Nanoscale Energy Conversion in the Quantum Well Solar Cell

Keith Barnham, Ian Ballard, Amanda Chatten, Dan Farrell,Markus Fuhrer, Andreas Ioannides, David Johnson,

Marianne Lynch, Massimo Mazzer, Tom Tibbits Experimental Solid State Physics, Imperial College London, London

SW7 2BW, UK k.barnham@ic.ac.uk http://www.sc.ic.ac.uk/~q_pv

Rob Airey, Geoff Hill, John Roberts, Cath Calder, EPSRC National Centre for III-V Technology, Sheffield S1 3JD, UK

Solarstructure , Permasteelisa, FULLSPECTRUM EU Framework VI,

Outline

First practical nanoscale photovoltaic cell

Enhanced spectral range of the strain-balanced quantum well solar cell (SB-QWSC)

Efficiency enhancement by photon recycling

Evidence for hot electron effects in the QW

Cell efficiency cell versus or Eg

GaAs cells - highest effic. single junction cells, Eg too high

lower Eg => higher efficiency

Can grow InyGa1-yAs bulk cells on virtual substrates but never dislocation free

Maximum at 1.1 m ~ 1.1 eV

Multi-junction cells need 4th band-gap ~ 1.1 m ~ 1.1 eV

Enhancing GaAs Cell Efficiency

From 30x – 1000x AM1.5 optimum single junction efficiency band-gap ~ 1.1 eV

Multi-junction approaches going for GaInNAs cell

No ternary alloy with lower Eg

than GaAs lattice matched to GaAs/Ge

GaAs1-yPy (y ~ 0.1) + InxGa1-xAs, (x~ 0.1 – 0.2)strain-balanced to GaAs/Ge => novel PV material

GaAsP/InGaAs Strain-Balanced QWSC

Advantages:

Can vary absorption band- edge and absorb wider spectral range without strain-relaxation

no dislocations > 65 wells

single junction with wide spectral range

ability to vary Eg gives higher tandem effic.

Balance stress between layers to match lattice parameter of the

substrate

SB-QWSC – Ideal Dark-Currents at High Concentration

Dark current of 50 well QWSC

Low current fits one parameter Shockley-Read-Hall model

High (concentrator) current slope changes

ideal Shockley current

+ radiative recombination in QW

Minimum recombination radiative at concentrator current levels

Investigation of Photon Cavity Effects 50 well SB- QWSC In0.1Ga0.9As wells

GaAs0.91P0.09 barriers

Control and distributed

Bragg reflector (DBR)

devices grown

side-by-side

n-Substra te

n-G aAs

p-G aAs

w-A lG aAs

MQW

BSF

Ta2O5 / SiNX

n-Substra te

DBR

Processed as concentrator, fully metalised, and photodiode devices 11 finger concentrator mask, 3.6% shading

n-G aAs

p-G aAs

w-A lG aAs

MQW

Ta2O5 / SiNX

BSF

400 500 600 700 800 900 10000

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100

0

20

40

60

80

100

Inte

rnal

qua

ntum

eff

icie

ncy

(%)

Wavelength (nm)

Ref

lect

ivity

(%

)

400 500 600 700 800 900 10000

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40

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100

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20

40

60

80

100

Inte

rnal

qua

ntum

eff

icie

ncy

(%)

Wavelength (nm)

Ref

lect

ivity

(%

)

400 500 600 700 800 900 10000

20

40

60

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100

0

20

40

60

80

100

DBR IQE Non-DBR IQE

Inte

rnal

qua

ntum

eff

icie

ncy

(%)

Wavelength (nm)

DBR reflectivity

Ref

lect

ivity

(%

)

Increase photon

absorption Increase photocurrent No series resistance In-situ growth

Distributed Bragg Reflectors

[3] D.C. Johnson et al. Solar Energy

Materials and Solar Cells, 2005

JSC (mA/cm2)

Device AM1.5d 1000W/m2 AOD 913W/m2

Non-DBR

28.0 26.3

DBR 28.6 26.9

Concentrator Measurements27% efficiency at 328x

low-AOD spectrumSingle junction record is

(27.6 +/-1)% at 255x

[3] Vernon S.M., et al. “High-efficiency concentrator cells from GaAs on Si”, 22nd IEEE PVSC 1991 pp53–35

Efficiency increase higher than expect from double pass in QWs

Enhanced Voc

10 10022

23

24

25

26 Non-DBR DBR

Eff

icie

ncy

(%)

Concentration (suns)

AM1.5d 1000W/m2

D.Johnson et al. WCPEC4, Hawaii May 06

Why the Efficiency Enhancement?Aim of DBR was to absorb photons on second pass

Some photons from radiative recombination at high bias trapped in the device

MQW

DBR

MQW

DBR

Photons reabsorbed in the QWs reduce dark current

Generalised Plank model for EL shows reduction consistent with dark current suppression

Photon recycling could take cell to 30% efficiency

820 840 860 880 900 920 940 960 980

0.1

1

Lum

ines

cenc

e (a

.u.)

