Neutron Scattering Studies of Hybrid Perovskites for ... · Hybrid Perovskites for Photovoltaic Applications ... – Thin films harvest light effectively ... at POWGEN/SNS • Fully

Neutron Scattering Studies of Hybrid Perovskites for Photovoltaic Applications

M.K. Crawford

DuPont CR&D

40 Years of Neutron Scattering Symposium

NIST Center for Neutron Research

February 18, 2016

• R.J. Smalley, N. Herron, L. Johnson, I. Milas, W.E.

Guise DuPont CR&D

• P. Whitfield, N. Jalarvo, Y.Q. Cheng, A. Ramirez-

Cuesta, L. Daemen, G. Ehlers, K. Page, X. Wang Oak

Ridge National Laboratory

• M. Tyagi NIST Center for Neutron Research and

University of Maryland

Acknowledgements

2/18/2016 40 Years of Neutron Scattering 2

• John Copley – For DCS

– For your help to members of the PAC Committee

• And BTAC as well

• Bill Kamitakahara – For your long dedication to improving the user program, proposal

system and the reviewers, and your service to the user group and its executive committee

• You have both contributed greatly to the science and culture of the NCNR – And I hope you continue to do so in retirement

Thanks!


http://www.google.com/url?sa=i&rct=j&q=&esrc=s&source=images&cd=&cad=rja&uact=8&ved=0ahUKEwj3taqlofXKAhUIyyYKHSvrASUQjRwIBw&url=http://www.ncnr.nist.gov/acns/reception.html&psig=AFQjCNH-rnLTcGs3OMXg7_n6qkuvFZ97iw&ust=1455470631421641

• GeNi2O4

– Antiferromagnetic cubic spinel

– Two Neel transitions: 12.1 K and 11.4 K

• DCS S(Q,w) beautifully shows

– 0.3 meV spin wave gap

– Nicely dispersing spin waves

My One DCS Experiment: Spin Waves in GeNi2O4


Lashley et al, Phys. Rev. B 78, 104406 (2008)

The Rise of Hybrid Perovskites for Photovoltaics

Unprecedented rate of improvement . . .

Efficiencies as high as 20.1% reported.

“The Perovskite Storm”

Exceptional Properties

• Ambipolar conductivity: p- or n-type semiconductor

• Carrier diffusion lengths : > 1 mm

• High defect tolerance

• Ideal bandgap of 1.55 eV for MAPbI3 (tunable)

• High Voc (>1 V)

• Low Voc Deficit (69% of bandgap)

650


http://www.google.com/url?sa=i&rct=j&q=&esrc=s&source=images&cd=&cad=rja&uact=8&docid=M_4XjuZiM0rwWM&tbnid=A9nXjKYjSik5XM:&ved=0CAUQjRw&url=http://www.1-material.com/toward-20-pce-perovskite-materials-from-1m/&ei=ZM-KU6TQIOrlsATtwYDADQ&psig=AFQjCNG-Aj4XbokNkGnBde6jKTeDQJrV7g&ust=1401690960766066

Perovskites

ABX3 • Hundreds of perovskites: Properties vary and include insulating,

antiferromagnetic, piezoelectric, thermoelectric, semiconducting, conducting, and superconducting materials

• (MeNH3)PbI3 (MAPbI3): “Standard” composition for perovskite-based PV, including record devices, with bandgap of 1.6 eV

Structure • 3D network of corner sharing PbI6

3- octahedra with charge-compensating cations in the gaps

Bandgap of MAPbI3 is readily tuned by chemical substitution:

• Halide substitution, e.g. MAPb(BrxI1-x)3: 1.6 to 2.2 eV

• Metal substitution, e.g., MASnI3: 1.3 eV Sn oxidation to Sn(IV) leads to degradation

