Novel Semiconductor Materials for High-Efficiency ...

1 R. R. King, University Seminar, 2014

Novel Semiconductor Materials for High-Efficiency Multijunction Photovoltaics

Richard R. King

Spectrolab, Inc. Sylmar, CA


Novel Semiconductor Materials for High-Efficiency Multijunction Photovoltaics

Richard R. King

Spectrolab, Inc. Sylmar, CA

Seminar University of California, Santa Barbara

Jan. 24, 2014

3 R. R. King, UCSB Seminar, Jan. 24, 2014

• Manuel Romero, Sarah Kurtz, Dan Friedman, Daryl Myers, Tom Moriarty, Keith Emery – NREL • Angus Rockett – Univ. of Illinois • Gerald Siefer – CalLab, Fraunhofer ISE • Geoff Kinsey – Fraunhofer CSE • Rosina Bierbaum – Univ. of Michigan • Russ Jones, Jim Ermer, Chris Fetzer, Abdallah Zakaria, Xing-Quan Liu, Daniel Law, Philip Chiu, Shoghig Mesropian, Xiaogang Bai, Dimitri Krut, Kent Barbour, Mark Takahashi, Andrey Masalykin, John Frost, Nasser Karam ...and the entire multijunction solar cell team at Spectrolab

Acknowledgments


• Big Picture

• Solar cell efficiency limits

• Unifying behavior in semiconductor energy levels

• Electronic activity of defects in different semiconductor families

• Multijunction solar cells and concentrator photovoltaics (CPV)

Outline

0

100

200

300

400

500

600

700

0 0.5 1 1.5 2 2.5 3 3.5 4Photon Energy (eV)

Inte

nsity

per

Uni

t Pho

ton

Ener

gy(W

/m 2

. eV

)

0

0.2

0.4

0.6

0.8

1

1.2

1.4

Phot

on u

tiliz

atio

n ef

ficie

ncy

.

AM1.5D, ASTM G173-03, 1000 W/m2Utilization efficiency of photon energy 1-junction cell 3-junction cell 6-junction cell


Big Picture


IPCC (2001) scenarios to 2100 IPCC (2001) scenarios to 2100

1000 years of Earth temperature history… and 100 years of projection

Climate Change – Temperature Anomaly by Year

• Fossil fuels are contributing to global climate change at alarming rate

• Further, dependence on imported fuels has a high toll in terms of political stability and national security

Rosina Bierbaum, Univ. of Michigan Intergovernmental Panel on Climate Change (IPCC)


Cogentrix, Alamosa, CO – 30 MW, III-V multijunction cells

Courtesy Amonix

Concentrator Photovoltaics (CPV) using high-efficiency III-V

3-junction solar cells

• Photovoltaic solar electricity combined with...

– power storage in plug-in hybrid vehicles

– long-distance power transmission from sunny locales to high-demand areas

offer a major part of a solution to these problems


Solar cell efficiency limits


Detailed Balance Limit of Solar Cell Efficiency

• 30% efficient single-gap solar cell at one sun, for 1 e-/photon

• 44% ultimate efficiency for device with single cutoff energy

Shockley and Queisser (1961)

Single- junction solar cell


0

100

200

300

400

500

600

700

0 0.5 1 1.5 2 2.5 3 3.5 4Photon Energy (eV)

Inte

nsity

per

Uni

t Pho

ton

Ener

gy(W

/m 2

. eV

)

0

0.2

0.4

0.6

0.8

1

1.2

1.4

Phot

on u

tiliz

atio

n ef

ficie

ncy

.

AM1.5D, ASTM G173-03, 1000 W/m2Utilization efficiency of photon energy to bandgap Eg to Voc at 1000 suns to Voc at 1 sun

Photon Utilization Efficiency 1-Junction Solar Cells


• Assumptions in Shockley and Quiesser (1961)

• Viewed from a different angle, these limitations represent opportunities for higher efficiency devices

Assumption limiting solar cell efficiency Device principle overcoming this limitation Single band gap energy Multijunction solar cells

Quantum well, quantum dot solar cells One e--h+ pair per photon Down conversion

Multiple exciton generation Avalanche multiplication

Non-use of sub-band-gap photons Up conversion

Single population of each charge carrier type Hot carrier solar cells Intermediate-band solar cells Quantum well, quantum dot solar cells

One-sun incident intensity Concentrator solar cells

Assumptions → Opportunities


C. H. Henry (1980) Theoretical Efficiency of Multijunction Solar Cells

3-junction solar cell

Theo. efficiency at 1000 suns 1J: 37% 2J: 50% 3J: 56% 36J: 72%


0

100

200

300

400

500

600

700

0 0.5 1 1.5 2 2.5 3 3.5 4Photon Energy (eV)

Inte

nsity

per

Uni

t Pho

ton

Ener

gy(W

/m 2

. eV

)

0

0.2

0.4

0.6

0.8

1

1.2

1.4

Phot

on u

tiliz

atio

n ef

ficie

ncy

.

