David Mitzi at BASF Science Symposium 2015

IBM Research

© 2012 IBM Corporation

150 Years BASFMarch 10, 2015

Current Status and Future Prospects of Earth-Abundant

Kesterite Photovoltaics

David MitziDuke University

IBM Research

© 2012 IBM Corporation

• Thin-film solar cells and kesterites• Solution processed Cu2ZnSn(S,Se)4 (CZTS) devices• Device/materials understanding and limitations• Concluding remarks and prospects for future

Goal: Low-cost high efficiency thin-film PV devices using abundant or readily-available elements

Commercial solar technologies

n-type

Photon

Built-in electric field

Conduction band

Fermi level

Valence band

e-

h+

p-type

~90% ofthe market

Growth in TF PV Led by First Solar and Solar Frontier

4.5GW of thin-film panels forecast to be shipped during 2015 CIGS and CdTe dominating growth

Accelerated Technology Roadmap Scenario Forecast by PV Technology Type

Two primary chalcogenide-based PV technologies:

Some issues:

1. Issues with heavy metalCd… Restricted in certain markets.

2. Use of scarce elements:In, Te

Advantages:

1. Direct band gap… Thinner absorber layer

2. Grain boundaries relatively benign

3. Compatible with monolithic integration

4. Flexible substrates possible.

Cadmium Telluride

back metal contact

CdTe (p-type)(3 to 10 μm thick)

CdS (n-type)TCO

glass substrate

Photons(sunlight)

CIGS= Cu(In,Ga)(S,Se)2

substrate

Metal (molybdenum)

CIS/CIGS (p-type)(1 to 5 μm thick)

CdS (n-type)TCO

encapsulant

Photons(sunlight)

Performance• >1GW cumulative production• 14% commercial modules• 21.7% laboratory cell

Major Players• Solar Frontier• Avancis • Global Solar

Performance• $<0.70/W as of Q2 2013• >7GW cumulative production• 14% commercial modules• 21.0% laboratory cell

Major Players• First Solar• PrimeStar Solar / GE • Calyxo

January 31, 2013

Cu(In,Ga)(S,Se)2 Cu2ZnSn(S,Se)4

Kesterite vs Chalcopyrite Structures:

3+ 4+

2+

CZTS… Progress up to 2009

• 1988: Ito and Nakazawa, PV effect demonstrated in CZTS thin film (In 1977 S.Wagner demonstrated PV effect in related Cu2CdSnS4

#)• 1996: First CZTS devices by sequential evaporation/sulfurization…0.66% efficiency*• 1996-2009: Steady progress in device performance for vacuum-deposited CZTS• 2008: CZTS device prepared by sputtering/sulfurization with 6.77% efficiency

# S. Wagner et. al., J. Cryst. Growth 39, 151 (1977)* H. Katagiri et. al., Thin Solid Films 517, 2455 (2009)

p-typeCu2ZnSn(S,Se)4

ITO + i-ZnO

Ni/Al contacts

n-type CdS

Mo

Glass

Mo

p-CZTSSe

n-CdS

i-ZnO

ITO

Ni/Al

Basic device structure:0.45 cm2 device area

Glass

Some issues for processing… Complex multi-element compound

with narrow stoichiometry range

Optimal composition is: [Cu]/([Zn]+[Sn]) ≈ 0.9 and [Zn]/[Sn] ≈ 1.2

Sn is volatile… Difficult to control stoichiometry

I.D. Olekseyuk et. al., J. Alloys and Compounds 368, 135 (2004)H. Katagiri et. al., Mater. Res. Symp. Proc. 1165, M04-01 (2009)

SnS2

ZnSCu2S

A. Weber et. al., J. Appl. Phys. 107, 013516 (2010)

kesterite(i.e., CZTSSe)

(I4)

S, Se

Cu

ZnSn

Why solution processing?

Solution/Suspension

Spin Coating

Anneal

Doctor blade

Slit Casting

• No vacuum required; No high temperature sources; High throughput (>1000m/min for flexible substrates established)

Spray coating

How to get CZTS into solution?

