Current Status of U.S. R&D

in PhotovoltaicsDr. Robert J. Walters

U.S. Naval Research LaboratoryHead, Solid State Devices Branch, Code 6810

Washington, DCphone: (202) 767-2533

cell: (703) 861-2484fax: (202) 404-7194

Email: Robert.Walters@nrl.navy.mil

Outline

• Introduction

• Overview of solar cell physics

• Discussion of standard solar cell characterization methodologies

• Highlights of major PV R&D areas

Introduction• My R&D focuses on space power, while this

symposium is focused on terrestrial power• I am Technical Program Chair for the 34th IEEE

Photovoltaic Specialist Conference (PVSC)– To be held June 2009, Philadelphia, PA (www.34pvsc.org) – 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

Photovoltaic Effect in a p-n Junction

• 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

Ef

Ec

Ev

n-side p-side

(-)(+)

back metal

n-type

p-type

anti-reflective coating

Top metal

substrate

Top metal

Top metal

grid

Solar Cell Electrical Measurements• In the dark, a solar cell is simply a diode

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Voltage (V)

10-6

10-5

10-4

10-3

10-2

10-1

Current (A

/cm

2)

Measured Data

Shunt Current

Recombination Current

Diffusion Current

Total Current

I(V) = Ishunt(Rsh) + Irec(I02,Et, p/ n) + Idiff(I01)

V A

• Need 4-wire measurement

• Typically achieved with a source-measure-unit (SMU) set to remote sense

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

0.00.10.20.30.40.50.60.70.80.91.0

Depth into Sample (m)

-150

-130

-110

-90

-70

-50

-30

-10

10

30

50

Current (mA))

Dark current

Under illumination

current at

short circuit, Isc

voltage at

open circuit, Voc

maximum

power, Pmp

Isc 34.39 mA/cm2

Voc 0.873 V

Pmp 25.08 mW/cm2

FF 0.835

Fill FactorFF = Pmp/(Isc*Voc)

V A

• Need 4-wire measurement

• Typically achieved with a source-measure-unit (SMU) set to remote sense

Solar Spectra

• Amount of atmospheric absorption referred to as Air Mass (AM)– Space is zero: AM0, 136.7 mW/cm2

– Terrestrial varies with geo-location, generally accepted calibration value is AM1.5 • Global – 100 mW/cm2

• Direct – 769 mW/cm2

0

500

1000

1500

2000

250 750 1250 1750 2250 2750

Inte

nsi

ty (

mW

/(m

2 n

m))

Wavelength (nm)

AM0

AM 1.5 - DIRECT

AM 1.5 - GLOBAL

Simulated Solar Spectra

• 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)

0

500

1000

1500

2000

250 750 1250 1750 2250 2750

Inte

nsi

ty (

mW

/(m

2 n

m))

Wavelength (nm)

AM0

AM 1.5 - DIRECT

AM 1.5 - GLOBAL

Xenon Lamp Simulator

Test Plane

Solar Simulator

Simulated Solar Spectra

• 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

0

500

1000

1500

2000

250 750 1250 1750 2250 2750

Inte

nsi

ty (

mW

/(m

2 n

m))

Wavelength (nm)

AM0

AM 1.5 - DIRECT

AM 1.5 - GLOBAL

Multi-zone Simulator

3 Xe bulbs for 280-800nm

2 incandescent bulbs for 1150-3000 nm

2 incandescent bulbs for 800-1150 nm

Sample plane

Calibrating Solar Cell Intensity

• 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.

Test Cell Area Various calibration cells

Calibration solar cellsSolarimeter

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)

200400600800100012001400160018002000

Wavelength (m)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

External QE InGaP

GaAs

Ge

Quantum Efficiency Measurements

Xenon Lamp

Monochromator

Sample Plane

Xenon LampFor light bias

Pyro-electric detector

Lock-in amp

Power supply for electrical bias

200400600800100012001400160018002000

Wavelength (m)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

External QE InGaP

GaAs

Ge

11%

12%

12%21%

12%

9%

10% 8%5%

32nd PVSC/4th World Conference Paper Distribution

1 Novel Materials

2 Thin Films

3 III-Vs

4 Si

5 Amorphous

6 Space

7 Modules

8 Systems

9 Programs

R&D Technology Areas• 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

20%

15%

11%19%

12%

8%15%

33rd PVSC Paper Distribution

Area 1

Area 2

Area 3

Area 4

Area 5

Area 6

Area 7

Solar Cell Technolgies

• Borrowed from Tim Anderson, Univ. of FL

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

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

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).

Crystalline Si Technology

• PV began with Vanguard at ~3% eff.

• Evolved to present <20% eff. in production– Sunpower, Sanyo, Sharp, BP, …

• Si wafer production capacity serious concern

Sunpower rear contact Si solar cell designC. Z. Zhou, 26th PVSC; Sept. 30-0ct. 3,1997; Anaheim, CA

50 yr Anniversary

~9% to <20% in 50yrs

Multi-junction Solar Cells• 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

AM0 Solar Spectrum

(1350 W/m2)

2-2.5eV

1.4-2eV

1-1.4eV

0.68-1.1eV

Theoretical calculations for 2-junction cells

1.11.21.31.41.51.61.71.81.92.02.12.2

Top Cell Bandgap (eV)

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

Bottom Cell Bandgap (eV)InGaP/GaAs (26%)

InGaP/InGaAs (32%)

20 - 23

23 - 26

26 - 29

29 - 32

32 - 35

AM0 Efficiency (%)

200400600800100012001400160018002000

Wavelength (m)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

External QE InGaP

GaAs

Ge

Multi-junction Solar Cells under Concentration

• 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., 33rd PVSC San Diego, CA 2008

Incident light is focused onto a small solar cell

Frank Dimroth, Fraunhofer ISE

Next MJ Technology – Metamorphic

• 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

Geisz et al., Appl. Phys. Lett. 93, 123505 2008

Novel Inorganic Materials – Intermediate Band• Major emphasis on intermediate-

band (IB) cells

– Allows for two, sub-band-gap photon absorption

– Calculations show efficiency exceeding ideal limits (>~60%)

– IB achieved via introduction of quantum-dots

L. Cuadra, A. Marti, A. Luque, Physica E 14 , 162 (2002)

(a) Energy gaps of the conventional solar cell(b) The intermediate band solar cell

(c) Energy band diagram of a row of dots(d) Resulting energy band diagram in equilibrium.

A.G. Norman, M.C. Hanna, P. Dippo, D.H. Levi, R.C. Reedy, J.S. Ward, and M.M. Al-Jassim , 31st IEEE PVSC, January 3–7, 2005

Novel Inorganic Materials – Nano-structures

InP substrate

n-InP

p-InP

InAsP MQW Region

Potential QE for single junction solar cell ~ 65% because of multi-exiton-generation

No MEG

MEG

Nano-crystals absorption controlled by size

Schaller et al Nanoletters 6, 424(2006)

• 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

Simplified schematic of nano-crystal PV device

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

Thin-Film Technologies – CuIn(Ga)Se2

• Thin film PV technologies promises low-cost fabrication on wide range of substrates• CuIn(Ga)Se2 (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

Front Contact

ZnO WindowCdS

CIGS Absorber

Back Contact

Glass Substrate

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

p-i-n a-Si(Ge), 40%

stainless steel – 5 mils

p-i-n a-Si(Ge), 20%

p-i-n a-Si

ITO - TCO

http://www.uni-solar.com/

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