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ARCPHOTOVOLTAICSCENTRE OFEXCELLENCE2010/11ANNUAL REPORT4. RESEARCH“First-generation” wafer-basedtechnology (BP Solar SaturnModule, the photovoltaic productmanufactured in the highestvolume by BP in Europe, usingUNSW buried-contact technology).Figure 4.1.1Example of “second-generation”thin-film technology (modulefabricated on CSG Solar’s Germanproduction line, based on thin-filmsof polycrystalline silicon depositedonto glass, again UNSW-pioneeredtechnology).Figure 4.1.24.1 Introduction to ResearchPhotovoltaics, the direct conversion of sunlight toelectricity using solar cells, is recognised as one ofthe most promising options for a sustainable energyfuture with the photoovltaics industry poisedto become one of the world’s largest of the 21 stcentury. The ARC Photovoltaics Centre of Excellencecommenced in mid-2003, combining previousdisparate strands of work supported under a varietyof programs, into a coherent whole addressingthe key challenges facing photovoltaics, as wellas “spin-off” applications in microelectronics andoptoelectronics. The Centre was funded by the ARCuntil December 2010 and has since been fundedunder a variety of other schemes.The Centre’s photovoltaics research isdivided into three interlinked strandsaddressing near-term, medium-termand long-term needs, respectively. Thepresent photovoltaic market is dominatedby “first-generation” product based onsilicon wafers, either single-crystallineas in microelectronics (Fig. 4.1.1) or alower-grade multicrystalline wafer. Thismarket dominance is likely to continue forat least the next decade. First-generationproduction volume is growing rapidly,with the technological emphasis uponstreamlining manufacturing to reduce costswhile, at the same time, improving theenergy conversion efficiency of the product. Alsoimportant is the reduction of the thickness of thestarting silicon wafer without losing performance,to save on material use.The Centre’s first-generation research is focussedon these key issues. Building upon the success of“buried-contact” solar cell, the first of the modernhigh-efficiency cell technologies to be successfullycommercialised (Fig. 4.1.1), the Centre hasdeveloped several other high-efficiency processesin commercial production or close to this, based onCentre innovations in laser and ink-jet processing.Wafers are expensive and need quite carefulencapsulation, since brittle and also thermallymismatched to the glass coversheet, makingfirst-generation technology reasonably materialintensive.Several companies worldwide arecommercialising “second-generation” thin-film celltechnology based on depositing thin layers of thephotoactive material onto supporting substratesor superstrates, usually sheets of glass (Fig. 4.1.2).Although materials other than silicon are of interestfor these films, silicon avoids problems that canarise with these more complex compounds dueto stability, manufacturability, moisture sensitivity,toxicity and resource availability issues. CSG Solar,a partner in the Centre, has commercialised anapproach pioneered by Centre researchers thatis unique in that it is based on the use of thesame high quality silicon used for first-generationproduction, but deposited as a thin layer onto glass.As well as its collaborative activities with CSGSolar, the Centre currently maintains a largelyindependent program addressing alternativesolutions to those adopted by CSG Solar forproducing high-performance “silicon-on-glass”solar cells. The main emphasis of both is thedevelopment of lower-cost approaches (such asdeposition by evaporation rather than PECVD). TheCentre also commenced activities on carbon-based,organic solar cells during 2009 and a program on14
Efficiency and cost projections forfirst-, second- and third-generationphotovoltaic technology (wafers,thin-films and advanced thinfilms,respectively).Second-generation thin-filmtechnology has a different coststructure as evident from this figure.Production costs per unit area area lot lower, since glass or plasticsheets are a lot less expensivethan silicon wafers. However, likelyenergy-conversion efficiencies arelower (6-15%). Overall, this trade-offproduces costs/watt estimated asabout 2 times lower than thoseof the wafer product, in largeproduction volumes.ARCPHOTOVOLTAICSCENTRE OFEXCELLENCE2010/11ANNUAL REPORTFigure 4.1.3other “earth abundant” materials, particularly CZTS(Cu 2ZnSnS 4), in 2010.At the present time, second-generation thin-filmsare entering the market in increasing quantities.Large-scale commercialisation of thin-film productleads to a completely different manufacturing coststructure compared to the wafer-based product.However, costs again are increasingly becomingdominated by material cost as production increases,for example, by the cost of the glass sheet on whichthe cells are deposited.More power from a given investment in materialcan be obtained by increasing energy-conversionefficiency. This leads to the possibility of a thirdgenerationof solar cell distinguished by the factthat it is both high-efficiency and thin-film. Toillustrate the cost leverage provided by efficiency,Fig. 4.1.3 shows the relative cost structures of thethree generations being studied by the Centre. Thisfigure plots efficiency against manufacturing cost,expressed in US$/square metre. First-generationtechnology has relatively high production cost perunit area and moderate likely efficiencies at themodule level (13-20%). The dotted lines in Fig. 4.1.3show the corresponding cost/watt, the marketmetric. Values below US$1/watt seem increasinglyfeasible by improving the efficiency while reducingmanufacturing cost, but this is the likely limit of thefirst-generation approach.The third-generation is specified as a thin-filmtechnology, which therefore has manufacturingcosts per unit area similar to second-generation,but is based on operating principles that donot constrain efficiency to the same limits asconventional cells (31% for non-concentratedsunlight for the latter). Unconstrainedthermodynamic limits for solar conversion are muchhigher (74% for non-concentrated light, giving anidea of the scope for improvement). If a reasonablefraction of this potential can be realised, Fig. 4.1.3suggests that third-generation costs could be lowerthan second-generation by another factor of 2 to 3.Of the third-generation options surveyed by Centreresearchers, “all-silicon” tandem cells based onbandgap-engineering using nanostructureswas selected as the most promising forrelatively near-term implementation (Fig. 4.1.4).This involves the engineering of a new class ofmixed-phase semiconductor material basedon partly-ordered silicon quantum-dots inan insulating amorphous matrix. The generalCZTS material system may also have somepotential here due to the range of bandgapsaccessible. Even tandem cells built on siliconwafers could fall into the targetted cost zonein Fig. 4.1.3. Photon up-conversion as a way of“supercharging” the performance of relativelystandard cells forms a second line of research.A third is the investigation of schemes forimplementing hot-carrier cells. Both of thelatter may prove to be particularly well suitedto organic solar cells.The fourth Centre strand of silicon photonicsdraws upon elements of all three of thephotovoltaic strands. A by-product of this workhas been the development of techniques basedon silicon light-emission for characterising bothcompleted devices, particularly solar cells, as well assilicon wafers at different stages of processing (Fig.4.1.5). Developing this approach to its full potentialhas formed an increasingly large part of the Centre’sphotonics program.Conceptual design of an all-silicontandem cell based on Si-SiO 2(orSi-Si 3N 4or Si-SiC) quantum dotsuperlattices. Two solar cells ofdifferent bandgap controlled byquantum dot size are stackedon top of a third cell made frombulk silicon.Figure 4.1.4Schematic representation of aPL imaging system. An externallight source is used to illuminatethe silicon wafer or solar cellhomogeneously. The luminescentemission (red arrows) from thesample is captured with a sensitiveCCD camera. This technology wascommercialised by Centre “spin-off”BT Imaging during 2008.Figure 4.1.515
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