Preparation Of Boron Doped Silicon Nano Paste And Its Fundamental Research For Application In Back Surface Field Of High-efficiency Silicon Solar Cells

Among various new types of clean energy,solar energy is considered to be one of the most preponderant choices.Si-based solar cells have dominated the market.Improving conversion and reducing costs are very crucial to the future success of solar industry.Compared to other solar cell structures,passivated emitter and rear contact(PERC)based solar cells only need to add two processes(rear passivated and laser opening),as cost slightly increased but efficiency significantly improved.Due to all these advantages,this type of solar cells is attracting more and more attention in academic research.Currently there are two obstacles for further improving PERC’s efficiency.First,it is difficult to achieve a high doping concentration in the p+ layer of local aluminum-back surface field(Al-LBSF).Second,thin p+ layer thicknesses within the contact regions and formation of Kirkendall voids at the rear contact between the Si substrates are very important issues.One of the solutions available is to use local boron-back surface field(B-LBSF)instead of Al-LBSF,which has higher doping concentration than BSF.Meanwhile,after improving the sintering process,the contradiction between the voids and junction depth can be alleviated.However,the B source and its diffusion process have many problems,industrialization is difficult.And only adjusting the sintering process is hard to significantly improve the voids and the doping depth.Therefore,in order to further improve the performance of solar cells,we developed a new type of B source and designed a new type of efficient back field structure to carry out boron diffusion and reduce the number of voids.This will be very beneficial to the preparation of low cost,high efficiency solar cells.This paper proposes a new method of preparation of local B/Al-back field structure(B/Al LBSF),and develops a new process for preparing low cost,high efficient Si solar cells.The main research results are as follows:(1)A low-cost,high efficient standardization system is set up for preparing the B doped Si nanoparticles..Process includes material selecting,testing,machine commissioning,clamping,tool setting,machining programming,machining,electrode replacing,product collection,purification,centrifugation,vacuum,drying-testing,weighting and storing.Testing shows Si nanoparticles are crystalline and about 30 nm in size.The yield in single equipment is more than 15 g/h,and the cost is about ￥3000 /Kg.(2)We developed a low-cost B doped Si nano paste and studied the influence of the composition of organic carrier formula on printing performance.The organic carrier composition of the organic solvents,surfactants,thickening agent,plasticizer,and thixotropic agent are studied.In order to choose the right chemicals,we characterized the chemical and physical properties of each component,and the proportion of each component is reasonably deployed according to their characters.Organic carrier formula has different viscosity and thixotropyand will have significant influence on the screen printing properties,and finally impacts boron doping process.We obtained a set of optimal formula of organic vehicle after testing.A good printing performance and accurate transfer pattern are achieved after using this paste.The cost of the organic carrier formula is about ￥130 /Kg.(3)The diffusion mechanism of boron element of Si nano paste is studied.Because the Si nanoparticles are self-doped,when the proportion of the Si nanoparticles in paste is selected,B element content in the paste will remain the same.Under this condition,the B diffusion belongs to the limited surface source diffusion,and the impurity distribution in the process of diffusion can be characterized by the Gaussian function.Researches show that when the solid content and temperature remain the same,as the diffusion time increases,the B concentration in the paste attached on Si wafer surface will continue to decrease,and B element continuously diffuses into the silicon substrate.When diffusion time remains unchanged,the increase of diffusion temperature,a similar phenomenon will occur.Studying the boron element diffusion mechanism can guide the selection of diffusion process and improve the performance of the Si paste.(4)The diffusion performances in the Si substrate of B element in Si paste are studied.The thermal and picosecond laser diffusion process are investigated,and their diffusion performance of B element within silicon substrate are analyzed.First of all,the thermal diffusion(800℃)experiment is carried out.Studies have shown that under the same diffusion conditions,as solid content increases,the concentration of B element in the silicon substrate is gradually increasing.And when the solid content and temperature keep the same,as the diffusion time increases,the sheet resistance on the Si surface is gradually reducing.Secondly,the picosecond laser diffusion experiment is carried out.Under the suitable laser parameters,B diffuses into the substrate through the picosecond laser cladding.The test results show that the Si cladding layer formed by PS laser are uniform and dense without cracks and voids.The highest B concentration in cladding layer reached about 3× 1019 atoms/cm3,and doped into the Si substrate for the depth about 0.5~1μm.Compared to the high thermal diffusion(above 1000℃)and nanosecond laser diffusion,picosecond laser diffusion can reduce the damage of silicon at the same time to gain good diffusion performance.This process provides a new way for producing low-cost and highly effective boron diffusion in the BSF.(5)The B/Al LBSF is prepared by B doped Si paste and thermal diffusion process,and a process for PW-PERC solar cells is developed.The characteristics of the process are: First,Si paste is used as the B source,the B element shallow diffuses into the Si substrate by high temperature.During metallization,B element continuously diffuses into the substrate assisted by Al paste,and at this time B/Al LBSF is formed.Second,the pre–deposited Si paste above the openings will supplement the Si element during the Al–Si alloy eutectic formation,which can decrease the amount of voids in the BSF.This process only adds three steps based on the PERC solar cells,and is fully compatible with the PERC cell process.The experimental results show that Si paste and thermal diffusion process match better with domestic Al paste,the efficiency of PW-PERC cells can be improved 0.2~0.3%,the average efficiency is close to 20%,back-surface recombination velocity(BSRV)and fill factor(FF)have different degrees of the enhancement.(6)The new type and high-efficiency B/Al LBSF is prepared by B doped Si paste and picosecond laser cladding process,and a process for SP-PERC high-efficiency solar cells is developed.The characteristics of the process are: First,Si paste is used as the B source,the smaller heat affected zone is obtained after picosecond laser cladding process.Meanwhile,B diffuses into the Si substrate.After being co-fired with Al paste,a new type and high-efficiency B/Al LBSF is formed.This B/Al LBSF can improve the carrier concentration in the p+ layer,and reduce the BSRV.Second,a homogenous heavily doped Si cladding layer is formed on the surface of the Si wafers by picosecond laser cladding.The Si cladding layer itself is a heavily doped layer and forms a metallurgical bond between Si substrate.The heavily doped Si cladding layer and localized B doping can decrease the contact resistance,reduce the minor carrier recombination at the local metal contacts,and avoid Kirkendall void formation.This process only adds four steps based on the PERC solar cells,but reduces the laser opening process.The test results showed that the BSRV of SP-PERC solar cells falls within 100cm/s.This optimized rear surface design finally contributes to push the overall average cell efficiency over 20.3% on p-type CZ wafers,and cell efficiency can improve 0.3~0.4% and FF can increase 0.8% at most.