Study On The Key Technologies Of Industrial Silicon-based Solar Cells&Modules

The world is now facing severe challenges relating to the energy shortage crisis and global warming.As a promising renewable energy,photovoltaic(PV)energy is gaining great attentions and supports from governments around the world to be one of the effective solutions to energy depletion issues.The prerequisite for a wide adaptation of PV energy is that its cost is becoming competitive to the conventional fossil fuel energy sources and is reaching grid parity worldwide.The key to lowering the cost per kilowatt hour of electricity generated by PV lies in improving solar cell conversion efficiency continuously,developing highly reliable and durable PV modules and solving the key technical problems encountered in the large-scale manufacturing and application.This work is a typical interdisciplinary project involving industry,academia and the research.Key technologies such as producing a low defect-density polycrystalline casting ingot,precisely aligned selective emitter(SE),surface-damage free reactive ion etch(RIE),spectrum-management based module assembly and hot spot failure which were seen in large-scale production and application were systematically analyzed and studied.Appropriate solutions were proposed and implemented.After introducing several advanced technologies to the 100MW production line,the highest total-area cell efficiency was increased to 18.84%while an average cell efficiency to 18.65%for a large size(156×156mm2)multi-crystalline silicon(mc-Si)solar cell.This is the highest value reported to date for large-scale industrial production Al-BSF standard mc-Si solar cells.With spectrum-management based module assembly technology,the corresponding module maximum power(Pmax)was 270.3W which is a successful transfer of advanced technologies developed in the laboratory to large-scale mass production.With an innovative progressive pressurization technology.the breakage issue of solar cells during the double-glass module manufacturing,was solved.After a systematical long-term reliability study,a strong correlation was found between Water Vapor Transmission Rate(WVTR)of backsheet and the Pmax degradation.It was also discovered that the key to the excellent performance of Si wafer-based double-glass PV module is replacing polymer back sheet by an impermeable glass panel.A feasibility study of hot spot failure due to an internal arcing within modules was presented for the first time.All of the experiments and data were acquired from a large-scale commercial production workshop and the innovative results are shown as follows:1.Low-defect density mc-Si ingots were successfully grown by optimizing the hot-zone and solidification process recipes.A strong association between crystallization defects and the cell conversion efficiency was discovered.The longer lifetime of ingot was,the higher solar cell conversion efficiency was.By optimizing the hot-zone,solidification process recipes and controlling the grain size with a seeded nucleation technology,the minority carrier lifetime and its uniformity across a wafer were improved and the defect density was reduced more than 30%on average.Compared to the solar cells with conventional growth wafers,solar cells with low defect-density wafers showed an additional improvement of 3%to 5%in internal quantum efficiency(IQE).an increase of 5.6 mV for open circuit voltage(Voc)and 0.4%increase in absolute efficiency on average respectively.A positive correlation between the crystal lifetime and the cell conversion efficiency was discovered-the longer the lifetime was,the higher the conversion efficiency was.The defect density at the bottom and top of an ingot was very high and the solar cells made with wafers from these sections had a range of low cell efficiencies between 14.5%and 15.5%.In contrast,solar cells made with wafers from the middle section of an ingot which had lower defect density had a high cell efficiency ranging from 16.5%to 17.5%.Different types of crystal defects demonstrated different behavior during the cell processing steps-dislocations were still found after completing the cell processing steps while interstitial metal impurities were removed during the cell processing steps.2.SE structure with a precise alignment between highly doping areas and metal grids,surface-damage free RIE texturing were successfully implemented on a large-scale manufacturing mass production line,resulting in an average cell efficiency of 18.65%,maximum up to 18.84%.With spectrum-management based module assembly technology,the corresponding module Pmax reached 270.3W.Selective emitter structure was used to decrease the surface concentration of a phosphorus dopant from 8.2×1020 to 1.2×1020 atom/cm3,resulting in an improvement of IQE from 38%to 67%at the wavelength of 300 nm.an increase of Voc by 3 mV and an extra gain of 0.35%absolute conversion efficiency.It was also found that the cell efficiency rose initially then decreased as the initial sheet resistance increased from 40?