T. K. Todorov et. al., Adv. Mater. 22, E156 (2010)

Selected Properties of N2H4:• Basic; Powerful reducing agent

• Good solubility of S and Se, as well as manymetal chalcogenides… Not Zn(S,Se)!

• Cleanly decomposes into N2 + H2 + NH3

• Hygroscopic; absorbs CO2 and O2 from atmosphere inert atm. processing

• highly toxic / combustible safety!

Hybrid particle/solution hydrazine-based deposition

MnXmN2H4 + X

N2H5+

N2H5+

N2H5+

MX-M = Cu, Sn X = S, Se

Zn add as powder

ZnX

MX-MX-

ZnX

ZnX

Particle-based slurry

MnXmN2H4 + X

N2H5+

N2H5+

N2H5+

MX-M = Cu, Sn X = S, Se

Zn add as powder

ZnX

MX-MX-

ZnX

ZnX


Particle-based slurry


Filtered particle component

N2H4ZnX (X=S, Se)

MnXmN2H4 + X

N2H5+

N2H5+

N2H5+

MX-

dry

heatX + N2H4decomp. products

M = Cu, Sn X = S, Se

Zn add as powder

ZnX

MX-MX-

ZnX

ZnX

heat

Cu2ZnSn(S,Se)4



Record CZTS device from 2009:

CZTSSe

Efficiency = 9.7 %Voc = 0.516 VJsc = 28.6 mA/cm2

Fill Factor = 65.4%Area = 0.44 cm2

Eg = 1.19 eV

Certified by NREL—Aug 11, 2009

Todorov et. al., Adv. Energy Mater. 3, 34 (2013)

Further champion CZTSSe devices (11.1% efficiency):Certified by Newport Corp.—Feb 24, 2012

Efficiency = 11.1 %Voc = 0.460 VJsc = 34.5 mA/cm2

Fill Factor = 69.8 %Area = 0.45 cm2

Eg = 1.13 eV

Todorov et. al., Adv. Energy Mater. 3, 34 (2013)

Compare 11.1% record device to SQ limit…

Jsc – 34.5 vs 43.4 mA/cm2: ↑ 26% FF – 70% vs 86% ↑ 23% Voc – 460 vs 820 mV: ↑ 78%

ηrec = 11.1%

ηSQ = ~31%

Define Voc deficit:

Voc = Eg/q – Voc

Voc < 0.5 V for best CIGS

Voc > 0.6 V for best CZTS

def

def

def

Can we improve the top stack to reduce the short wavelength losses?

11% WR Device (Jsc/JSQ = 0.79)

Jsc enhancement through optical stack optimization…


Planar Optical Model(scattering matrix)

Should be optimizing the transmitted light rather than just reflected light

M. Winkler et. al., Energy and Environ. Sci. 7, 1029 (2014)

New device design leads to ~10% enhancement in Jsc

Device Verification

1

2

2

1 baselineimproved



New WR and first CZTS device over 12% efficiency



Influences on Fill Factor:

M. A. Green, Solid State Electronics 24, 788 (1981)

Voltage

Cur

rent

VMAX

IMAX

VOC

ISC

Maximum Power Rectangle

SCOC

MAXMAX

IVIVFF =Fill factor,

RL

Cell Eff FF Jsc Voc

% % A/cm2 V

IBM-CIGSSe 15.2 75.0 32.6 0.623

IBM-CZTSSe 11.1 69.8 34.5 0.460

Neglecting Rsh, FF can be approximated:

Using voc for our CIGS(15%) and CZTS(11%) devices and rs = 0.04, we get:

FF(CIGS) = 75.0%FF(CZTS) = 70.2% Difference is ~5%

FF deficit in CZTS can to a large extent be accounted for through the Voc deficit…

voc – ln(voc + 0.72)FF = (1-rs) voc + 1

M. A. Green, Solid State Electronics 24, 788 (1981)

Influences on Fill Factor:

Large database of device results available for Voc

Band Gap (eV)

V oc

defic

it (V

) • For all devices, Voc deficit >0.6 V (for CIGS it is ~0.5 V)

