Crystalline Silicon Photovoltaic Cells

By Martin A. Green*

1. Introduction

Photovoltaics remains one of the most benign ways yet sug-gested for meeting future energy requirements in both urbanand the rural third world.[1] Recognizing this, many countries,particularly Japan, Germany, the Netherlands, Italy, Switzer-land, Spain, and the USA have programs under way aimed atincreasing the use of the cells and thereby reducing their cost.The most effective recent programs have been those encour-aging the urban residential use of the cells, generally involvinga three-way subsidization of cell costs between the govern-ment, the home owner, and the local electricity company. Forexample, since 1998 over 30 000 grid-connected residentialsystems have been installed in Japan with a 33-1/3 % subsidyfor the home owner. The electricity grid essentially providesfree storage for the electricity by buying back surplus gener-ated electricity at the same rate it charges for power sent inthe other direction.

Most present systems use cells fabricated using thin(0.3 mm) silicon wafers similarly to those used in the pres-ently much larger microelectronics industry. These are slicedfrom crystals grown either by the Czochralski crystal growthtechnique used in that industry or from much larger ingotsprepared by cruder directional solidification techniques thatresult in ªmulticrystallineº (large grain polycrystalline) mate-rial. The reduced requirements for material perfection com-pared to microelectronics means that off-specification materi-al prepared for the latter industry is used as relativelyinexpensive feedstock for these crystal growth processes.

2. Recent Performance Improvements

The first silicon solar cells were made at Bell Laboratoriesin the 1940s and were partly responsible for the interest in Siand Ge that led to the explosion of the semiconductor indus-try in the 1950s.[2] Figure 1a shows the subsequent evolutionin laboratory silicon solar cell performance. A recent burst ofactivity, spearheaded by the author's group, has taken solarenergy conversion efficiency to 25 %, close to the fundamen-tal limit of 29 % for silicon.[3]

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The photovoltaic industry is in a phase of rapid expansion, growing at over 30 %per annum over recent years. Although technologies based on thin-film compoundand alloy solar cells are under active development, most commercial solar cellspresently use self-supporting bulk crystalline or multicrystalline silicon wafers, simi-lar to those used in microelectronics. The laboratory performance of these cells, at 25 % solar energy conversionefficiency, is now approaching thermodynamic limits, with the challenge being to incorporate these improvementsinto low-cost commercial products. Improvements in cell optical design, particularly in their ability to ªtrapºweakly absorbed light, has also led to a growing interest in thin-film cells based on polycrystalline silicon, havingadvantages over other thin film photovoltaic candidates.

±[*] Prof. M. A. Green

Centre for Third Generation PhotovoltaicsUniversity of New South WalesSydney, 2052 (Australia)E-mail: m.green@unsw.edu.au

Fig. 1. a) Evolution of silicon laboratory cell performance; b) structure of a25 % efficient silicon solar cell.

The structure of a 25 % efficiency cell is shown in Figure 1b.The cell is made on a high-carrier-lifetime, boron-dopedp-type floatzone wafer, with a carrier lifetime in the com-pleted device well above 1 ms. Cells of similar performancehave been fabricated on wafers produced by the magneticallyconfined Czochralski (MCZ) technique, where similar post-processing carrier lifetimes can be obtained.[4]

Electronically, the cell is basically a very large area p±n junc-tion diode with avenues for carrier recombination within thecell minimized. Apart from the high carrier lifetimes, the cellis almost completely enshrouded in a layer of thermally grownsilicon dioxide that ªpassivatesº device surfaces, thus minimiz-ing recombination in these areas. Contact to the n-type andp-type regions is through either slots or holes in this oxide thatkeep high-recombination, metal±semiconductor contact areasto a minimum. Recombination in contact areas is furtherreduced by having them heavily doped, while keeping the totalvolume of poorer quality, heavily doped silicon small.

