Current Status of U.S. R&D in Photovoltaics

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1 Current Status of U.S. R&D in Photovoltaics Dr. Robert J. Walters U.S. Naval Research Laboratory Head, Solid State Devices Branch, Code 6810 Washington, DC phone: (202) cell: (703) fax: (202)

2 Introduction Outline Overview of solar cell physics Discussion of standard solar cell characterization methodologies Highlights of major PV R&D areas

3 Introduction My R&D focuses on space power, while this symposium is focused on terrestrial power I am Technical Program Chair for the 34 th IEEE Photovoltaic Specialist Conference (PVSC) To be held June 2009, Philadelphia, PA ( 4.5 days of technical sessions plus full day of tutorials dedicated solely to PV technology Co-located with Solar Energy Industries Association (SEIA) PV America Exhibition - ~400 solar industrial exhibits I will use my knowledge of the PVSC program to give an overview of PV R&D in the US I will begin with a discussion of how one measures PV performance in a lab

4 Photovoltaic Effect in a p-n Junction Top metal grid n-side p-side E c (-) E f (+) Top metal Top metal E v anti-reflective coating p-type n-type substrate back metal Given semiconductor with band-gap Eg (ev), addition of energy Eg promotes electron from bound to free state Energy can come from absorption of photons, solar photons in particular, of proper wavelength Must extract the photogenerated carriers Junction electric field separates the photogenerated charges, which can then be collected at the front and back electrodes

5 Current (A/cm 2 ) Solar Cell Electrical Measurements In the dark, a solar cell is simply a diode 10-1 Measured Data 10-2 Shunt Current Recombination Current V A Diffusion Current Need 4-wire measurement 10-3 Total Current Typically achieved with a source-measure-unit (SMU) set to remote sense I(V) 10-4 = I shunt (R sh ) + I rec (I 02,E t, p / n ) + I diff (I 01 )

6 Solar Cell Electrical Measurements In the dark, a solar cell is simply a diode Under illumination, the photogenerated current is superimposed upon the diode dark current The current vs. voltage (IV) curve is the primary solar cell characteristic measured under white light illumination I sc ma/cm 2 V oc V P mp mw/cm 2 FF Darkcurent Underilumination voltageat opencircuit, V oc V Fill Factor FF = P mp /( Isc *V oc ) A Need 4-wire measurement Typically achieved with a source-measure-unit (SMU) set to remote sense

7 Intensity (mw/(m2 nm)) Solar Spectra AM0 AM DIRECT AM GLOBAL Wavelength (nm) Amount of atmospheric absorption referred to as Air Mass (AM) Space is zero: AM0, mw/cm 2 Terrestrial varies with geo-location, generally accepted calibration value is AM1.5 Global 100 mw/cm 2 Direct 769 mw/cm 2

8 Intensity (mw/(m2 nm)) Simulated Solar Spectra 2000 AM AM DIRECT AM GLOBAL Xenon Lamp Simulator Wavelength (nm) Test Plane Solar Simulator Xe arc lamp is work-horse of simulator industry Good UV/VIS spectral match adequate for Si Single lamp gives low complexity Large area illumination - ~ 12 inch diameter Significant spectral lines (spikes) in IR inadequate for advanced (specifically, multi-junction technologies)

9 Intensity (mw/(m2 nm)) Simulated Solar Spectra 2000 AM AM DIRECT AM GLOBAL Multi-zone Simulator 3 Xe bulbs for nm 2 incandescent bulbs for nm 2 incandescent bulbs for nm Wavelength (nm) Multi-zone simulators use different lamps to simulate different sections of the spectrum Excellent spectral fidelity throughout the spectral range Significantly more complex - 9 lamps so 9 power supplies and optical packages Significantly smaller illumination area - ~6 inch diameter at most Sample plane

10 Calibrating Solar Cell Intensity Calibration solar cells Test Cell Area Various calibration cells Solarimeter Primary method for calibrating solar cell intensity is to use calibrated solar cells Calibration cells must have spectral response similar to solar cells under test A solarimeter is also often used This is a pyranometer that measures solar radiation energy based on the absorption of heat by ablack body.

