Current Status of Solar Photovoltaic Technology Platforms, Manufacturing Issues and Research

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Speaker: Dr. Steven S. Hegedus, Institute of Energy Conversion, University of Delaware

In his talk, Dr. Hegedus, a 30-year solar cell research veteran, provides an overview of the existing status of today's solar technology platforms and manufacturing issues, as well as provide viewers with his perspective looking 3 to 5 years into the future. He discusses cadmium telluride (CdTe) and copper indium gallium selenide (CIGS) thin-film technology. He also provides up-to-date results for advanced crystalline silicon (c-Si) high efficiency cell technology concepts such as the amorphous/c-Si heterojunction, all-back-contact cells, selective emitters and laser-fired contacts. Finally, he briefly describes his lab's current work on addressing critical issues in CIGS and c-Si cell technology.

Dr. Hegedus has been a member of the research staff at IEC at the University of Delaware, the world's oldest photovoltaic research laboratory, since 1982. He co-edited the 1st and 2nd editions of the "Handbook of Photovoltaic Science and Engineering" (Wiley 2003, 2011) and is a co-editor of the journal "Progress in Photovoltaics."
Thursday, Sept. 27, 1 p.m. EDT
Source: http://www.photonics.com/Webinar.aspx?WebinarID=24

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Current Status of Solar Photovoltaic Technology Platforms, Manufacturing Issues and Research

  1. 1. Current Status of Solar PhotovoltaicTechnology Platforms, Manufacturing Issues and Research Steve Hegedus Institute of Energy Conversion University of Delaware With assistance from IEC staff: Brian McCandless (CdTe), Bill Shafarman (CIGS) Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #1
  2. 2. Outline Introduction to IEC at U of Delaware PV trends, growth in scale, contribution to energy production Crystalline Si PV status, baseline technology, near term focus:  New methods emitter formation, passivation and device architecture Thin Film PV status, baseline technology and near term focus:  Cu(InGa)Se2 : wide gap alloys, improved 2-step selenization  CdTe: higher deposition T , substrate  a-Si/nc-Si (briefly) : multijunction, collapse of the a-Si industry Two common PV myths Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #2
  3. 3. Institute of Energy Conversion at U of Delaware Founded in 1972 to perform thin-film PV research World’s oldest continuously operating solar research facility First 10% efficient thin film solar cell (1980) Dept of Energy University Center of Excellence for Photovoltaic Research and Education (1992) First flexible Soft funded - government and industry contracts 10% cell 2012 staff: 11 professional, 3 tech, 2 admin, 5 post doc, >20 grad students (4 depts) 4x4 inch Recently rec’d $8.4M from DOE (3 year grants) minimodule Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #3
  4. 4. IEC Research Program Goals Expand the fundamental science and engineering base for thin film and c-Si photovoltaics to improve performance Transfer these technologies to large-scale manufacturing  IEC has been responsible for growth of several PV start-ups through technology transfer and validation Provide workforce with PV scientists and engineers  >40 graduates since 1992 (PV Center of Excellence) Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #4
  5. 5. IEC Technology Thrust Areas Thin film polycrystalline CuInGaSe2-based (CIGS) solar cells Thin film polycrystalline CdTe solar cells Silicon-based solar cells  Front and back contact heterojunction (a-Si/c-Si)  Thin film tandem a-Si and nc-Si Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #5
  6. 6. IEC Facilities: complete capability for fabrication and characterization of thin film and c-Si solar cells Over 20 thin-film deposition systems: PECVD (vhf/rf/dc), HWCVD, PVD, Vapor Transport, sputtering, H2S/H2Se reaction, chemical bath Materials characterization: XRD, GIXRD, VASE, EDS, SEM, AFM, AAS, XPS, FTIR, Raman, optical trans+refl, Hall effect Device fabrication: complete capability for high efficiency solar cells: c-Si (front heterojunction and IBC), CdTe, Cu(InGa)Se2 , a-Si Laser and mechanical scribing for monolithic module fabrication Device characterization: J-V, J-V-T, QE, C-V, OBIC, accelerated life stress (damp-heat, ambient, light) Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #6
