1

Annamaria Petrozza

Hybrid Perovskite Solar Cells

Milan, Novembre 27th, 2015

"ORGANIC ELECTRONICS : principles, devices and applications"

2

B X

A

Perovskite Crystal with ABX3 stoichiometry

I-V-O3, II-IV-O3 and III-III-O3 e.g KTaO3, SrTiO3 and GdFeO3

I-II-X3, e.g. CsSnI3, CH3NH3PbI3,

3

B X

A

It is not only a matter of stoichiometry….

The Goldschmidt tolerance factor:

)(2 XB RR

RRt XA

t< 0.7 the perovskite falls apart t >1 towards 2D structures…

4

CH3NH3PbI3 (CH3NH3)2PbI4

(C10H21NH3)2PbI4 (CH3NH3)4PbI6*H2O

Playing with Dimensionality

5

CH3NH3PbI3 (CH3NH3)2PbI4

(C10H21NH3)2PbI4 (CH3NH3)4PbI6*H2O

5 Ishihara, J. Lum, 1994.

Dielectric Confinement

6

Organo-Metal Halide Crystalline Perovskite

A= Pb2+

X= I-, Cl-, Br-

B = (CH3NH3+)x

ABX3

7

Organo-Metal Halide Crystalline Perovskite

Eric Hoke et al, Chem Sci, 2015

Filip et al, Nat Comm, 2014

8

Organo-Metal Halide Crystalline Perovskite

9

CH3NH3+ :Orientational disorder and Polarizability

Hydrogen Bonds Dipole “nearly” free to move

10

Modular Structure

11

Prone to a variety of processing

methods

Type II Hetero-Junction

Excitonic Solar Cells

13

Intrinsic Loss in Excitonic Solar Cells

1.0 1.2 1.4 1.6 1.80.4

0.6

0.8

1.0

1.2-0.6eV-0.4eV-0.2eVVoc = BG

CIGS

CdTe

ncSi

CZTSS

aSi

Vo

c (V

)

Bandgap (eV)

OPV

DSC

MSSCGaAs

Si

Hybrid Crystals in DSSC Devices

h u h

Hole transfer

e- injection

H. S. Kim et al. Sci Rep. 2: 591 (2012)

CH3NH3PbI3 perovskite as light

antenna in the DSSC concept

Glass

FTO

HTL

Silver Perovskite role:

• Absorber

• Electron-transporter

M.Lee et al. Science 338, 643 (2012)

TiO2

Al2O3

Hybrid Crystals in Hybrid Solar Cells

16

Which is their strength?

17

18

Optoelectronic Devices

Tan et al, Nat Nanotech, 2014

Zhu et al, Nat Mat, 2015

19

Designing the Device Architecture

TiO2/Al2O3 scaffold+ Perovskite

Perovkite capping layer Perovkite capping

layer

HTM HTM HTM

Nano-structured vs Thin Film J. Ball, EES, 2013

Designing the Device Architecture

• Light Absorption • Charge Generation • Photo-carriers Transport

Designing the Device Architecture

• Light Absorption • Charge Generation • Photo-carriers Transport

Light Absorption

Silicon

In direct band-gap Phonon assisted

um range sun light penetration depth

thick solar cells

no light emission

GaAs

direct band-gap

excitonic effects at absorption edge good light penetration depth

Dyes/conjugated polymers

localized states excitonic effects large absorption cross-section/ efficient carrier recombination

Silicon

In direct band-gapPhonon assisted um range visible light penetration dept thick solar cells no light emission

GaAs Dyes/conjugated polymers

direct band-gap

excitonic effects at absorption edge good light penetration depth

Light Absorption

localized states excitonic effects large absorption cross-section/ efficient carrier recombination

Silicon

In direct band-gapPhonon assisted um range visible light penetration dept thick solar cells no light emission

Hybrid Perovskites

Dyes/conjugated polymers

1.50 1.53 1.56 1.59 1.62 1.65 1.68 1.71 1.74 1.77 1.800.5

1.0

Energy (eV)

4.2K

290K

Op

tica

l D

en

sity (

a.u

.)

direct band-gap

excitonic effects at absorption edge good light penetration depth

Light Absorption

localized states excitonic effects large absorption cross-section/ efficient carrier recombination

25

“ ..is a quasi-particle that represents a collective excited state of an ensable of atoms or molecules. It is represented by a wavepacket for which we can define mass and speed which transports Energy”

Exciton

26

Binding energy

Oscillator strength

Delocalization

Lifetime

Singlet

Triplet

How many kind of excitons??

