Passivation And Gettering Effect Of Tunnel Oxide Passivated Contact(TOPCon)

The tunnel oxide passivation contact?TOPCon?is a passivation structure for crystalline silicon solar cells,which consists of an ultra-thin silicon oxide film and a highly doped polycrystalline silicon layer.It is mainly used for the passivation of the rear surfaces of solar cells.TOPCon structure features excellent surface passivation effect and selective collection of carriers.The conversion efficiency of crystalline silicon cells with TOPCon has exceeded 25.7%.But compared with the theoretical limit efficiency,there is still much space for improvement.Impurities and defects inside crystalline silicon,as well as metal-semiconductor contact recombination losses on the front surface of solar cells are the main factors that restrict the improvement of efficiency.Due to its unique structure,TOPCon has the possibility of gettering impurities,and changing its optical properties by doping some chemical elements in the polysilicon layer.So it can be used in the front surface of solar cells,which has become the research focus recently.In view of these aspects,experiments on the gettering effect and the passivation and electrical properties after carbon doping of TOPCon structure were carried out in this paper.In terms of the gettering effect of TOPCon,firstly,Fe impurities were introduced into the silicon wafer by the solution immersion method.Then the ultrathin silicon oxide was prepared by the nitric acid oxidation and the doped layer was deposited on top of the silicon oxide layer by plasma-enhanced chemical vapor deposition?PECVD?.A series of parameters including the annealing temperature,the holding time,the thickness of silicon oxide,the thickness of polysilicon,the doping concentration of polysilicon and initial Fe concentration of silicon wafer were set in the preparation process of TOPCon.The Fe concentration was measured and calculated using the minority carrier lifetime device to characterize the effect of each parameter on the gettering.In terms of passivation performance of TOPCon,the different concentrations of C were doped into the polysilicon layer,and the crystallization annealing temperature and ways of heating up were studied.Its passivation performance was tested by minority carrier lifetime device.The square resistance of the passivation layer was analyzed by four-probe square resistance tester.Crystallization rate was characterized by Raman spectrometer.The activation and diffusion distribution of P were measured by the electrochemical capacitance-voltage profiler?ECV?.The contact resistivity after C doping was tested by semiconductor parametric instrument.The effect of carbon doping on the passivation performance and electrical properties of the passivation structure were discussed and analyzed in detail.The main conclusions are as the following:?1?The gettering effect of TOPCon structure is promoted by both annealing temperature and holding time.The residual Fe concentration decreases from 2.2×10¹¹cm^-33 to 8.4×10⁹ cm^-3 as the annealing temperature raising from 720?to 920?.Moreover,the residual Fe concentration decreases from 1×10¹¹ cm^-3 to 1.5×10⁹ cm^-3as the holding time increasing from 10 min to 180 min.The effect of the silicon oxide layer and the polysilicon layer is opposite.The former inhibits the gettering process while the latter is the main gettering area and the gettering effect is enhanced when its thickness is increased.The increase of the P concentration of the polysilicon layer enhances the gettering effect,too.The main reasons are that the increase of the P concentration enhances the ability of the polysilicon layer to contain more Fe atoms and the internal diffusion of P causes a certain degree of damage to the silicon oxide.?2?The difference in the initial Fe concentration of the silicon wafer makes differences in gettering effect of TOPCon.When the initial Fe concentration is lower,the passivation performance of the silicon wafer after gettering will be better and the minority carrier lifetime and iV_oc will also be greatly improved.The hydrogenation treatment of Al₂O₃ can promote the gettering effect and passivation performance.Results show that TOPCon can significantly increase the bulk lifetime of silicon wafers and achieve excellent surface passivation.In practical industrial production,the impurity concentration of silicon wafers is much lower than 1×10¹¹ cm^-3.Therefore,the low-quality silicon wafers have the chance to be used for solar cell fabrication with notably improved performance by using the excellent gettering and surface passivation of the TOPCon.This is very meaningful for the solar cells industry.?3?The TOPCon structure can withstand higher crystallization annealing temperature after C doping to the passivation layer.In the temperature ranging from 880? to 940?,the iV_oc of C-doped samples can be stabilized at about 735 mV.When CH₄/SiH₄ is more than 1.5,the passivation of the samples heating from 600?is consistent with that of the samples heating from room temperature.The iV_oc exceeds 735 mV in both cases.Moreover,the diffusion distribution of P at this situation is not affected by the heating method.Both the doping of C and the heating method have no effect on the crystallization rate of the passivation layer.Whether or not C is doped,the crystallization rate of the polysilicon layer reaches about 70%.?4?The square resistance of the passivation layer is increased by the doping of C,which is not good for carrier transport.More P atoms can be activated and the square resistance of the passivation layer can be reduced by increasing the annealing temperature.The contact resistivity of the passivation layer can be proportionally improved by the doping of C.