Fundamental Processes in   Organic and Hybrid Solar Cells

www.tu-chemnitz.de/physik/OPKMLHP15, Kloster Banz · 12. März 2015 · deibel@physik.tu-chemnitz.de
Carsten Deibel
Optik und Photonik kondensierter Materie
Institut für Physik
Technische Universität Chemnitz
Fundamental Processes in  
Organic and Hybrid Solar Cells

www.tu-chemnitz.de/physik/OPKMdeibel@physik.tu-chemnitz.de
How Do Organic Solar Cells Work?
Step 1: Light Absorption
➟ Exciton Generation in Polymer
Fullerene
Aluminium Cathode
Transparent Anode
Polymer
Voltage
Current

Step 2: Exciton Diffusion
➟ to Acceptor Interface
Fullerene
Aluminium Cathode
Transparent Anode
Polymer
Voltage
Current
singlet losses

Step 3: Exciton Dissociation
➟ Polaron Pair Generation
Fullerene
Aluminium Cathode
Transparent Anode
Polymer
charge transfer:
very fast and
very efﬁcient
Voltage
Current
singlet losses

Step 4: Polaron Pair Dissociation
➟ Free Electron–Hole Pairs!
Fullerene
Aluminium Cathode
Transparent Anode
Polymer
Voltage
Current
singlet losses
geminate losses

Step 5: Charge Transport
➟ Photocurrent
Fullerene
Aluminium Cathode
Transparent Anode
Polymer
Voltage
Current
singlet losses
geminate losses
nongeminate losses

Outline
photogeneration vs geminate recombination
experiment: effect of intercalation
nongeminate recombination
simulation: effect of phase separation
radiative efﬁciency
experiment: perovskite photovoltaics

Model System: Intercalation
OCH3
O
S
S
S
S
R
R
n
pBTTT pBTTT:PCBM
1:4
1:0.7
1:1
OCH3
O
H3CO
O
pBTTT:bisPCBM
1:1
PCBM
c.f. Cates et al, Nano Lett. 9, 4153 (2009) and Nano Lett.12, 1566 (2012)

Intercalation→ Microstructure
pBTTT:PCBM
1:4
1:1
100
80
60
40
20
Counts/a.u.
4.54.03.53.02.52.0
2θ / °
neat pBTTT
pBTTT:PCBM, 1:1
pBTTT:PCBM, 1:4
pBTTT:bisPCBM, 1:1
3.1 nm
2.3 nm
85 °C, 30 min
pBTTT:bisPCBM
1:1
OCH3
O
OCH3
O
H3CO
O
S
S
S
S
R
R
n

Formation of Charge Transfer Complex
pBTTT:PCBM
1:4
1:1
pBTTT:bisPCBM
1:1
10
1
10
2
10
3
10
4
10
5
10
6
10
7
Absorption/cm
-1
1000 800 700 600 500
Wavelength / nm
3.02.52.01.51.0
Energy / eV
neat pBTTT
pBTTT:PCBM, 1:1
pBTTT:PCBM, 1:4
pBTTT:bisPCBM, 1:1
CT
ground 
state
singlet 
excitons
CT 
free 
carriers
c
h

Time Delayed Collection Field (TDCF)
0
td tcoll
-V
Vpre
Vcoll
t
laser
pulse
0
j
Qpre
Qcoll
t
Qtot = Qpre+Qcoll
Laser excitation: 532 nm, 80 ps
typical delay time td=20ns
c.f. Mort et al, PRL 45, 1348 (1980)

Field Dependent Photogeneration
In Dependence of Intercalation
1.0
0.8
0.6
0.4
0.2
0.0
Normalizedchargeqtot
-4 -3 -2 -1 0
Prebias / V
pBTTT:PCBM
1:1
1:4
532nm
pBTTT:PCBM
1:4
1:1

Hot or not? Scenarios:
ground 
state
singlet 
excitons
CT 
complexes
free 
carriers
fast 
relaxation
ground 
state
singlet 
excitons
CT 
complexes
free 
carriers
hot 
dissociation
Photon energy dependence
1.0
0.8
0.6
0.4
0.2
0.0
-4 -3 -2 -1 0
Prebias / V
pBTTT:PCBM
1:1
1:4
532nm

Hot or not?
P=P(λ)
P≠P(λ)
ground 
state
singlet 
excitons
CT 
free 
carriers
fast 
relaxation
ground 
state
singlet 
excitons
CT 
free 
carriers
hot 
dissociation
532nm532nm
1064
nm
1064
nm
1.0
0.8
0.6
0.4
0.2
0.0
-4 -3 -2 -1 0
Prebias / V
pBTTT:PCBM
1:1
1:4
532nm

Hot or not?
Only weak photon energy
dependence on photogeneration yield
P=P(λ)
P≠P(λ)
ground 
state
singlet 
excitons
CT 
free 
carriers
fast 
relaxation
ground 
state
singlet 
excitons
CT 
free 
carriers
hot 
dissociation
532nm532nm
1064
nm
1064
nm
1.0
0.8
0.6
0.4
0.2
0.0
-4 -3 -2 -1 0
Prebias / V
pBTTT:PCBM
1:1
1:4
532nm
1064nm

Subsequent Charge Extraction
1.00
0.95
0.90
0.85
0.80
Extractionefficiency
-4 -3 -2 -1 0
Prebias / V
1.00
0.95
0.90
0.85
0.80
0.75
0.70
pBTTT:PCBM
1:4
pBTTT:bisPCBM
1:1
pBTTT:PCBM
1:4
pBTTT:bisPCBM
1:1
A. Zusan, K. Vandewal, …, C. Deibel. Adv. Ener. Mater. 4, 1400922 (2014)