Wavelength (nm)

0.92V < Vapp

< 0.98V

Single QW Electroluminescence low bias

Bulk Well

820 840 860 880 900 920 940 960 980

0.1

1

Lu

min

esc

en

ce (

a.u

.)

Wavelength (nm)

Vapp

= 1.00VV

app= 1.10V

0.92V < Vapp

< 0.98V

Single QW EL at high bias

Bulk Well

840 850 860 870 880 890 900 910 920 930 940 950 960 970 980 990

0.1

1

Lu

min

esc

en

ce (

a.u

.)

wavelength (nm)

0.84V < Vapp

< 1.02

10 QW Electroluminescence low bias

Bulk

Well

840 850 860 870 880 890 900 910 920 930 940 950 960 970 980 990

0.1

1

Lu

min

esc

en

ce (

a.u

.)

wavelength (nm)

0.84V < Vapp

< 1.02

Vapp

= 1.16VV

app= 1.04V

10 QW EL at high bias

Bulk

Well

Model EL (radiative recombination)

Detailed Balance leads to generalised Planck:1

(E) (use measured QE) and T determine shape

EF requires absolute calibration

L(E,F)dE 2n2LW

h3c 2

(E)E 2

e(E EF ) kBT 1dE

(E) = absorption coefficientT = temperature of recombining carriers EF = quasi-Fermi level separation

where

J.Nelson et al., J.Appl.Phys., 82, 6240, (1997)M.Fuhrer et at Proc. EU PVSEC Dresden,Sept 06

EL - model and experiment

data model

920 930 940 950 960 970 980 990

0.1

1

Lu

min

esc

en

ce (

a.u

.)

wavelength (nm)

increasing V

920 930 940 950 960 970 980

0.1

1

lum

ine

sce

nce

wavelength

T=300.0K T=320.0K T=340.0K T=360.0K T=380.0K

Increasing T

(nm)

(a.u

.)

EL - Bulk Peak

840 850 860 870 880 890

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.91

1.1

Lu

min

esc

en

ce (

a.u

.)

Wavelength (nm)

Fits T = 299 K

Conclusions SB-QWSC concentrator cells (near) highest efficiency and

widest spectral range of single junction cells

Radiative recombination dominates at high current levels and photon recycling observed with DBR

EL reduction with DBR consistent with dark-current

Evidence for hot carrier effects at high current levels in EL shape consistent with generalised Planck

These nanoscale properties occur at the high current levels to be expected in terrestrial concentrator systems

Advantages of the SB-QWSC

Approximately double the efficiency of current cellsWidest spectral range in a single junction cell so

keeps high efficiency as sunlight spectrum variesNano-scale effectss – photon cavity, hot electronsSmall size ~ mm – optoelectronic fabrication.Need high concentration to bring price down

What application?

Building integrated concentrator photovoltaics (BICPV)

Novel Application - Building Integrated Concentrators

SMART WINDOWS No transmission of direct sunlight

Reduce glare and a/c requirement

Max diffuse sunlight - for illumination

No need for lights when blinds working

(2 – 3) x power from Silicon BIPV

Electricity at time of peak demand

Cell cooling in frame - hot water Barnham, Mazzer, Clive, Nature Materials, 5, 161 (2006).

SB-QWSC - highest efficiency single junction cell, ~ 1mm size

UK – over 60% electricity used in buildings over 7 x as much solar energy falls on those buildings

0%

20%

40%

60%

80%

100%

120%

0 30 60 90 120 150 180 210 240 270 300 330 360

Day

1.2%

25.6%7.3%

8.6%

17.9%

9.3%2.1%

5.0%

23.1%

Space Heating

Water Heating

Cooking

Lighting

Cooling

Ventilation

Refrigeration

Office Equipment

Other

Electricity

100

110

120

130

140

150

160

170

180

1.0 10.0 100.0

Larger side/Smaller side

kWh

/m2

Calculated output : San Francisco

L 3L

6L

Fraction of electricity consumption provided by photovoltaic cells

Consumption = 145 kWh/m2

Savings

Average electricity generated by 1 m2 of façade over 1 year

Luminescent Concentrators for Diffuse Component of Sunlight

Dye-doped luminescent concentrators (1977): Advantages

no tracking requiredaccept diffuse sunlightstacks absorb different Eg ~ E, gives max. effic. thermalisation in sheet

Disadvantagesdyes degrade in sunlight loss from overlap of

absorption/luminescencenarrow absorption band

A Goetzberger and W Greubel, Appl. Phys. 14, 1977, p123.