Wannier-Mott Frenkel

27

Frenkel Exciton

• Solids made by weakly interacting units (e.g organic crystals, inter-molecular coupling weaker than the intra-molecular ones).

• General case of a Molecular Excitation (DSSC)

• The wavefunction squared amplitude is the probability of finding the excited state in the lattice.

G. Lanzani, “The Photophysics behind Photovoltaics and Photonics” Wiley-VCH

De e

E

g

u

Wannier-Mott • Solids with tightly bounded atoms

• Low Screening Coulomb attraction does not allow the generation of FREE e-h pairs

• Hydrogenoid system.

• Center of Mass/Exciton radius (LARGER than the lattice constant)

E1

Eg

K

E

rh

re

RX kh

ke

O

he

XbgnK

mm

K

n

EEE

X

22

2,

G. Lanzani, “The Photophysics behind Photovoltaics and Photonics” Wiley-VCH

• Energy spent in the exciton dissociation

Why do we need to know the details?

• Energy spent in the exciton dissociation • Absoprtion cross section enhancement

Why do we need to know the details?

3)(

1

B

Xa

Elliott’s Theory (1963)

• Energy spent in the exciton dissociation • Absoprtion cross section enhancement

Why do we need to know the details?

n

nx

eh

Tk

n

n

nnn

FC

Tk

E

B

exc

FC

excFC

B

b

2/3

2)

2(

32

Exciton Vs Free Charges

at the Thermodynamic Equilibrium

Saha-Langmiur equation

@ RT PV regime

Total excitation density Excitons

Free charges

D’Innocenzo et al, Nat Comm. 5, 3586, 2014

Law of mass action

4 2

2 2 22 2B

B

eE

a

e

2

2

eaB

e

Bohr Radius

Exciton Reduced mass

Dielectric constant

Exciton Binding Energy

The golden rule:

“the Bohr orbital frequency (Eb/h) must be compared with the optical phonon frequency”

he

he

mm

mm

Dielectric constant (ε):

A measure of a substance’s ability to insulate charges from

each other.

Screening in solids: basic concept

E0

E=0 Ed

𝐸𝑑 =𝐸 0𝜀

METAL DIELECTRIC

Screening mechanisms

Electronic Polarizability (Electron Cloud Distortion) 1015 s-1

Dipole Re Orientation (Langevin Mechanism) 108- 1010 s-1

Ions Displacement (Optical Phonons)

1010- 100 s-1

Screening mechanisms

Dielectric constant of GaAs. Why it is so easy

e0 (T) = 12.4 + 0.00012*T

LO = 36meV TO = 38meV

20 e

e

Dielectric constants of CH3NH3PbI3, real and imaginary parts

Dielectric constants of CH3NH3PbI3, real and imaginary parts

1KHz

41

)exp(1Tk

Ek

B

bD uu

Eb (upper limit)= 50meV

Exciton Binding Energy

(I) Direct Measurement of Exciton Binding Energy in CH3NH3PbI3

hD

D’Innocenzo et al, Nat Comm. 5, 3586, 2014

Limitations: It is assumed that exciton-phonon interaction induces only exciton dissociation though in a limited range – it assumes the exciton binding energy constant in T

