| With the emergence of new materials,new structures,and new mechanisms,the performance of new generation of micro/nano photovoltaic devices has been greatly improved,and it is expected to play an important role in many fields such as healthcare,communications,and optoelectronic integration.However,the performances of photovoltaic devices are still limited by many factors such as optics,electricity,and thermal.Moreover,the physical mechanisms of carrier transportation and energy losses are not thoroughly studied,and the coupled multi-physical mechanisms within photovoltaics are difficult to be quantitatively analyzed.Aiming these scientific issues,from the perspective of optics,carrier transportation and carrier thermodynamic behaviors,this thesis combines high-precision opto-electro-thermal(OET)theoretical models with the experimental preparation to quantitatively analyze the multi-physical loss mechanisms of micro/nano photovoltaic devices,which provides a comprehensive guidance scheme for the optimization design of high-performance photovoltaic devices.The main research results of this thesis are as follows:(1)Regulating and optimizing the optoelectronic performance of perovskite solar cells:Perovskite solar cells(PSCs)have achieved a power conversion efficiency of more than 25%,but the device performances are still limited by many physical factors,as well,there is still much room for improvement from the efficiency limit.In order to explore the physical reasons behind the limitations of the perovskite photovoltaic performance[including complete,free-electron transportation layer(free-ETL)and free-hole transportation layer(free-HTL)configurations],this thesis couples electromagnetic and carrier transportation to construct an optoelectronic(OE)model of PSC.The influences of electrical parameters such as the doping type,doping concentration and thickness of the perovskite layer,and doping concentration of the ETL(HTL)on the optoelectronic response of PSCs are discussed.Using the energy band diagram to analyze the energy band bending,the built-in electric field to quantify the carrier separation and transportation abilities,and relying on the body/surface component to characterize the current loss of the device,the study found that the self-doping effect of perovskite has a significant effect on regulating the performance of the device,for example,for PSCs with the nip configuration,the p-type doping perovskite layer shows a stronger separation ability for photogenerated carriers,so as to obtain higher conversion efficiency.On the contrary,n-type doping can make pin-type PSCs obtain higher performance.By optimizing the carrier dynamics characteristics,the efficiencies of PSCs under complete,free-HTL,and free-ETL configurations were theoretically boosted from 16.6%,10.7%,and 13.3%to 19.0%,15.9%,and 18.5%,respectively.(2)Opto-electro-thermal characteristics of radiative cooling homojunction photovoltaic devices:Passive radiative cooling technology can cool the photovoltaics without the any-energy input,which has attracted great attention.However,the high-performance radiative cooling of photovoltaic systems need to accurately consider the multi-band spectral filtering characteristics,which depends on the comprehensive consideration for the fundamental optical,electrical and thermal characteristics.Based on the photonic crystal principle,the designed radiative cooler for the silicon solar cell(SC)has obtained the near-perfect spectral selection characteristics from the visible light to the infrared band(0.3-25 μm)in this study,which features a large angle-and polarization-insensitive characteristics.In order to obtian the temperature effects on the performance of photovoltaic devices,this thesis constructs a homojunction OET model based on the radiative cooling of photovoltaic systems.We quantify the OET physical mechanisms of raditative cooling SC system from the macroscopic(including the absorption of sunlight,the radiative power of the cooler/atmosphere,the non-radiative heat exchange process,and the electrical output)and the microscopic(carrier generation/recombination/transportation/collection and thermalization/Joule/recombination and Peltier heats)perspctives.Besides,by investigating the temperature effect on the performance of photovoltaic devices,it is found that the working temperature of SCs can be reduced by 10.16℃ and the absolute efficiency can be increased by 0.45%with integrating the designed radiative cooler.Based on the new mechanism of radiative cooling,this study establishes a basic theory and model for analyzing photovoltaic devices from the macroscopic and carrier thermodynamics perspectives,and provides theoretical guidance for analyzing the OET characteristics of the micro/nano photovoltaics and related devices.(3)Experimental preparation and opto-electro-thermal theory construction of heterojunction photovoltaic devices:Advanced heterojunction is the core of photovoltaic devices to obtain high efficiency.Therefore,how to construct high-performance heterostructures has become the research focus in the photovoltaic fields.Different from the theory of coupled homojunction OET,the heterojunction photovoltaic devices contain more abundant light absorption,electrical transportation and thermal conversion mechanisms,but the theory of coupled OET within heterojunction has not been broken through,which is highly challenging.This dissertation first experimentally prepared a perovskite solar cell with an efficiency of more than 21%.In order to further optimize the performance of heterojunction photovoltaic devices,we constructed a coupled OET model of heterojunction photovoltaics.It is found that there are six microscopic energy loss processes within the heterojunction device,among which the sum of Joule and Peltier heats is a constant and can be regarded as the intrinsic losses within the heterojunction photovoltaic device.It is the first time to achieve a breakthrough for the heterojunction OET theory and simulation.By analyzing the physical mechanism of energy losses,we effectively regulate the energy conversion process within photovoltaics,such as the regulations of doping concentration and energy level alignment.Based on the OET theory,multi-optimization strategies are provided for guiding the design of high-efficiency heterojunction devices,thus,the efficiency of PSCs is theoretically boosted to 23.84%from 21.37%.(4)Analysis of opto-electro-thermal energy losses and optimization design of photovoltaic devices:The research of coupled OE/OET theory provides a guidance for quantifying OET losses of homojunction and heterojunction devices.However,the physical origin of energy losses and how to further approach the efficiency limit of photovoltaic device has not been resolved.This thesis accurately analyzes the energy and voltage losses pathways and related physical mechanisms for the pn homojunction and pn/pin heterojunction photovoltaic devices from the carrier thermodynamic perspective.It is found that there are six intrinsic energy losses and three electrical potential losses channels within SCs.In addition,the energy tracking method is used to quantify the physical reason that limits the efficiency of tandem Si/perovskite and perovskite/perovskite SCs,which is the first realization of the analysis of the carrier thermodynamic characteristics for tandem SCs.By optimizing the double junction Si/perovskite SCs,it is predicted that the efficiency can be reached to 37.5%.Based on the detailed balance theory,the efficiency limit model of photovoltaic devices coupled with temperature effect is constructed.It is found that the single junction photovoltaic efficiency limit is 32.2%,and the optimal band gap is 1.38 eV,which is 1.5%lower than the ideal case.This research based on energy analysis can not only promote the understanding of the fundamental science of OET,but also provide the guide for the design of high-performance optoelectronic devices. |
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