• Strong Eg dependence of Voc deficit (Eg/q – Voc)NREL CZTSe

IBM 11.1% CZTSSe

Vac. Deposit CZTS

IBM 15.2% CIGSSe

Data represents >1000 CZTSSe devices

MnXmN2H4 + X

N2H5+

N2H5+

N2H5+

MX-

dry



Zn add as powder

ZnX

MX-MX-

ZnX

ZnX

heat

Cu2ZnSn(S,Se)4



MnXmN2H4 + X

N2H5+

N2H5+

N2H5+

MX-

dry



Zn add as powder

ZnX

MX-MX-

ZnXZnX

heat

Cu2ZnSn(S,Se)4


MnXmN2H4 + X

N2H5+

N2H5+ MX-

dry

X + N2H4decomp. products


MX-

MX-

heat

Cu2ZnSn(S,Se)4

All-solution hydrazine-based deposition

T. K. Todorov et. al., IEEE Journal of Photovoltaics 4, 483 (2014)

Zinc salt

N2H5+

N2H5+

MX-N2H5+

MX- N2H5+

MX-N2H5+

MX-

IBM pure solution approach for CZTSSe

Superior smoothness/uniformity of solution-processed layers.

λ = 950 nmλ = 404nm

λ = 950 nmλ = 404nm

Solution process

Slurry process

LBIC Study


Solution process reaching slurry record benchmark

Slurry

Pure solution

Eff. (%)

Voc(V)

Jsc(mA/cm2)

FF (%)

Slurry 11.1 460 34.5 70

Solution 11.1 489 33.0 69


W. Wang et. al., Adv. Energy Mater. 4, 1301465 (2014)

• Get both increase in Jsc (2%) and Voc (12%)relative to previous 11% champion of same Eg

• Best Voc deficit for band gap — Eg = 1.13eV(i.e., 0.62 vs 0.67 V for previous 11% device)

• While Voc is improved in the current device…Still a long way to go until Voc deficits of< 0.5V can be achieved (as for high-performance CIGS).

Combining with new optical stack approach… 12.6%!

Champion CZTSSe Solar Cells – Still large Voc deficit

Jsc – 35.3 vs 43.4 mA/cm2: ↑ 23% FF – 70% vs 86% ↑ 23% Voc – 513 vs 820 mV: ↑ 60%

Voc deficit = 0.62

Voc is better, but nowhere near good enough!

Where is this Voc deficit coming from…

-- Interface / grain boundaries?-- Bulk defects (deep trap and

band tail states)?

Grain boundaries in high-performance CZTS…

K. Sardashti et. al., Adv. Energy Mater. in press (2015)

Air annealing used for high-performance devices SnOx and Cu poor at GBs

SnOx can serve to passivate GBs and might even be beneficial if not too thick

Causes : Charged defects

Electrostatic Potential FluctuationsBand gap Fluctuations

Causes : Competition between kesterite and stannite phase, secondary phases, non-uniform S/(S+Se), non-uniform strain

h+

e-γOPT

EV

EC

h+

e-σg

EV

EC

Band tailing can reduce Voc

TqkVV

B

goc

inoc 2

2homhom σ

−=

Voc Deficit : (Rau et al., APL 2004)

Band tailing reduces the effective Eg and therefore reduces achievable Voc

CuZn+, VCu

+, ZnCu-, SnZn

2-

T. Gokmen et. al., Appl. Phys. Lett. 103, 103506 (2013)

Chen et. al., Phys. Rev. B 81, 245204 (2010)Chen et. al., Appl. Phys. Lett. 101, 223901 (2012).

Defects in CZTSSe:

400 600 800 1000 1200 14000.0

0.2

0.4

0.6

0.8

1.0

λ

(nm)

EQ

E (%

)

PLEg

dEQE/dλ

arb.

uni

t

dEQE/dλ

Eg PL

arb.

uni

t

EQ

E (%

)

0.0

0.2

0.4

0.6

0.8

1.0

400 600 800 1000 1200 1400

λ

(nm)

15% CIGSSe 11% CZTSSe

Tailing in EQE below band gap suggest tailing in density of states (DOS)

CZTSSe has roughly twice more tailing compared to CIGSSe

PL spectrum has a wider peak at much lower energy than Eg for CZTSSe

)())(1ln()()( λλλαλ EQEEQEDOS ∝−−∝∝

Evidence for band tailing…


300 K

0 10 20 30 40 501E-4

1E-3

0.01

0.1

1

TRP

L (n

orm

aliz

ed)

t (ns)

