Silicon Radial Junction Thin Film Solar Cells And Optoelectronic Detection Applications

Developing renewable and clean energy has always been an important way for human society to address the current energy crisis and the accompanied environmental challenges.As an important source of renewable energy,solar energy is inexhaustible,safe,reliable and environmentally friendly,and thus has become a hotly competed research region among all the leading countries.Particularly,silicon-based solar cells dominate the photovoltaic market and occupy 90%of the market share.Silicon-based thin film solar cells have received widespread research attention because of its low cost,large-scale production and suitability for distributed applications.However,hydrogenated amorphous silicon(a-Si:H)thin film is limited by its disordered atomic arrangements,light-induced degradation and other shortcomings,resulting in a relatively low power conversion efficiency.In order to solve these problems,constructing three-dimensional(3D)radial junction structure upon silicon nano wires(SiNWs)is an innovative and promising direction.Based on the low-melting-point metal catalyzed SiNW growth technology established by our research group,we stage a systematic investigation,in this Thesis,of the structural design,optimization,fabrication and characterization of radial junction a-Si:H thin film solar cells built upon SiNWs grown via a plasma assisted vapor-liquid-solid(VLS)process.We demonstrate the advantages of the three-dimensional radial junction structure include a strong light-trapping effect,unique cavity-mode resonant coupling,largely reduced light induced degradation(LID)and et al.The major innovative contributions of this thesis work are as follows:1)We have studied and optimized the method of low-melting point metal(tin)catalyzed the VLS growth of SiNWs in PECVD system.The effects of the key control parameters,such as the hydrogen treatment process,initial metal catalyst thin film thickness,growth during,plasma condition and doping strategy were systematically studied.Our research group has been able to fabricate optimized nanowire arrays with length of～1 μm,mean diameter of 30～40 nn with a well controlled density of～3 × 108/cm.Based on the successful morphology control of SiNWs,we fabricated the radial junction thin film solar cells and optimize the doping conditions in SiNWs and the thickness variations in the window layers and the intrinsic absorber layer,as well as the annealing condition on the performance of the cells.These efforts lead to a high-performance radial single junction a-Si:H thin film solar cell with an open circuit voltage of 0.8 V,fill factor of 66%,short current density of 16 mA/cm2 and overall power conversion efficiency of 8%,which also laid a solid foundation for further optoelectronic device applications.2)We carried out systematic finite element method analysis and numerical simulation of the radial junction solar cells,and revealed the unique light incoupling,absorption and field distribution in single radial junction configuration.Especially for the structural characteristics of the random-oriented nanowire array,the effect of tilting angle of the radial junction cell arrays on the whole photoelectric conversion and absorption characteristics is studied in detail.We show that given a properly controlled nanowire density,the random orientation and self-shadowing of the radial junction structures with diameter being comparable to the incident wavelength is not a limiting issue.These simulation results provide profound understanding and theoretical guidance for experimental fabrication and optimization of the radial junction solar cells constructed over low cost randomly distributed SiNW arrays.3)A novel inorganic cesium-based perovskite quantum dot(IPQDs)and SiNW radial pin hybrid solar blind photo-detector is proposed,where the IPQDs have excellent UV photoelectric absorption and high quantum efficiency to UV photons.By uniformly arranging the IPQDs on the surface of the silicon radial junction structure,the solar-blind signal can be directly converted to the optimized wavelength(-500 nm)for a-Si:H pin units,where an efficient and ultra-fast photoresponse is guaranteed,thanks to the optimized very thin intrinsic layer thickness of only 80 nm(compared to 300 nm in planar junction).It is shown that the IPQDs radial junction hybrid detector is capable of detecting ultraviolet light in solar blind spectrum(<280 nm)with a response speed of 0.48/1.03 ms and a peak responsivity of 54 mA/W @ 200 nm.These results pave the way toward large area manufacturing of high performance Si-based perovskite UV detectors in a scalable and low cost procedure.