Review of Photovoltaic Energy Production Using CdTe Thin-Film Modules Timothy A. Gessert

Transcription

1 Review of Photovoltaic Energy Production Using CdTe Thin-Film Modules Timothy A. Gessert Principal Scientist, Group Manager - Polycrystalline Thin Film PV Devices National Center for Photovoltaics, NREL tim_gessert@nrel.gov, Research supported by United States Department of Energy Contract No. DE-AC36-99GO1337

2 Overview US Energy 11 Thin-Film PV Market and Industry Operation of Thin-Film CdTe PV Devices

3 24 US Total Energy Consumption by Fuel Type 99.6 Quadrillion BTU Energy (~1 Quads) (~1x1 15 BTU, 28x1 12 KWH) 3.3 TW (Constant) Power Coal 22% (225 Projected 3.3 TW - Power Quads) Natural Gas 23% US Energy US Renewable Energy 6.1 Quadrillion BTU (.2 TW Power) Wind 2% Waste (LFG) 9% Wood 33% Geothermal 6% Hydro 44% Solar 1% Alcohol 5% Petroleum 41% Renewable 6% e 24 US Solar Energy.63 Quadrillion BTU -.2 TW Power ( % Hot Water 6% Thermal 3% PV) 24 US PV Module Production ~15 MW/Year 24 Cumulative World Production ~4 MW (4 GW,.4 TW) Nuclear 8% Thin Films 17% Silicon 83% Source: 24 Annual Energy Review Energy Information Administration Thin-Film PV produced ~.3% of 24 US Energy Consumption

4 Historical and Projected US Energy Consumption by Major Fuel Type - to 225 Total Energy Consumption projected to increase by 1.4%/year Petroleum consumption projected at 1.5% year Electricity consumption projected to increase by 1.8%/year US Energy 11 Quadrillion BTU Source: Annual Energy Outlook 25 - With Projections to 225 Energy Information Administration

5 US Energy Consumption by Sector (24) Electricity Sector is largest and coal generates most of electricity Total US Energy Consumption (1 Quads) US Energy 11 Electric Power Generation By Fuel Type Industrial 22% Transport 27% Nuclear 2% Residential 11% Electricity 39% Coal 5% Hydro. 7% Natural Gas 18% Petroleum 3% Other Renewables 2% The Dilemma: The US needs to plan to double its electricity consumption in ~35 years (~1.8% growth) - while acknowledging issues associated with burning more coal, natural gas, and petroleum - and acknowledging issues associated with building more dams and nuclear power plants Source: Energy Information Administration

6 Solar Land Area Requirements For 24 US Electricity Requirements Caribou, ME 155 KWH/m 2 -yr Nebraska ~18 KWH/m 2 Madison, WI 16 KWH/m 2 -yr Boulder, CO 19 KWH/m 2 -yr Phoenix, AZ 21 KWH/m 2 -yr Photovoltaics Sources: K. Zweibel, PV Past the Tipping Point ( PVWATTS Calculator, Version 1 ( Tilt Note: Munich, Germany ~11 KWH/m 2

7 US Energy 11 MWp/yr % Growth in PV Module Production US a-si US CdTe US CIGS US World Series World Growth in PV Module Production Source: PV News, Paul Maycock, Vol. 27, No. 3, April 28 (and previous volumes) 35% Could we deploy 3.3 TW of PV in US - In my lifetime? - Assumptions: 35% Annual Exponential Growth in Production 24 US Installed PV ~1 MW (=.1 TW) Installed PV = ~ Twice Present Production Capacity US PV Production Deployed (=Twice Capacity) 24.5 TW.1 TW 26.1 TW TW TW TW TW TW TW TW TW ~2 GW/Year DOE 25 TF Goal TW TW TW ~.47 TW p = Total 24 US Nuclear TW TW TW TW TW ~13 TW p = ~4 TW Constant Power (I ll be only 78 years old!)