AM1.5D, ASTM G173-03, 1000 W/m2Utilization efficiency of photon energy 1-junction cell 3-junction cell



Unifying behavior in semiconductor energy levels


Unifying trends of defect energy levels in different semiconductors

• A number of phenomena have been observed indicating the tendency for some defect levels to form at a nearly constant energy in different semiconductors with respect to the vacuum level

Universal alignment of hydrogen levels in semiconductors Van de Walle and Neugebauer, Nature (2003)

Bulk reference level in Cu(GaxIn1-x)(SySe1-y)2 semiconductors Turcu, Kötschau, and Rau,, J. Appl. Phys. (2002)

Fermi-level stabilization energy Walukiewicz, Phys. Rev. B (1988)

• These observations indicate a common defect configuration at the atomic scale may be responsible for the near constancy of energy level in different semiconductors

• The position of common defect energies has the power to explain and predict doping properties and defect recombination activity – in the same semiconductor family ( e.g., CuInSe2 and Cu(GaIn)(SSe)2 ) – and in different families ( e.g., Cu(GaIn)(SSe)2 and GaInN )


Hydrogen energy levels in different host semiconductors

• Hydrogen energy level (red bars) is at nearly constant level with respect to vacuum level when incorporated into a wide variety of semiconductors

• Applies to group-IV, III-V, and II-VI semiconductors, and even to electrolytes Van de Walle and Neugebauer, Nature, 2003



• Questions: → To what extent and in what circumstances do the energy levels

of certain defects mirror the constancy of hydrogen atom states in different semiconductors?

→ Does this behavior establish the tendency for some types of defects to form near a specified energy from the vacuum level?


Admittance Spectroscopy

• Changes in capacitance with frequency and temperature → extraction of defect energies and densities through admittance spectroscopy

• Leads to a clear picture of the evolution of spontaneously forming defect energies in Cu(GaxIn1-x)(SySe1-y)2 chalcopyrites with changing S and Ga composition

Turcu, Kötschau, Rau, JAP, 2002


Similarity of defect energy levels in different chalcopyrites

• Trap energies due to native defects in Cu(GaxIn1-x)(SySe1-y)2 are approximately constant with respect to group-VI composition in semiconductor

• Trap energies are closer to midgap for higher bandgap compositions → leads to higher recombination at higher Eg

→ challenge for finding high Eg top cell material for chalcopyrite-based multijunction cell Turcu, Kötschau, Rau, JAP, 2002



• Questions: → To what extent and in what circumstances do the energy levels

of certain defects mirror the constancy of hydrogen atom states in different semiconductors?

→ Does this behavior establish the tendency for some types of defects to form near a specified energy from the vacuum level?

→ To what degree does the reference bulk defect energy level observed in I-III-VI chalcopyrites extend to other semiconductor families?

→ What does this behavior say about the recombination at defects in different host semiconductors, especially the low recombination activity observed in materials such as CuInSe2 and GaInN?


Fermi-level stabilization energy

• Fermi level stabilizes at nearly constant level with respect to vacuum level in a wide variety of semiconductors and semiconductor families → Both for damage induced by radiation and at semiconductor surfaces → Close to energy of hydrogen incorporated in semiconductors

• Fermi-level stabilization energy EFS is near midgap in GaAs and GaInP, but near conduction band in GaInN → reduces recombination activity of states at EFS as In content goes up

Walukiewicz, PRB, 1988; Li et al., PRB, 2005


Experimental and Theoretical Bandgap-Voltage Offset Woc

0.0

0.5

1.0

1.5

2.0

0.6 1 1.4 1.8 2.2Band Gap Eg (eV)

E g/q

, Voc

, and

(Eg/q

) - V

oc (

V)measured Vocmeas. Eg from EQEWoc = (Eg/q) - Vocradiative recomb. onlydetailed balance model

d-A

lGaI

nP

GaA

s1.

4 - e

V G

aInA

s

o-G

aInP

AlG

aInA

s

d-A

lGaI

nP d

-GaI

nP

d-A

lGaI

nP

0.97

-eV

GaI

nAs

GaI

nNA

s

1.10

-eV

GaI

nAs

1.24

-eV

GaI

nAs

1.30

-eV

GaI

nAs

Ge

(ind

irect

gap

)

AlG

aInA

s

Voc and band gap-voltage offset Woc = (Eg/q) - Voc

of solar cells with wide range of band gaps

Si

(indi

rect

gap

)

0.79

-eV

GaI

nAs

• Difference between bandgap and steady-state quasi-Fermi level splitting (open-circuit voltage) in solar cells is strikingly similar across wide range of III-V and group-IV semiconductors (as well as II-VI, I-III-VI, and other classes of semiconductors)

Ref.: R. R. King et al., Prog. in PV, doi: 10.1002/pip.1044 (2010)


Semiconductor Eqns. Formulated in Terms of Band Gap-Voltage Offset W

• Woc formulation has more physical basis, related to NC , NV rather than ni2

• Far more invariant with respect to Eg , good for multiple subcells in MJ cells

• Makes Woc a convenient indicator of solar cell quality – amount of SRH recombination vs. radiative recombination – for wide range of Eg

