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First Principles Studies of the Electronic Structure of Organic Semiconductors and Interfaces
Sahar Sharifzadeh
Molecular Foundry, Lawrence Berkeley National Lab
2010 User Meeting
Jeff Neaton
Leeor Kronik (Weizmann)
Ariel Biller (Weizmann)
MF theory group
Computational resources:
National Energy Research Scientific Computing Center (NERSC) Scientific cluster support (SCS) at LBL MF IT division
Financial support DOE NanoHub NSF BASF ISF
Eric Isaacs
Biwu Ma
Electronic Structure and Device Performance
B. Ma, et al., Proc. SPIE , (2009);
C.E. Mauldin, et al., ACS Appl. Mater. Interfaces, 4, 2627(2010).
Donor Layer
PEDOT:PSS
Ag
ITO Glass
C60BCP
∆∆∆∆
HOMO
HOMO
LUMO
LUMO
Donor C60
Computational Methods
Kohn-Sham DFT within standard approximations
Shortcomings for the study of organics
Poor description of highly localized electrons
No VdW interactions Ground state theory -- No electron-hole interactions
Excited states
Ionization potential and electron affinity of molecules obtained as
total energy differences within DFT (∆SCF)
Many-body perturbation theory
Geometry optimization with GGA-PBE
Lattice vectors of crystals kept at experimental value
Many-body Perturbation Theory
Addition/removal energies via the GW approximation
Excitation energies obtained as first order correction to DFT
ε
V==Σ WGW,knVknknEknEE DFT
xc
GWA
n
DFT
n
GWA
n
vvvv−Σ+= )(
kkk
Hybertsen and Louie, Phys. Rev. 34, 5390 (1986).
Many-body Perturbation Theory
Addition/removal energies via the GW approximation
S.G. Louie, in Topics in
Computational Materials Science, (World Scientific, Singapore, 1997)
Many-body Perturbation Theory
Rohlfing and Louie, PRL 81 2312 (1998).
Optical excitation energies via the solution of the BSE
Solution for two-particle correlation function:
Electron-hole interactions explicitly accounted for
can get exciton binding energy
(Eck− Evk
)Avck
S+ vck
′ v ′ c ′ k
∑ Keh
′ v ′ c ′ k A′ v ′ c ′ k
S=Ω
SAvck
S
Addition/removal energies via the GW approximation
Excitation energies obtained as first order correction to DFT
ε
V==Σ WGW,knVknknEknEE DFT
xc
GWA
n
DFT
n
GWA
n
vvvv−Σ+= )(
kkk
)( BSE
gap
GW
gap EE −=∆
Outline
Electronic Level alignment at donor-acceptor interfaces
Excited states in bulk organic crystals
S.S., A. Biller, L. Kronik, J.B. Neaton
E.B. Isaacs, S.S., B. Ma, J.B. Neaton
Practical Applications
HORIBA Scientific
Thin-film transistors
Shimada et al., Jap. J. App. Phys. 47, 184 (2008)
Spintronics
Pentacene PTCDA
Photovoltaics
Derouiche and Djara, Solar Energy Materials & Solar Cells 91, 1163 (2007)
Existing ExperimentsPentacene PTCDA
Hill, et al., Chem. Phys. Lett. 327, 181 (2000).,Amy, et al., Organic electronics 6, 85 (2005).
Pentacene on Au
Spectroscopy
Park, et al. APL 80, 2872 (2002), Bulović, et al., Chem. Phys. 210, 1 (1996)
UPS/IPES
Optical
Absorption
Previous Theoretical StudiesPentacene PTCDA
Tiago,et al., PRB 67, 115212 (2003)]
Pentacene crystal
Dori,et al., PRB 73, 195208 (2006)
PTCDA molecule
Hummer et al., Modern Physics Letters B 20, 261 (2006)]
Pentacene crystal
GW Bandstructure GW DOS Absorption Spectra
Prototypical Organic Crystals
Pentacene (C22H14)
Triclinic, space group
2 molecules/unit cell
1P
PTCDA (C24H8O6)
Monoclinic, P2/m space group
2 molecules/unit cell
C
H
O
HOMO/LUMO Energies of Molecules and GW
Pentacene
HOMO
LUMO
Evac = 0
DFT-PBE GW ∆SCF expt.
HOMO
LUMO
HOMO/LUMO Energies of Molecules and GW
PTCDA
DFT GW ∆SCF expt.
Evac = 0
HOMO
LUMOLUMO
HOMO
Calculated gap agrees well with reported results: Dori,et al., PRB 73, 195208 (2006)
Agrees well with previous studies: Tiago,et al., PRB 67, 115212 (2003)]
Energ
y (e
V)
Pentacene Crystal Band-structure and
Density of States
2.2 eV2.4 eV
GW
-Corre
cte
d D
ensity
of S
tate
s
0.7
0.4
Along k
Γ X Y Γ Z E Z
HOMO-LUMO Gaps and Polarization
)1
(2
~2
ε
ε −
R
qP
PEEmolecule
gap
Crystal
gap *2−=Molecule represented by
a sphere in dielectric medium
3/1
cellunit
4
3*
2
V
=
πR
4.7 eV 2.7 eV
Isolated molecule Bulk crystal
HOMO
LUMO
HOMO
LUMO
4.5 eV 2.2 eV
HOMO
LUMO
Isolated molecule Bulk crystal
HOMO
LUMO
(2.1) (2.3)+P
-P
Comparison with Photoemission
De
nsity o
f sta
tes Broadened GW-corrected DOS
Uncertainties in interpreting photoemission data
• For PTCDA, definition of gap varies from 2.5 - 4.0 eV•Correction to gap for surface v. bulk polarization:0.6 eV•Controversy on whether the edge or peak value of orbitals should be taken • Correction to gap for vibrational effects: 0.2 eV
Energy (eV)
2.88
4.0
UPS IPESHill, et al., Chem. Phys. Lett. 327, 181 (2000).