8 Applications for Web-Based PV How large is the PV Roof Shingle Market? US Residential Roof Area = 163 x19ft2 = ~16x19 m2 US Commercial Roof Area = 5 x19 ft2 = ~5x19 m2 Shading Factor Residential = 78% Commercial = 5% Roof Area Appropriate for PV US Residential = 3.6x19 m2 Commercial = 2.5x19 m2 How much energy would this produce? Assume 18 KWH/m2/year (US Average) 1% efficient PV modules Residential + Commercial = 1.1x112 KWH/year Or 1%of U.S. Electricity Consumption in 24 Note: US BIPV market alone is 24X more pies Amount of peak module power ~6x111 Watts - not just 24X - 24 times the 26 World Production of PV!! the 2% piece! Source for Roof Areas and Shading Factors: Ron Judkoff, Director Buildings and Thermal Systems, NREL Rigid Glass Flexible

9 US Produced vs. End-Used Energy (Exajoules) 2/3-3/4 of energy used to produce electricity is ultimately Rejected BIPV application embodies very low rejection factor, so 1% is much higher

10 Example - Size of PV Market for Commercial Glass Area of PV Installed Background 1.E+13 1.E+12 Area (sq. meters) 1.E+11 1.E+1 1.E+9 1.E+8 1.E+7 1.E+6 PV Use of Glass Could Exceed 25 World Glass Production By ~22! 2 TW-yr 1% efficient Actual Area to Install USA Roofing Production Roofs in USA World Glass Production World Paved Roads 1.E Year Source: Brent Nelson, NREL

11 The Best One-of-a-Kind Laboratory Cell Efficiencies for Thin Films (Standard Conditions) 2 Efficiency (%) CuInGaSe 2 CdTe Amorphous silicon (stabilized) Matsushita Boeing Monosolar Kodak Boeing Univ. of Maine Kodak Boeing ECD ARCO Univ. of So. Florida Boeing BP Solar EuroCIS Boeing Univ. of So. FL Photon Energy AMETEK NREL United Solar NREL NREL RCA

12 Some CIGS-alloy, CdTe, & a-si Laboratory Cell NREL-Confirmed Record Efficiencies Area (cm 2 ) CIGSe CIGSe CIGS CIAS CdTe CdTe a-si Area (cm 2 ) V OC (V) J SC (ma/cm 2 ) FF (%) Efficiency (%) Comments CIGSe/CdS/Cell NREL, 3-stage process CIGSe/ZnS (O,OH) NREL, Nakada et al Cu(In,Ga)S 2 /CdS Dhere, FSEC Cu(In,Al)Se 2 /CdS IEC, Eg = 1.15eV CTO/ZTO/CdS/CdTe NREL, CSS ZnO/CdS/CdTe/Metal U. of Toledo, sputtered United Solar, Stabilized Efficiency Sources: Updated from R. Noufi and K. Zweibel, Proc. 4th WCPEC, Waikola, Hawaii, 5/26, Photon International, October 24 Updated

13 Polycrystalline Thin Film PV Modules (Standard conditions, Aperture-area, *NREL Confirmed) Ranked by Power Company Device Aperture 2 Area (cm ) Efficiency (%) Power (W) Date GlobalSolar CIGS * 88.9* 5/5 ShellSolar CIGSS * 86.1* 1/5 WürthSolar CIGS /4 FirstSolar CdTe (6623) 12.6* * 8/8 ShellSolarGmbH CIGSS /3 Antec Solar CdTe /4 ShellSolar CIGSS * 46.5* 3/3 United Solar a-si /97 Sources: R. Noufi and K. Zweibel, Proc. 4th WCPEC, Waikola, Hawaii, 5/26, Photon International October 24, Updated.

14 Present Strengths of Each Thin-film PV Technology a-si Demonstrated Efficiency Perceived Production Advantage Perceived Materials Abundance/ Low Toxicity Strength CdTe CIS Strength Strength Thoughts: Each technology has different advantages Its not clear which (if any) advantage will yield a long-term product advantage This situation could remain true for many years!

