Preparation Of Ultra-thin Silicon Oxide Tunneling Layer Based On PECVD In-situ Oxidation Method And Its Application In High Efficiency TOPCon Solar Cells

Based on the world's energy shortage,we need to develop new energy sources.Solar energy has received wide attention as the most cost-effective renewable energy source.In 2017,it accounted for 66%of China's total renewable energy installed capacity.At present,solar cells are mainly based on conventional crystalline silicon cells,but crystalline silicon technology is highly competitive,it is necessary to develop more efficient solar cells.In high efficiency solar cells,Tunnel Oxide Passivated Contact solar cell?TOPCon?has attracted the attention of the international photovoltaic market due to its good surface passivation and compatibility with conventional solar cells production lines.The solar cell is made of high quality ultra-thin silicon oxide and doped polysilicon layer for efficient full-area passivation and carrier selective collection.The full-area passivation surface provides no silicon/metal contact interface,which is beneficial to increase the open circuit voltage(V_oc).Collecting carriers in a full area can reduce the lifetime sensitivity and improve the fill factor?FF?.In addition,the solar cell also has other advantages 1)no laser opening required;2)no photo-induced attenuation using N-type silicon;3)compatible with medium-high temperature sintering;4)strong technical scalability.In the conventional TOPCon solar cell,phosphorus-doped crystalline silicon?n-cSi?is used as substrates,and ultra-thin silicon oxide is prepared by thermal concentrated nitric acid oxidation,and the amorphous silicon is deposited by plasma ehanced chemical vapor deposition?PECVD?method,then subjected to high-temperature annealing for crystallization to achieve good passivation effect.The disadvantage of the traditional method is that the nitric acid is polluted to the environment,and the acid mist is too large during use,which may cause instability between solar cells and high liquid exchange cost,it is not conducive to industrial mass production.The paper is based on solving the above problems.The research contents and corresponding results are as follows:1.To replace the 1²nm high-quality ultra-thin silicon oxide layer grown by concentrated nitric acid,and simplify the process to achieve one-time PECVD deposition of the passivation layer and the back field layer,we use plasma-assisted laughing gas?nitrous oxide,N₂O?oxidation treatment to grow a high quality ultra-thin silicon oxide layer on the silicon wafer,we call it laughing gas silicon oxide?N₂O SiO_x?.Firstly,the effect of process parameters?deposition time,RF power,deposition temperature?of PECVD on the thickness of silicon oxide was studied systematically.The thickness of N₂O SiO_x is 2³nm,is thicker then the SiO_x grown by concentrated nitric acid,it can ensure that the thickness of the silicon oxide can satisfy the carrier transport conditions.On the basis of the thickness,the quality of N₂O SiO_x was investigated and compared with SiO_x grown with concentrated nitric acid.It can be seen from comparison that the content of tetravalent SiO_x in N₂O SiO_x is higher than that of HNO₃ SiO_x,and the quality is better than that of SiO_x grown by nitric acid.The thickness of SiO_x was measured by an ellipsometry spectrometer.The quality of SiO_x was obtained by X-ray photoelectron spectroscopy.2.Based on the thickness and quality of N₂O SiO_x to meet the tunneling conditions,the double-sided passivation structure poly-Si/SiO_x/n-cSi/SiO_x/poly-Si was prepared to replace SiO_x grown by hot concentrated nitric acid.Firstly,the effects of different annealing temperatures on the passivation structure were investigated.Then the passivation layer was characterized and analyzed by Raman spectroscopy and electrochemical capacitance-voltage profiler?ECV?to improve the passivation effect.As a result,the optimum annealing temperature of N₂O SiO_x passivation structure is 880°C,and the optimum annealing temperature of concentrated nitric acid SiO_x passivation structure is 820°C.From the ECV profile,the optimum annealing temperature of N₂O SiO_x is high because of the density of SiO_x grown by plasma is higher than that of wet growth of nitric acid,so a higher annealing temperature is required to ensure proper diffusion concentration of the doped layer.3.In order to meet the requirements of solar cell preparation,besides the good passivation,the contact resistance of poly-Si/SiO_x layer was also explored.The sample structure for measuring contact resistance was GaIn/n-cSi/N₂O SiO_x/n⁺-poly-Si:H/Al dot.The effects of different N₂O SiO_x preparation temperatures and thicknesses on the structure were investigated.In addition,the effects of different annealing temperatures on the contact resistance of poly-Si and SiO_x layers were investigated.As a result,the higher temperature,the thicker SiO_x layer,the larger the contact resistance,and the higher annealing temperature,the smaller the contact resistance.However,the contact resistance values are all less than 10 m?·cm²,and the contact resistance of the passivation layer for solar cell preparation is 4.9 m?·cm²,which satisfies the requirements for solar cell preparation.4.Boron is diffused on the substrate of n-type solar grade silicon,then the TOPCon solar cell having a back surface passivation structure of poly-Si/SiO_x and a front surface passivation structure of aluminum oxide and silicon nitride is prepared,and the electrode is prepared by photolithography.At present,the efficiency of solar cell with N₂O SiO_x is up to 19.38%,which is comparable to the results of the same batch of NAOS SiO_x.