Studies On The Surface And Interface Of Semiconductor Nanostructures And Related Modulation Of Optoelectronic Properties

Solar cells,a device of generating electrical power by converting solar radiation energy,have attracted much attention since it was created at Bell laboratory in 1954.In particular,the great development of the nanostructure in optoelectronic properties provides a real possibility for achieving the economy of the third generation solar cells in recent decade.Compared to the traditional photovoltaic devices,nanostructure solar cells can not only reduce costs and simplify the preparation process,but also enhance the internal/external quantum efficiency and optoelectronic properties effectively due to its fascinating physical and chemical properties emerging from the surface and interface effects.However,although the nanostructure has great potential to improve the optoelectronic properties,or even increase the conversion efficiency of solar cell beyond the Shockley-Queisser limit,the conversion efficiency of single junction solar cells is still lower than this limit due to the size have a significant impact on optoelectronic properties.In fact,when the size of the specimen down to nanometer,the bond of the edge atoms will become shorter and stronger spontaneously induced by the high surface-to-volume ratio and coordination defects of the surface/interface atoms.As a result,the nanostructure will be in self-equilibrium state,and change the related performances such as optoelectronic and transport properties.Moreover,as the size of nanostructure shrinks below the critical size,the surface barrier and band curvature decrease due to the reduction of depletion region,which can enhance the surface recombination rate.Therefore,how to modulate the optoelectronic and transport properties as well as suppress the recombination between electrons and hole in nanostructures have become one of the significant challenges in photovoltaics industry and optoelectronic device.In this paper,we develop an analytical method to explore the influence of surface and interface effect on the optoelectronic and transport properties of nanostructure solar cells based on atomic-bond-relaxation consideration.First of all,we study the size-and temperature-dependent conversion efficiency in nanostructure solar cells.Then,we take the core-shell nanostructure and nanocone as an example to explore the influence of the epitaxial layer and shape on suppressing the recombination rate and light reflection as well as enhancing the carriers transport.Moreover,we study the size-dependent multiple cxction generation and Auger recombination process in nanostructures based on the Fermi statistical theory and Fermi's golden rule.The achievements are shown as follows:?1?We address the influence of the size,shape and temperature on the nanostructure conversion efficiency from the perspective of atomistic origin based on atomic-bond-relaxation consideration.It is found that the physical origin of optoelectronic properties change can be attributed to the self-equilibrium state induced by the bond contract spontaneously in surface atom and thermal effect.We demonstrate that the conversion efficiency dependents on the shape and it decreases first-slowly with reducing size and then decreases rapidly as the size of nanowires beyond the critical dimension.?2?We investigate the recombination theory and light absorption in the nanostructure solar cells.We take the core-shell nanostructure and nanocone as an example to explore the influence of epitaxial layer thickness and shape on the surface recombination and light absorption.Our result show that the epitaxial layer can not only reduce the surface effect and improve the stability of the nanostructure,but also suppress the surface state and electron-hole pairs recombination,resulting in enhancement the optoelectronic properties.Moreover,nanocone is of great benefit to enhance light absorption and suppress the surface recombination due to the gradually change of the diameter from top to bottom,which can dramatically promote the conversion efficiency.In addition,we found the competition between light absorption and the recombination rate determines the superior photoelectric properties of nanostructures.?3?We study the influence of the size and shape on the multiple cxction generation and Auger recombination process in nanostructures based on the Fermi statistical theory and Fermi's golden rule.Our prediction shows that nanocone and thin size of the nanostructure are of great benefit to the utilization rate of high energy photons,which can transform the loss energy to excite a new exciton efficiently,resulting in enhance the MEG and suppress the threshold energy.Strikingly,nanowire has an obvious effects on enhancing the optoelectronic properties and suppressing the Auger process due to its low band gap and quantum confinement.Moreover,we also show that AR rate of the nanostructure is determined by the coupling effect between the band gap and diameter and it approximately has aE_g^7/2D⁷ dependence,resulting in the log-log dependence of the AR lifetime versus diameter is approximately linear withD^p and the p changes with the size of the nanostructures.?4?We explore the effect of epitaxial layer thickness and on the carrier mobility in semiconductor nanowires based on the atomic-bond-relaxation consideration.Our results show that modulation the size and epitaxial layer thickness of the nanowires is great benefit to reduce phonon scattering,surface roughness scattering as well as self-equilibrium strain,and thereby allowing for the enhancement of carrier mobility efficiently.Furthermore,we found that the competition among the phonon scattering,surface roughness scattering and subbands determines a maximum value for the hole mobility.