  7. 7. The Big Picture:PV applications and achievements Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #7
  8. 8. PV can be installed anywhere, 10’s Watts to 100’s Megawatts Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #8
  9. 9. Recent worldwide achievements Installation: 17 GW in 2010 (100% growth), 30 GW in 2011 (70% growth)  Average annual growth >50% p/y for decade EU: PV providing 2-4% of annual electricity in Spain, Germany, Italy  May 2012 Germany received >10% from PV  On one day >40% (22 Gigawatts peak supply out of 27 GW installed) US: 5.7 GW installed, 2 GW in CA  Over 70% of 2011 installations are ‘utility scale’ or > 100 kW  Worlds largest PV power plant 250 MW Aqua Caliente Project (CA, thin film) Creating hundreds of thousands of jobs  400,000 in Germany; 100,000 in US  R&D, manufacturing, supply chain (materials), system design, installation Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #9
  10. 10. Trend in PV applications: 1990-2009  1995: PV demand driven by off-grid applications  After 1995: Innovative policy in Japan, Germany stimulated market for grid-connected residential and commercial  >2008: Asian Si modules drove down prices, increased installations  >2010: Significant growth in utility scale > 1 MW projects  2012: First year of flat or  negative growth in decades, projected to recover 2013 Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #10
  11. 11. Industry in turmoil: ‘roller coaster ride’ Significant consolidation, bankruptcy, closures in past 2 years Top companies for years suddenly quit PV or bankrupt Worldwide capacity ~ twice demand yet demand still growing Huge excess inventory Shrinking profits - many companies selling at loss to compete c-Si done much better in price and efficiency than many expected, squeezing thin film start ups Renewed emphasis on improving performance since costs so low Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #11
  12. 12. Brief Overview of PV BasicsPhotonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #12
  13. 13. What is a PV device? Direct converter of light into electricity: photons in, electrical current (DC) out Three critical processes:Light Absorption + Carrier Generation + Carrier Collection (current flow) (deliver P to load) e- V+ e- h+ load h+ V- charge separation Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #13
  14. 14. Cell efficiencies vs. bandgap EG Record performance single junction cells vs. theoretical limit  Expect maximum performance with EG ≈ 1.5 eV  But theor eff >25% possible EG ≈ 1 – 1.8 eV a-Si/nc-Si 2J  Many thin film, III-V options Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #14
  15. 15. Commercial Scale PV Devices  Single crystal or multicrystalline Si wafers  Dominate market: 85-90% of sales  Solar grade Si, lower qual than IC  Module efficiency: 14-20%  Low cost Asian Si driving prices down  Thin films (1-3 µm polycrystalline or amor)  Ultimately lower cost than Silicon wafers (??)  On glass, metal or plastic foils  Diverse materials, techniques  Lower quality, imperfect crystallization, more defects  Module efficiency: 8-14%  Unique advantages in building integrated products  10-15% of market, #2 PV company is TF CdTe (First Solar)Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #15
  16. 16. One common PV challenge: reducing gapbetween champion cell and module efficiency Multi c-Si and TF CIGS both ~20% cell efficiency. But mc-Si has more mature module technology. Wolden et al, JVST-A 29 (2011) 030801 Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #16