• Ko ≡ Jo / ni2 has nearly all band gap dependence taken out

kTqVi enpn /2=

= 2ln

inpn

qkTV

kTqWVC eNNpn /−=

( ) VqEW g −≡

=

pnNN

qkTW VCln

kTqW

VC

ph

i

oo

oceNN

JnJK /

2 =≡kTqVpho

oceJJ /−=



• Each of these observations reveals a profound, unifying aspect of the fundamental nature of these materials

• Connections between defect energies in very different semiconductor systems

→ may help identify fundamental principles behind the remarkably low defect recombination activity in some types of semiconductors, such as CuInSe2 and GaInN

• Finding answers to these questions will help us understand at a deeper level why we observe the semiconductor properties that we do


Electronic activity of defects in different

semiconductor families


Electronic activity of defects in different semiconductor families

• Semiconductor growth and characterization

• Some interesting materials systems – Metamorphic III-Vs

– Dilute nitride GaInNAs(Sb)

– Polycrystalline I-III-VIs and related materials Chalcopyrites (CuvAg1-v)(AlxGayIn1-x-y)(SzSe1-z)2 e.g., CIGS

Kesterite Cu2ZnSn(S,Se)4

– Perovskites CH3NH3Pb(X)3 where X = Cl, Br, I


Organometallic Vapor-Phase Epitaxy (OMVPE)

As

Arsine, AsH3

H

Hydrogen, H2

In

Trimethylindium TMIn, In(CH3)3

P

Phosphine, PH3 Diethyltelluride

DETe, Te(C2H5)2

Te

Ga

Trimethylgallium or TMGa, Ga(CH3)3

C

Typical Precursor Chemicals:

H2 + TMGa

H2 AsH3/Ph3

Sealed “Bubbler” containing MO source precursor

Chemical Reactor Chamber Low Pressure & High Temperature

Exhaust Gasses

Point Of Use Gas Scrubber

Scrubbed Hydrogen

Crystal growth on substrates on heated rotating graphite

H2


Molecular Beam Epitaxy


Selenization and Sulfidation CIGS and CZTS

www.unk.edu/nss/chemistry.aspx?id=41606

www.beltfurnaces.com/efficiency_of_CIGS.html

www.prweb.com/releases/Smit_ovens/CIGS/prweb2039344.htm

auo.com/print.php?sn=192&lang=en-US

onlin

elib

rary

.wile

y.co

m/d

oi/1

0.10

02/c

phc.

v14.

9/is

suet

oc


Recombination activity at defects

• Characterization of energy levels, capture cross sections, density, and recombination activity of defects: – deep-level transient spectroscopy (DLTS) – admittance spectroscopy, both in the dark and with photoexcitation – time-resolved photoluminescence (TRPL) – among others

• Atomic reconstruction and bonding configuration of defects: – transmission electron microscopy (TEM) – atom probe tomography (APT) – atomic force microscopy (AFM) – Fourier transform infrared spectroscopy (FTIR) – other imaging and characterization tools


Metamorphic III-Vs


Terrestrial Conc. Cell Designs from 40% to 50%

(a) (b) (c) (d) (e) (f) (g) (h) (i) (j)3J Lattice-

Matched (LM)C3MJ+

3J Meta-morphic (MM)low mismatch

C4MJ

3J Meta-morphic (MM)high mismatch

3J InvertedMetamorphic

(IMM)


4J Double-Grade InvertedMetamorphic

(MMX2)

5J Lattice-Matched (LM)

w. epitaxial Ge subcell

5J Lattice-Matched (LM)w. GaInNAsSb

subcell


SemiconductorBonded (SBT)

6J Triple-GradeInverted

Metamorphic(MMX3)

MJ Cell 39.42% 40.00% 40.54% 43.26% 44.44% 47.87% 43.25% 47.43% 47.64% 50.91% EfficiencyChange 0.0% 1.5% 2.8% 9.7% 12.7% 21.4% 9.7% 20.3% 20.9% 29.2% in Power from C3MJ+ Efficiencies for AM1.5D, ASTM G173-03 spectrum, 50.0 W/cm2 (500 suns), 25°C

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

Subc

ell B

and

Gap

s (e

V)

34%

36%

38%

40%

42%

44%

46%

48%

50%

52%

MJ

Cel

l Effi

cien

cy (

%)

C1 Eg

C2 Eg

C3 Eg

C4 Eg

C5 Eg

C6 Eg

MJ CellEfficiency

transparent buffer

1.35-eV GaInAs cell 2

1.83-eV GaInP cell 1

metal gridline

0.67-eV Ge cell 3and substrate

• Modeled production avg. efficiency of 40.0% at 500 suns (50.0 W/cm2)


• Low dislocation density in active cell layers in top portion of epilayer stack: ~ 2 x 105 cm-2 from EBIC and CL meas. • Dislocations confined to graded buffer layers in bottom portion of epilayer stack