15 CdTe Thin-Film Solar Cells Process Direction

16 TCO and Buffer Layer CdTe bandgap TCO Layer Alternatives: SnO 2 :F (First Solar, AVA, others, US) In 2 O 3 :Sn/SnO (Antec Solar, Germany) Cd 2 SnO 4 (Primestar Solar, US) Buffer Buffer Layer Alternatives Undoped SnO 2 Zn 2 SnO 4 Data from X. Wu et. al, Proc. 28th Photovoltaics Specialists Conf. pp (2))

17 Effect of TCO on PV Module Performance 1 CdTe PV Module (~85 nm Bandgap) Loss of ~1.5 ma/cm 2 for Commercial Glass 8 ~3 ma/cm2 Unscaled QE, T+R, A (%) ~4 ma/cm2 T+R, Soda-Lime Glass -1 ppm Fe2O3 T+R, Soda-Lime Glass - 1 ppm Fe2O3 T+R, 759 Technical Glass Absorption, Commercial TCO Drude Model 7e2 cm-3, 3 cm2/v-sec QE, Commercial CdTe Device QE, NREL CdTe Device Loss of ~1.5 ma/cm 2 for Commercial TCO Model Parameters: Mobility ~3 cm 2 /V-sec Carrier Concentration ~7x1 2 cm -3 TCO Absorption Wavelength (nm) 7 8 9

18 Effect of TCO on PV Module Performance.6 Absorptance (%).4 Visible.3.2 n (1/cm 3 ) 1 x x x x 1 19 µ = 1 cm 2 V -1 s -1 Absorptance (%) n = 5 x 1 2 cm -3 µ (cm 2 /V-s) Wavelength (nm) Wavelength (nm) General Conclusion High TCO Mobility is a Good Thing µ = qτ m *

19 CdS Layer Thickness: ~3 nm (Industry), ~5-1 nm (Research) Buffer Solution Growth (NREL, BP Solar) CdSO 4, NH 4 OH, N 2 H 4 CS (Thiorea), H 2 O Close-Space Sublimation (Antec, SSI, AVA) Gas-Phase Transport (First Solar) Sputtering CdS:O (NREL) CdZnS CdS:In With ZTO Buffer Layer Future Trend (Near) Elimination of CdS (Photon Energy/Golden Photon)

20 CdTe Layer (Congruent Sublimation)

21 CdTe Layer (Research System Designs) Close-Space Sublimation (CSS) (Present NREL Design) Gas-Phase-Transport (GPT, VTD ) (Univ. Delaware [IEC] Design Shown) Substrate Halogen Lamp Ta Wire Confined in Boron Nitride Heater/Enclosure (Hot-Press boron nitride (BN) with borate binder) CdTe CdTe Source 4 x6 Pre-Heater, 6 C 4 x4 Substrate 2 x6 Post-Heater 6 C Halogen Lamp Source: D. Rose et. al., Prog. In Photovoltaics; Res. and Appl. 7, (1999) Halogen Lamp Heaters Non-Heated Region Constrains Deposit

22 CdTe Back Contact Historic Back Contact Te-enriched interface Produced by chemical Etching processes ZnTe:Cu CdTe 2µm CdS All-Dry Contact Process Glass/SnO 2 :F/CdS/CdTe/ZnTe:Cu/Ti Device (Ti removed) Sources: Gessert et. al., 31 th IEEE PVSC, pp Romero et. al., MRS Proc. 719 F8.4.1 (22)

23 Motivation How do Thin-Film CdTe PV Junctions Form?? Current Density (ma/cm 2 ) A-1, Å 614A-2, 5Å 615A-4, 1Å' 616A-1L, 2Å 613A-2, 3Å 611A-2, 42Å 617A-3, 5Å Voltage (V)

24 The ZnTe:Cu/Ti Contact Process (All-Dry, High-Temperature [~3 C]) ZnTe:Cu/Ti Contact Process Higher Temp. Upstream Processes Lower Temp. Downstream Processes Photolithography Ti ZnTe:Cu CdTe Glass Some Research Advantages Precise control of junction performance High device stability Can achieve very low Cu incorporation Easy to make large, identical sample sets