  17. 17. Why efficiency matters – fixed BOS costs Levelized cost of Energy: • Lifecycle costs/energy • LCOE costs include Balance of System which scale with # modules, area • Lower eff = higher BOS$ • More rack, wiring, install $ • y-intercept is system price without module • Si modules ‘selling’ at $0.9/W Or $120/m2 Wang et al Renewable and Sustainable Energy Rev (2011) Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #17
  18. 18. Crystalline Si (c-Si) Technology and Advanced ConceptsPhotonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #18
  19. 19. Standard commercial Si PV cell process start 900°C 900°C 400°C finish Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #19
  20. 20. Commercial Si Solar Cell, Eff ~ 15-17% Front contact (Screen printed Ag fired through SiN) Random textured F and R (~ 0.3 µm) Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #20
  21. 21. World record Si solar cell: PERL• PERL cell: Passivated Emitter, Rear contact Locally diffused• 2-step emitter (thin n between contact and thick n+ under contact)• UNSW, AU, 1998; very complex design, not manufacturable Cell Voc (V) Isc (mA/cm2) FF(%) Eff(%) PERL 0.70 42 81 24.7 Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #21
  22. 22. Conflicting emitter properties: pn junction vs resistance vs absorbing ‘dead layer’property Advantage DisadvantageIncrease thickness •Reduce lateral R for •Increase absorption in current flow to Ag highly defect layer contact (photons not converted •Prevent melting Ag SP to e-h pair), lower blue metal penetrate to base QE and JscIncrease doping •Reduce lateral R •Increase defects and •Reduce contact R with recombination (Io) so Ag or other metal grid decrease VocIncrease bandgap •Decrease absorp loss •Increase lateral •Increase band bending, resistance significantly, reduce recomb (Io) req second conductive layer ($$) Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #22
  23. 23. Why not ‘tune’ the emitter to have properties it needs only where it needs them? Why not a 2 step emitter – spatially specific?  different thickness and doping where needed?  acknowledge that current flow is 2D not 1D Why not replace with wider bandgap material? Why not get rid of the emitter all together** On the front of the device Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #23
  24. 24. Industrially proven high efficiency Si solar cell concepts Three commercial proven enhancements (full size wafer results)  PERL/SE: passivated+selective emitter, rear localized contact  HIT: ‘HJ intrinsic thin’ a-Si/c-Si heterojunction  IBC: ‘interdigitated back contact’ rear emitter and base contact Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #24
  25. 25. Hi Eff concept #1: the 2-step ‘selective emitter’ Conventional 1 step 2 step emitter: thinner n+ everywhere thick emitter except under metal n++Increased blue Appliedresponse with Materialsthinner n+ website Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #25
  26. 26. SE option 1: laser doping + plating metal http://www.photonics.com/Article.aspx?AID=40098 Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #26
  27. 27. SE option 2: depositthicker n++ then etch back• requires alignment of front metalto thicker n++ mesa Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #27
  28. 28. Thinner vs selective emitter: Voc, Isc, FF, Eff Gauthier “Industrial Approaches of Selective Emitter on Multicrystalline Silicon Solar Cells” 24th Eu-PVSEC (2009) 2-CV.5.46 Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #28
  29. 29. Hi Eff #2 Heterojunction Solar Cell: deposited a-Si passivation layers reduce surface recombination 10nm (p)a-Si:H 10nm (i)a-Si:H 30nm (n)a-Si:H Rear  Device: n-type c-Si wafer and 5-10 nm 60nm ITO Contact PECVD a-Si layers (EG=1.7-1.8 eV) (Al) hυ (i) a-Si:H surface passivation layers (both sides) (p)a-Si:H emitter (front)FrontContacts (n)a-Si:H back contact (rear)(Ag) 300μm n-c-Si wafer All a-Si and contacts deposited <200°C (low $, Electron Current less defects, no warping thin wafers)  High efficiency and VOC (Sanyo/Panasonic): EF EC η = 23% champion cells, 19% modules VOC = 740mV EV Hole Current Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #29