• Cation substitution: Cs, 1.7 eV; FA, 1.5 eV

CsPbI3 readily decomposes Cation must be small enough to fit within the gap (tolerance factor)

a

c

Cs+ CH3NH3+ HC(NH2)2

+

MA FA 2/18/2016 40 Years of Neutron Scattering 6

MAPbI3 Brillouin Zone and Band Structure

• Cubic perovskite Pm-3m

• Direct bandgap semiconductor – High optical absorption coefficient

– Thin films harvest light effectively

• High symmetry Brillouin zone boundary points – R, M, X

– Optical bandgap is located at the R point

– R point becomes the point in the tetragonal phase

• T. Umebayashi et al, PRB 67, 155405 (2003)


Hybrid perovskite crystal structures are complex

• Presence of organic cations generates large amounts of disorder at high temperatures where PV devices operate

• Structural phase transitions involving rotations of PbI6 octahedra, coupled to organic cation reorientations through hydrogen bonds, have significant impact on physical properties such as

– Charge carrier mobility

– Ionic conductivity

– Exciton binding energies

– Optical absorption

– Thermal conductivity

– Heat capacity


Structures: Neutron and X-Ray Diffraction

• Neutron diffraction data collected at POWGEN/SNS

• Fully and partially deuterated samples

– Reduce incoherent scattering

– Measure isotope effects on phase transitions

– C, N and H/D atom positions

• X-ray diffraction data collected at Advanced Photon Source

– DND-CAT

– 0.4 Å wavelength (31 keV)

– High resolution

2/18/2016 40 Years of Neutron Scattering

Element Neutron Coherent

Cross-section (barn)

X-Ray Cross-

section (barn)

H 1.76 0.6

D 5.59 0.6

C 5.56 5

N 11.0 6.5

Pb 11.12 9392

I 3.5 1622

Spallation

Neutron

Source

ORNL

9

Orthorhombic structure: T = 10 K

• Pnma structure refinement

– T = 10 K to minimize diffuse background contribution

– Scattering to high Q – some diffuse background even at 10K

– Full un-constrained refinement carried out.

– Same structure as MAPbBr3 at 10 K (Swainson et al.)

• Three strong hydrogen bonds between D and the I(2) sites of 1 x 2.625 Å/179.5º and 2 x 2.696 Å/150.6º

• Deuterium ADPs as expected for such a structure

• Pb and I ADPs small with little directional motion by the iodines

95% ellipsoids


Tetragonal structure: T = 190K

• I4/mcm symmetry

– Same as Weller structure: Chem Commun (2015)

– C and N ¼ occupancy, D 1/8 occupancy; 8 positions

• D-I distances on order of 3.0 Å: weaker H-bonds

95% ellipsoids


Cubic structure: T = 350K

• Pm-3m symmetry

• Despite extreme disorder

– Only 5 atomic positions so no constraints needed

– Very significant diffuse background which was partially modelled using a sin(q)/q curve to reduce number of background parameters

– Structure agrees with literature structure

• MA is completely disordered as a nearly free rotor.

• Iodine ADPs similar size to the deuterium atoms

• Closest centroid D-I distance 3.09 Å

– Weak hydrogen bonds

95% ellipsoids 2/18/2016 40 Years of Neutron Scattering 12

• Only showing the PbI6 octahedra

• Cubic-tetragonal transition involves rotations of octahedra around a single cubic axis

– Order parameter is rotation angle

• Tetragonal-orthorhombic transition involves tilts around additional cubic axes

MAPbI3 Structures


Cubic Pm-3m Tetragonal I4/mcm Orthorhombic Pnma

327 K 160 K

Structural phase transitions in MAPbI3

Pm3m

I4/mcm

Space Group Glazer Notation for Octahedra Tilts and Rotations

Pm3m a0a0a0

I4/mcm a0a0c-

Pnma a+b-b-

Howard and Stokes, Acta Cryst B54, 782 (1998)


Hybrid Perovskite Phase Transitions

• Cubic-tetragonal phase transition of MAPbI3

– Similar to cubic-tetragonal transition in SrTiO3 at 110 K

– Driven by condensation of single triply degenerate R-point phonon (out-of-phase rotation around cubic c axis)

