Design,fabrication And Performance Of Silicon TOPCon And IBC Solar Cells

Development and utilization of energy is not only the cornerstone of human social development,but also an important driving force for world economic growth.The global energy industry is accelerating its transformation to efficient,clean and diversified energy structure.As a part of new energy,photovoltaic(PV)has made important contributions to the energy conservation,emission reduction and improvement of climate environment.Although China’s PV industry started late,R&D investment and installed capacity are increasing year by year.The proportion of PV power in total power generation has increased from almost zero ten years ago to3.1% in 2019.PV cells are mainly divided into crystalline silicon(c-Si),compound,organic and dye-sensitized solar cells.Among them,c-Si solar cells have the advantages of abundant raw materials and mature technology,dominating the market share for more than 85% and maintaining a leading position in the next decade or even longer.Conversion efficiency improvment and production cost reduction for solar cells have been the target of PV peers.Silicon cell structure has experienced early aluminum back surface field(Al-BSF)with conversion efficiency of less than 20%,and now the main market entity is passivated emitter and rear contact(PERC)structure with efficiency of more than 22%.However,PERC cell is facing a bottleneck in efficiency,and the next generation of high efficiency c-Si solar cell needs to be studied.Tunnel oxide passivated contact(TOPCon)and interdigitated back contact(IBC)are two of the most promising solar cells,both of which have the higest laboratory efficiency of about 26% and have the advantages of being compatible with production-line high-temperature processes.This dissertation focuses on the research of these two kinds of solar cells,including theoretical calculation,fabrication technology and photoelectrical properties,etc.The main work is divided into the following parts:Firstly,in view of the advantages of n-type c-Si with high minority carrier lifetime and matte attenuation,front selective emitter and rear TOPCon structures are innovatively introduced into n-type passivated emitter and rear totally-diffused(n-PERT)solar cells.The front side is doped by nanosecond pulse laser to rapidly drive boron ions from borosilicate glass layer into p+ emitter region,forming a p++selective emitter.After polishing the backside,the ultra-thin Si Ox and n+ poly-Si layers were deposited by nitric acid oxidation and plasma enhanced chemical vapor deposition(PECVD),respectively.Suitable energy laser doping is beneficial to improve the spectral response and interface passivation of solar cells.The decreasing trend of open-circuit voltage(VOC)is accelerated with the decrease of band gap of Si Ox,while short-circuit current remains unchanged,indicating that the oxygen content in Si Ox has different influences on interface passivation.In our experiment,compared with bifacial c-Si solar cell without TOPCon,the maximum VOC of the cell with TOPCon will be increased by 19 m V,resulting in an absolute increase for conversion efficiency of 0.9%,which proves that TOPCon plays an important role in interface passivation and carrier transport.In addition to improvement of poly-Si doping,the quality of front emitter and silicon substrate also had an important influence on VOC.The doping concentration of front emitter(Np+)and densities of bulk defect state(Ntr)of silicon substrate were further optimized.The results showed that VOC will increase further to ~0.720 V on the basis of 0.670 V when Ntr and Np+change from 1×1011 cm2/e V and 2×1019 cm-3 to 1×108 cm2/e V and 2×1021 cm-3,respectively.At the same time,the influence of backside morphology on TOPCon battery was studied.Influence of the rear morphology on TOPCon solar cells was also studied.Smooth surface is conducive to good passivation.But,the contact resistance of smooth surface is generally higher,which can be optimized by adjusting the doping of contact area and changing the composition of metal slurry.Secondly,the industrial application of p-type PERC c-Si solar cell is more extensive.After the efficiency of PERC solar cell is improved to a certain extent,front surface passivation becomes more and more important.For this reason,TOPCon structure was prepared on the front texturing surface of p-type PERC solar cell.Because the thickness of Si Ox is much smaller than the size of pyramid,it is difficult to control the uniformity of Si Ox,which greatly affects the interface passivation.In this disseration,textured ultrathin Si Ox was grown with thermal oxidation and intrinsic poly-Si contact layer was then deposited through thermal decomposition of silane based on the same low-pressure chemical vapor deposition(LPCVD)tube furnace with short intervals to avoid the natural oxidation of cell precursor.Electron microscopy showed that the uniformity and compactness of Si Ox and poly-Si layers were very good,in which the thickness of Si Ox was 1.5 nm.With the activation of hydrogen passivation,the average implied-VOC of sintered sample increased by 15-25 m V compared with that before sintering.The lowering of drive-in temperature during doping will increase the effective minority lifetime(τeff)and implied-VOC values while reduce crystallization of the mixed poly-Si layer.The optimized temperature is 780 ℃,which can not only demonstrate the advantages of interface hydrogen passivation but also avoid the serious parasitic absorbtion loss.An appropriate drive-in time is also important.With a poly-Si deposition time of 5 min,when drive-in time is 900 s,the implied-VOC can reach above 690 m V.The test results of elliptic polarization spectrometer showed that the thickness of poly-Si deposited for 5 min was about 25 nm thick.Conversion efficiency of the solar cells can be further improved from21.3% to 23.4% with the total recombination current desnity is reduced from 280 f A/cm2 to 33 f A/cm2.Finally,TOPCon and IBC structures have the advantages of interface passivation(electrical properties)and more photon absorption(optical properties),respectively,the integration of the two structures into a solar cell is one of the future research trends.Since TOPCon has been studied previously,we have focused on a new structure of IBC solar cell here,combining a front floating emitter(FFE)for carrier extraction(i.e.,“pumping effect”)with a design of rear shallow groove.This enables the staggered positions for back emitter and back surface field(BSF),avoiding the negative effects of non-diffused gap and surface p-n junction between emitter and BSF,and ensuring effective collection of carriers by contact electrodes.FFE with low doping concentration can improve the utilization ratio of c-Si absorption layer to incident photons,but the lateral voltage difference also weakens which stem from the difference in the built-in voltage between FFE and the base area.Therefore,optimization of FFE doping concentration is one of the ways to obtain high efficient IBC solar cells.The optimal order of magnitude is 1~2×1019 cm-3.According to the doping depth of emitter,the maximum carrier collection can be obtained with groove depth of less than 2 μm.When doping concentrations of emitter and BSF were 8×1019cm-3 and 2×1020 cm-3,respectively,the conversion efficiencies of shallow groove FFE-IBC solar cells could be increased by 6.5% and 6.1%,respectively,compared with conventional FFE-IBC structure.More importantly,the ratio of BSF width to emitter width Wn+/Wp+ dropped from 1/4 to 4/1,the conversion efficiency of front surface field FSF-IBC and FFE-IBC solar cells decreased by 21.3% and 9.5%,respectively,clearly demonstrating that the high-efficiency FFE-IBC solar cells can improve,significantly,the tolerability for the variation of Wn+/Wp+ in IBC solar cells.