  30. 30. Two modes of c-Si Surface Passivation by a-Si:H Field effect passivation:Defect neutralization by H atoms: Increased band bending at junction Reduce c-Si surface dangling bonds Repel/separate majority or minority carrier Reduce recombination (IO), increase VOC Reduce Io, increase VOC EC EC EF EF EG,c-Si EG,a-Si:H EG,a-Si:H EG,c-Si EV EV c-Si Substrate a-Si:H film (n)c-Si Substrate (i)a-Si:H film PECVD a-Si:H provides best passivation and processing < 300 °C, high VOC Si surface cleaning critical to good passivation Well-established, slow deposition rate, easily control thickness ~ 5-10 nm Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #30
  31. 31. Hi Eff Si #3: Interdigited back contact (IBC) cell Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #31
  32. 32. Standard front junction vs all back contact Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #32
  33. 33. IBC spectral response higher inshort (blue) and long (IR) wavelengths Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #33
  34. 34. Sunpower IBC: highest efficiency module Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #34
  35. 35. Integration of both device concepts: SHJ-IBC Cell TCO p-type a-Si intrinsic a-Si n-type c-Si intrinsic a-Si n-type a-Si TCO Silicon heterojunction (SHJ) solar cell. Interdigitated back contact (IBC) solar cell.First published •intrinsic a-Si bufferresults on SHJ-IBC •separate a-Si pBy IEC (APL 2007) and n regions IBC-SHJ solar cell (IEC structure) Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #35
  36. 36. IEC Multichamber PECVD for HIT and IBC-SHJ 4 chambers plus 2 load lock, DC/RF/VHF plasma Multiple substrate sizes (1x1 up to 12x12 inch) Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #36
  37. 37. Thin Film PV Common Features  Monolithic Integration via laser patterning: enabling technology  Lower efficiency: TF PV best suited for BIPV, large power plants Status and Critical Issues  Cu(InGa)Se2  CdTe  A-Si/nc-Si (briefly) Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #37
  38. 38. Monolithic Series Interconnection : laser scribe Allows structuring of large area uniform thin film layers into series connected junction diodes; critical technology for TF PV Three laser scribing steps (patterning steps P1, P2, P3)  P1) bottom conductor; P2) semiconductor junction; P3) top conductor Width of cells determines module current (ISC), # in series determines VOC Chapter 12, Handbook of Photovoltaic Science and Eng (Luque, Hegedus), Wiley 2011 Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #38
  39. 39. TFPV applications: BIPV  Appearance preferred for building-integrated PV Semitransparent a-Si Architectural skylight85kW Shell SolarCu(InGa)Se2 in Wales Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #39
  40. 40. Flexible a-Si on SS: BIPV (flex laminate)  Flexible PV for roll-out rooftop installation (USSC triple junction/SS) Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #40
  41. 41. 4 MW of CdTe installed by Tucson Electric Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #41
  42. 42. 15 MW of 3Sun Tandem Thin Si in Altomonte, Calabria ItalyPhotonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #42
  43. 43. Cu(InGa)Se2 Thin Film Solar Cells Help from Bill Shafarman Institute of Energy Conversion University of DelawarePhotonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #43
  44. 44. Why thin film CuInSe2 alloys for PV? Direct bandgap chalcopyrite materials with high absorption coefficient Extraordinary compositional tolerance Alloy with Ga, Al, Ag, S to engineer bandgap  improve performance, TF tandem Can be deposited on glass or light flexible substrates: polymer, foils Highest device and module efficiency of any TF PV technology Multiple deposition technologies with promise of scalability Attracted considerable private investor funding Outdoor module stability demonstrated Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #44
