Blakers, Andrew

Everett, Vernie

Franklin, Evan

Weber, Klaus

English

The Australian National University

20% efficient sliver cells fabricated with a simplified processing sequence  E.T. Franklin, A. Blakers, K. Weber and V. Everett College of Engineering and Computer Science Australian National University Acton, ACT 0200 AUSTRALIA E-mail: evan.franklin@anu.edu.au    Sliver technology, first developed at the ANU [1-8], offers large reductions in silicon consumption and wafer throughput per MW. However, sliver technology requires more processing steps than conventional silicon solar cell fabrication. The additional processing costs per wafer are easily justified because the cells are highly efficient and because far less silicon is used and far fewer wafers require processing for a given module area when compared to conventional technologies. Current research at the ANU is aimed at delivering a simplified processing sequence capable of producing sliver cells with a reduced manufacturing cost and with better performance than the originally developed fabrication process. With a simplified process it should be possible to reliably manufacture highly efficient sliver cells with a high yield and a very low cost compared to conventional solar cells. Cells have been manufactured at ANU using the new processing sequence with recorded efficiencies of over 20%. 1. INTRODUCTION A sliver cell is a very thin, monocrystalline silicon solar cell. Large decreases in silicon usage, by a factor of 10-20, and large reductions in the numbers of wafers processed per module, by a factor of 20-50, can be achieved compared with standard silicon technology. Typical sliver dimensions are 50-100mm long, 0.5-2mm wide and 20-60µm thick.   The essential step in forming slivers is the micromachining of deep, narrow grooves all the way through thick silicon wafers. The wafer is processed using standard techniques to turn each sliver into a solar cell. Completed cells are removed from the wafer and rotated 90 degrees to give a large area multiplication. The large face of the sliver cell, the sidewall formed by grooving, becomes the sun-facing surface of the cell, whilst the metallised contacts run along either edge. The processing treatment of both sidewalls of a sliver is identical and so the cell is perfectly bifacial. Texturing is quite easily added to the process, though standard texturing techniques cannot be applied to anisotropically etched sliver cell’s <111> sidewalls. Excellent texturing and light-trapping is instead achieved via an acid etch technique through a very thin LPCVD deposited silicon nitride layer. Incorporation of texturing and an optimal ARC will result in sliver cells capable of delivering efficiencies greater than 20% at one sun.  There is virtually no shading due to metallisation since the metal contacts are on the side of the sliver. The metal contacts occupy only a small fraction of the total surface of the cells, and doping below the contacts can be made quite heavy. Excellent, low resistivity contacts and minimal recombination are thus achieved. Good short-circuit currents, open-circuit voltages and efficiencies are observed as a result.  The long and narrow shape of sliver cells confers unusual possibilities for module designs. One such design, utilises a very simple lambertian reflector, and has cells occupying only a fraction (typically half) of the module surface area. This allows for a further reduction in silicon usage. 20% efficient sliver cells  Franklin Sliver process. Each wafer can contain thousands of slivers with an effective area of 20-50 times the wafer surface area. Figure 1: The sliver solar cell 2. SIMPLIFIED SLIVER PROCESSING Complex wafer processing is more expensive because it entails a larger fabrication facility, more processing equipment, higher maintenance costs and larger consumables and waste disposal costs. Also, the longer the processing sequence the lower the yield will be for a similar but shorter sequence. A further disadvantage of a long process compared with a short process is that development and refinement of the process is more difficult: feedback takes longer, and the problem of lower yields commonly encountered in R&D is exacerbated.  The original sliver cell processing sequence consists of 59 separate processing steps [7], where a single processing step is defined as a set of operations that take place with the assistance of a particular piece of process equipment (such as a phosphorus diffusion) or which are similar and occur sequentially (such as a wafer-washing step). Recent research at ANU has focused on arriving at a simplified processing sequence capable of delivering higher efficiency cells and a higher yield. The simplified processing sequence contains far fewer processing steps, around 32 of the same type of steps, and utilises fewer pieces of equipment.  3. SLIVER CELL DESIGN REQUIREMENTS Sliver cells have several specialised design features and requirements. Since the cells are very thin (around 50µm) with emitter diffusions on both the front and rear surfaces, very high internal quantum efficiencies and also high open-circuit voltages are possible with only relatively small minority carrier diffusion lengths in the bulk. This is most fortuitous since the distance between sliver cell terminals is large enough (in the order of 1000µm) for majority carrier drift in the bulk region to contribute significantly to series resistance losses unless low resistivity substrates  (which have reduced diffusion Clean Energy? – Can Do!  – ANZSES 2006  2 20% efficient sliver cells  Franklin length) are used. Modelling has been used to demonstrate that 0.1!.cm boron doped FZ wafers give good results in respect of minimising both bulk series resistance and bulk recombination [8].   A major loss mechanism for sliver cells is related to the resistance of the emitter region. This manifests itself as a distributed series resistance and generally has the largest impact upon performance for an otherwise well made cell. Modelling has previously been used to show that this is the case, and that it is particularly prevalent for either wide sliver cells or for sliver cells operating under concentrated illumination. Even at an illumination intensity of one sun for 1mm wide cells, the series resistance of the emitter regions alone can account for some 3 or 4 fill-factor points  A closely related loss mechanism which has been observed in sliver cells fabricated with the simplified process is attributable to a region of heavily compensated silicon which arises due to the overlap of the phosphorus sidewall emitter diffusion with the sliver cell edge boron diffusion. The consequence is a reduced fill-factor and open-circuit voltage, corresponding to a distinct non-ideal recombination component. This is readily observed on a measured JSC-VOC curve. On the other hand, the close proximity of the two diffusions provides a low reverse breakdown voltage owing to tunnelling, which confers robust tolerance of partial shading of a module, without the need for bypass diodes.  4. MONITORING SIDEWALL EMITTER DIFFUSIONS Effective monitoring and tight control over the sidewall emitter diffusion is crucial in achieving optimal sliver cells. Diffusion doping is traditionally measured by including one or more lightly doped, undiffused monitor wafers in each diffusion. The sheet resistance is then simply measured using the four point probe method. If the diffusion process is not limited by the arrival of diffusion reactant gases then it can be reasonably assumed that every surface receives the same dose as the monitor wafers. However, this proves not to be the case for the doping of sliver cell sidewalls. In this case, the arrival of reactant gases during the deposition stage of the diffusion somewhat limits the available supply of dopant atoms at the silicon surface and results in lighter diffusions. The sidewall emitter diffusion therefore requires separate monitoring. We include a specially fabricated sidewall monitor wafer with the phosphorus emitter diffusion. This undiffused p-type wafer has etched grooves the same as those wafers containing sliver cells. The individual strips of silicon are, at the end of the diffusion and drive-in, removed and the sheet resistance measured using a four-point probe as shown in figure 2.    050100150200250300350400450760 770 780 790 800 810 820Phosphorus deposition temperature (°C)Sheet Resistance ("/#)I V           Fig. 2. Sidewall and planar diffusion sheet resistances, and sidewall measurement technique (inset). SidewallP lanars w Vs IwkR ..!! Clean Energy? – Can Do!  – ANZSES 2006  3 20% efficient sliver cells  Franklin  Also plotted in figure 2 are sidewall sheet resistance measurements for experimental 1mm thick wafers with approximately 80µm wide grooves, along with the accompanying measurements for planar wafers. The wafers are from six separate diffusions, conducted at varying deposition temperatures (two were done at 790°C). These, and other measurements, indicate an approximate two-fold increase in emitter sheet resistance on sliver sidewalls compared to planar wafers. The additional sheet resistance increases for thicker wafers and for narrower grooves.  To complete the picture it is necessary to measure how the doping, or sheet resistance, varies across the 1mm width of the diffused sidewall. A lengthy process, involving the creation of a number of very small channels at various positions on the sidewall of the monitor strips, is used to isolate parts of the sidewall surface. The sheet resistance is thereby measured at specific locations. Observed sheet resistance profiles for four different diffusions are plotted in figure 3. The dashed line accompanying each set of measurement points corresponds to the sheet resistance measured for the complete sidewall surface. Doping at the middle is marginally lighter than at the edges, but generally the dose is quite uniform across the sliver cell sidewall for the particular diffusion conditions that we use.  Accurate monitoring and control of the emitter diffusion is especially important for wider cells (1.5mm or 2mm wide for example) and for sliver cells that are designed for operation at low to medium concentration ratios [8].  0501001502002503003504000 200 400 600 800 1000Sidew all position ($m)Sheet resistance ("/sq)  Fig. 3. Measured emitter doping profile across sliver sidewalls. 5. CELL PERFORMANCE FROM SIMPLIFIED PROCESSING During the development of the simplified processing sequence, cells produced were all un-textured and most had a (non-optimal) silicon dioxide AR coating. This was partly for ease of processing and then later was done out of necessity whilst the laboratory’s LPCVD furnace was completely re-built. A history of the development of the sequence is presented in figure 4. This plots the achieved efficiency, for each completed batch, on a time scale spanning the development of the simplified processing sequence.   