GaInAs cap

GaInAs MC

GaInP TC

0.2 µm

Tunnel junction

Pre-grade buffer

Misfit dislocations

GaInAs graded buffer to 8%-In

0.2 µm

Ge substrate

Cross sectional TEM Ga0.44In0.56P/ Ga0.92In0.08As/ Ge

Cell


Line of 0%relaxation


Qy

(Stra

in) Å

-1

Qx (Tilt) Å-1

Ge

Ga0.92In0.08As MC

GaInP TC

GradedBuffer

(115) glancing exit XRD



Qy

(Stra

in) Å

-1

Qx (Tilt) Å-1

Ge

Ga0.92In0.08As MC

GaInP TC

GradedBuffer

(115) glancing exit XRD

• GaInP/ 8%-In GaInAs/ Ge metamorphic (MM) cell structure

• Nearly 100% relaxed step-graded buffer → removes driving force for dislocations to propagate into active cell layers

• 56%-In GaInP top cell pseudomorphic with respect to GaInAs middle cell

High-Resolution XRD Reciprocal Space Map (RSM)


0

10

20

30

40

50

60

70

80

90

100

300 400 500 600 700 800 900 1000 1100 1200 1300 1400Wavelength (nm)

Inte

rnal

Qua

ntum

Effi

cien

cy (%

)

0.96

eV1.40

1.08

1.26

1.38

1.30

GaInAs single-junction solar cells

2.4% lattice mismatch

1.6% lattice mismatch

Internal QE of Metamorphic GaInAs Cells on Ge

Metamorphic = "changed form"


1 µm 20 µm

Cathodoluminescence (CL)

disloc. density = 4.4 x 106 cm-2

Plan-View Transmission Electron Microscopy (TEM)

disloc. density = 3.1 x 106 cm-2

23%-In GaInAs double heterostructure on Ge

Dislocation Imaging in 23%-In GaInAs


Ge or GaAs substrateGe or GaAs substrate

nucleationnucleation

buffer layerbuffer layerbuffer layerbuffer layer

emitter

1.39-eV GaInAsinverted LM subcell

base

emitter

1.39-eV GaInAsinverted LM subcell

base

contactcontact

metalmetal

50 µm 8e-9766-1

1.39-eV ILM subcell GaInAs comp. 2% In Latt. mismatch 0.1% Disloc. density 2.5 x 105 cm-2

50 µm 8e-9756-1

50 µm 8e-9760-1

50 µm 8e-9783-11

EBIC images and dislocation density of inverted metamorphic cell test structures

1.39-eV GaInAs

1.10-eV IMM subcell 23% In 1.6% 3.9 x 106 cm-2



Ge substrateGe substrate

nucleation and pre-grade buffernucleation and pre-grade buffer

transparent MM transparent MM graded buffer layersgraded buffer layers


emitter

1.10-eV GaInAsinverted MM subcell

base

emitter

1.10-eV GaInAsinverted MM subcell

base

contactcontact

metalmetal

1.10-eV GaInAs





emitteremitter

0.970.97--eV GaInAseV GaInAsinverted MM subcellinverted MM subcell

basebase

emitteremitter


basebase

contactcontactcontactcontact

metalmetal

0.97-eV GaInAs





emitteremitter


basebase

emitteremitter


basebase

contactcontactcontactcontact

metalmetal

0.84-eV GaInAs

Dislocations in Inverted Metamorphic Cells – EBIC


0

1

2

3

4

5

6

7

8

9

0 10 20 30 40 50In Composition for GaxIn1-xAs (%)

Dis

loca

tion

Den

sity

from

EB

IC (

106 c

m-2

)

and

Pho

ton

Inte

nsity

from

CL

(10 3

cps

)

0

5

10

15

20

25

30

35

40

45

50

Car

rier L

oss

(%)

Dislocation density from EBICOverall % photon intensity from CL% carrier loss at each dislocation from CL

-0.07 0.64 1.36 2.07 2.79 3.51Lattice Mismatch Relative to Ge (%)

Inverted metamorphic (MM)GaxIn1-xAs solar cells

Dislocations in Inverted Metamorphic Cells

Ref.: R. R. King et al., 23rd European Photovoltaic Solar Energy Conf., Valencia, Spain, Sep. 2008.


