigate the effects of the exposure conditions, development time, etching temperature, catalyst type, thickness of the metal film, infiltrating layer, the deposition rate of metal film and the concentration of etching liquor on the fabrication of the microwire arrays. By optimizing the MaCE conditions, high-quality vertical Si microwires with length up to ~80μm are fabricated. |
