In this paper, we examine the areas in c-Si growth, materials, and processing that require improvement through research to overcome barriers to the implementation of the PV Roadmap's Si goals

Ciszek, T.

Ruby, D. S.

Sopori, B.

UNT Digital Library

27Research Needs of c-Si Technology Required to Meet Roadmap MilestonesABSTRACTIn this paper, we examine the areas in c-Si growth,materials, and processing that require improvement throughresearch to overcome barriers to the implementation of thePV Roadmaps Si goals.1. C-Si Materials GrowthThe key issues in c-Si materials growth for reaching theRoadmaps module production goals are throughput of thegrowth processes, control of impurities and defects, andeffective utilization of silicon-feedstock material.  Trade-offs are required in order to achieve an optimum balancebetween these key issues.  Lets look at them in more detailand examine the associated R&D needs.Throughput of the Growth ProcessesTo improve PV module throughput to the Roadmaptarget of 200 MW/factory/year, the typical Si-PV factorymust produce >4,000 m2/day of silicon.  This assumes 15%modules capable of generating 150 Wp/m2, and round-the-clock operation with 10% down time and/or yield losses.For the growth methods currently in use, the throughput permachine and total number of growth machines (growers)required is approximately:GrowthMethodThroughput(m2/day)No. ofGrowersDendritic Web 1 4,000ESP/String Ribbon 1.7 2,350Capillary Die Growth 20 200Czochralski (CZ) Growth 30 133Directional Solidification 70 57Float-Zone (FZ) Growth 80 50Electromagnetic Casting 600 7Substrate Melt Shaping >1000 <4Thin-Layer Si Growth ? ?For ingots, the throughput numbers assume concomitantwire-sawing yielding 20 wafers/cm of ingot.  Most thin-layer growth approaches are not at a mature enough stage toestimate throughputs. Research is required to improve thethroughput of existing growth methods, to evolve higher-throughput methods, and to demonstrate comparablethroughputs with thin-layer growth approaches.Control of Impurities and Defects during GrowthThe growth methods that can sustain a factory withfewer than ten growth machines will generate ingot materialwith smaller grains than other approaches.  Or they generatesheet material with still smaller grains as well as impuritiesfrom melt/substrate and melt/ambient interactions.Research is required to improve grain size and perfection, toreduce  impurity   contents   during   growth   by   these  fastmethods, and to understand defect and impurity behavior inthe growth processes, with the goal of in-situ ameliorationof adverse effects.Effective Utilization of Silicon Feedstock MaterialIn the growth of the PV and integrated-circuit (IC)semiconductor silicon industries, we have, on average,reached the point where the PV demand for low-cost Sifeedstock exceeds the IC supply of off-specification andreject Si.  One approach that can be taken is to developalternatives to the silane and chlorosilane processes forcreating IC-purity silicon from metallurgical-grade (MG) Si,with the goal of obtaining sufficient, but not excessive,impurity levels for solar-grade silicon at a lower cost.Another approach is to focus on thin-layer Si growthmethods that utilize less silicon.  Both are daunting researchtasks in light of the throughput and module performancerequirements of the Roadmap.2. Material Quality Enhancement in Si Solar CellsLow-cost silicon wafers used in commercial productionof solar cells contain high concentrations of impurities anddefects in the as-grown form.  The PV industry useseconomical processes that have been optimized to upgradethe material quality by incorporating impurity gettering andpassivation during cell fabrication.  Current solar cells haveefficiencies in the 14%-15% range, which is a remarkableachievement.   However, higher cell-efficiencies must bereached to meet the future goals of the PV IndustryRoadmap.  It is imperative that advanced processingmethods be developed that can overcome current limitationsin the material quality of solar cells.  Key issues in thedevice processing are: developing techniques that canfurther improve the device performance through qualityenhancement of the substrate; implementing these advancedapproaches as high-throughput production processes;ameliorating the impurity and defect issues through use ofthinner devices; and improving process reliability throughefficient monitoring.Impurity getteringIn solar cell fabrication, impurity gettering isaccomplished through phosphorous diffusion and Alalloying employed for junction and contact formation,respectively.  These processes are able to remove a majorityof dissolved transition-metal impurities from the wafer.Recent research has shown that although the majority of thecell area exhibits strong gettering, large regions (~20%)show low performance.  These regions, which have beenidentified as defect clusters, are decorated withprecipitated impurities. Because precipitated impurities areimmobile, it is difficult to getter them by standard solar- cellD. S. RubySandia National LaboratoriesMS 0752, Albuquerque NMT. F. Ciszek and B. L. SoporiNational Renewable Energy Laboratory1617 Cole Blvd, Golden CO 8040128fabrication processes.  Modeling results have shown thatthese regions act as shunts to degrade voltage-related cellparameters.   Their removal from the wafer necessitates theirdissolution with concomitantly long, high-temperatureanneals.  Thus, the challenge is to invent processes to reducethe time and/or temperature for gettering such precipitates.One approach is to inject point defects during the process.Modeling results show that successful gettering ofprecipitated impurities will result in 18%-20% efficient cellson the low-cost material.  