1

10

100

1000

10000

0 100 200 300 400 500

Time (ns)

Phot

olum

ines

cenc

e In

tens

ity (

arb.

uni

ts)

τeff = 47 ns

AlGaInP/ GaInP/ AlGaInP double heterostructure

1

10

100

1000

0 2000 4000 6000 8000 10000

Time (ns)

Phot

olum

ines

cenc

e In

tens

ity (

arb.

uni

ts)

τeff = 2450 ns

GaInP/ GaInAs/ GaInP double heterostructure

• Double heterostructures grown with AlGaInP/GaInP and GaInP/GaInAs interfaces, in stack similar to MJ cells

• TRPL measurements at NREL

• Minority-carrier lifetime up to 2450 ns in 1%-In GaInAs on Ge substrate

Time-Resolved Photoluminescence:

GaInP- and GaInAs-base DHs


0.1

1

10

100

1000

10000

0 5 10 15 20 25 30 35Indium Mole Fraction of GaInAs Lattice-Matched to Base (%)

τ eff

Mea

sure

d by

TR

PL (

ns)

Base MaterialRecent data nid-GaInAs, recent data p-GaInP (disordered)Previous data nid-GaInAs nid-GaInP (ordered) nid-GaInP (disordered)

Eg = 1.407 eV

1.813 eV

1.887 eV 1.311 eV

1.736 eV

1.807 eV

1.114 eV

1.619 eV

0.994 eV

1.529 eV

Time-Resolved PL of LM & MM Double Heterostructures


Sublattice ordering


P Ga In

[001]

[100] [010]

(111) planes

all In

all Ga

all In

Group-III Sublattice Ordering

Ga0.5In0.5P fully ordered

(order parameter η = 1)

CuPtB ordering on [111] or [111] planes

In practice:

η = 0.4-0.5 , Eg ≈ 1.8 eV for GaInP lattice

matched to GaAs

Eg(η) = Eg(0) - (0.471 eV)η2


Ga0.5In0.5P fully disordered

(order parameter η = 0)

No CuPtB ordering

In practice: η = 0.0-0.1 , Eg ≈ 1.9 eV

for GaInP lattice matched to GaAs

Bandgap difference with respect

to ordered GaInP occurs in conduction band → ∆Ec

P Ga In

[001]

[100] [010]

(111) planes

random Ga & In random

Ga & In

random Ga & In

Group-III Sublattice Ordering


[110] cross-section of disordered GaInP epilayer showing [110]-oriented P dimers of the β(2 x 4) reconstruction. The stresses caused in in the growing crystal by surface phosphorus atoms provide the thermodynamic driving force for ordering.

Surface Reconstruction of GaInP


0

20

40

60

80

100

120

140

160

0 2 4 6 8 10 12 14Relative Omega, referenced to 1%-In GaInAs Peak (degrees)

1/2(

115)

XR

D In

tens

ity d

ue to

Gro

up-II

I O

rder

ing

in G

aInP

(co

unts

/s)

GaInP Ordering State andLattice Match to GaInAsordered, GaInP LM to 1%-In GaInAspartially disordered, "disordered, "ordered, GaInP LM to 8%-In GaInAsdisordered, "

-2 0 2

½(1

15) X

RD

Inte

nsity

Due

to G

roup

-III

Subl

attic

e O

rder

ing

in G

aInP

(co

unts

/s)

1.813 eV

1.867 eV 1.887 eV1.736 eV

1.807

-2 0 2 -2 0 2

Direct Meas. of GaInP Ordering from ½(115) XRD Peak


Dilute nitride GaInNAs(Sb)




Matched (LM)C3MJ+


C4MJ



(IMM)



(MMX2)




subcell




Metamorphic(MMX3)


0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

Subc

ell B

and

Gap

s (e

V)

34%

36%

38%

40%

42%

44%

46%

48%

50%

52%

MJ

Cel

l Effi

cien

cy (

%)

C1 Eg

C2 Eg

C3 Eg

C4 Eg

C5 Eg

C6 Eg

MJ CellEfficiency


1.12-eV GaInNAsSb cell 4


1.71-eV AlGaInAs cell 22.00-eV AlGaInP cell 1

metal gridline




• Two defect levels consistently seen in GaInNAs by DLTS

• Electron trap stays at 0.9-1.0 eV above valence band as N is added and Eg decreases

• Electron trap becomes quite shallow for N compositions near 1 eV

• 2nd defect level follows lowering of conduction band as N is added → stays near midgap and acts as effective recombination center

Ptak et al., NREL PRM, 2003



• Presence of H greatly decreases formation energy of (N-H-VGa)2- defect complex

• Acts as acceptor

• May be one of the main fundamental defects in GaInNAs(Sb)

• Positron annihilation spectroscopy shows increased VGa in MBE-grown nitride in the presence of H

Ptak et al., NREL PRM, 2003



• Quantum efficiencies well above 90% and comparable to those for GaAs can be achieved for a range of N compositions and bandgaps in GaInNAs(Sb) grown by MBE

• These current densities are high enough to contribute usefully to high-efficiency multijunction solar cells

Ptak et al., JAP, 2005



• Dilute nitride GaInNAs grown by MOVPE can have high measured quantum efficiencies

• Annealing and thermal history have a strong influence on the quality of dilute nitride GaInNAs(Sb)

Volz et al., JCG, 2008


• FTIR spectra of Ga0.975In0.025N0.002As0.998 (2.5% In) • N primarily bound to Ga in Ga4N configuration before anneal (467 cm-1) • Higher fraction of N bound to In in Ga3InN configuration after anneal → greater mass of In causes lower vibrational frequency signal (457 cm-1) • Evidence for H forming bonds with N in GaInNAs – change upon annealing may be due to change from NH to NH2 defect complex


