| Accompanied with the progress of modern society, in terms of a series of breakthroughs in science and technology, the environmental impacts and consequences are drawing more and more world-wide attentions and serious concerns about the sustainability of this growth pattern that inflicts heavy burden upon the environment and over-exploitation of the limited fossil energy resources. It has become urgent that new alternative and green energy sources and technologies have to be established and implemented in a world-wide large scale, in order to curb the green-gas emission and switch smoothly into a sustainable growth and development pattern. To this end, the development of clean energy source technologies of wind power, photovoltaics and bio-system are gaining remarkable momentums in recent year. Particularly, according to the prediction of international energy agency (IEA), the percentage of renewable energy will amount to more than 33% by 2030, wherein solar energy will account for a large portion thanks to its long-term and basically inexhaustible availability and zero-impact to local ecosystem.So far, the most appealing large scale and commercially profitable PV technology is based on bulk crystalline or polymorphous silicon (Si) technology, with a dominant market share> 85%. Among them, thin film hydrogenated amorphous Si (a-Si:H) can be deposited at a far low process temperature over large substrate, making them ideal candidate for low-cost, flexible and distributed PV applications. However, compared to the steady progress of bulk c-Si solar cells, a-Si:H thin film solar cell has been limited to a relatively low performance with a power conversion efficiency below<13% (or<10% in production lines). This has been largely limited by the higher defect density in the disorder a-Si:H material and the light-induced-degradation (LID) upon lasting exposure to sunlight. In order to counter these drawbacks, it is critical to explore a new architecture that will help to promote the performance of the a-Si:H thin film that is not limited by conventional parametric constraints in planar thin film solar cell structure.The fabrication of radial junction (RJ) solar cells upon well-defined Si nanowires (SiNWs) framework is emerging as a promising strategy to boost the PV performance of thin film solar cells. The SiNWs can be produced via a "top-down" lithography-etching procedure or a "bottom-up" self-assembly growth, while the latter boasts a really low-cost, scalable and nanoscale size manufacturing which are ideal to construct large area thin film solar cells. So far, the growth of SiNWs have been mostly achieved via a vapor-liquid-solid (VLS) mechanism, where molten gold (Au) catalyst droplets are adopted as catalyst to absorb gaseous precursor, like silane, and produce SiNWs with a diameter similar to the size of the droplet. However, to implement this technique for PV, a major concern arises from the incorporation of Au atoms into SiNWs which will introduce unfavorable recombination centers. Since 2008, we have proposed and worked in developing an alternative way to address this issue, by exploring a series of low-melting-point (LMP) metals of tin (Sn), indium (In) and bismuth (Bi) as catalysts for VLS growth in a conventional plasma enhanced chemical vapor deposition (PECVD) system.Despite of early prediction that these LMP catalysts have been deemed as unsuitable for VLS growth due to a low surface tension (only half of that of c-Si), we have demonstrated for the first time a stable and readily tunable VLS growth of c-SiNWs is indeed possible in a PECVD environment. Our results also discover the key role of a sidewall surface coating of the liquid droplet in stabilizing the VLS growth, and yet at a much lower growth temperature<300℃. This has also enabled us to establish a complete fabrication procedure of radial junction thin film solar cells, which are fully compatible to the mainstream Si thin film technology on top of low cost glass or metal foil substrates.In this work, we first concentrate on establishing a reliable fabrication of SiNW array with readily tunable density and morphology, and then investigate the strong light trapping effect among them and its unique benefits in boosting thin film light emission from a 3D nanostructured framework. We show that the light emission from SiN thin film can be largely enhanced compared to the planar references; In the next step, we construct radial PIN junction over doped SiNWs, where a strong light trapping effect allows us to decrease the absorber thickness from 300 nm to just 80 nm, while achieving still the same and even stronger light harvesting performance. Based on this radial junction configuration, we carry out also a systematic study of radial junction a-SiGe thin film solar cell, which help to extend the absorption spectrum to 700 nm in a-Si:H to above 900 run in a-SiGe:H. This result provides an important basis for future tandem radial junction thin film solar cells. On the other hand, we discover also a unique Sn-Bi alloy catalyst strategy to grow n-type doped SiNWs without the need of any dopant precursor gas, which helps us to improve largely the morphology and density of the VLS-grown SiNWs and achieve simultaneously a strong light trapping and good open circuit voltage.In summary, the major innovations in this thesis can be summarized as follows:1. Exploring a low temperature growth of Si nanowires, via a low-melting-point metal catalyzed VLS process, and demonstrate a significant PL emission enhancement of Si-based thin film materials deployed over such a 3D nanowire architecture;2. Fabricating high performance radial junction thin film solar cells, via a systematic parametric optimization and structural control, with a power conversion efficiency higher than 8.2%;3. Accomplishing the first radial junction a-SiGe thin film solar cells, with a largely extended absorption spectrum to 900 nm wavelength;4. Establishing a novel alloy-doping and catalyzing strategy to optimize Bi-Sn mediated VLS growth and simultaneously doping in the SiNW cores. |
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