11 Quantum Efficiency Measurements Quantum Efficiency (QE) is a measure of the response to monochromatic illumination QE is essentially the ratio of number of charge carriers collected to the number of photons absorbed Spectral response is related to QE and gives a quantitative measure of current out per energy absorbed (A/W)

12 Quantum Efficiency Measurements Xenon Lamp Xenon Lamp For light bias Monochromator Lock-in amp Sample Plane Power supply for electrical bias GaAs Ge External QEInGaP Wavelength(m) Pyro-electric detector

13 Novel Technologies Inorganic Materials and Devices Organic Materials and Devices II-IV and Related Technologies Thin Film Deposition and Characterization Transparent Conductors and Contacts Device Fabrication and Modeling/Characterization Concentrator Cells and Modules Materials and Devices Concentrator Receivers and Modules Crystalline Si Technologies Feedstock and crystallization Surface passivation and bulk defects Device fabrication Modeling, metrology, and characterization Amorphous, Nano and Film Si Technologies Amorphous Si Technology Micro/Nano-crystalline Si Technology Film-Si Technology Device characterization, light trapping and modeling Module performance and manufacturing Space Technologies Space materials and devices Space systems Flight performance and environmental effects PV Modules and System Components Manufacturing and Markets Reliability and Long-Term Performance Module Measurements and Ratings Inverters and BOS Components Module Packaging: Strategies, Materials, and Processing Terrestrial Systems Stand Alone Systems Grid connected systems Building integrated systems System integrated PV R&D Technology Areas 12% 32nd PVSC/4th World Conference Paper Distribution 9% 12% 8% 10% 8% 5% 21% 12% 11% 12% 33rd PVSC Paper Distribution 15% 19% 20% 11% 15% 1 Novel Materials 2 Thin Films 3 III-Vs 4 Si 5 Amorphous 6 Space 7 Modules 8 Systems 9 Programs Area 1 Area 2 Area 3 Area 4 Area 5 Area 6 Area 7

14 Solar Cell Technolgies Borrowed from Tim Anderson, Univ. of FL

15 PV Industry Growth Why now? we have seen solar revolutions before Today s social, economic, and scientific conditions, i.e. global warming, high oil prices, and PV technology maturity, put us at a tipping point Photon International, December, 2007

16 Issues Affecting Integration of PV Generation into the Energy Grid Our electrical grids are generally vertically connected with centralized generation, distributed consumption, and limited capability for interconnection between grid control areas Over the past decades, several factors have dictated a shift from central to distributed generation (DG): Liberalization of electricity markets and new technology have made construction of big power plants more economically risky than smaller ones Need for security and quality of supply, which is greatly influenced by our dependence on foreign oil and ageing infrastructure Influence of fossil fuel on Global Warming PV is well suited for DG, and grid interconnection issues are crucial for the large-scale integration of PV DG Inverters: key technology/hardware to ensure quality of supply and do-no-harm where new voltage control techniques are needed Standards development and uniform regulations to facilitate incorporation of PV

17 Solar Cell Technology Progress Note: This graph does not include the 40.8% multijunction concentrator cell confirmed in 2008, nor the re-evaluated CIGS cell at 20% (September, 2008).

18 Crystalline Si Technology 50 yr Anniversary ~9% to <20% in 50yrs Sunpower rear contact Si solar cell design C. Z. Zhou, 26th PVSC; Sept. 30-0ct. 3,1997; Anaheim, CA PV began with Vanguard at ~3% eff. Evolved to present <20% eff. in production Sunpower, Sanyo, Sharp, BP, Si wafer production capacity serious concern

19 Multi-junction Solar Cells AM0 Solar Spectrum (1350 W/m 2 ) 2-2.5eV 1.4-2eV 1-1.4eV eV GaAs Ge External QEInGaP Wavelength(m) Higher efficiencies can be attained by combining junctions of different band-gaps Current state of the art is 3J InGaP/GaAs/Ge Demonstrated ~22% around 1998, increased 30% in 5 years Technology development driven by space applications Majority of all spacecraft are powered by these cells Theoretical calculations for 2-junction cells AM0Eficiency(%) InGaP/GaAs(26%) InGaP/InGaAs(32%)

20 Multi-junction Solar Cells under Concentration Incident light is focused onto a small solar cell Frank Dimroth, Fraunhofer ISE Higher efficiencies can be attained under concentration Lower system costs are projected with CPV CPV has attained highest efficiencies of any technology, ~40% Significant production for terrestrial systems Limited application in space G. S. Kinsey et al., 33 rd PVSC San Diego, CA 2008