Quantum Dot ConcentratorQDs replace dyes in

luminescent concentrators:

QDs degrade less in sunlight

core/shell dots high QE

absorption edge tuned by dot

size

absorption continuous to short

red-shift tuned by spread in dot

size

spread fixed by growth conditions

(K.Barnham et al. App. Phys.Lett.,75,4195,(2000))

Thermodynamic Model for QDC The brightness, B(, of a radiation field that is in equilibrium with electronic degrees of freedom of the absorbing species:

Applying the principle of detailed balance within the slab:

IC = concentrated radiation field, Qe = quantum efficiency, e = absorption

cross section

Extend to 3-D fluxes + boundary conditions

0 BQ

NdINdF

e

eCe

1

182

22

hec

nB

n = refractive index= 1/kT = chemical potential

I1()

z = 0

z = D

x

y

z

c

2

c

A.J.Chatten et al, 3rd WCPEC, Osaka, 2003 E Yablonovitch, J. Opt. Soc. Am. 70, 1362, 1980.

Characterisation of ZnS/CdSe QDs in Acrylic with Thermodynamic Model

SD387 Red SD396 yellow

Thermodynamic model fits PL shape and red-shift

of Nanoco QDs assuming only absorption cross sectionFitting current measured at cell on edge gives

Qe(SD387) = 0.56 (c.f. Nanoco 0.4 – 0.6)

Thermodynamic Model confirms unexpected luminescent stack result

Incident lightLayer Experimental Jsc

(mA/m2) Predicted

Jsc (mA/m2)

Top 10.2 ± 2.0 9.1 ± 2.1

Bottom 35.1 ± 2.0 37.9 ± 1.3

Incident lightLayer Experimental Jsc

(mA/m2) Predicted

Jsc (mA/m2)

top 47.5 ± 2.0 46.9 ± 2.1

Bottom 4.8 ± 2.0 3.8 ± 1.3

Total output = 45.3 (mA/m2)

Total output = 52.3 (mA/m2)

EL Modeling Confirms Recycling 50 QW dark current show 33% reduction of J01

Model EL by detailed balance ~ 30% reductionSupports efficiency increase results from photon recycling

1.32 1.34 1.36 1.38 1.40 1.42 1.44

1

10

Calculated Measured

Nor

mal

ised

em

issi

on (

a.u.

)

Energy (eV)

London – Vertical South - East Facing Wall

A tandem cell 13% more efficient than a SB-QWSC harvests only 3% more electrical energy

Compare SB-QWSC with Tandem in Smart Windows

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 28 56 84 112 140 168 196 224 252 280 308 336 364

Day

Po

we

r/(k

Wh

/m2 )

P.Tandem P.Single

Series current constraint means tandem optimised

for one spectral condition (and one temperature)

Single Molecule Precursor ZnS/CdSe Core-Shell QDs

Core shell ZnS/CdSe dots by thermolysis at 270 °C of single-molecule precursors

in PLMA using with TOPO cap

Luminescence fit

is two-flux thermodynamic model.

Currently part of “FULLSPECTRUM”

Framework VI Integrated Project

(T.Trindade et al. Chemistry of Materials, 9, 523, (1997)) (A.J.Chatten et al, Proc. 3rd WCPEC, Osaka, 2003)

absorption and luminescence of Nanoco OMN29 QDs

0

50

100

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300

1.90 2.10 2.30 2.50 2.70 2.90 3.10 3.30 3.50

E/eV

Abs

orpt

ion

/a.u

.

0

0.2

0.4

0.6

0.8

1

1.2

Lum

ines

cenc

e/a.

u.

Experimental absorption with a linear background subtracted

Gaussian used to fit absorption threshold

Absorption fit used in predicting the luminescence

Normalised predicted luminescence

Normalised experimental luminescence

Absorption and emission data from Sarah Gallagher

0.06% by mass QDs in chloroform

Pathlength 1cm

BICPV – Smart Windows

Transparent Fresnel Lenses(300 – 500)x concentration

1.5 or 2-axis tracking

Novel secondaries

~ 1 mm solar cells

Cell efficiency ~ 30%

Adds ~ 20% to façade cost

Heat

Diffuse Daylight

Diffuse Daylight

Solar Cells

Direct Sunlight

Lenses

Front Glass

ElectricityHeat

Diffuse Daylight

Diffuse Daylight

Solar Cells

Direct Sunlight

Lenses

Front Glass

Electricity