• 0.2% N

• 2.5% In

Kurtz et al., APL, 2001



• Probability of N finding In-N nearest neighbor environment much higher for greater In compositions in GaInNAs

• Change in N bonding environment also thought to be cause of blueshift in fundamental bandgap of GaInNAs upon annealing → ~20 mV per additional indium atom in bonding configuration




• Chain-like columnar nitrogen ordering in the [001] direction causes local strain fluctuations as seen in strain sensitive (202) dark field TEM images

• Nitrogen chains are broken up resulting in more homogeneous strain in GaInNAs with increasing anneal schedule in images (a), (b), (c)



Chalcopyrites

Cu(In,Ga)Se2 (“CIGS”) and more


The CIGS Device

Light absorbing layer: a Cu(In,Ga)Se2 alloy Remarkably low recombination at extended defects Alternate window (emitter) layers available Back contact: nearly always Mo for stability

Angus Rockett – U. Illinois


TEM image Schematic

Devices are thought to be limited by recombination in the depletion region, not by heterojunction recombination.

• What is the major recombination center?

• What do grain boundaries do?

• Why does CuGaSe2 not work well?

• Why do some growth processes work better than others?

The Real Device



c

a

In

SeYet:

• Extended defects inactive

• Polar surfaces most stable

• Hole mobility phonon limited for p to >1019 cm-3

• Polycrystalline devices work better than single crystals

• Disordering energy is low so there are many point defects

• A polar compound so charged surfaces could be a problem

Chalcopyrite CIGS



• Layers facet spontaneously into polar (112) type planes

• Smooth facets alternate with rough facets

• Indexing surface planes shows smooth planes are metal terminated

(220)/(204) epitaxial layer AFM image

Red: metal terminated Blue: Se terminated

(112

) A (112)B

Conclusion: Somehow the polar surfaces are stabilized, giving a very strong preference for these.

(220)/(204) Oriented CIGS



CIGS solar cells • Are heterojunction devices with a very strongly inverted

junction (Cd doping overwhelms Fermi level pinning).

• Do not mind grain boundaries because they are highly faceted to extremely passive (112) surfaces.

• Heterojunction is made to these surfaces regardless of grain orientation.

• Point defects control doping in the bulk and are very consistent.

• Edge dislocations do not matter because they turn into (112) surfaces.

Conclusions of all of this…



Perovskites


Perovskite solar cells

• Present perovskite-based solar cells evolved from dye-sensitized solar cell (DSSC) technology

• Perovskite CH3NH3PbClxI3-x absorbers have very high absorption coefficients allowing thin, practical layers to be used

• Very simple processing



Snaith, JPCL, 2013

• Much of electron and hole transport can take place through the perovskite light absorber material itself, rather than through the porous TiO2 scaffold used in DSSCs

• Perovskite cells work even better with insulating porous Al2O3 scaffold, avoiding voltage loss of 0.2-0.3 V from lower conduction band of TiO2

• External quantum efficiency (incident photon-to-electron conversion efficiency, or IPCE) of two types of perovskite absorbers, with bandgaps of ~2.2 and 1.55 eV

• >12% 1-sun eff. using porous Al2O3 scaffold, 15% with vapor-deposited perovskite



Snaith, JPCL, 2013

• High Eg of perovskites (2.2 eV for CH3NH3PbBr3 to 1.55 eV for CH3NH3PbI3 ) a good match for top cell of flat-plate, one-sun multijunction with silicon, Cu(GaxIn1-x)(SySe1-y)2 or kesterite Cu2ZnSn(S,Se)4 bottom cell


Multijunction solar cells and concentrator

photovoltaics (CPV)


Lattice-Matched and Metamorphic Cell Structure


Record efficiency III-V multijunction solar cells

68 R. R. King, UCSB Seminar, Jan. 24, 2014 Concentrator cell light I-V and efficiency independently verified by J. Kiehl, T. Moriarty, K. Emery – NREL

• First solar cell of any type to reach over 40% efficiency

Spectrolab Metamorphic GaInP/ GaInAs/ Ge Cell

Voc = 2.911 V Jsc = 3.832 A/cm2

FF = 87.50% Vmp = 2.589 V

Efficiency = 40.7% ± 2.4%

240 suns (24.0 W/cm2) intensity 0.2669 cm2 designated area 25 ± 1°C, AM1.5D, low-AOD spectrum Ref.: R. R. King et al., "40% efficient

metamorphic GaInP / GaInAs / Ge multijunction solar cells," Appl. Phys. Lett., 90, 183516, 4 May 2007.