/? to 60?/? However,the cell efficiency decreased as the post-etch back sheet resistance went from 90 ?/? to 130?/?.To align highly doped areas with metal grids precisely,the same edge-align algorithm was selected for both mask and metal grid printing to ensure that the printed patterns were located in the middle of the wafers.Compared to the conventional acidic texturing process for mc-Si wafers.RIE texturing reduced the optical reflectance on the front side of the wafer from 23%to 6%,resulting in an increase of short-circuit current density by 1.04 mA/cm2 and of conversion efficiency by 0.63%.Damage removal clean(DRC)was found to be an effective solution to remove physical damages induced during RIF.Longer DRC process time degraded the texturization,increased the front surface reflectance and made the sharp pyramidal-like structure round and smooth.which is helpful to high quality surface passivation after the SiNx ARC process and reduction of the recombination of photon-generated carriers and Voc loss.After incorporating several advanced technologies such as low defect-density mc-Si casting ingot,precisely aligned SE,surface-damage free RIE on a mass production line,the average efficiency of 18.65%was demonstrated while the maximum conversion efficiency was 18.84%over a batch of 10000 solar cells,equating to an absolute efficiency gain of 1.58%compared to a conventional solar cell.With spectrum-management based module assembly technology such as high transmission EVA,anti-reflection coating glass,the corresponding Pmax for a 60-cell mc-Si PV module was 270.3W.an extra 20W increased over a conventional 60-cell mc-Si PV module with the same dimension.To the best of our knowledge,these are the world record efficiency and power output to date for PV industry obtained on a mass production line.3.A large-scale double-glass module manufacturing yield was increased from 70%to 98%due to less solar cells breakage with an innovative progressive pressurization technology.After systemically investigation,it was inferred tha the higher WVTR was,the more Pmax degradation was,and the key to the excellent performance of double glass PV module is replacing polymer back sheet by impermeable glass panel.By replacing the single-step pressurization with an innovative progressive pressurization,the solar cells breakage issue during double-glass module lamination process was improved and the manufacturing yield was increased from 70%to 98%,which is the highest level of a large-scale double-glass module manufacturing.The difficulty of double-glass module installation was solved with an integrated structure which helped to promote the large-scale application of double-glass modules.It was demonstrated during extensive fire testing that the double-glass module offered high degree of fire protection,and received the highest fire resistance Class-A rating according to the criteria of UL790.Only 0.3%Pmax degradation for the double-glass module was observed and the electroluminescence(EL)picture taken after the 5400pa mechanical loading test showed no micro-cracks.An extensive series of extended sequential long-term reliability stress tests including thermal cycling(TC)600,damp heat(DH)3000,600 hours potential induced degradation(PID)and humidity freeze(HF)50 were performed on both the conventional back sheet modules and double-glass modules.The relative Pmax degradation of conventional back sheet modules was 3.87%,7.34%,13.3%,33.73%while that of double-glass modules was 2.78%,3.12%,2.27%,2.72%,respectively.Double-glass modules exhibited much less relative Pmax degradation under all kinds of stress tests.It was found that there was a strong positive correlation between WVTR of back sheet and the Pmax degradation.It is inferred that the conventional back sheet modules have a higher WVTR and more Pmax degradation,while double-glass modules are impermeable and have much less Pmax degradation.The key to a highly reliable and durable Si wafer-based double-glass PV module is replacing the polymer back sheet with a glass panel which does not allow water vapor to permeate.4.The impact of reverse-current leakage on a PV plant safety-related hot spot failure was systemically investigated.A positive correlation between the temperature of shaded areas and the reverse leakage current was discovered.A feasibility study of a hot spot failure due to an internal arcing was presented for the first time.Theoretical and empirical studies of modules with high leakage current cells showed a positive correlation between the temperature ofshaded areas and the reverse leakage current.The greater the leakage current was.the higher the temperature of the modules shaded area was.Through continuous analysis.simulation,and experiments on modules with hot spot failure in PV power plants.It was also discovered that even in the absence of shading,thermal stresses caused by variances in day-night temperature in combination with poor soldering connections may form small gaps and cause an arcing effect,which can lead to severe hot spot failure.