Study On All Back Contact Silicon Solar Cells With Dopant-free Heterojunctions

Because of the mature industrial system,high conversion efficiency,good cost performance and stability,crystalline silicon solar cells still occupy the main photovoltaic market.With the development of crystalline silicon solar cells,a large number of new technologies have been applied,including passivated emitterand rear cell?PERC?,amorphous silicon?a-Si?/crystalline silicon?c-Si?heterojunction?SHJ?,tunnel oxide passivated contact?TOPCon?and interdigitated back contact?IBC?technologies,which have helped the efficiency of crystalline silicon solar cells reach26.7%in laboratory and over 23%in industry.However,how to reduce the cost when keep a high efficiency is also the biggest problem in the whole area of the crystalline silicon solar cells.In this disseration,we have studied the dopant-free heterojunction all-back contact solar cell through using new technology with the low-cost and great potential?doped-free heterojunction technology?and IBC technology.For dopant-free heterojunction,it not only has a number of advantages of a-Si/c-Si heterojunction,such as low temperature process?<200??,simple process,high open circuit voltage(V_oc⁷50mV),as well as the symmetry structure,but also has more advantages,like cheap equipment,wide range of materials,higher band gap?lower parasitic absorption?,which are not only in solving the short circuit current(J_sc)loss caused by the parasitic absorption of doping amorphous silicon in conventional SHJ solar cells,but also have great potential in simplifing the fabrication process of IBC solar cells.At the same time,using IBC structure can achieve a higher J_sc because it moves the top electrode in conventional solar cells to the rear side of devices,which is expected to achieve a higher efficiency too.Firstly,through fabricating the front-back contact solar cells with poly?3,4-ethylenedioxythiophene?:Polystyrene?PEDOT:PSS?/Si heterojunction,we try to understand the factors,including the silicon oxide layer at the interface between PEDOT:PSS and Si,the shape design of top electrode,the work function of the rear electrode materials,the thickness of PEDOT:PSS film,the resistivity of silicon wafer,and the surface textures of silicon,is how to affect the efficiency of the dopant-free heterojunction solar cells.Through a series of studies,we not only understand how to improve the efficiency,but also point out the problems which must be faced in fabricating this kind of solar cells.For example,PEDOT:PSS film has a high parasitic absorption and a low refractive index,which will lead to a large optical loss,and PEDOT:PSS film is unable to well cover the surface of pyramid textures,resulting in a large recombination loss at the PEDOT:PSS/Si interface.Ultimately,because of above problems,it is difficult to make the J_sc and efficiency over 35mA/cm² and 16%,respectively.Secondly,in order to fully understand the characteristics of IBC solar cells and prepare for the further improvement of efficiency,we have used Quokka software to systematically simulate the parameters of the all-back contact solar cell.Through simulation and comparison with the front-back contact solar cells,we can fully understand the structural characteristics and carrier transport characteristics of all-back contact solar cells,as well as the influence of each parameter on the efficiency of IBC solar cells.Through specific quantitative calculation,detailed theoretical guidance is provided for subsequent experiments.Among them,the research shows that,in IBC devices,both of the front surface passivation and silicon wafer quality?bulk lifetime?are more important than that of front-back contact solar cells.The poor passivation of electronic transport layer?ETL?will lead to a strong electronic shading phenomenon,so,compared with hole transport layer?HTL?,ETL has a greater impact on the efficiency of IBC solar cells.In the simulation of passivation and contact resistance properties of carries transport layer,we can see that both factors play an equally important role in obtaining high efficiency solar cells.Thirdly,aiming at the difficulties faced by PEDOT:PSS/Si heterojunction front-back contact solar cells and combining with the theoretical study of all-back contact solar cells,we have carried out the research of PEDOT:PSS/Si heterojunction all-back contact solar cells.In the experiment,the process procedure which is suitable for fabricating heterojunction by spin-coating method were successfully developed after adopting the buried ETL electrode technology.Through studying the ETL materials'properties in passivation and contact resistance,a medium passivation material,MgO_x,was selected to fabricate the all-back contact solar cells with PEDOT:PSS/Si heterojunction,and finally the efficiency of this solar cells reached to 16.3%.In the simulation,through using DECIVE software to calculate,it is found that,even if without changing the passivation properties of HTL,it can also make the J_sc of IBC solar cells reach at 41.6mA/cm² and the efficiency exceeds 22.4%when the surface recombination rate of ETL reduced to 10cm/s.Finally,we have investigated the dopant-free heterojunction all-back contact solar cells with intrinsic amorphous silicon passivation layer.Through lifetime measurement and Cox&Strack method?CSM?,we have extracted passivation quality?J₀?and contact resistance??_c?from various materials including amorphous silicon with different thickness,and find that the passivation quality and contact resistance will be increase with the growth of the thickness of intrinsic amorphous silicon thin film,where the increase of the HTL's contact resistance is the most obviously,increasing from14.1m?·cm² at 0nm to 3.4m?·cm² at 8nm.Through using above data,the theoretical efficiency of IBC cells with different thickness intrinsic amorphous silicon film can be calculated.The results show that,with the growth of the thickness of amorphous silicon film,the corresponding IBC solar cells'efficiency will increase at first and then decreases.The maximum value of efficiency occurs at 2-4nm,and the corresponding theoretical efficiency is>24%.From the electrical power loss analysis at 4nm,it indicates that,through improving the quality of silicon wafers and reducing the contact resistance of HTL,it is expected to improve the efficiency to>26%.In experiments,we have studied two different methods to fabricate IBC cells with intrinsic amorphous silicon.One is a direct metal mask method,which is a simple process,while the other is a conventional lithography method.We first used the direct metal mask method to fabricate IBC solar cells with different thicknesses of intrinsic amorphous silicon film.It showed similar trends as that of the simulation.The IBC solar cells with 4nm a-Si:H layer obtained the best efficiency,but the value of efficiency was relatively low compared to that of simulation.From a further careful study of thermal evaporation used in the direct mask method,we found the question of possible leakage channel.Through adjusting the position of evaporation source and depositing SiN_x thin film under the main grid electrode to reduce the probability of current leakage,it eventually raised the efficiency of solar cells to 20.1%(V_oc=659mV,J_sc=41.6mA/cm²,fill factor FF=73.2%).On the other hand,we also used the lithography method,which is similar as that of IBC-PEDOT solar cells'fabrication produce.The results showed that the lithography method basically eliminated the power loss caused by current leakage in direct metal mask method,and at last,it achieved a higher conversion efficiency of 20.6%(V_oc=658mV,J_sc=41.4mA/cm²,FF=75.6%).