Record 40.7%-Efficient Concentrator Solar Cell

• Efficiencies have now reached 41.6% for both metamorphic and lattice-matched 3-junction cells


External QE of LM and MM 3-Junction Cells



Solar Junction 3-junction cell with dilute nitride bottom cell


3-junction record efficiency cell with dilute nitride bottom cell

Solar Junction 44.0% eff. AM1.5D under concentration NREL confirmed

~1-eV GaInNAs(Sb) cell 3

1.42-eV GaAs cell 2

1.89-eV GaInP cell 1

metal gridline

www.semiconductor-today.com/news_items/2012/OCT/SOLARJUNCTION_151012.html



Soitec/Fraunhofer ISE 4-junction semiconductor bonded cell


4-junction record efficiency semiconductor bonded cell

Fraunhofer ISE/ Soitec 44.7% eff. AM1.5D under concentration Fraunhofer ISE confirmed

GaInAs cell 4

GaInPAs cell 3

GaAs cell 2(Al)GaInP cell 1

metal gridline

bondedinterface

www.ise.fraunhofer.de/en/press-and-media/press-releases/presseinformationen-2013/world-record-solar-cell-with-44.7-efficiency


Record Efficiency One-Sun SBT 5-Junction Cell

D. Law, P. Chiu et al., to be published

38.8%

Spectrolab 5-junction semiconductor bonded cell, 1-sun, AM1.5G


5-junction record 1-sun efficiency semiconductor

bonded cell

Spectrolab 38.8% eff. AM1.5G, 1-sun cell NREL confirmed


1.1-eV GaInPAs cell 4

1.7-eV AlGaInAs cell 2


2.0-eV AlGaInP cell 1

metal gridline

semiconductorbondedinterface


InP growth substrate GaAs or Ge growth substrate



2.0-eV AlGaInP cell 1 0.75-eV GaInAs cell 5


GaAs or Ge growth substrate












semi-conductor

bonded interface

metal gridline






semi-conductor

bonded interface

metal gridline

– Both high-bandgap and low-bandgap cell sets use high-quality, lattice-matched materials

– Atomically abrupt semiconductor bonded interface

– Both small-lattice (GaAs) and large-lattice (InP) growth substrates can be reused after substrate removal

• Direct semiconductor bonding for multijunction solar cells

Semiconductor-Bonded Technology (SBT) Terrestrial Concentrator Cell


• Fabricated “preliminary” SBT AM1.5G cell under IR&D leveraging knowledge from SBT space cells.

• Spectrolab’s SBT terrestrial cell achieved efficiency of 37.8%, 1-sun, AM1.5G (then world record) in April 2013.

• Recently achieved new world record efficiency of 38.8%, 1-sun, AM1.5G in August 2013

• Highest efficiency 1-sun terrestrial solar cell of any type.

• Expect Eff. > 47% at moderate concentrations

Record Efficiency = 38.83%

Record Efficiency One-Sun SBT 5-Junction Cell

Chiu et al., PVSC, 2013




Matched (LM)C3MJ+


C4MJ



(IMM)



(MMX2)




subcell




Metamorphic(MMX3)


0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

Subc

ell B

and

Gap

s (e

V)

34%

36%

38%

40%

42%

44%

46%

48%

50%

52%

MJ

Cel

l Effi

cien

cy (

%)

C1 Eg

C2 Eg

C3 Eg

C4 Eg

C5 Eg

C6 Eg

MJ CellEfficiency


1.12-eV GaInNAsSb cell 4


1.71-eV AlGaInAs cell 22.00-eV AlGaInP cell 1

metal gridline



5-junction nitride solar cell measured light I-V

0

2

4

6

8

10

12

14

0 1 2 3 4 5

Voltage (V)

Cur

rent

Den

sity

(m

A/c

m2 )

4J, no nitride, EQE Jsc, no AR 20.2%

4J, no nitride, IQE Jsc 29.8% eff.

5J nitride, EQE Jsc, no AR 22.2%

5J nitride, IQE Jsc 32.4% eff.

AM0 efficiency projected from QP cells, no AR

• Addition of dilute nitride GaInNAs cell to 5-junction stack adds ~400 mV open-circuit voltage to cell at one sun


5-junction nitride solar cell measured quantum efficiency

0

10

20

30

40

50

60

70

80

90

100

300 500 700 900 1100 1300 1500 1700 1900Wavelength (nm)

Qua

ntum

Eff

icie

ncy

(%)

12.312.0 12.1 13.3 17.6 mA/cm2

IQE




Matched (LM)C3MJ+


C4MJ



(IMM)



(MMX2)




subcell




Metamorphic(MMX3)


0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

Subc

ell B

and

Gap

s (e

V)

34%

36%

38%

40%

42%

44%

46%

48%

50%

52%

MJ

Cel

l Effi

cien

cy (

%)

C1 Eg

C2 Eg

C3 Eg

C4 Eg

C5 Eg

C6 Eg

MJ CellEfficiency

transparent buffer


0.97-eV GaInAs cell 5transparent buffer

1.20-eV GaInAs cell 4transparent buffer

1.77-eV AlGaAs cell 21.465-eV AlGaAs cell 3


metal gridline

Ge or GaAs

growth substrate



0

100

200

300

400

500

600

700

0 0.5 1 1.5 2 2.5 3 3.5 4Photon Energy (eV)

Inte

nsity

per

Uni

t Pho

ton

Ener

gy(W

/m 2

. eV

)

0

0.2

0.4

0.6

0.8

1

1.2

1.4

Phot

on u

tiliz

atio

n ef

ficie

ncy

.