  45. 45. Cu(InGa)Se2 Thin Film PV Performance  Highest cell efficiency = 20.3% (ZSW 2010)  Efficiency ≥ 18% from several laboratories  Sub-module eff. = 17.8% with area > 800 cm2 (Solar Frontier 2012)  12–14% module efficiency from companies worldwide Grid ZnO:Al Manufacturing CdS  Many companies with various approaches Cu(InGa)Se2 1. Reaction of metal precursors (2-step)  Low cost deposition of metals Mo  Batch process: “selenization” Substrate 2. Multi-source evaporation (1-step)  In-line process, high temp Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #45
  46. 46. Cu(InGa)Se2 Optical Absorption High optical absorption of sunlight  Direct bandgap  Complete absorption in ~ 1 µm thickness (CdTe very similar)  Reduces requirements for minority carrier transport 1 bsorption 0.8 CIGS 0.6 Si elative A 0.4 0.2 R 0 0.001 0.01 0.1 1 10 100 1000 thickness (µm) Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #46
  47. 47. Cu(InGa)Se2 Grain Structure Films are polycrystalline with rough surface  Grain size ~ 0.1 – 1 µm depends on deposition conditions  e.g. substrate temperature during evaporation  But device performance is remarkably insensitive to grain size, morphology Cu(InGa)Se21µm Mo deposited at 400°C deposited at 550°C Wilson, Birkmire, Shafarman Proc. 33rd IEEE PVSC (2008) Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #47
  48. 48. CuInSe2 Alloys with wider EG: Al, Ga, S CuInSe2 has EG=1.0 eV, limits eff., major focus 20 yrs is to raise EG Wide range of ternary, quaternary alloy options  Recent focus on alloys with Ga/(Ga+In), S/(S+Se), Ag/(Ag+Cu) Alloying changes EG also lattice constants (no epi!), band alignment x = alloy fraction: Al/(In+Al) Ga/(In+Ga) Ga/(In+Ga) S/(Se+S) a x=0 b x=0.24 c x=0.61 Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #48
  49. 49. Increasing efficiency and VOC at higher EGIncreasing EG with alloy (Ga, Ag, S) Failure to capture benefit of larger EGto push efficiency at EG >1.3 eV due to VOC/ EG<1 is critical issue Contreras et al, 37th IEEEE PVSC, Seattle 2011 Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #49
  50. 50. Process 1: Elemental Co-evaporation Simultaneous delivery of elemental vapors to hot substrate Makes higher efficiency devices than 2-step Independent control of each element  Ga/(In+Ga) gradient  bandgap gradient Substrate Heater Substrate at 400-600°C Film Growth Monitor To vacuum pump Thermal Evaporation Pbase ≈ 1x10-6 Torr Sources for Cu, In, Ga, Se, (S) Prun ≈ 2x10-5 Torr Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #50
  51. 51. IEC Cu(InGa)(SeS)2 Co-evaporation Five source system for Cu-In-Ga-Se-S  Boron nitride Knudsen cells  Typical temperatures T(Cu) = 1350°C T(In) = 1000°C T(Ga) = 1100°C T(Se) = 300°C  Source design and T control are critical features Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #51
  52. 52. Process 2: 2-step Precursor Reaction Advantages: lower cost, higher uniformity + materials utilization Many precursor deposition options with Cu, In, Ga –  sputtering – commercially available  electrodeposition – high utilization, non-vacuum, batch  ink printing – high utilization, non-vacuum, continuous Reaction Mo/Cu(InGa)Se2 Mo/Cu/Ga/In H2Se/H2S or Se/S 400 - 600°C Reaction in hydride gases (H2Se, H2S) or elemental vapors (Se,S) Multi-step reaction pathway to form Cu(InGa)Se2 Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #52
  53. 53. Cu(InGa)Se2 Precursor Reaction IEC H2Se / H2S reactor  reaction in quartz tube of glass/Mo/Cu-In-Ga precursor layers  atmospheric pressure with flowing H2Se / H2S / Ar / O2  Temperature-time cycle critical to uniformity, manufacturibility Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #53
  54. 54. CIGS Module Process Flow Clean Mo dc P1 laser CIGSSubstrate sputtering scribe deposition ZnO:Al P2 mech. HR ZnO CdS bathdeposition scribe deposition depositionP3 mech. Attach EVA/glass Module Scribe buss bars lamination completion Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #54