25 Some Insight from Past Studies Cu Diffusion Less Than Optimum Cu Diffusion Greater Than Optimum Current Density (ma cm 2 ) ~24 C (24V) ~28 C (28V) ~32 C ( 32V, Optimum) Current Density (ma cm 2 ) ~32 C (32V, Optimum) ~34 C (34V) ~36 C (36V) Effect of Contacting Temperature Voltage (Volts) Voltage (Volts).8 1. Current Density (ma cm -2 ) ZnTe:Cu Thickness Devices Contacted at 36 C 1 µm.5 µm.2 µm.1 µm.4 µm Effect of ZnTe:Cu Thickness Insight It s not as simple as just accounting for Cu from the contact! Voltage (Volts).8 1. Gessert, et. al, 4th World Conf. PV Energy Conversion, pp

26 Some Insight from Past Studies Comparison of C-V Analysis Vary Contact Temperature Vary ZnTe:Cu Thickness N A -N D (cm -3 ) ZnTe:Cu Depositon Temperature 24 C 3 C 32 C 34 C 36 C 36 C 34 C 32 C N a -N d (cm -3 ) ZnTe:Cu Thickness Contact Temperature 36 C 1 KHz, 1 mv Sweep -8. V Volts 1 µm.5 µm.2 µm.1 µm V Bias 1, Å V Bias 2, Å V Bias 5, Å V Bias 1, Å C Depletion Width (µm) 3 C Depletion Width (um) 3 4 (1 khz, 1 mv, -8. to +.6 Sweep) Gessert, et. al, 4th World Conf. PV Energy Conversion, pp

27 Some Insight from Past Studies Cu Concentration (cm -3 ) ZnTe:Cu CdTe CdS 24 C 3 C 32 C 34 C 36 C 1 1. µm, 36 C.5 µm, 36 C.2 µm, 36 C Previous, 1. µm, 36 Previous, 1. µm, 32 ZnTe:Cu CdTe CdS 2 3 Depth (µm) Cu Concentration (cm -3 ) Vary Contact Temperature Excessive Cu Optimum Cu Insufficient Cu Vary Contact Thickness Excessive Cu?? Optimum Cu High-Resolution Compositional Analysis (SIMS) Depth (microns) Insufficient Cu Gessert, et. al, 33 rd IEEE Photovoltaics Specialists Conf., Paper No. 14

28 Some Insight from Past Studies 1 Quantum Efficiency (%) Sun Bias +.4 Volts 24 Volt Heater Insufficient Cu 32 Volt Heater Optimum Cu 36 Volt Heater Excessive Cu Red-Light Bias QE Confirms Photoconductive CdS Only For Excessive Cu Devices Wavelength (nm) 8 QE Comparison Suggests Narrowing Junction for Excessive Cu Devices 9 Apparent Quantum Efficienty (%) Uncorrected for Actual J sc Wavelength (nm) 8 9 Gessert, et. al, 33 rd IEEE Photovoltaics Specialists Conf., Paper No. 14

29 SCAPS1D Simulation 4th World Conf. PV Energy Conversion, pp N A -N D (cm -3 ) ZnTe:Cu Depositon Temperature 24 C 3 C 32 C 34 C 36 C 36 C 34 C 32 C N A (cm -3 ) e14 1e11 SCAPS Simulation 1 khz, Sweep -.7V to +.1 Volt NA Nd NA ND CC46_2e13_1e17 CC46_1e14_1e14 CC46_4e13_1e16 CC46_1e14_1e13 CC46_8e13_1e15 CC46_1e14_1e11 1e14 1e13 1e14 1e14 CdTe N A CdS N D 8e13 1e15 4e13 1e16 2e13 1e C Depletion Width (µm) 3 C Depletion Width (µm) 4 Current Density (ma cm 2 ) ~32 C (32V, Optimum) ~34 C (34V) ~36 C (36V) Current Density (ma cm -2 ) SCAPS Modeled LIV/DIV CdTe N A _CdS N D LIV_2e13_1e17, 844 mv, 22.9, 83.1% LIV_1e14_1e14, 893 mv, 24., 72.4% LIV_1e14_1e12, 894 mv, 23.9, 64.7% LIV_1e14_1e11, 887 mv, 23.8, 6.8% DIV46_1e14_1e14 DIV46_1e14_1e12 DIV46_1e14_1e Voltage (Volts) Voltage (Volts).8 1. Gessert, et. al, 4th World Conf. PV Energy Conversion, pp

