Study Of The Theoretical Simulation And Mechanism Of High-Efficiency Crystalline Silicon Solar Cell

The current research hotspots for high-efficiency crystalline silicon cells at home and abroad are HIT cells and TOPCon cells.They use intrinsic hydrogenated amorphous silicon/doped amorphous silicon and silicon oxide/doped polycrystalline silicon,respectively Comprehensive passivation and selective carrier transport,this kind of selective full passivation contact plays an important role in increasing the open circuit voltage(VOC)and fill factor(FF)of the cell.Although the efficiency of HIT cells has reached 26.7%,people have limited understanding of their physical mechanism.Many parameters exist in the manufacturing process of HIT,such as thickness,doping concentration,minority lifetime,defect recombination,resistivity,etc.,playing an important role in cell performance.In order to take advantage of HIT in surface passivation and overcome the problem of incompatibility with the traditional crystalline silicon cell manufacturing process.In 2013,some people developed a new structured cell-TOPCon cell.Although the efficiency of TOPCon cells has been rapidly improved and has now reached 25.8%,some basic physical mechanisms of the devices have not been fully understood.By using AFORS-HET software for theoretical simulations of HIT and TOPCon solar cells,this paper provides reference for the design and optimization of high-efficiency crystalline silicon solar cellsThe simulation optimization structure is the optimal value of the thickness,doping concentration and bandgap width of each layer of HIT cell of TCO/n-a-si H/i-a-si:H/p-c-si/i-a-si:H/p+-a-si:H/TCO.The results show as follows.(i)the optimal thickness of the emission layer n-a-si:H is 5nm,the doping concentration is 1.0×1020cm-3,and the bandgap width is 1.8eV.With the increase of emission layer thickness,the absorption of light will be improved and the absorption ratio of substrate will be reduced.The increase of doping concentration will increase the ionized electron concentration and increase the electric field intensity in the junction area,which is beneficial to the separation of photogenic carriers.The larger width of band gap is beneficial to reduce the absorption of long-wave photons and improve the absorption of low-energy photons in the base region.(ii)The optimal thickness of the intrinsic buffer layer i-a-si:H is 5nm,and the bandgap width is 1.6ev,so the cell efficiency can reach the best performance,because the parasitic absorption and parasitic resistance of the intrinsic buffer layer thickness are the lowest.(iii)The optimal doping concentration of substrate p+-c-si is of the order of 1017cm-3.If the doping is too large,a large number of B-O bonds will be generated and the lifetime of fewer particles will be reduced.So,the substrate must be low doping.(iv)The optimal thickness of the back-field p-c-Si:H is 5nm,the optimal doping concentration is 1.0×1020cm-3,and the bandgap width is 1.8eV.This is similar to the case of the emission layer,because the addition of back-field increases the field strength,which is conducive to the separation of electron hole pairs and the current densityThe simulation analyzes the effects and difference of TCO work function on n-type and p-type HIT cells.The results show that the size of WTCO can affect the degree of band bending,and the optimal range of WTCO contact before p-type HIT is 3.9-4.4eV.When the WTCO is larger,the n-a-si:H band will bend upward,and the interface potential barrier will be increased,which will hinder the accumulated electrons and have a negative effect on Jsc,while the selection of back-contact WTCO greater than 5.1eV has better matching.Similarly,the optimal range of WTCO before and after n-type HIT is 5.4-6.3eV and 3.6-4.0eV,respectively.By comparison,it is found that p-type pre-hit contact WTCO is less than n-type pre-contact WTCO,but back-contact WTCO is larger than n-type back-contact WTCOThe interface state and hanging bond density are the key factors affecting the HIT cell.It is found that the Ditl and Dit2 states can cause a large number of defects to compound and seriously deteriorate the performance output,which must be passivated lower than 1012cm-2 The larger the Ntr,the larger the space charge zone will be transferred from the crystalline silicon c-Si surface to amorphous silicon a-Si:H,resulting in the widening of the depletion layer of amorphous silicon,which will lead to the accumulation of holes at the interface,significantly enhancing composite effect and reducing cell efficiencyThe two-sided TOPCon cell generates more power than the single-sided TOPCon cell,because the two-sided TOPCon cell increases the utilization rate of photons and the separation of photo-generated carriers.The addition of silicon oxide and the perfect structure formedwith the heavily doped polysilicon layer have superior passivation capacity and carrier selectivity,which makes multi-particle tunneling easier,and the few particles will be blocked,reducing the recombination probability.The optional thickness of SiO2 is 1.2nm,the density of interface states being significantly reduced at this point and the distance between quasi-fermi energy levels of electrons enlarged with the increasing thickness,which is conducive to the increase of open circuit voltage and current density.However,the efficiency will be significantly reduced when it is larger than 1.2nm,which indicates that the multi-particle tunneling is closely related to the thickness of SiO2 and must be a high-quality thin layer.The best back-field polysilicon doping is of the order of 1020cm-3,which is clearly heavy doping.The thin layer of SiO2 and the heavily doped polysilicon structure can reduce the recombination of front and rear surface carriers,among which the recombination of backside carriers is stronger than that of front surface.This is the passivation mechanism of the field effect.The strong passivation effect of SiO2 thin layer can be attributed to the presence of mesoporous structure of SiO2,which provides favorable conditions for passivation and more possibilities for multi-particle tunnelingThe simulation analyzes the effects and differences of temperature and light intensity on HIT and TOPCon cells.Studies have shown that Voc decreases and Jsc increases with increasing temperature,and the degree of influence of Voc is significantly greater than that of Jsc,because increasing temperature reduces the size of the band gap.The same change of temperature is more stable for HIT cell Jsc than TOPCon cell Jsc,because HIT cell has lower temperature coefficient than TOPCon cell.With the increase of light intensity,both Jsc and Voc are increased,but the influence on Jsc is significantly greater than that on Voc.At this time,FF changes do not have a simple functional relationship,mainly because Jsc is directly proportional to the light intensity.With the same variation of light intensity,Voc of TOPCon cell is more stable than Voc of HIT cell,the reason lies in the fact that TOPCon cell has lower light intensity coefficient than Voc of HIT cell.