  55. 55. CIGS thin film manufacturing status: > 30MW real capacityManu- Deposition Champion Current Deposition Deposition Process commentsfacturer Technology product % nameplate Process Con´s apa capacty MW/a Pro´sManz 1-stage co- 15.1 30 Simpler, more Sacrifices efficiency Glass-glass(Würth) evaporation 120 advanced process Cd- bufferSolibro 14.4Global Solar 3-stage co- 15 (cell) Highest known TF Complex process SS substrate.Energy evaporation 13 (mod) module efficiency glass/polymerMiaSolé Reactive sputter 15.7 >40? good efficiency Complex process SS substrate, cut + potential, rel. stitch, glass-glass small capex exp. CdS-drySolar 2-step: Sputter+ SF: 17.8 980 advanced Sacrifices efficiency glass-glassFrontier, H2Se/S- SF: 14.1 process, Higher OpEx than CdSStion/ Selenization (manu) 5 + 135 potentially higher evap. glass-glassTSMC Stion: 14.5 +300 CapEx Cd-free TSMC 15.1Avancis, 2-step: 30 + 100 Glass-glassHyundai Sputter+Se- +100 CdS (Cd-free) evap.+ RTP- cryst./H2SSolo Power 2-step 15.1% cell 30 + 400 Good metal Sacrifices efficiency SS substrate electroplate- 13.5% mod utilization, rel. low Polymer Selenization CapEx CdS Source: MarkusSpectra/ Webinar San Francisco‚ Feb. 2012 plus adds from09/27/12 #55 Photonic Beck Photon “PV manufacturing and research” Hegedus the author
  56. 56. Two Current research issues: Cu(InGa)Se2 Fundamental understanding of relation between wide EG alloys and device performance with multisource evaporation Control uniformity and reduce process time with Se/S reaction Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #56
  57. 57. 1. New wide gap alloy (AgCu)(InGa)Se2 Ag addition to Cu(InGa)Se2 lowers melting temperature, better surface mobility, potential for improved structural hence electronic quality Ag increases bandgap by up to 0.25 eV, Single phase over entire range of Ag–Cu and Ga–In alloying Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #57
  58. 58. Notable wide EG cell results with (AgCu) Cells: SLG/Mo/(AgCu)(InGa)Se2/CdS/ZnO/ITO/grid/MgF2 High Eff = 17.6% with Eg ≈ 1.3 eV High VOC = 890 mV with Eg ≈ 1.6 eV Hanket, et al., Proc. 34th IEEE PVSC Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #58
  59. 59. 2. H2Se/H2S reaction of Cu-Ga-In Precursors: Ga segregation at rear limits EG and Voc• 30’ H2Se@450°C, 15’ H2S@450°C • 15’ H2Se@450°C, 15’ H2S@450°C• Complete H2Se reaction prior to H2S • Partial H2Se reaction prior to H2S• Ga segregated at back, none at front • Ga uniform, higher at front• Low EG at front junction, low Voc • Higher EG at front junction, higher Voc 60 60 50 Se 50 ) ) com osition (% Se positio (% 40 40 n 30 Cu Mo 30 Cu Mo 20 In p 20 In com 10 10 S Ga S Ga 0 0 0 20 40 60 80 0 20 40 60 80 sputter time (min) sputter time (min) Hankett et al, Proc 4th World PVSEC, 2006 Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #59
  60. 60. Single step H2Se process vs. Three-step H2Se/H2S process 20 Single step process Eff Voc Jsc FFTemp, 2 Single-step : 8.3% 0.383 V 37.0 mA/cm 58.7%°C 450 2 3-step : 14.2% 0.599 V 32.2 mA/cm 73.5% Single step: 0 J (mA/cm ) H2Se 2 20 60 ~ 90 Time, min Mo Three-step process -20 Ga accumulation 550 Temp, Mo °C 400 -40 Ga homogenization 1st step : 2nd step : 3rd step : H2Se Ar H2S -0.2 0.0 0.2 0.4 0.6 0.8 20 50 10 10 20 10 V (V) Time, min Process optimization  Ga distributed uniformly with 3 step H2Se/H2S  Major improvement in VOC and Eff. Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #60
  61. 61. CdTe Thin Film Solar Cells Help from Brian McCandless Institute of Energy Conversion University of DelawarePhotonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #61
  62. 62. Why CdTe TF solar cells? Chemically stable, simple phase diagram, easy surface passivation Film deposition by a variety of scalable techniques Optimized cells require post-dep halide (Cl) exposure at ~ 400C Easily adapted to monolithic integration Low cost: thin absorber, low cap ex equipment costs, high dep rate Best laboratory cell efficiency >17%, best module 14% (First Solar) Presently lowest price PV available (First Solar) • First Solar <$0.75/W manufacturing cost • 2-3 hours from glass to module with 13% efficiency • Responsible for US lead in module manufacturing • Thin film success story! Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #62
  63. 63. Superstrate CdTe/CdS cell configuration Glass superstrate (SLG or advanced) SnO2 (comm or more complex TCO) HR layer (intrinsic TCO, ~ 20 nm) CdS (n-type, 30-50 nm) CdTe (p-type, 2-4µm) + CdCl2 step Contact (contains Cu as dopant)Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #63