30 Some Insight from Past Studies Constant Thickness ~.5 µm Vary ZnTe:Cu Temperature Constant Temperature ~3 C Vary ZnTe:Cu Thickness Cu Cu But Assumes Minority Carrier Lifetime Does Not Change

31 Minority Carrier Lifetime Problem - Previous TRPL studies indicate minority carrier lifetime decreases with Cu Metzger, Albin, Levi, Sheldon, Li, Keyes, Ahrenkeil, JAP 94(6) (23) Wu, Zhou, Duda, Yan, Teeter, Asher, Metzger, Demtsu, Wei, Noufi, TSF, 515, 5798 (27) Demtsu, Albin, Sites, Metzger, Duda, TSF, 516 p (28)

32 Minority Carrier Lifetime Why a Problem? If both carrier lifetime and space charge width decreases with Cu incorporation, why does voltagedependant collection decrease? Glass CdTe Current Density (ma cm 2 ) ~24 C (24V) ~28 C (28V) ~32 C ( 32V, Optimum) Glass Voltage (Volts).8 1. Gessert, et. al, 33 rd IEEE Photovoltaics Specialists Conf., Paper No. 14

33 Minority Carrier Lifetime Time Resolved Photoluinescence (TRPL) 65 nm Excitation Two Power Levels 25 µw and 25 µw Glass CdTe 82 nm PL Measurement Room Temperature

34 TRPL Study of Cu Diffusion (As a Function of Contact Temperature) Minority Carrier Lifetime 25 µw Power 25 µw Power PL counts 1 1 UV7 (25 C) UC699 (2 C) UC697 (24 C) UC694 (28 C) UC72 (3 C) UV696 (32 C) UC695 (34 C) UC698 (36 C) PL counts 1 1 UC7 (25 C) UC699 (2 C) UC697 (24 C) UC694 (28 C) UC72 (3 C) UC696 (32 C) UC695 (34 C) UC698 (36 C) Time 3 4 5n(s) 1 2 Time 3 4 5n(s)

35 Minority Carrier Lifetime Time Resolved Photoluinescence (TRPL) (As a Function of Contact Temperature) 1 Optimum Cu Minority Carrier Lifetime (ps) Insufficient Cu t mw t 1.25 mw t mw t 2.25 mw Excessive Cu Approximate Substrate Temperature During Contact ( C) 35 Gessert, et. al, 33 rd IEEE Photovoltaics Specialists Conf., Paper No. 14

36 Minority Carrier Lifetime Low-Temperature Photoluinescence (LTPL) Increasing Contact Temperature produces peak that that has been associated with a defect complex Cu i -O Te, and an unidentified peak at ~1.53 ev Reference for Cu i -O Te Corwine, Sites, Gessert, Metzger, and Duda, Appl. Phys Lett. 86 (1) (25) PL (Arb. Units) ev Cu i -O Te Energy (ev) ~1.53 ev 4.25K, nm, 1 mw PL from Glass Side UC7 (25 C) UC697 (24 C) UC694 (28 C) UC696 (32 C) UC698 (36 C) Gessert, et. al, 28 EMRS Meeting, Strassbourg, France To Be Published Thin Solid Films, 29.

37 Minority Carrier Lifetime Time Resolved Photoluinescence (TRPL) (As a Function of Contact Thickness) Minority Carrier Lifetime (ps) NREL Tau 1 (2.5 mw, 36 C) CSU Tau 1 (2.5 mw, 32 C) VTD Tau 1 (2.5 mw, 32 C) VTD (5 Å, 25 C) Wu (No Cu) Demtsu (No Cu) ZnTe:Cu Thickness (Å) Gessert, et. al, 33 rd IEEE Photovoltaics Specialists Conf., Paper No. 14