42

TkE

TTB

B

ek

Tkk 0

1T

1

12 2

1

T

1

T

2T

1

D

(I) Direct Measurement of Exciton Binding Energy in CH3NH3PbI3

43

(II) Direct Measurement of Exciton Binding Energy in CH3NH3PbI3

Numerical modelling of band-edge absorption by using Elliot’s theory of Wannier excitons

43

Eb = 25meV

Saba et al, Nat Comm. 5, 5049, 2014

Note: This simple formalism does not consider the frequency dependance of the exciton-phonon interaction

44

m*= 0.1m ; Eb = 16meV

Miyata et al, Nature Physics 11, 582–587 (2015)

n

REE gn

)*

()2/1()(m

eBNEBE g

At 4K

e 9

(III) Direct Measurement of Exciton Binding Energy in CH3NH3PbI3

45

Eb 10 meV

Miyata et al, Nature Physics 11, 582–587 (2015)

n

REE gn

)*

()2/1()(m

eBNEBE g

At 161K

e > 9

(III) Direct Measurement of Exciton Binding Energy in CH3NH3PbI3

Organic-Inorganic Interplay: Raman Spectroscopy as a probe

60 80 100 120 140 160 180 200 220 240 260 280 3000.0

0.2

0.4

0.6

0.8

1.0 Meso

Flat

Ra

ma

n I

(n

orm

.un

its)

Raman shift (cm-1)

Grancini G et al, JPC Letters, 5, 3836, 2014

• Light Absorption • Charge Generation • Photo-carriers Transport

48

Exciton Vs Free Charges

at the Thermodynamic Equilibrium

Saha-Langmuir equation n

nx

eh

k

n

n

nnn

FC

kB

exc

FC

excFC

B

T

E

2/3

2

b

)T2

(

Total excitation density Excitons

Free charges

Are there excitons around? MAPbI3

Room Temperature, in the typical PV regime ph <1016 cm-3 :

free carriers only

THz conductivity spectra are Drude-like in accordance with the presence of free charge carriers. Wehrefenning et al, Adv Mater, 26, 1584, 2014, Milot et al, Adv Funct Mater, 2015 DOI: 10.1002/adfm.201502340, PL dynamics are dictated by bimolecular recombination processes. Yamada, Y J. Am. Chem. Soc. 2014, 136, 11610. Stranks, Phys. Rev. Appl. 2014, 2 034007. fs-TA spectra show band filling effect and Varnshi –shift. Kamat et al, Nature Photonics, 2014; Grancini&Kandada et al, Nature Photonics 2015

The deal for PV is: we exploit the presence of the exciton transition for light harvesting without paying in charge dissociation

Are there excitons around? MAPbI3

Low Temperature, Exciton population is detectable.

THz spectra probe localization effects as a consequence of exciton formation below 80K. Milot et al, Adv Funct Mater, 2015 DOI: 10.1002/adfm.201502340, fs-TA spectra show a modulation of the photo-bleach as a result of exciton-exciton interaction. Grancini&Kandada et al, Nature Photonics 2015

1014

1015

1016

1017

1018

1019

1020

1021

1022

0.0

0.2

0.4

0.6

0.8

1.0

290K

170K

70K

Excitation Density (cm-3)

x (

nF

C/n

TO

T)

5 meV

WARNING: The Exciton binding energy increases (the dielectric constant decreases) when cooling down! Miyata et al, Nature Physics 11, 582–587 (2015)

MAPbI3 Local order/ microstructure

D’Innocenzo et al, Nat Comm. 5, 3586, 2014 Grancini&Kandada et al, Nature Photonics 2015

Designing the Device Architecture

• Light Absorption • Charge Generation • Photo-carriers Transport

53

500 600 700 8000.0

0.2

0.4

0.6

0.8

O.D

.

Wavelength (nm)

CH3NH

3PbI

3-xCl

x/PMMA

0.0

0.5

1.0

PL (

norm

.)

