| Photoconversion devices play a crucial role in the fields of energy,photodetection,biosensing,and medicine.In recent years,with the progress of nanotechnology,photoconversion devices are developing in the direction of nanocrystallization.However,the miniaturization of the device leads to a weak light-matter interaction and a degradation of optoelectric performance.With the advanced principle of nanophotonic,the introduction of micro/nano structure to design and control the device working process can effectively improve our light management ability,increase light conversion efficiency,and reduce cost.In this dissertation,a series of work has been done on the micro/nano photovoltaics and hotelectron photoconversion,including the investigation of new mechanism and scientific principle of novel structures,the design of high-performance devices as well as the thermodynamic loss analysis and design strategy of hot-electron devices.The research achievements in this dissertation mainly include the following contents:(1)Broadband absorption enhancement in thin-film solar cells: It is proposed that nanopatterning the amorphous silicon layer into a two-dimensional photonic crystal in aSi:H/?c-Si:H tandem solar cells increases the optical resonance density and strength,contributing to an almost ideal absorption of incident light.The introduction of a suitable thickness of the intermediate layer solves the problem of photocurrent mismatch resulting from the uneven absorption enhancement between the top and bottom cells.The lightconversion efficiency of 12.67% is predicted with an enhancement ratio of 27.72% compared to the planar reference.We propose a dual-diameter nanohole arrays(NHs)to further improve the absorption efficiency of NHs photovoltaic system.The large NHs in the top layer induce a low effective refractive index and a better impedance match with air;while the small NHs in the bottom layer induce the strengthened resonant cavity modes;together contributing to an ultra-broadband absorption enhancement and 17.39% improved ultimate efficiency compared to that of the bset single NHs system.(2)Advanced micro/nano light trapping structures in single nanowire solar cells(SNSCs): we introduce the concept of asymmetric design for the core/shell silicon SNSCs,which enables a substantially enhanced optical absorption under an improved coupling between the photoactive material and the focus of the dielectric shell.By properly deviating the silicon core away from the dielectric shell center,the light-trapping capability of the device is significantly improved in almost the whole spectral band without degrading the carrier collection performance.The photocurrent density as well as the light-conversion efficiency is enhanced by ~ 40% compared to that of the system under the coaxial core/shell nanowire(NW)design.It is proposed that integrating a nanovoid resonator into the single NW can substantially enhance the absorption to sunlight by exciting highly strengthened optical resonances in the nanovoid-deformed NW cavity.Different from the conventional morphological engineering of semiconductor nanostructures,the asymmetric nanovoid design substantially increases the radiative losses of leaky modes without dramatically affecting the local density of optical states in the absorber.Without considering the factor of the reduced photoactive material,the photocurrent density of the single NW solar absorber under the asymmetrical nanovoid design can be improved by ~ 37.5% over that based on the conventional solid NW devices.(3)Hot-electron photodetection based on surface plasmons(SPs): an architecture of conformal TCO/semiconductor/metal(TCO: transparent conductive oxide)NW array is proposed for hot-electron photodetection with the near-perfect,polarizarion-insensitive,ultra-narrow-band optical absorption and a tunable optical response across the visible and near-infrared bands.The wavelength,strength and bandwidth of the plasmonic resonance are tailored by controlling the lattice periodicity and topology.By the excitation of localized SPs,a strong field concentrates at the top corner of NWs with a high hot-electron generation rate.The photoresponsivity of the device is more than five times larger than that of the flat reference.(4)Planar hot-electron photodetector: a planar microcavity-integrated hot-electron photodetector is proposed,in which the TCO/semiconductor/metal structure is sandwiched between two asymmetrically distributed Bragg reflectors(DBRs)and a lossless buffer layer.At the resonance,the absorption efficiency in the metal is 92%,21-fold enhanced compared to the reference without a microcavity,which leads to an order of magnitude improved unbiased responsivity.For the first time,the concept of Tamm plasmons(TPs)is introduced in hot-electron photoconversion and the detailed analysis is carried out from the transient dynamics after hot-electron excitation to the detailed hot-electron transport process.By the excitation of TPs in the planar multilayers of DBR/metal/semiconductor/metal,light incidence can be strongly confined in the localized region between the top metal and the adjacent dielectric layer,so that more than 87% of the light incidence can be absorbed by the top ultra-thin metal layer.This enables a strongly asymmetric absorption and a strong & unidirectional photocurrent in the system.Moreover,the two planar devices not only significantly simplify the conventional SP-based systems,but also have a photoresponsivity higher than that of the conventional metallic nanostructured system and the multiband photodetection in the infrared band.(5)Loss mechanisms and strategies for efficient hot-electron photoconversion: The device quantum yield is fundamentally low due to the existences of various hot-electron loss channels;moreover,the strongly nanostructured plasmonic/metamaterial are generally required,which bring challenges to the low-cost and large-scale fabrication.In this study,we systematically explore the fundamental thermodynamic losses in hot-electron devices,elucidate the inherent physical limitations leading to the low photoconversion efficiency,provide the corresponding solutions to highlight the opportunities for breaking these limitations as well as provide the potential solutions,and explore the potential strategies to realize the high-efficiency hot-electron photoconversion.It is shown that the low optical absorption,resistive dissipation,nearly uniform hot-electron energy distribution,rapid hotelectron thermalization,and momentum conservation in the interfacial electron transfer process are the key reasons leading to the extremely low hot-electron photoconversion efficiency observed previously.Conquering the corresponding performance-limiting factors,several realistic systems are proposed in this study to show the possibilities of improving the system performance.The system efficiency after multi-domain optimizations can in principle reach 60% in the near-infrared band. |
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