38 What Does it all Mean?? Current Density (ma cm 2 ) Minority Carrier Lifetime (ps) Constant Cu, Vary Temp.4.6 Voltage (Volts) Insufficient Cu ~24 C (24V) ~28 C (28V) ~32 C ( 32V, Optimum) Current Density (ma cm 2 ) t mw 1. Optimum Cu Excessive Cu Approximate Substrate Temperature During Contact ( C) ~32 C (32V, Optimum) ~34 C (34V) ~36 C (36V) Minority Carrier Lifetime (ps) Current Density (ma cm -2 ) Constant Temp, Vary Cu Thickness ZnTe:Cu Thickness (Å) ZnTe:Cu Thickness Devices Contacted at 36 C 1 µm.5 µm.2 µm.1 µm.4 µm.4.6 Voltage (Volts) NREL Tau 1 (2.5 mw, 36 C) CSU Tau 1 (2.5 mw, 32 C) VTD Tau 1 (2.5 mw, 32 C) VTD (5 Å, 25 C) Wu (No Cu) Demtsu (No Cu) Voltage (Volts).8 1.

39 Conclusions Some CdTe PV Devices Conclusions Net acceptor level increases during contacting, producing optimum W depletion Minority carrier lifetime can increase and/or decrease during contacting Primarily depends on temperature during Cu diffusion Also depends on amount of Cu diffused Reason(s) remain uncertain For the ZnTe:Cu/Ti contact, the longest lifetimes occur at contact temperature of ~28-32 C This may explain various contact-process functionalities! Coincident with formation of ev peak (~4K) Anti-correlated with ~1.53 ev Pl peak? (~4K) Strategy for optimization of Cu-containing contacts: Understand effect of temperature on lifetime Adjust Cu to optimize depletion width and V oc (and limit Cu diffusion into CdS)

40 Life-Cycle & Mineral Resource Considerations For Thin Film PV

41 PV Life Cycle Comparisons - Energy Pay Back Energy Payback Time (EPBT) Considers Energy Consumed During: Extracting, refining, and purifying raw materials Production, associated materials, and transportation of modules Installation, balance of systems (BOS) components, and use of modules Module disposal or recycling Ribbon Si Poly Si Mono Thin Films Si a-si CIS CdTe Modeled Efficiency 11.5% % 1 14.% 14.% 3 1 6% 2 1% 3 9% 1 8% 3 EPBT (Years) EPBT (Years) EPBT (Years) V. Fthenakis and H. Kim, Proc. 26 European MRS Meeting, Nice France (6/6) 2. PV FAQs, U.S. Dept. Energy Pub. No. DOE/GO (and internal references, 12/4). 3. M. Raugie et. al., 2th European PVSC, Barcelona, Spain (6/5) Ref 1&3: Ref 2,

42 PV Life Cycle Comparisons - Cadmium Emissions Emissions During Electricity Production from typical US coal-fired power plant plant (per GWhr = 36x1 9 Joules) 2-7 grams Cd (+14 grams collected) 1 tons CO 2, 8 tons SO 2, 3 tons NO x,.4 tons particulate Cd emissions for oil-fired power plants higher than coal Cd emission also associated with natural gas and nuclear plants Source: V. Fthenakis and H. Kim, Proc. 26 European MRS Meeting, Nice France (6/6)

43 PV Life Cycle Comparisons - EcoToxicity Other Si Wafers Electricity Glass EVA Frameless Polycrystalline Si Modules Frameless CdTe Modules Total Eco-Toxicity Potential Source: M. Raugie et. al., 2th European PVSC, Barcelona, Spain (6/5)

44 Mineral Resource Considerations for Thin Films (Si, a-si, CuInGaSe 2, CdTe) Source: PV FAQs, DOE/GO (1/4),

45 Some Other Conclusions Understanding CdTe total eco-toxicity continues to evolve Performance of CdTe PV modules would benefit immediately from improved glass and TCO products Fastest actual pathways to reduced CdTe PV eco-toxicity 1. Reduce amount of glass used in module 2. Reduce electricity used to produce module 3. Reduce other material inputs (primarily EVA) Fastest pathway to reduced CdTe PV political toxicity 1. Reduce amount of CdTe and Pb-solder used in module

46 Conclusions Thank You!