Study On Optical And Electrical Dynamics Of Low-Dimensional Photoconversion Devices

In recent years,the low-dimensional photoconversion devices,such as nanowire solar cells and two-dimensional-materials-based devices,have attracted much attention due to their unique optical and electrical properties,and are expected to play an important role in the next generation of flexible,micro-nano and wearable photoelectric devices.However,there are still some problems,such as insufficient research on the intrinsic physical mechanism of low-dimensional photoconversion devices,relatively low photoconversion efficiency(PCE)of low-dimensional photovoltaic devices,and the physical mechanism behind the high response gain of two-dimensional-materials-based photodetectors(2DMPDs)cannot be quantitatively analyzed.For these problems,starting from the basic light-matter interaction and semiconductor theories,this dissertation has deeply explored the new mechanism of low-dimensional photoelectric devices,established accurately photoelectric coupled models,and designed optimally micro-nano photoconversion devices with high efficiency of light absorption and electrical response.The research achievements in this dissertation mainly include the following contents:(1)Quantitative analysis of high gain mechanism for 2DMPDs:2DMPDs have attracted a broad interest due to their many unique benefits(e.g.,the giant photoresponsivity).However,the device-level simulation,which takes account of the optical absorption,electrical transportation and semiconductor material properties,is still challenging.Moreover,the quantitative analysis of the physical mechanism behind high response is relatively rare.This work establishes a comprehensive multidimensional optoelectronic simulation for the 2DMPDs via coupling the electromagnetic response and the carrier transport in spatial and frequency/time domains.The operating mechanism of photodetectors,internal photoelectric theory,semiconductor trap effect and advanced simulation technology are deeply explored.The trap effect is quantified and introduced into the model to reproduce the response gain in the experimental observation.The simulation provides a convenient platform for exploring the underlying sciences and intrinsic operations of this new kind of device,and can guide effectively the development and fabrication.(2)Optimal design for high performance nanowire photovoltaic devices:In view of the relatively low PCE of single nanowire solar cells,two designs are proposed to enhance the PCE from both optical and electrical aspects.A design of hydrogenated amorphous silicon(α-Si:H)single nanowire solar cells(SNSCs)with asymmetric front-opening crescent cross-section morphology is proposed.The photoelectric performance of the crescent-shaped α-Si:H SNSCs system is evaluated by coupling electromagnetic response and carrier transport.Due to the improved impedance matching conditions as well as enhanced optical antenna effect,the proposed structure obtains broadband and enhanced optical absorption with well angle response.Electrical simulation predicts that the PCE can reach 15.70%,showing an enhancement ratio of 43.77%relative to the circular structure.Further,Considering that the PCE of existing SNSCs is still limited by low operating voltage,a design of radial tandem hydrogenated microcrystalline silicon(μc-Si:H)(core)/α-Si:H(shell)SNSCs is proposed.The absorption tunability of the core and shell cells is investigated to realize current matching of the tandem battery.The result shows that there is a good linear relationship between core and shell radius under current matching conditions,enabling a convenient design of tandem SNSCs with a maximized and matched photocurrent.Considering the carrier loss duo to the recombination,the photoelectric coupled simulation of the device is carried out.It is found that the PCE of the device is increased by 34%due to the increase of the open-circuit voltage(Voc)of the tandem battery.(3)Study on 2D-materials based van der Waals heterojunction photovoltaic devices:Atomically thin transition metal dichalcogenides(TMDs)have gained much attention due to their unique optoelectronic properties.However,besides the plenty of experimental attempts,the high performence device-level optoelectronic simulation has seldom been reported.A comprehensive and accurate photoelectric simulation for the 2D van der Waals heterojunction photovoltaic devices is established,including light capture behavior and detailed light conversion behavior in ultrathin pn junction.The unique carrier operating mechanism of 2D-materials based van der Waals heterojunction is researched deeply.The optoelectronic simulation provides a convenient way to study the multi-domain responses of the extremely thin optoelectronic devices.Based on this simulation technique,a metal micro-cavity structure consisting of the silver nanorod array on the upper surface and the silver reflecting layer on the back is designed to enhance the light trapping ability of the monolayer MoS2/WSe2 heterojunction.The results show that the short-circuit current density(Jsc)and PCE of the monolayer MoS2/WSe2 heterojunction photovoltaic devices with the proposed design is increased from 2.5 mA/cm2 and 1.13%to 5.21 mA/cm2 and 2.42%,respectively.Further,ultrathin film optical interference theory is applied in new high quality 2D materials to enhance the PCE of multilayer 2D materials heterojunctions.Firstly,the interference effects of ultrathin films system consisting of strong absorbing medium and metal back reflector are comprehensively studied from the underlying physics,photonic manipulation,optical performance optimization,and also the photoconversion application.Then an ultrathin MoS2/WSe2(total thickness less than 20 nm)heterojunction/silver substrate photovoltaic application system is constructed.Under AM1.5G solar illumination,the Jsc and PCE of the system reach 20 mA/cm2 and 9.5%,which are about 200%higher than the traditional SiO2 substrate system.In summary,this dissertation establishes device-level photoelectric simulation models for nanowire photovoltaics and 2D-materials based photoconversion devices,explore deeply the new intrinsic mechanism,and propose a variety of optimal design to enhance the optical absorption and electrical response of the devices.The device-level simulation can effectively guide the development and fabrication of the low-dimensional photoconversion devices,and promote their application in the fields of highly integrated micro-nanostructure photovoltaics and high-performance photodetections.