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• W37057070 abstract "Long term concerns about climate change and fossil fuel depletion will require a transition towards energy systems powered by solar radiation or other renewable sources. Novel concepts based on silicon materials and devices are investigated for applications in the next generation photovoltaic (PV) devices and photoelectrochemical (PEC) water splitting for solar energy conversion and storage. Expanding thermal plasma chemical vapor deposition (ETP-CVD), a remote plasma synthesis method, is verified as an efficient process for the fabrication of free-standing silicon nanocrystals (Si NCs) on an industrial scale. The unique physical, mechanical and electrical properties of Si NCs might open routes to new PV concepts to breach the so-called Shockley-Queisser limit using mechanisms like multiple exciton generation and up- or down-conversion of the incident spectrum. Under intensive laser illumination conditions, the thermal heating effects of Si NCs become the dominant mechanism for the transverse optical (TO) mode red-shifts of the first order Si-Si peak in reference to the bulk c-Si in the Raman spectrum. The free-standing Si NCs can be heated to their melting points by a well-focused laser, and the temperature can be determined by the measured ratio of Anti-Stokes-to-Stokes TO mode intensities. In contrast, Si NCs in various matrices can hardly be heated using the same amount of laser power due to good thermal conductivity. If the free-standing Si NCs are further heated, the intensity of the blackbody radiation in Raman spectrum starts to compete with that of the TO mode. Various PEC/PV configurations for solar water splitting structures are discussed in this thesis to directly store the solar energy in the form of hydrogen fuels. In Chapter 4, the a-Si:H/a-Si:H double-junction solar cell is demonstrated as the simplest and easiest option to meet the requirements for the integration with gradient-doped W:BiVO4 photoanode, by considering the stability in aqueous solutions, simple fabrication process, matching spectral response, voltage and current density. The optimization steps of the a-Si:H/a-Si:H solar cells are carried out in both experiments and simulations, by varying the top i-layer thickness in reference to the AM 1.5 spectrum and the spectrum transmitted through the BiVO4 photoanode respectively. The stability of the a-Si:H/a-Si:H solar cell shows less sensitive light-induced degradation kinetics under the spectrum transmitted through the BiVO4 photoanode from that under the standard AM 1.5 spectrum. In Chapter 5, the performance of the front BiVO4 photoanode is further improved, comparing with the studies in Chapter 4, by improving photon absorption and carrier collection. Photon absorption is enhanced by the application of light trapping techniques on the BiVO4 photoanode using textured TCO glass substrates. The carrier collection is optimized based on our new findings on diffusion length of the photogenerated charge carriers in an undoped BiVO4. By ingenious design of the gradient W-dopant profile, the thickness of the film is extended without deteriorating the carrier separation efficiency. The catalytic limitation is overcome by electrodepositing a thin film of cobalt phosphate as water oxidation catalysts on the surface of BiVO4. The optimized front photoanode is combined with three types of solar cells to form a hybrid PEC/PV solar water-splitting. The collaboration of a BiVO4/a-Si:H/nc-Si:H photoanode demonstrates the best performance concerning the better solar spectrum utilization of nc-Si:H up to 1100 nm near-infra-red region. A 5.2% solar-to-hydrogen conversion efficiency, which is the highest ratio of metal-oxide based photoanodes ever reported, has been achieved by this PEC/PV configuration. Besides the photoanode device for oxygen evolution reaction, a photocathode based on thin-film silicon technology is designed and optimized as well, to form an unbiased photoanode /photocathode aiming to revolutionize solar water splitting. Photon absorption is enhanced by state-of-the-art implementation of light trapping techniques on the a-SiC:H photoanode using a glass substrate with integrated micro-textured photonic structures. The light traveling length is prolonged in the high quality grown nc-Si:H benefiting from the scattering morphology on the glass substrate. The carrier collection is boosted by our unprecedented design of the gradient boron dopant profile from the a-SiC:H p-layer to the i-layer. Novel spectral utilization techniques are applied in the device for the integrated PV junctions, supported by a theoretical optical model. A benchmark photocurrent density of -5.1 mA cm-2 at 0 V vs. RHE is achieved in the a-SiC:H/a-Si:H/nc-Si:H configuration. It is note-worthy to address that this photocathode does not contain a passivation layer nor any catalyst. The efficient operation of a photocathode also requires metal catalysts to facilitate charge-transfer reactions at the interface between the semiconducting light absorbers and the electrolyte. Atomic layer deposition (ALD) is employed to fabricate the Pt nanoparticles and thin films as the hydrogen-evolution catalysts under varied conditions. Using MeCpPtMe3 and ozone as the precursors and substrate temperatures as low as 200 °C, a growth rate as fast as 1.1 A/cycle is achieved. The electro-catalytic activity of ALD-grown Pt thin films on glassy carbon electrodes shows comparable performance for the hydrogen-evolution reaction as that of Pt films deposited using electron-beam evaporation." @default.
• W37057070 created "2016-06-24" @default.
• W37057070 creator A5009533156 @default.
• W37057070 date "2015-01-15" @default.
• W37057070 modified "2023-09-23" @default.
• W37057070 title "Novel Concepts for Silicon Based Photovoltaics and Photoelectrochemistry" @default.
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• W37057070 doi "https://doi.org/10.4233/uuid:1f3a0f5f-4036-44bf-97d3-5f7d483209aa" @default.
• W37057070 hasPublicationYear "2015" @default.
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