– Superlattice Bragg peaks appear with cubic indices ½ (hkl) with h, k, l odd

– I4/mcm

• Low temperature transition involves cubic M and X-point phonons

– Rotations around different cubic axes

– But coupled to order-disorder transition of MA cations through hydrogen bonds

FAPbI3 MAPbI3


X-Ray Bragg Reflections and PbI6 Rotations: d6-MAPbI3

• Cubic-tetragonal phase transition at T = 330 K – First-order (phase coexistence)

• (200) cubic Bragg reflection – Splits into (220) and (004)

tetragonal Bragg peaks – Tetragonal strain is a secondary

order parameter

• R-point ½ (311) Bragg reflection – Pseudo-cubic unit cell – Same as (211) in tetragonal cell – Shows the presence of out-of-

phase PbI6 rotations – I4/mcm space group


MAPbI3 Lattice Parameters: Neutron and X-Ray Diffraction

• Two structural phase transitions

– High temperature transition nearly continuous , but XRD shows phase coexistence

– Lattice parameters show discontinuities at low temperature structural transition at 160 K


150 175 200 225 250 275 300 325 350

6.20

6.25

6.30

6.35

acu

b(Å

)

Temperature (K)

acub

atet

ctet

(atet

+ctet

)/2

Neutron X-Ray

Order Parameter for Cubic-Tetragonal Phase Transition: d6-MAPbI3

• X-ray and neutron diffraction both show phase transition is close to tricritical (intersection of first and second order transitions)

– Tetragonal distortion from x-ray diffraction

– Distortion mode analysis from neutron diffraction

– Order parameter scales with (Tc-T)0.25


150 200 250 300 350

0.000

0.005

0.010

0.015

0.020

0.025T

etr

agonal D

isto

rtio

n, (c

-a)/

a

T(K)

(c-a)/a

A*(T-Tc)2

d6-MAPbI

3

Tc = 333.1(8) K

= 0.278(5)

300 305 310 315 320 325 3300.0

0.2

0.4

0.6

0.8

1.0

R4

+ (

a,0

,0)

Mo

de

Am

plit

ud

e

Temperature (K)

= 0.26(1)

Tc = 329.05(5)

18

• Cubic 200 Bragg peak – Splits into (220) and (004)

Bragg peaks in tetragonal phase

– Phase coexistence • Narrower region than d6-

MAPbI3

• Tetragonal (211) superlattice Bragg peak – ½ (311) in cubic cell

– Pnma phase appears via first-order transition at T = 160 K


Bragg Reflections and PbI6 Rotations: h6-MAPbI3

19

• Tetragonal distortion vs T

• Small region of cub0c-tetragonal coexistence

• Tetragonal distortion vs T

– (Tc – T)2

– = 0.248

– Tricritical


100 150 200 250 300 350

0.005

0.010

0.015

0.020

0.025

Tetr

agonal D

isto

rtio

n, (c

-a)/

a

T(K)

(c-a)/a

A*(T-Tc)2

h6-MAPbI

3

Tc = 332.9(3) K

= 0.248(2)

160 180 200 220 240 260 280 300 320 340

6.20

6.22

6.24

6.26

6.28

6.30

6.32

6.34

6.36P

seudocubic

Lattic

e P

ara

mete

r (Å

)

T(K)

acub

atet

ctet

Order Parameter for Cubic-Tetragonal Phase Transition: h6-MAPbI3

20

d6-MAPbI3 Single Crystal Diffraction on TOPAZ at SNS


• d6-MAPbI3 single crystal

– 1-2 mm in size

• Tetragonal (211) Bragg reflection

– Superlattice ½ (311) in pseudocubic cell

– Power law fit again consistent with near-tricritical behavior

MAPbI3 phase transitions: MA disorder grows with temperature

Pnma T = 10 K Pnma T = 130 K I4/mcm T = 190 K

Pm-3m T = 350 K I4/mcm T = 300 K

Orthorhombic Tetragonal

Tetragonal Cubic 330 K

160 K


Hydrogen bonds play a role in structural transitions

• Hydrogen bond strengths are directly correlated with structural phase transition at 160 K