Photo-carriers Diffusion Length

54

Glass

+ - + -

Silica Selective Quencher

Diffusion constant Natural decay rate (no quencher)

Electron-Hole Diffusion Lengths

By Photoluminescence Quenching

55

Electron-Hole Diffusion Lengths

By Photoluminescence Quenching

Science , 342,341, 2013

DT (ne+nh)

CB

VB

56

Perovskite Species D (cm2s-1) LD (nm)

CH3NH3PbI3-xClx Electrons 0.042 ± 0.016 1094 ± 210

Holes 0.054 ± 0.022 1242 ± 250

CH3NH3PbI3 Electrons 0.017 ± 0.009 117 ± 29

Holes 0.012 ± 0.005 96 ± 22

0 50 100 150 20010

-2

10-1

100

PL

(norm

.)

Time after excitation (ns)

PMMA

Spiro-OMeTAD

PCBM

CH3NH3PbI3-xClx

0 10 20 30 40 5010

-2

10-1

100

PL

(n

orm

.)

Time after excitation (ns)

PMMA

Spiro-OMeTAD

PCBM

CH3NH3PbI3

Electron-Hole Diffusion Lengths

By Photoluminescence Quenching

Science , 3

42

,34

1, 2

01

3

57

Time-resolved Photoluminescence

Yamada, Y J. Am. Chem. Soc. 2014, 136, 11610. Stranks, Phys. Rev. Appl. 2014, 2 034007. Saba, M.; Nat. Commun. 2014, 5 No. 5049. D’Innocenzo, J. Am. Chem. Soc. 2014, 136 (51), pp 17730–1773 Deschler, J. Phys. Chem. Lett. 2014, 5, 1421.

Stranks et al

Deschler, et al

58

Time-resolved Photoluminescence

D’Innocenzo et al. JACS, 2014, 136 (51), pp 17730–1773

59

Time-resolved Photoluminescence

D’Innocenzo et al. JACS, 2014, 136 (51), pp 17730–1773

100 1000 10000

760

762

764

766

768

770

772

774

776

778

780

782

Band Edge Position

cwPL Peak Position

Avarage crystallite size (nm)

Spe

ctra

l po

siti

on

(n

m)

CBBB

G

E TkGi

EphphGrad

de

En

dvER

ee

eeee

e )(1

1)()(

)()()()(

Filippetti, et al. J.Phys.Chem, 2014, doi:10.1021/jp507430x

Measuring Charge-Transport in Perovskites Terahertz/Microwave Spectroscopy

Simple sample preparation Only informative on short length-scale

Time-of-flight Relevant over longer length-scale

Time-scales difficult to measure in thin-film

Space-charge limited current Relevant to optoelectronic devices

Needs highly-selective non-limiting contacts

Hall-effect Simultaneously get free-carrier density Like THz, only band mobility obtained

Q. Dong et al., Science (2015)

Q. Dong et al., Science (2015) C. Wehrenfennig et al., Adv. Mat. (2013)

C. Stoumpos et al., Inorg. Chem. (2013)

μ ~ 10 cm2/Vs

μ ~ 25 cm2/Vs

μ ~ 25-165 cm2/Vs

μ ~ 66 cm2/Vs

The room temperature structure of MAPbX3 is a fluctuating structure where titling and

distortion of the octahedral networks and rotations and polarizability of the molecular dipole can strongly affect the optoelectronic

properties of the semiconductor.

Take-home Message

Open Questions

Which is the secret of the low recombination rate Elucidation of the photo-carriers cooling Role of phonons Nature of carriers, localization vs delocalization Carriers transport mechanism

Vs Structural Properties

Technology

Technology

Zhang et al, Mater. Horiz., 2015, 2, 315–322

Technology

Perovskite PV at IIT

p I n

TiOx/PCBM

MAPbI3

Spiro MeOTAD

Aumento efficienza fotovoltaico tradizionale

Fotovoltaico leggero e flessibile, a basso costo di produzione e

energetico

17.6 %

p I n

Processi a bassa temperatura < 120 °C

Celle «TANDEM»

67

Technology Development

Perovskite PV at IIT

Further Developments

Interface Engineering Stability Toxicity ..