AM1.5D, ASTM G173-03, 1000 W/m2Utilization efficiency of photon energy 1-junction cell 3-junction cell



0

100

200

300

400

500

600

700

0 0.5 1 1.5 2 2.5 3 3.5 4Photon Energy (eV)

Inte

nsity

per

Uni

t Pho

ton

Ener

gy(W

/m 2

. eV

)

0

0.2

0.4

0.6

0.8

1

1.2

1.4

Phot

on u

tiliz

atio

n ef

ficie

ncy

.

AM1.5D, ASTM G173-03, 1000 W/m2Utilization efficiency of photon energy 1-junction cell 3-junction cell 6-junction cell



0

20

40

60

80

100

0.5 1.0 1.5 2.0 2.5 3.0 3.5

Photon Energy (eV)

Qua

ntum

Effi

cien

cy (

%)

1.91-eV GaInP Cell 1 EQE

Quantum efficiency of subcells in 6-junction cell


0

20

40

60

80

100

0.5 1.0 1.5 2.0 2.5 3.0 3.5

Photon Energy (eV)

Qua

ntum

Effi

cien

cy (

%)

1.91-eV GaInP Cell 1 EQE1.81-eV GaInP Cell 2 EQE



0

20

40

60

80

100

0.5 1.0 1.5 2.0 2.5 3.0 3.5

Photon Energy (eV)

Qua

ntum

Effi

cien

cy (

%)

1.91-eV GaInP Cell 1 EQE1.81-eV GaInP Cell 2 EQE1.57-eV AlGaInAs Cell 3 EQE



0

20

40

60

80

100

0.5 1.0 1.5 2.0 2.5 3.0 3.5

Photon Energy (eV)

Qua

ntum

Effi

cien

cy (

%)

1.91-eV GaInP Cell 1 EQE1.81-eV GaInP Cell 2 EQE1.57-eV AlGaInAs Cell 3 EQE1.39-eV GaInAs Cell 4 EQE



0

20

40

60

80

100

0.5 1.0 1.5 2.0 2.5 3.0 3.5

Photon Energy (eV)

Qua

ntum

Effi

cien

cy (

%)

1.91-eV GaInP Cell 1 EQE1.81-eV GaInP Cell 2 EQE1.57-eV AlGaInAs Cell 3 EQE1.39-eV GaInAs Cell 4 EQE1.05-eV GaInNAs Cell 5 EQE



0

20

40

60

80

100

0.5 1.0 1.5 2.0 2.5 3.0 3.5

Photon Energy (eV)

Qua

ntum

Effi

cien

cy (

%)

1.91-eV GaInP Cell 1 EQE1.81-eV GaInP Cell 2 EQE1.57-eV AlGaInAs Cell 3 EQE1.39-eV GaInAs Cell 4 EQE1.05-eV GaInNAs Cell 5 EQE0.67-eV Ge Cell 6 EQE6J, cumulative EQE



MJ Solar Cell

Bypass Diode

Metallized Substrate

• Individual cells mounted on individual substrates, each with a bypass diode

• Suitable for point focus modules

Concentrator Cell Receivers


Case Study 1: Soitec Concentrix™ Technology

Courtesy Soitec


Soitec Concentrix™ Technology

• Concentration ratio ~500

• High efficiency cells based on III-V materials

• Fresnel lens as primary optics

Courtesy Soitec


Case Study 2: Solar Systems

Courtesy Solar Systems Pty Ltd, Australia

Dense array, active cooling


Solar Systems

Courtesy Solar Systems Pty Ltd, Australia, Photo: Pierre Verlinden


• Fossil fuels contributing to global climate change at alarming rate

• Multijunction cells break Shockley-Queisser single-junction efficiency limits

• Wide range of semiconductor bandgaps needed with low recombination

• Unifying behavior in semiconductor energy levels offers deeper experimental and theoretical understanding of universal patterns in formation of defect energy levels across broad classes of semiconductors

• Will provide framework to understand and better use inherently low defect recombination activity in certain polar covalent semiconductors, in families represented by CuInSe2, GaInN, and Cu2ZnSn(S,Se)4

• Novel semiconductor materials enable a zoo of new multijunction solar cells with efficiencies ranging over 50%

• Path to 50% efficiency promises to open wide geographic regions for cost-effective photovoltaics

• Efficiency advantage of 4, 5, and 6J cells outweighs the effect of variable spectrum on current balance

• New understanding of defect structure in semiconductor families such as chalcopyrite, kesterite, and perovskite materials will enable advances in wide-bandgap top cells for flat-plate multijunction cells, bringing together the high efficiency of multijunctions with thin-film technology for low-cost solar electricity

Summary and Future Prospects

Novel Semiconductor Materials for High-Efficiency ...

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