The parameter that benefits from this attention to minimiz-ing recombination is the dark saturation current density, Thecells act as almost ideal diodes with values below 50 fA/cm2

obtained in high-performance cells. The almost perfect elec-tronic and optical properties of the cells have made them idealfor evaluating silicon material parameters to previouslyunprecedented accuracy, notably silicon's intrinsic carrier con-centration[5] and long wavelength band-to-band absorptioncoefficient.[6]

Optically, the most notable feature is the inverted pyramidsover the cell's top surface. These are typically 10±15 lm insize, formed by anisotropic etching of the silicon to exposeintersecting {111} equivalent planes below the originally (100)orientated wafer surface plane. One function is to reducereflection from the top surface by a ªdouble bounceº effect,aided by a double-layer MgF/ZnS antireflection coating ontop of the oxide (not shown). Although about half the sunlightis absorbed within the first few micrometers of entering sili-con, the other half is quite weakly absorbed with much pass-ing obliquely across the cell and reaching the rear. By havingthe rear metal contact displaced from the cell by the lowrefractive index oxide, its reflectance is enhanced, redirectingmost of the light back towards the top surface. Here the sec-ond role of the inverted pyramids becomes apparent. Mostredirected light will strike a pyramid side other than the singleside from which it can escape, trapping the light into the cellby total internal reflection. This increases the cell's opticalthickness to about 50 times the physical thickness (ideally 4n2,where n is silicon's refractive index). Virtually every photo-generated electron±hole pair contributes to the cell output,regardless of where it was created in the cell.

3. Low-Cost Processing

As opposed to values of 25 % for laboratory cells, commer-cial wafer cells generally have more modest energy conver-sion efficiency in the 10±15 % range. One reason is the

relatively simple standard cell structure shown in Figure 2a.This type of cell is based on screen-printing front and rearmetal contacts onto the cell surface using metal pastes. Since

similar technology is used in hybrid microelectronics, thisapproach has benefited from the considerable infrastructurealready developed for this industry. However, the pastes arenot ideal for producing low-resistance contacts to the diffusedcell junction. Moreover, the silver-based pastes are inherentlyexpensive, cannot produce fine linewidths and have a relative-ly low conductance compared to bulk silver. The consequen-tial compromises in cell design account largely for therelatively low efficiencies.

Over the last decade, two new higher performance celldesigns have come into commercial use. The first is theªburied contactº approach developed by the author's group,shown in Figure 2b. In this approach, a laser is used to blastnarrow grooves in the cell surface, which are subsequently

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Fig. 2. a) Commercial screen-printed silicon wafer solar cell; b) improvedªburied contactº cell design; c) HIT cell, combining amorphous and crystallinesilicon cell technologies.

filled with electrolessly plated metal (Ni/Cu/Ag trilayer). Thisovercomes many limitations of the screen-printed approach,resulting in commercial efficiencies in the 16±18 % range. Themain remaining limitation is the less than ideal properties ofthe cell rear contact. The improved cell performance resultsin 10±20 % lower production costs per unit power output thanthe screen-printed approach.[7]

A second high-efficiency commercial cell is the HIT cell(heterojunction with intrinsic thin-layer) of Figure 2c. Thiscombines the bulk wafer approach with thin-film amorphoussilicon±hydrogen alloy cell technology, as widely used inconsumer products such as pocket calculators.[8] Since theamorphous silicon alloy has a much wider bandgap than bulksilicon, this material is ideal for low-recombination termina-tion of the bulk silicon wafer, probably even more so than thethermal oxides used in the laboratory cell of Figure 1b. Thisgives HIT cells very good electronic properties apparent ashigh-output voltages. However, the absorbing properties ofthe amorphous layers combined with those of the transparentconducting oxides used to contact them results in a poor re-sponse to blue light, accounting for about 10 % loss in poweroutput. Commercial cell efficiencies in the 17±18 % range arereported.[9] No costing studies appear to have been reported.