– Low temperature orthorhombic structure has strongest H-bonds

– Order-disorder transition of MA cations

• H-bonds decrease in strength with increasing temperature

– Thermal expansion

– MA cation disorder

Hydrogen bonds

2.50

2.60

2.70

2.80

2.90

3.00

3.10

3.20

0 50 100 150 200 250 300 350 400

MAPbI3 Hydrogen Bond Length

D-I distance 1

D-I distance 2

D-I

dis

tan

ce (

Å)

T(K)


Neutron Spectroscopy of h6-MAPbI3: dispersion

• CNCS spectrometer at SNS – T = 1.7 K

• MA vibrations have little dispersion – Molecular vibrations

– Intermolecular coupling is weak

• Peak assignments (from DFT) – 10-25 meV peaks correspond to

CH3 torsions, and librations and translations of MA cations

– Peak at 38 meV is intramolecular vibration, mostly NH3 torsion


MAPbI3: Neutron Vibrational Spectra and DFT Calculations

• VISON spectrometer at SNS

– T = 10 K

• Neutron vibrational spectrum

– Similar to IR and Raman, but no selection rules

– MA vibrations dominate due to large incoherent neutron scattering cross-section for H

• Density functional theory

– Vibrational eigenvectors

• Use as input to calculate neutron spectrum

– Provides vibrational mode assignments

• I. Milas (DuPont) and Y.Q. Cheng (ORNL)

• Measurements of vibrations and phonons are important for understanding

– Electron-phonon coupling (charge transport)

– Thermal conductivity (heat capacity)

Comparison of neutron and DFT spectra provides a demanding test of accuracy of DFT calculation


0 100 200 300 400 500 600 700 800 900 1000

0

10

20

30

40

50

60

305

Inte

nsity

Energy (cm-1)

h6-MAPbI3

Neutron

T = 5 K

0

2

4

6

8

10

12CH

3NH

3 librations

NH3 torsion

355

h6-MAPbI3

DFT

Fundamentals

1st overtones

MAPbI3 neutron vibrational spectra vs DFT

• Partially and fully deuterated samples

– Establish vibrational mode assignments

• “NH3 torsion” at 37.7 meV shifts significantly upon deuteration

– H/D = 1.26 (vs 1.41)

• “CH3 torsion” at 11.5 meV shifts to 9 meV upon deuteration

– H/D = 1.26 (vs 1.41)

– Test accuracy of vibrational eigenvectors from DFT calculations


0

20

40

60

CH3NH

3PbI

3

VISION

CH3NH

3PbI

3

DFT

0

20

40

60

CH3ND

3PbI

3

CH3ND

3PbI

3

0

20

40

60

CD3NH

3PbI

3

CD3NH

3PbI

3

0 200 400 600 800 10000

5

10

15

Inte

nsity (

arb

units)

Energy (cm-1)

CD3ND

3PbI

3

0 200 400 600 800 1000

Energy (cm-1)

CD3ND

3PbI

3

QENS: Structural transitions affect dielectric properties

Mean square displacement (MSD) measured using HFBS at NIST Center for Neutron Research

Calculated from elastic peak intensity vs T

Measure of mobility of methylammonium (MA) cations

Dielectric constant tracks mobility of the MA cations at structural phase transitions

T < 160 K – Low dielectric constant

MA cations are fully ordered, dipoles no longer reorient in response to electric field

Dynamics due to rotations around C-N bond

T > 160 K – High dielectric constant

Nearly free rotation of MA cations above transition leads to large dielectric constant

Cubic-tetragonal transition at 330 K does not affect MA cation dynamics or dielectric properties

High dielectric constant in cubic and tetragonal phases reduces exciton binding energy increases charge separation efficiency Onoda-Yamamuro et al, J Phys Chem Solids 53, 935 (1992)

0 50 100 150 200 250 300 350

0.0

0.5

1.0

1.5

2.0

2.5

CH3NH

3PbI

3

MS

D (

A2)