Zahn, et al., Chem. Phys. 325, 99 (2006).
Krause, et al. New J. Phys. 10, 085001 (2008).
The Optical Gap and Delocalized Nature of the
Exciton
Lowest-energy excitation is π π*
Optical gap (eV) Molecule Crystal
Pentacene
GW/BSE 2.2 1.7*
Expt. 2.3 1.8
PTCDA
GW/BSE 2.7 2.2
Expt. 2.6 2.2
GW/BSE within 0.1 eV of experiment!
*Agrees well with previous studies: Tiago,et al., PRB 67, 115212 (2003)]
X
|Ψe|2
Pentacene Crystal
Previous theoretical predictions ∆∆∆∆ = 0.1- 0.6 eV for pentacene crystal
Nayak and Periasamy, Organic electronics 10, 1396 (2009); M.L. Tiago,et al., PRB 67, 115212 (2003);K. Hummer et al., Modern Physics Letters B 20, 261 (2006)
∆∆∆∆ = 0.6 eV for PTCDA crystalNayak and Periasamy, Organic electronics 10, 1396 (2009)
Experimental estimates
∆∆∆∆ = 0.4-1.6 eV for pentacene crystalAmy, et al., Organic electronics 6, 85 (2005)
∆∆∆∆ = 0.3-1.8 eV for PTCDA crystalHill, et al., Chem. Phys. Lett. 327, 181 (2000); Zahn, et al., Chem. Phys. 325, 99 (2006);
Krause, et al. New J. Phys. 10, 085001 (2008)
The Exciton Binding Energy
PTCDA:
• Molecule: 2.0 eV
• Crystal: 0.5 eV
Pentacene:
• Molecule: 2.3 eV
• Crystal: 0.5 eV ~ 1/ε * 2.3 eV ~ 1/ε * 2.0 eV
Exciton binding energy (∆) = Eopt – EHOMO-LUMO
Conclusions I
GW/BSE accurate in describing excited states in organics
Electrostatic model for addition/removal energies is reasonably accurate for describing bulk polarization
Exciton binding energy
Predicted to be ~0.5 eV for both pentacene and PTCDA
~1/ε * exciton binding energy of the molecule
Simple electrostatics can be applied to extrapolate energetics of other solid-state organic systems from single molecule calculations
Can use ∆SCF to get HOMO/LUMO gaps of molecules
4.7 2.7
Can We Explain Variations in VOC?
Voc ~ ECT?
B. Ma, et al., Proc. SPIE 21, 1413 (2009);
C.E. Mauldin, et al., ACS Appl. Mater.
Interfaces 4, 2627(2010).4Ta-SubPc
4Tp-SubPc
2Ta-SubPc
2Tp-SubPc
SubPc-A
The Charge Transfer Energy and Voc
crystal
Acceptor
erface
Donor
erfaceE ∆−−= intintCT EAIP
HOMO
HOMO
LUMO
LUMO
Donor Acceptor
ECT
A Simple Estimate of the Charge Transfer Energy
Non-linear relationship IP-EA one order of magnitude larger than VOC
HOMO
LUMO
Acceptor
HOMO
LUMO
Donor
~Voc?
Incorporation of Crystal Effects with Electrostatics
?EAIP~VOC
Acceptor
crystal
Donor
crystal −
PIPIP moleculecrystal −= PEAEA moleculecrystal −=
The Exciton Binding Energy is Morphology-Dependant
rcrystal
ε
1~∆
crystal
Acceptor
crystal
Donor
crystalCTE ∆−−= EAIP~VOC
The Exciton Binding Energy is Morphology-Dependant
The Exciton Binding Energy is Morphology-Dependant
rcrystal
ε
1~∆
crystal
Acceptor
crystal
Donor
crystalCTE ∆−−= EAIP~VOC
The Exciton Binding Energy is Morphology-Dependant
The Exciton Binding Energy is Morphology-Dependant
crystal
Acceptor
crystal
Donor
crystalCTE ∆−−= EAIP~VOC
rcrystal
ε
1~∆
r ~ 6.9 Å
∆ ~ 0.52 eV
4~
LUMO and HOMObetween charge ofCenter ~
ε
r
Correlation Between VOC and ECT
The Exciton Binding Energy is Morphology-Dependant
crystal
Acceptor
crystal
Donor
crystalCTE ∆−−= EAIP~VOC
rcrystal
ε
1~∆
r ~ 12.2Å
∆ ~ 0.27 eV
r ~ 13.3 Å
∆ ~ 0.30 eVr ~ 6.9 Å
∆ ~ 0.52 eV
4~
LUMO and HOMObetween charge ofCenter ~
ε
r
pentacene/C60 interfaceYuanping Yi, et al., JACS, 131, 15777 (2009).
Excellent Correlation Between VOC and ECT
Excellent correlation between ECT and Voc Need model for interface morphology
Conclusions II Studied the interface between C60 and a variety of novel donor materials
for OPV
Calculated charge-transfer excitation energy correlates well with measured open-circuit voltage once morphological considerations are incorporated
We predict that crystalline systems have a smaller Voc than amorphous ones due to larger exciton binding energy
Future work Beyond the Mulliken limit description: neutral excitation energies with TDDFT
Better understanding of interface morphology either experimentally or through theoretical models
HOMO
HOMO
LUMO
LUMO