Carrier Selective Materials In Crystalline Silicon Solar Cells

Crystalline silicon(c-Si) solar cells have occupied a considerable portion of the photovoltaic market due to their advantages of superior efficiency,excellent stability and satisfied cost performance.Optimization of structure and application of novel materials have always been pivotal approaches to improve the efficiency of solar cells.In the past few years,a large number of novel structures and technology have been developed,including passivated emitter and rear cell(PERC),amorphous silicon(a-Si)/c-Si heterojunction(SHJ),tunneling oxidation passivation contact(TOPCon)and interdigital all back contact(IBC),etc.,which have helped the efficiency of c-Si solar cells in laboratory to 26.7%and industrial to>24%.With the consecutive improvement of silicon wafer quality,the composite loss caused by Si wafer is being compressed.Therefore,the limiting factor of efficiency has been the interface between various fuctional layers,such as the recombination losses caused by the direct contact between the front/rear metal electrodes and Si in the PERC.For a long time,the selective contact of carriers has always been obtained with heavily doped silicon.These heavily doped regions introduced not only high Auger recombination,but also a large of the manufacturing cost.In addition,multiple toxic and dangerous gas sources have to been used in these doped processes.Therefore,it is one of the internationally recognized development directions of c-Si solar cells to obtain the interface with high passivation and low contact resistance in a low-cost way and realize the selective contact of carriers.Dopant free silicon heterojunction solar cells are representative products of this strategy.The dopant-free carrier selective contact,that is,deposited a film on the Si surface using suitable compound materials by low-cost methods(such as solution method or thermal evaporation,etc.).The efficient selective transmission of holes and electrons are approached by matching work function and conduction/valence band,which formed an asymmetric energy gradient.Dopant-free heterojunction have great potential for highly efficient solar cells obtained with using low-cost materials and fabrication methods,which has benefited from the advantages of widely materials library,no toxic dopants,and low-temperature processing.In this paper,we aim at the dopant-free heterojunction solar cells based on n-Si substrate,comprehensively explore and optimize the two mainstream hole selection contact materials PEDOT:PSS and Mo O_x,and develop a series of new electronic selection contact materials.Moreover,a set of theoretical experience of screening and improving the carrier selective contact materials have been obtained according to the experiment summarized.Firstly,the similarities and differences between PEDOT:PSS/Si heterojunction and traditional p-n homojunction have been described by the investigation of PEDOT:PSS/Si heterojunction solar cells.The effects of additives on the conductivity and work function of PEDOT:PSS films,and then on devices efficiency were discussed.Furthermore,the thickness of PEDOT:PSS films,the resistivity of Si substrate,etc.are also adjusted to improve the cells performance.There are many imperfection for PEDOT:PSS/Si heterojunction,such as the huge optical losses for planar-Si/PEDOT:PSS contact and the recombination losses of non-conformal contact for textured-Si/PEDOT:PSS.This paper will provide approaches to solve the issues from two directions.The first approach is adjusting the reaction process to obtain the light trapping structure suitable for penetration of organic chain.Heterojunction solar cells based on this structure can maintain a high short circuit current density(J_sc>32 m A/cm²)while increasing the fill factor(FF)by 10%compared with the devices based on traditional silicon nanoholes substructure.Furthermore,two silane coupling agents(GOPS and TMOS)were introduced as additives of PEDOT:PSS.The adhesion between Si and PEDOT:PSS was enhanced by the bonding property of the additive with Si and PEDOT:PSS simultaneously.The perfect conform coating on the pyramid structure have been demonstrated with the additive concentration of 3 wt.%(GOPS)and 0.5 wt.%(TMOS),which suppress the recombination in pyramid-Si/PEDOT:PSS interface,and then increase the open circuit voltage(V_oc)and FF of solar cells.Then,the MoO_x were investigated as hole selective contact material.There are key problem that work function and conductivity could not be increased synchronous for Mo O_x films.In this article,the source of the work function of Mo O_x and the internal mechanism of the decline of the effective work function were researched by theoretically analyzed.A novel hot wire oxidation-sublimation deposition(HWOSD)system was designed and built,as an optional technology,to prepare Mo O_x and doped-Mo O_x thin films.The hydrogenated Mo O_x(H_yMo O_x)film exhibited high conductivity while almost no loss of work function,resulting in a significantly diminution of contact resistivity.Moreover,the stability is significantly improved in the high humidity environment.The passivation performance(implied open-circuit voltage i V_oc)remains above 97%when placed at 70%humidity for more than 10 h.In addition,a novel hole selective contact of WO_x/Mo O_x stack have been introduced to solve the problem that the thermal stability of Mo O_x/Si contact resistivity is unsatisfactory.The WO_x/Mo O_xstack contact demonstrated a better stability of passivation and contact resistivity,which can be attributed in slower diffusion of oxygen in WO_x films.Finally,a series of novel electron selective contact materials have been demonstrated,including alkali metal acetate(AMAc)and its composite structure with amorphous silicon[AMAc/a-Si:H(i)].AMAc/Si contact shows a ultro-low contact resistivity of 9 m??cm²,and AMAc/a-Si:H(i)contact provides a low contact resistivity(11 m??cm²)and an excellent interface passivation(J_0c?12 f A/cm²).A combination of p⁺n junction and these electron selective contacts demonstrated a high power conversion efficiency of above 20%.The maximum attainable efficiency of n-type solar cells with vary electron selective contacts performance,i.e.,contact recombination J_0cand contact resistivity was visualized using Quokka simulations.The simulation demonstrated a result that interface passivation and suited conduction band are critical factor for high efficiency solar cells.Furthermore,Zn O was selected as the candidate materials for electron selective contact.By doping with lithium,the Li-Zn O films show a work function of 3.85 e V.In addition,a negligible conduction band order of less than0.2 e V between Li-Zn O and n-Si were obtained.The power conversion efficiency of over 15%was verified in PEDOT:PSS/n-Si heterojunction cells with the Li-ZnO rear contact.