T(K)

warming

cooling

(1 kHz)


Dielectric constant and low T phase transitions: other halides

0 50 100 150 200 250 300 350

0.0

0.5

1.0

1.5

2.0

2.5

CH3NH

3PbBr

3

MS

D (

A2)

T(K)

warming

cooling

0 50 100 150 200 250 300

0.0

0.5

1.0

1.5

2.0 CH3NH

3PbCl

3

MS

D (

A2)

T(K)

warming

cooling

CH3NH3PbBr3 CH3NH3PbCl3

(1 kHz)


MAPbI3: QENS and Partial Deuteration

• MSD is characteristic of H atoms of MA cations: rotations around the C-N bond at low T and rotations of the bond at high T

• No significant isotope effect on phase transition or MSD values

0 50 100 150 200 250 300 350

0.0

0.5

1.0

1.5

2.0

2.5

CD3NH

3PbI

3

MS

D (

Å2)

T(K)

Warming

Cooling

0 50 100 150 200 250 300 350

0.0

0.5

1.0

1.5

2.0

2.5

CH3NH

3PbI

3

MS

D (

A2)

T(K)

warming

cooling


0 50 100 150 200 250 300 350

0.0

0.5

1.0

1.5

2.0

2.5

CH3ND

3PbI

3

MS

D (

Å2)

T(K)

Cooling

Warming

Structural transitions affect electronic properties

• Photoluminescence measured as a function of temperature

• PL is sensitive to electronic structure of PbI3 sublattice

– PbI6 octahedra undergo additional tilts/distortions at the 160 K tetragonal orthorhombic phase transition

– Tilts change Pb-I-Pb bond angles, and this affects electronic structure

– PL shifts to higher energy in orthorhombic phase Sargent group, U. Toronto

B.R. Sutherland et al, Adv. Mater. 27, 53 (2015)


MAPbI3: high temperature transition affects ionic conductivity

• Electrical conductivity vs temperature – Cubic tetragonal phase

transition occurs at 327 K

– Conductivity is sensitive to Pb-I-Pb bond angles and lengths

• Activation energy for conduction in cubic phase is 0.38 eV – Consistent with ionic

conductivity in other halides

– Detailed understanding is still lacking

T. Baikie et al, J Mater Chem A (2015)


Does high temperature phase transition affect electronic properties?

• PL shifts with temperature

– Associated with change of electronic band structure

– Band gap should shift due to lattice expansion

– Separating thermal effect from structural effect is difficult

• Short-range cubic structure could be tetragonal?

– Single crystal diffraction shows R-point Bragg reflections in cubic phase, but broadened

– Electronic structure would not change significantly across phase transition


Some remaining questions

• Lead-halogen phonons – Nature of phase transitions: displacive vs order-disorder

• How do energies of PbI6 rotational modes change with temperature? Are the modes overdamped?

– Anharmonicity and ultralow thermal conductivity – What is magnitude of electron-phonon coupling? – Is tricritical nature of cubic-tetragonal transition important for PV

properties?

• Electronic properties – Why are these materials so efficient for PV?

• High dielectric constants? Defect and exciton screening? • Is Pb necessary for high efficiency?

– Reason(s) for high carrier mobilities? – Can these materials be doped with electrons or holes? – Understand the effects of phase transitions on electronic

properties?


Conclusions

• Hybrid perovskites are complex

– Static and dynamic disorder due to organic cations

– Structural phase transitions

– Hydrogen bonds

• Structural complexity impacts properties important for photovoltaic applications

– Dielectric constants

– Optical properties

– Thermal conductivity

– Heat capacity

– Thermal stability

• Neutron (and x-ray) scattering studies can provide a detailed microscopic understanding of the structures and dynamics of hybrid perovskites and other advanced materials


Thanks! Questions?

Documents

Neutron Scattering Studies of Hybrid Perovskites for ... · Hybrid Perovskites for Photovoltaic Applications ... – Thin films harvest light effectively ... at POWGEN/SNS • Fully