  64. 64. CdTe/CdS heterojunction cell structure:grain boundaries, S-Te interdiffusion, QE CdS depletion Contact CdTe1‐xSx alloy (Cu+other) 1.0 Quantum efficiency Bandgap shift 0.8 p-CdTe 0.6 0.4 n-CdS CdCl2 HT 0.2 as-deposited Zn2SnO4 Cd2SnO4 glass 0 400 500 600 700 800 900 Wavelength (nm) Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #64
  65. 65. CdTe/CdS module processing Cleanglass/SnO2 First Deposit Post Dep Substrate Scribe CdS, CdTe CdCl2 Treat CdTe Surface Second Third Back Etch Scribe Scribe Contact Tabbing, Junct box Test Encapsulate Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #65
  66. 66. CdTe deposition technology T > 500°C T < 400°CAtmospheric Screen Spray High Cl Golden Photon Print O2 Matsushita Vacuum Abound, USF PVD Canrom CSS No Cl First Solar, Calyxo ED BP Solar VT PrimeStar/GE, Low O2 NREL, IEC XunLight 26 rf Sp Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #66
  67. 67. IEC CdTe Research using Vapor Transport DepDevelop process for Eff > 15% on moving glass substrate ( 3 cm/min) to provide basis for in-line manufacturingOptimize: CdTe deposition, substrate, contacts, CdCl2 annealing  Surface chemistry, interdiffusion, impurities, grain growthCurrently IEC CdTe work is proprietary; evaluating alternative glass, TCO, buffer layers CdTe Vapor Transport System Source CdCl2 Reactor Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #67
  68. 68. CdTe film growth: TSS, thickness, rateSubstrate temp 500ºC 570ºC AFM 20 x 20 m 5 m, 8 m/min 5 m, 8 m/minFaceted morphology, grain size increases with substrate temperatureFilm thickness 550ºC 550ºC 1 m, 8 m/min 23 m, 8 m/min Grain size increases proportional with CdTe thicknessGrowth rate 550ºC 550ºC 6 m, 8 m/min 6 m, 81 m/min Grain size inversely proportional to CdTe growth rate Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #68
  69. 69. Fundamental CdTe issues Compensating defects → doping and defect control difficult  Difficult to achieve NA>5E14 cm3 High hole affinity → non-ohmic back contact (needs p+ Cu2Te) Unable to increase Voc>0.85V which is only 60% of EG  Highest efficiencies with non-commercial substrate  Replace SLG with BSG glass ($) : more transparent, higher T deposition  Replace SnO2:F (FTO) with Cd2SnO4/Zn2SnO4 (CTO/i-ZnO)  Higher CdTe dep T allows improved grain structure (higher Voc, FF) Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #69
  70. 70. New substrates enable higher CdTe dep T  higher EffCommercial Tec10 SLG/SnO2:F/i-ZTO vs R&D GL/Cd2SnO4/i-ZTO Record CdTe FF>81% With new substrates 13.5% 16.5% [McCandless et al, to be submitted  (2012)] Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #70
  71. 71. CdTe thin film manufacturing statusManu- Deposit Champion Current Depo Depo Process modulefacturer Tech. product % apa nameplate Process Con´s capacity MW/a Pro´sFirst Solar Low Press 14.4 mod 2,700 Simple and Global player, Glass- Vapor Champ lab cell: closing matured Quality ?? glass 17.3% process, high Transport several lines thruputPrimestar/ “Thermal evap.“ 12.8 mod 30, started ? Techn. Status Glass-GE construct ? glass 400 MW, on holdAbound Low Press 15.7 cell Started Glass in, Lower Glass-Solar Thermal evap. construct, module out. efficiency? glass(CSU) Closed 2012Calyxo Atmos. press. 13.4% 80 Potential low Quality? Glass-(Q-cell) thermal evap. Champion lab cost, high rate Techn. Status glass cell: 16.2% ? Source: Schock / SNEC Shanghai‚ May 2012 plus adds from Dimmler 38th IEEE PVSC Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #71
  72. 72. Brief discussion of a-Si based multijunction PV Steven Hegedus Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #72