Much of the development of the sequence involved dealing with parasitic series and shunt resistance, altering diffusion recipes, devising an improved metallisation scheme and changing around the order of process steps in order to maximise yield. The gains in efficiency were made via incremental improvements in open-circuit voltage (now 660mV – 670mV at one sun) and significant improvements in fill-factor (now at 0.81 or above). What the plot does not show is that, on top of the gradual improvement in efficiency, there was also a large improvement in reliability and consistency of results both within and between batches. Batches now have very high yield, with performance of all cells Clean Energy? – Can Do!  – ANZSES 2006  4 20% efficient sliver cells  Franklin matching very closely. This accomplishment within an R&D setting bodes well for the robustness of the process sequence in a manufacturing environment.  10%11%12%13%14%15%16%17%18%19%20%Oct-04 Jan-05 M ay-05 A ug-05 No v-05 M ar-06Measured efficiency Fig. 4. Achieved efficiencies, without texturing, over the course of streamlined fabrication process development.  The best untextured 1mm wide one-sun cells produced with the simplified processing sequence have efficiencies of up to 19.3% (calibrated in-house measurement). A short-circuit current density greater than 36.5mA/cm2 would be required to satisfy the 20%+ efficiency mark. The best untextured cell efficiencies at high (5-30 suns) illumination intensities are found for narrow cells with heavy emitter doping. Cells with efficiencies above 18.6% over an intensity range of 5 – 15 suns and cell with efficiencies above 18.3% between 30 and 50 suns have been produced. These should also translate to >20% efficiency with the addition of texturing. -40-35-30-25-20-15-10-505-200 0 200 400 600 800 V (mV) JSC = 37.1 mA/cm2VOC = 666mV FF = 81.8% Efficiency = 20.2%  J (mA/cm2 )    Figure 5: 20% efficient sliver solar cell Progress over more than a year of process development has recently led to the demonstration of 20% efficient sliver cells (figure 5) under non-concentrated illumination. Areas for substantial further improvement have also been identified, and efficiencies above 21% are likely to be obtained in the future. Clean Energy? – Can Do!  – ANZSES 2006  5 20% efficient sliver cells  Franklin SLIVER WAFERS TO SOLAR MODULES Near the end of wafer processing, each 15cm wafer contains up to several thousand slivers, enough to cover up to 1m2 of module area. The small size of each sliver necessitates a novel module construction method.  A novel method of packaging Sliver solar cells has been devised that appears to have considerable advantages over previous techniques. As far as possible the method takes advantage of standard PV modularisation technology. The only materials used in the module are those found in the great majority of conventional PV modules. The module design is sufficiently close to conventional module designs that we have confidence that it will have similar reliability to conventional modules. ACKNOWLEDGEMENTS Support from Origin Energy for the development of Sliver technology at ANU, particularly in the period 1998-2003, is gratefully acknowledged. Origin Energy is now undertaking commercialisation of Sliver technology at its new factory in Adelaide [http://sliver.com.au]. REFERENCES [1] Weber & Blakers, Semiconductor Processing PCT/AU01/01546 (2001) [2] K.J. Weber, A.W. Blakers, M.J. Stocks, J. H. Babaei, V.A. Everett, A.J Neuendorf, and P.J. Verlinden, “A Novel Low Cost, High Efficiency Micromachined Silicon Solar Cell”, Electron Device Letters 25, 37 (2004) [3] M.J. Stocks, K.J. Weber, A.W. Blakers, J. Babaei, V. Everett, A. Neuendorf, Mark Kerr and P.J. Verlinden, “65-Micron Thin Monocrystalline Silicon Solar Cell Technology allowing 12 Fold Reduction in Si Usage”, 3rd World Conference on Photovoltaic Solar Energy Conversion, May 2003, Osaka, Japan [4] A.W. Blakers, M.J. Stocks, K.J. Weber, V. Everett, J. Babaei, P. Verlinden, M. Kerr, M. Stuckings and P. Mackey, “Sliver Solar Cells”, 13th NREL workshop on Crystalline Si Materials and Processing, Vail Colorado, August 2003 [5] K.J. Weber, A.W. Blakers, V.A. Everett, P. Deenapanray and E.T. Franklin, “Sliver solar cells”, 31st IEEE PVSC, Florida 2005 [6] K.J. Weber, J. MacDonald, V.A. Everett, P.N.K. Deenapanray, M.J. Stocks and A.W. Blakers, “Modelling of Sliver® Modules incorporating a lambertian rear reflector”, 19th European Photovoltaic Solar Energy Conference, Paris, 2004 [7] A.W. Blakers, V. Everett. P.N.K Deenapanray, E. Franklin and K.J. Weber, Recent developments in Sliver solar cell technology, 20th EC Photovoltaic Solar Energy Conference, Barcelona, June 2005 [8] E.T. Franklin and A.W. Blakers, “Sliver cells for concentrator systems”, 19th European Photovoltaic Solar Energy Conference, Paris, 2004  Clean Energy? – Can Do!  – ANZSES 2006  6 

20% efficient Sliver cells fabricated with a simplified processing sequence

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20% efficient Sliver cells fabricated with a simplified processing sequence

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