300 K

0 10 20 30 40 50t (ns)

1E-4

1E-3

0.01

0.1

1

TRP

L (n

orm

aliz

ed)

Interesting TR-PL data for CZTSSe…

15% CIGSSe 11% CZTSSe

Room temperature : Comparable lifetimes (~5 ns)

Low temperature : Lifetime increases 3 orders of magnitude for CZTSSe ( ~10µs)

0 2000 4000 6000 8000 10000

4 K

300 K

0 10 20 30 40 501E-4

1E-3

0.01

0.1

1

TRP

L (n

orm

aliz

ed)

t (ns)

4 K

300 K

0 10 20 30 40 50t (ns)

1E-4

1E-3

0.01

0.1

1

TRP

L (n

orm

aliz

ed)


0 2000 4000 6000 8000 10000

4 K

300 K

0 10 20 30 40 50t (ns)

1E-4

1E-3

0.01

0.1

1

TRPL

(nor

mal

ized

)Evidence for Electrostatic Potential Fluctuations

CZTSSe

Low temperature lifetime increases 3 orders of magnitude for CZTSSe Can be understood in terms of electrostatic potential fluctuations We propose that these electrostatic potential fluctuations (amplitude ~

60 meV) and associated band tailing are responsible for the bulk of the Voc deficit issue.

h+

e-γOPT

EV

EC

e- and h+ localized at low T

Electrostatic Potential Fluctuations


Impact of double In2S3/CdS emitter on Voc:

quick anneal

New Voc deficit record

Indium diffusion into CZTSSe increases carrierdensity and improves Voc for given Eg

New record low Voc deficit achieved (593 mV)

J. Kim, H. Hiroi, et. al., DOI: 10.1002/adma.201402373 (2014)

• Thermal co-evaporation of CZTS in a high vacuum chamber with base pressure 10-9 Torr

• Cu, Zn, and Sn: Knudsen-type cells• S, Se: valved thermal crackers• Target composition: Cu/Sn ~1.8, Zn/Sn ~1.2

(Cu poor and Zn rich conditions)• Substrate heated to ~ 150oC and rotate at 10-20

rpm during growth• For a complete solar device: hot plate annealing

at 540C for 5 minCu

Zn Sn SSe

substrate

K. Wang et. al., Appl. Phys. Lett. 97, 143508 (2010).

High-performance kesterite films made by evaporation…

Y. S. Lee et. al., Adv. Energy Mater. 1401372 (2014).

Very uniform films & very low [S]

Voc deficit = 578 mV

High-performance kesterite films made by evaporation…

Conclusions…

?

• Continued promising progress on efficiency of both small cells and submodules for CZTSSe

• Progress has come mostly from Jsc and FF, but somerecent progress on Voc as well(new Voc deficit record of 578 mV)

• Voc deficit remains thedominant issue for CZTSSe

• The data supports the idea that this Voc deficit is predominantlyarising from band tailing associatedwith electrostatic potential fluctuations… Need to find a way to resolve this.

IBM:Santanu Bag Aaron R. BarkhouseS. Jay CheyRichard FerlitaThomas Goislard de MonsabertTayfun GokmenSupratik GuhaOki GunawanRichard Haight

Many Thanks / Co-Workers…

IBM - Tokyo Ohka Kogyo Co. - Solar Frontier Joint Development Project

TOK Corp.:Akimasa NakamuraMasaru KuwaharaKouichi MisumiHidenori MiyamotoYubun Kikuchi

Solar Frontier:Hiroki SugimotoHomare Hiroi

Marinus HopstakenSunit MahajanXiaofeng QiuSean SeefeldJiang TangSathish ThiruvengadamTeodor TodorovWei WangMark WinklerYu Zhu

IBM Watson

Contact information for speaker:

Dept. of Mech. Eng. and Materials Science Edmund T. Pratt Jr. School of EngineeringDuke UniversityBox 90300 Hudson HallDurham, NC 27708-0300

[email protected]

Thank you for your attention!

mailto:david.mitzi@