  73. 73. Why amorphous Si (a-Si:H) for thin film PV?  Unique among thin film PV technologies  Easily vary doping (p or n), bandgap, crystallinity (a-nc), thickness,  Well-established commercially viable multijunction process (>20 yrs)  Minimal deposition steps: PECVD plus sputter back contact (<200°C)  Highest cell efficiency (3-junction a-Si/a-SiGe/nc-Si) : 14% (stabilized)  Commercial module stabilized efficiency (2J a-Si/nc-Si) : 9-10%  Least difference between best cell and typical module efficiency: Oerlikon, Sharp, AMAT have tandem 12% cell and 10% module  Challenge: native and light induced defects low mobility+lifetime  limit efficiency; even as lowest cost PV in market, cannot compete Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #73
  74. 74. Multijunction multibandgap a-Si solar cells‘micromorph’a-Si/nc-Si tandemoptimum trade-offbetween cost andefficiency Eff=6-7% Eff=9-11% Eff=12-13-% Eff=12-13% Chapter 12, Handbook of Photovoltaic Science and Eng (Luque, Hegedus), Wiley 2011 Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #74
  75. 75. Current a-Si PV commercial status: deathwatch  ~ 10-15 companies bought Oerlikon or AMAT turn-key fab lines 2007- 2009 during Si feedstock shortage and PV price increase  Low efficiency, high cap ex, new a-Si PV at disadvantage  Most now closed (bankrupt), few operating in Asia  Subsequent c-Si overcapacity and PV module price collapse squeezed a- Si from the cost side (its strength) now multi-Si comparable cost  Appl Matl closed their a-Si fab line 2010, Oerlikon Solar closed 2012  United Solar (USSC/ECD) flex roofing product: closed 2012 after 30 yr  Sharp (?), Panasonic, Kaneka, NexPower, Astraenergy-Chint; 3Sun (Italian JV Sharp-Enel) are leading players, ~9-10% efficient 2J  58 MW Sharp micromorph tandem installed in CA in 2012 Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #75
  76. 76. Dispell Two PV MythsPhotonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #76
  77. 77. I heard it takes more energy to make a solar module than they can ever produce? NO!!!! How many years of operation before reach energy break-even point? Energy payback is <1.5 years for today’s crystalline Si wafer modules, even less for next generation thin film modules With 25 year warrantees, today’s PV modules will be net producers of clean, CO2-free electricity for at least 23 years! Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #77
  78. 78. I heard you would have to cover the country with solar modules to make any ‘real’ energy?Total energy:200x200 miles(50% coverage)Electricity only:100x100 miles(50% coverage) Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #78
  79. 79. Questions? Then buy this book! 2nd Edition (2011) of the most comprehensive PV book -1100 pages -6 new chapters -8 different cell technologies -TCO’s -performance characterization -batteries, inverters, system -policy, economics Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #79
  80. 80. Back-up slidesPhotonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #80
  81. 81. 2012 TF PV industry expected productionVery fluid, obtained from Renewable Energy World on-line 6/4/2012 2102 Top Thin Film PV manufacturers Solibro - CIGS 20 3Sun - thin Si 30 T-Solar - thin Si 35 Miasole - CIGS 40 Trony - thin Si 80 NexPower - thin Si 90 Astroenergy - thin Si 120 Sharp - thin Si 180 Solar Frontier - CIGS 620 First Solar - CdTe 1530 0 200 400 600 800 1000 1200 1400 1600 2012 MW produced Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #81
  82. 82. Double vs Triple Junction: United Solar/ECD Guha, SPIE Photovoltaics 2009 Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #82
  83. 83. Cadmium toxicity: perception problem Widely studied by Brookhaven Natl Lab and NREL Cd: lung, kidney, bone carcinogen CdTe: less soluble, more stable, less toxic Cd is by-product of Zn mining Choices: react Cd with Te and encapsulate behind glass in controlled environment and generate clean energy; or leave it in exposed ore tailings Not released to environment during roof-top residential fire EU: granted CdTe PV an exemption; politically vulnerable US: CdTe PV not classified as toxic waste (EPA) Japan, Korea: banned CdTe PV and closed research First Solar recycling/insurance program, “behind the fence” www.nrel.gov/cdte Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #83

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