Research On The Loss Mechanism Of Energy Conversion Process And Hot-spot Phenomena In Solar Cells

As one of the main technology to utilize the solar energy,the solar photovoltaic(PV)technology has achieved rapid development in the last decades and researchers all over the world have made large efforts to increase the device efficiency and reduce the cost of the PV systems.Solar cells,the most important devices to utilize the solar energy via solar PV technology,only convert a small part of incident solar energy into electricity that can be utilized directly and a big part of the energy loss contributes to the self-heating phenomenon of the cells,which further exacerbates the energy loss processes.Meanwhile,due to the nonuniform illumination and defects in the cells,uneven heat source may exist inside the cell and bring more risks to the operation of PV systems.In conclusion,the problems of energy loss and temperature rise in solar cells not only affect the electrical performance of the cells but also do harm to the stabilization of the PV modules.And thus,it is very important to conduct the study of energy loss processes through the processes of energy conversion in solar cells.Based on the mechanisms of energy conversion and loss,this study conducts the coupling analysis of light absorption,energy conversion and heat generation in solar cells from the perspectives of generation,recombination and transport of carriers.The effects of heat generation processes on solar cells are revealed.The specific research contents mainly include four parts:1.Study of energy loss mechanisms in single-junction solar cellsBased on the improved model of energy conversion and heat generation in single-junction solar cells,the full loss processes,including below E_g loss,thermalization loss,angle mismatch loss,non-radiative recombination loss,etc.,are studied quantitatively and the effects of these loss processes on cell performance and self-heating phenomenon are analyzed systematically and respectively.The key loss processes that restrict the cell from higher efficiency are revealed.According to the coupling analysis of interactions between loss processes and electrical characteristics,effects of self-heating phenomenon on energy loss processes and output performance of solar cells are discussed,and it is proved that the self-heating phenomenon in solar cells leads to the serious efficiency drop.Depending on the mechanisms of different loss processes,the control methods of different energy loss processes in single-junction solar cells are summarized,which may help improve the power conversion efficiency of single-junction solar cells.2.Analysis of energy loss processes in multijunction solar cellsBased on the analysis of energy loss processes in single-junction solar cells,the model of energy conversion and heat generation in multijunction solar cells is established and the dominant loss processes are effectively suppressed.The below E_g loss and thermalization loss,which are difficult to deal with in single-junction solar cells,are well controlled in double-junction and treble-junction solar cells,remarkably enhancing the final power conversion efficiency.And the temperature-coupled analysis result between single-junction and multijunction solar cells also demonstrates the practical advantage of multijunction solar cells.3.Multiscale prediction of localized hot-spot phenomena in solar cellsBy establishing the opto-electro-thermal coupled solar cell model,the energy loss processes in a microstructure surface solar cell are studied quantitatively and the nonuniform volume heat source distribution of the cell is obtained.Based on the volume heat source distribution,the transient temperature-rise process of the microstructure surface solar cell is simulated and the weak localized hot-spot phenomena at microscale are revealed theoretically.Proceeding from the nonuniform volume heat source distribution at microscale,the model of a unit cell in a photovoltaic module at macroscale is established and the significant localized hot-spot phenomena are successfully found in concentrator PV solar cells.Effects of environmental conditions on localized hot spots are analyzed and the probable hazard hot spots may bring about is predicted theoretically.4.Close examination of localized hot spots within solar cellsBased on the theoretical work,we design and construct the experimental system to detect and analyze the localized hot-spot phenomena within solar cells.A xenon lamp is applied to simulate the sunlight and a Fresnel lens is installed to focus the light onto the cell surface.Some environmental conditions such as bulk obstructions and dust are also considered to simulate the unexpected effects the cells may suffer outdoors.In the tests,the localized hot-spot phenomena are successfully detected,which verifies our theoretical prediction.The transient forming and variation processes of hot spots resulted from different mechanisms are explored with an infrared thermal camera.Meanwhile,effects of hot-spot phenomena on the electrical properties of a cell under different irradiation conditions are analyzed with a DC electronic load.This work reveals the significant performance degradation in solar cells with hot spots.Depending on different forming mechanisms,the prevention and elimination methods of localized hot spots are summarized.The experimental result indicates that the localized hot spots must be dealt with timely before they grow into the large-scale hot spots and expand to the surrounding cells.