21 Next MJ Technology Metamorphic Geisz et al., Appl. Phys. Lett. 93, Higher efficiencies gained by combining optimized band-gaps Limited by availability of suitable substrates for high quality growth Strain-balanced lattice-mismatched growth has enabled 1 sun, 30% AlInGaP/InGaAs/Ge Inverted metamorphic technology is enabling break-though of 30% barrier 40.8% achieved with cell shown in image

22 Novel Inorganic Materials Intermediate Band Major emphasis on intermediateband (IB) cells (a) Energy gaps of the conventional solar cell (b) The intermediate band solar cell Allows for two, sub-band-gap photon absorption Calculations show efficiency exceeding ideal limits (>~60%) IB achieved via introduction of quantum-dots (c) Energy band diagram of a row of dots (d) Resulting energy band diagram in equilibrium. L. Cuadra, A. Marti, A. Luque, Physica E 14, 162 (2002) A.G. Norman, M.C. Hanna, P. Dippo, D.H. Levi, R.C. Reedy, J.S. Ward, and M.M. Al-Jassim, 31 st IEEE PVSC, January 3 7, 2005

23 Novel Inorganic Materials Nano-structures Quantum-well solar cells (MQW) QWs increase absorption range Must balance current increase w/voltage decrease due to increased dark current Nano-crystals Size of crystals controls absorption edge, so incorporating crystals enables tailoring of spectral response MEG in crytals may enable dramatic increase in SJ cell efficiency Colloidal crystals offer possibility of spray-on solar cells p-inp InAsP MQW Region n-inp InP substrate Nano-crystals absorption controlled by size No MEG MEG Potential QE for single junction solar cell ~ 65% because of multi-exiton-generation Simplified schematic of nanocrystal PV device Schaller et al Nanoletters 6, 424(2006)

24 Novel Organic Materials Photon absorption creates exitons in electro-chemical material Photoactive materials employed can be semiconducting polymers, molecules or a combination Exiton dissociation achieved by dissolving donor and acceptor molecules in the same solution Solution deposited on a substrate like ITO coated glass Advantages Low material cost Low temperature processing Compatibility with flexible substrates and roll-to-roll processing Tunability of material properties through chemical synthesis Challenges Low device efficiency (~5% at best now) Stability and reliability

25 Thin-Film Technologies CuIn(Ga)Se 2 Front Contact ZnO Window CdS CIGS Absorber Back Contact Glass Substrate Thin film PV technologies promises low-cost fabrication on wide range of substrates CuIn(Ga)Se 2 (CIGS) has shown great potential for over two decades Small area lab cells have demonstrate upwards of 20% eff. Results are not repeatable and high eff. not achievable in production Many CIGS production houses presently exist/in development Low cost modules at ~10-12% Primary technical road-blocks are Lack of understanding of basic physics of operation of the devices Material quality control over large area

26 Thin Film Technologies Amorphous Si Thin-film deposition of a-si used to form PV devices Material band-gap controlled deposition temperature and Ge alloy fraction Established commercial product from United Solar Ovionics Technology tops out at about 12% eff. with triple junction a-si experiences degradation in performance during first ~100 hrs of exposure (Stabler-Wronski effect) Commercial product sold after initial light soaking ITO - TCO p-i-n a-si p-i-n a-si(ge), 20% p-i-n a-si(ge), 40% stainless steel 5 mils

27 SUMMARY Measurements Solar cell is basically a p/n diode IV curve measurement both dark and illuminated Requires 4-wire measurement Spectral fidelity dictates complexity of simulator technology Spectral response Illumination source with monochromator Appropriate light and electrical biasing for MJ measurement PV technology maturity - global warming, high oil prices, and PV technology maturity put us at a tipping point for a true PV revolution PV R&D areas Si is most mature and panel production is out-stripping wafer production capacity MJ concentrators are taking efficiencies to incredible levels (>40%!!) and offering a new facet to terrestrial power possibilities Thin film, mainly CIGS, experiencing tremendous growth in production capacity offering low cost modules, but technology development well below laboratory demonstrated efficiencies (11% compared to ~20%) Significant R&D in next generation technologies Intermediate band solar cells to increase photon to electrical conversion efficiency Nano-structures to extend spectral absorption range and increase carrier generation Organic solar cells to produce low cost, substrate-agnostic devices for ubiquitous application