An apparently fortuitous feature is that these cells usen-type, phosphorus-doped wafers, since this produces the nor-mal polarity scheme for amorphous silicon technology. Recentwork, however, suggests that this type of wafer may be a bet-ter choice than the p-type, boron-doped wafers normally used.It has been found that, under illumination, oxygen introducedfrom the crucible during the Czochralski growth processforms an active complex with boron dopants, reducing carrierlifetimes.[10] Since no similar detrimental complexes form inphosphorus-doped material, the latter would appear moresuited to the routine, low-cost production of the high-carrier-lifetime wafers, providing a path to higher production cell effi-ciencies in the future.

4. Ribbon Silicon

One undesirable cost in the traditional wafer approach isthat of slicing wafers from ingots, regardless of whether theseare prepared by the Czochralski or directional solidificationapproaches. Not only are there direct costs, but over half theingot material is wasted as kerf or powder loss during slicing.Over the last decade, a switch from inner diameter sawing tocontinuous wire cutting has improved the overall slicingeconomics. However, the direct growth of silicon in sheet orribbon form is an option with clearly superior economics, ifsuccessfully implemented.[7]

The most established approach is the EFG method (edge-defined film-fed growth), where the silicon is grown from acarbon die. Rather than the planar ribbon shown in Figure 3a,the approach has evolved to the growth of silicon tubes, elimi-nating edge effects during growth.[11] Wafers are cut from thethin-walled tubes using lasers.

Another ribbon approach with an even longer history is thedendritic web approach of Figure 3b. Close control on tem-perature is used to solidify the growing ribbon first as den-drites defining the ribbon edge. As these are drawn from themelt, molten material is trapped between them, solidifying toform the final ribbon. A more robust variation based on theuse of carbon strings rather than dendrites to support thegrowing ribbon appears likely to beat the original approachinto large-scale production.[12]

There has been a recent surge of research interest inpeeled-layer technologies for photovoltaics, lower cost varia-tions of microelectronics ªsmart-cutº technology. The basicadvantage sought is to obtain many silicon sheets from thestarting wafer. Economics are unclear at present.

5. Supported-Film Approaches

Another approach to eliminating slicing is to deposit silicondirectly onto a supporting substrate. Developments withªlight trappingº mentioned earlier allow these films to be asthin as a few microns, in principle, while still maintaining goodperformance.

The most developed approach is the silicon thin-film onceramic approach.[13] Although published details are sketchy,the approach may involve the low-temperature deposition ofsilicon onto an expansion-matched, tape-cast, electricallyconducting ceramic substrate followed by high-temperaturerecrystallization. Product appearance and performance aresimilar to a directionally solidified, multicrystalline waferproduct, although there are prospects for capturing additionalbenefits, such as the fabrication of an interconnected moduleof cells on a single ceramic sheet.

A completely different, lower temperature approach uses avariation of amorphous silicon alloy technology to produce aªnanocrystallineº silicon phase. Unlike standard alloy materi-al, this phase resembles crystalline silicon in its optical andelectronic properties. Plans are in progress to produce astacked-cell module with a cell of this material underlying onebased on an all-amorphous silicon alloy, giving better perfor-mance and stability than amorphous silicon alone.[14]

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Fig. 3. a) EFG (edge-defined film-fed growth) ribbon; b) dendritic web ribbon.

A third approach developed by the author's group, usesintermediate temperatures to produce polycrystalline siliconfilms a few micrometers thick directly onto glass.[15] Pilotproduction efficiencies are presently within the range ob-tained for thin-film solar-cell products based on other semi-conductors (amorphous silicon alloy, cadmium telluride, cop-per indium diselenide, and dye-sensitized titanium dioxide).Ultimately, very high efficiency is expected from thisapproach, but without the stability, toxicity, or resource avail-ability issues associated with other options.

6. Conclusion

Crystalline or polycrystalline silicon cells are expected tosatisfy most of the rapidly growing demand for solar photovol-taic product over the coming decade. Cells are expected tobecome thinner and higher performing, with possibly a shift inpreferred wafer type from boron- to phosphorus-doped.

