Application Of Novel Silicon-dielectric Thin Films And Rapid-thermal Anneal (RTA) For High-efficiency Passivated-contact Si Solar Cells

With the increasing depletion of traditional energy sources such as coal and petroleum,mankind is facing the dual problems of energy crisis and energy pollution.Therefore,it is extremely urgent to develop the renewable and environmentally friendly energy sources.Solar energy,as a renewable and clean energy,is of great significance to human sustainable development,and the core problem of the research is to improve the conversion efficiency of solar cells.This article contains three studies on tunnel oxide passivated contact(TOPCon)solar cells.The first study is on the preparation of phosphorus-doped polysilicon with rapid thermal anneal(RTA).The RTA is used to crystallize the plasma-enhanced chemical vapor deposition(PECVD)deposited hydrogenated amorphous silicon(a-Si:H)thin film to form the polysilicon passivated contact in TOPCon solar cells.The effects of annealing temperature,annealing time,cooling time,and the polysilicon thickness on the surface passivation are investigated.The primary advantage of the RTA is reducing the whole crystallization period to?15min,shorter than the conventional tube-furnace annealing period of>60 min.The optimized RTA process leads to an implied open-circuit voltage(i V_oc)of 712 m V and a single-sided dark saturation current density(J_0,s)of 12.5 f A/cm² in the as-annealed state,which is inferior to the surface passivation of the controlled one prepared by a tube furnace annealing.Fortunately,a subsequent Al O_x:H capping hydrogenation improves the i V_oc and J_0,s to 727 m V and 4.7 f A/cm²,respectively.The best surface passivation is obtained at 820?with 5?10 min annealing.The cooling time also has an effect on i V_oc,but the reduction in the passivation quality by fast cooling can be compensated by the hydrogenation.We find that the RTA is a robust method to prepare high-quality polysilicon passivated contact without introducing blistering when the thickness of the a-Si:H is controlled within 40 nm,with which the open circuit voltage(V_oc)increased from 590 m V to 660 m V and the efficiency increased from 17.0%to 21.1%as the polysilicon film thickness decreased from 70 nm to 40 nm.The champion conversion efficiency of 23.04%(V_oc=679.0 m V,J_sc=41.97 m A/cm² and FF=80.86%)is achieved,which demonstrates the effectiveness of RTA for preparing a high-efficiency polysilicon passivated-contact solar cell.The second study is on the preparation of phosphorus-doped polysilicon nitride(poly-Si N_x)passivated contact structure.The TOPCon structure is generally consist of an ultrathin oxide layer(SiO_x)and a highly doped polycrystalline silicon(poly-Si),or polysilicon carbide(poly-Si C_x)and polysilicon oxide(poly-SiO_x)since the addition of C,O can effectively reduce the possibility of blistering during annealing and the parasitic absorption losses of the poly-Si on the front side,which helps to improve the short circuit current of dual-sided TOPCon solar cells.In this work,the phosphorous-doped poly-Si N_x is prepared to replace the poly-Si and form a new passivated contact with an ultrathin SiO_x layer,achieving an excellent surface passivation with proper electrical contact and conductive characteristics.The nitrogen(N)content,crystallinity,passivation quality and photoelectrical properties of phosphorous-doped poly-Si N_x are investigated as a function of[NH₃]/([Si H₄]+[NH₃]+[H₂])gas flow ratio(R).From the experimental results,five conclusions are drawn as followed:1)The N content of poly-Si N_x is proportional to the R value;2)The N addition has a retarding effect on the crystallization of a-Si N_x:H,increasing the phase transition temperature from 820?to940?and improving the thermal stability of SiO_x;3)An Al O_x:H capping hydrogenation raises the i V_oc to a stable level of around 740 m V with the annealing temperature not exceed 900?,once the temperature raises to more than 900?,the i V_ocquickly decreases as both the temperature and R increase;4)The contact resistivity and sheet resistance of poly-Si N_x are slightly higher than that of poly-Si,but still meet the requirements of TOPCon solar cells fabrication.The third study is on the numerical simulation of the application of the p-type and n-type polysilicon passivated contact on the industrial-level p-type silicon wafer.The effects of(i)the front-structure design,(ii)the bulk lifetime and resistivity of the p-type wafer,and(iii)the carrier selectivity of polysilicon passivated contact on cell performances are investigated.Furthermore,the corresponding energy-loss pathways are classified by using free energy loss analysis(FELA).In essence,the rear-junction solar cell with the n-type polysilicon passivated-contact generates more internal power because of the better surface passivation and less front metallization shading,but the efficiency potential is limited by the low lifetime of the state-of-the-art p-type Czochralski(Cz)wafer.Thus,the p-type polysilicon passivated-contact serving as the back-surface field would be more favorable if the lifetime of the p-type Cz silicon were less than 350?s.Over the long term,the lifetime of the p-type wafer possibly becomes the bottleneck of the high-efficiency polysilicon passivated-contact solar cells.Finally,we present the roadmap toward the 23%industrial p-type silicon solar cell with the p-type or n-type polysilicon passivated contact.In summary,the preparation of phosphorous-doped poly-Si with RTA and phosphorous-doped poly-Si N_x,the numerical exploration for polysiliocn passivated-contact p-type silicon solar cell are investigated systematically.We believe the research could provide some new approaches to the photovoltaic industry for the cost reduction of the high efficiency TOPCon solar cell manufacturing.The RTA process is an efficient and energy-saving annealing method with the thickness modulation to avoid blistering;the N addition to the poly-Si enhances the transmittance of contact layer to reduce the parasitic absorption losses and improves the adhesion of the ploy-Si contact layer;the front-structure design and the bulk lifetime is the key to obtain high-efficiency polysilicon passivated-contact p-type silicon solar cell.