Alternately, it may be necessary toperform crystal growth with the objective of eliminatingimpurity precipitation.  These new considerations mayredefine the as-grown material quality and crystal growthconditions for low-cost Si.Hydrogen passivationH-passivation is used as a standard process by manycompanies in solar-cell manufacturing.  Typically, hydrogenis incorporated during plasma-enhanced chemical vapordeposition (PECVD) nitridation followed by a highertemperature drive-in process.  The detailed mechanisms ofH-diffusion and passivation are still not well understood.Because PECVD can be tailored to include surfacepassivation, such a process can be very valuable.  It isinteresting to note that effective passivation also requiresthat impurities be in a dissolved state.  The challenges are todevelop simpler, more effective passivation processes basedon a comprehensive understanding of passivationmechanisms.  An important issue is whether hydrogen canpassivate defect clusters.Processes for increased throughputIt is clear that the processes for upgrading materialquality must constitute standard procedures of cellfabrication that are compatible with high throughput,without incurring additional process costs.  Forming gasannealing, rapid thermal processing (RTP), continuousoptical furnaces, and low-temperature oxidation arepotential future processes.   New designs are needed toensure throughputs that are compatible with the needs of thePV industry.Thinner wafer processingFor the same material quality, thinner wafers can yieldhigher cell performance. Another important advantage ofthin cells is that their passivation and gettering processescan be less time consuming.  Thus, much emphasis will beplaced on thin cells.  However, thinner wafers raise a varietyof issues related to the need for:•  Significantly lower interface recombination throughsurface passivation.•  Reduced stresses due to metallization and asymmetricdepositions.•  Automated handling for minimum breakage.3. Crystalline-Si Cell ProcessingThe PV Roadmap efficiency goals for c-Si cells call forproduction-line average efficiencies of 16% in 3 years, 18%in 10 years, and 20% in 20 years.  Industrially-producedcells will need to incorporate many of the high-performancefeatures demonstrated by laboratory cells, but use low-cost,high-throughput fabrication techniques.Low-recombination emittersHigh performance requires the use of passivated emittersthat have a selectively-doped emitter profile.  This allowsthe Si to remain highly doped beneath the gridlines for lowcontact recombination and resistance, but more lightlydoped between gridlines for reduced emitter recombination.The lightly doped portion of the emitter needs to haveexcellent surface passivation to avoid losses due to surfacerecombination.  These selective emitter structures have beendemonstrated by several research groups, some using single-diffusion techniques amenable to mass production.Surface passivationPlasma-deposited silicon-nitride films have been shownto provide excellent emitter and rear-surface passivation.They also double as an effective antireflection coating.  Inaddition, they can provide good bulk-defect passivation byproviding a source of atomic hydrogen, which can diffuseinto the bulk to passivate defects and impurities.  This canallow use of lower-quality feedstock or growth methods.The critical need is to design a high-throughput plasmadeposition system with low operational cost.Thinner wafersBesides affording lower cost through better materialutilization, thin cells are more tolerant of lower-quality bulkSi.  However in order to maintain performance, thin cellsrequire excellent light absorbing properties and low back-surface recombination.TexturingLower-cost multicrystalline, sheet, and ribbon Si needaffordable texturing to reduce front-surface reflectance andredirect absorbed light obliquely within the cell.  Thisincreases optical path length, generates carriers closer to thejunction for increased collection, and can trap weakly-absorbed light through total internal reflection.Self-doping metallizationsMetals which contain appropriate dopants have thepotential to form self-aligned selective-emitters and back-surface fields at low cost by avoiding the need for additionaldiffusion steps.  Some new metal deposition techniques alsohave the potential to reduce contact and series resistance byavoiding the use of oxide frit and organic binders.Back-contactsCells with rear gridlines can benefit from the avoidanceof gridline shadowing and may allow significant cost-savings due to simplified cell-interconnection methods.4. SummaryThe challenges for c-Si cells to meet the Roadmap goalsare substantial.  At the projected production levels, it will benecessary to have sources of low-cost solar-grade silicon orto evolve growth processes that utilize much less material.High-throughput growth processes with low defect andimpurity concentrations will also be a necessity.  Novelhigh-throughput cell processing for higher device efficiencyand low-cost material upgrading will also be needed.

Research Needs of c-Si Technology Required to Meet Roadmap Milestones

In this paper, the authors examined the areas in c-Si growth, materials, and processing that require improvement through research to overcome barriers to the implementation of the photovoltaic road maps's Si goals. To obtain PV module throughput to the roadmap target of 200 MW/factory/year, the typical Si PV factory must produce &gt;4,000 m{sup 2}/day of silicon

Ruby, Douglas S.

Ciszek, T. F.

Sopori, B. L.

Research needs of c-Si technology required to meet roadmap milestones

https://digital.library.unt.edu/ark:/67531/metadc888276/m2/1/high_res_d/909216.pdf

Research Needs of c-Si Technology Required to Meet Roadmap Milestones

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