Silicon directly grown in the form of self-supporting, multi-crystalline ribbons is expected to also appear in larger volumeon the market. It appears likely that, by the end of the decade,polycrystalline silicon thin-film product deposited directlyonto glass or another supporting substrate may also have mar-ket impact.

±[1] M. A. Green, Power to the People, UNSW Press, Sydney 2000.[2] M. Riordan, L. Hoddeson, Crystal Fire, Norton, New York 1997.

[3] M. A. Green, Silicon Solar Cells: Advanced Principles and Practice,Bridge Printery, Sydney 1995.

[4] J. Zhao, A. Wang, M. A. Green, Prog. Photovoltaics 1999, 7, 471.[5] A. B. Sproul, M. A. Green, J. Appl. Phys. 1993, 73, 1214.[6] M. Keevers, M. A. Green, Appl. Phys. Lett. 1995, 66, 174.[7] T. M. Bruton, G. Luthardt, K.-D. Rasch, K. Roy, I. A. Dorrity, B. Garrard,

L. Teale, J. Alonso, U. Ugalde, K. Decleerq, J. Nijs, J. Szlufcik, A. Rauber,W. Wettling, A. Vallera, ªA Study of the Manufacture at 500 MWp p.a. ofCrystalline Silicon Photovoltaic Modulesº, Conf. Record, in 14thEuropean Photovoltaic Solar Energy Conference (Eds: H. A. Ossenbrink,P. Helm, H. Ehmann), H. S. Stevens, Bedford, UK 1997, pp. 11±16.

[8] J. Yang, A. Banerjii, T. Glatfelter, K. Hoffman, X. Xu, S. Guha, ªProgressin Triple-Junction Amorphous Silicon-Based Alloy Solar Cells and Mod-ules Using Hydrogen Dilutionº, Conf. Record, in 1st World Conference onPhotovoltaic Energy Conversion, Hawaii, December 5±9, 1994, pp. 380±385.

[9] T. Sawada, N. Terada, S. Tsuge, T. Baba, T. Takahama, S. Tsuda, S. Naka-no, ªHigh Efficiency a-Si/c-Si Heterojunction Solar Cellº, Conf. Record,in 1st World Conference on Photovoltaic Energy Conversion, Hawaii,December 5±9, 1994, pp. 1219±1225.

[10] S. W. Glunz, S. Rein, J. Knobloch, W. Wettling, T. Abe, Prog. Photovol-taics 1999, 7, 463.

[11] A. Eyer, A. Rauber, A. Boetzberger, Optoelectronics 1990, 5, 239.[12] R. L. Wallace, R. E. Janoch, J. I. Hanoka, ªString RibbonÐA New

Silicon Sheet Growth Methodº, Conf. Record, in 2nd World Conferenceon Photovoltaic Solar Energy Conversion (Eds: J. Schmid, H. A. Ossen-brink, P. Helm, H. Ehmann, E. D. Dunlop), European Commission, JointResearch Centre, Ispra, Italy 1998, pp. 1818±1821.

[13] J. E. Cotter, A. M. Barnett, D. H. Ford, M. A. Goetz, R. B. Hall,A. E. Igram, J. A. Rand, C. J. Thomas, ªAdvanced Silicon Film Solar CellDesign and Developmentº, in Conf. Proceedings, 13th EuropeanPhotovoltaic Solar Energy Conference (Eds: W. Freiesleben, W. Palz,H. A. Ossenbrink, P. Helm), H. S. Stevens, Bedford, UK 1995, p. 1732.

[14] K. Yamamoto, M. Yoshimi, Y. Tawada, Y. Okamoto, A. Nakajima, ªCostEffective and High Performance Thin Film Si Solar Cell Towards the 21stCenturyº, Tech. Digest, 11th International Photovoltaic Science and Engi-neering Conference, Sapporo, Sept. 20±24, 1999, pp. 225±228.

[15] Pacific Solar, 2000 Annual Report, Pacific Solar Pty. Ltd., Sydney 2000.

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