Intermediate Conclusions on
Photogeneration
Photogeneration depends on electric ﬁeld
• strong vs weak ﬁeld dependence  
(pBTTT 1:1→1:4): role of delocalisation!
Photogeneration is approx.  
photon energy independent
• Photogeneration via  
relaxed intermediate state  
(charge transfer complex)
ground 
state
singlet 
excitons
CT 
free 
carriers
fast 
relaxation
A. Zusan, K. Vandewal, …, C. Deibel. Adv. Ener. Mater. 4, 1400922 (2014)

Outline
photogeneration vs geminate recombination
experiment:
nongeminate recombination
simulation: effect of phase separation
radiative efﬁciency
experiment: perovskite photovoltaics

LUMO
HOMO
(1)
(2)
(1)
Nongeminate Recombination by Langevin
1st guess for Recombination in low-mobility materials
}R(n) / (µe + µh)n2
(1) finding of charge carriers → mobility μ
(2) recombination event (faster than (1))

Deviations from Langevin Recombination
300
200
100
0
-100
currentdensity[A/m
2
]
0.80.60.40.20.0
voltage [V]
dark 1 sun
w/o add
with add
PCE [%] FF [%]
w/o add 3.8 51
with add 7.1 69
• „reduced Langevin“, c.f. Pivrikas et al,
PRL 94, 176806 (2005)
• R(n) due to μe(n)+ μh(p)?
PC70BM
F
S
S
O
S
S
O
O
n
O
PTB7

(a) (b) (c)
Impact of Morphology
Phase separation
with bulk traps
Phase separation,
bulk & interface traps
Classical Langevin
case, R ∝ (μe+μh)⋅n2
R ∝ f(μe,μh)⋅n2 ? In addition, deep
interface traps?
Homogeneous,  
„effective medium“

Kinetic Monte Carlo Simulation
• hopping transport
• energetic disorder, e.g. Gaussian
density of states distribution
• blend morphology
• determine μe, μh, and R=k(μe, μh)npAcceptor
Donor
+
E-Field
−
c.f. Bässler, PSSB 175, 15 (1993)

Simulated Nongeminate Recombination
small domains: almost as homogeneous system, Langevin
R = knp
k =
e
✏
(µe + µh) = kL

large domains: min mobility, Koster et al, APL 88, 052104 ︎2006︎
R = knp
k =
e
✏
min (µe, µh)

general case: „mix of both“!

Result:
power law mean
R = knp
k =
e
✏
f1(d)2Mg(d) (µe, µh)
where Mg =
✓
µg
e + µg
h
2
◆1/g
d (nm) f1 g Case
small domains < 5nm ≈ 1 1 arithmetic mean
typical case 10–35 0,5–1 0 geometric mean
large domains > 50nm < 0,5 -1 harmonic mean k /
✓
1
µe
+
1
µh
◆ 1
k /
✓
µe + µh
2
◆
k / (µeµh)
1/2
M. Heiber, V. Dyakonov, C. Baumbach, C. Deibel. Accepted by PRL (2015). arXiv:1503.02848

Intermediate Conclusions on
Nongeminate Recombination
Charge carrier recombination in
dependence of phase separation
• most organic bulk heterojunction
solar cells:  
geometric mean of mobilities 
better approximation of
nongeminate losses than
Langevin’s arithmetic mean
• reduction factor partly explained
for asymmetric mobilities
M. Heiber, V. Dyakonov, C. Baumbach, C. Deibel. PRL, accepted (2015). arXiv:1503.02848

Absorption tail of Inorganics
De#Wolf.#et.#al.#JPCL%5%1035%(2014)%

…of an OPV
De#Wolf.#et.#al.#JPCL%5%1035%(2014)%
PPV:PCBM(
(EQE%shape)

De#Wolf.#et.#al.#JPCL%5%1035%(2014)%
EU=15#meV
…and a Perovskite

MAPI Perovskite cells
ayout of the studied solar cells. (A) Perovskite (MAPI) and (B) Organic (MDMO-PPV:PCBM) cells.
www.nature.com/scientificreports
Jsc 18,9 mA/cm2
Voc 1080 mV
FF 55 %
η 11,2 %
Solar cell processing including
perovskite coevaporation of the two
starting materials PbI2 and CH3NH3I
by Bolink group in Valencia
c.f. Reviews by Snaith, Grätzel, etc

Photovoltaic Quantum Efficiency
1.5 2.0 2.5 3.0 3.5
Energy (eV)
0.0
0.2
0.4
0.6
0.8
1.0EQE PV
EQEPV
normalised

and Emission (Electroluminescence)
1.5 2.0 2.5 3.0 3.5
0.0
5.0x10
17
1.0x10
18
1.5x10
18
Energy (eV)
EQEEL
=1.2E-4
PhotonFluxnr/(s*eV*m
2
) Emission Flux
(Jinj
=189 A/m2
)
0.0
0.2
0.4
0.6
0.8
1.0EQE PV
EQEPV
Electroluminescence Yield = 0,012 % for Jinj = Jsc (1 sun)

EQEPV vs EL

Black Body Radiation: Sun and Sample
EQEP V (E) = EL(E)E 2
exp
✓
E
kT
◆
Reciprocity: G. Kirchhoff (1860); P. Würfel (1982); U. Rau, PRB (2007)
SunEarth

EQEPV vs EL: Reciprocity Relation

A figure of merit to judge solar cell performance.
External Radiative Efficiencies (ERE)
M. A. Green, Prog. Photovolt: Res. Appl. 20:472 (2012)

A figure of merit to judge solar cell performance
External Radiative Efficiencies (ERE)
K. Tvingstedt, Malinkiewicz, Baumann, Deibel, Snaith, Dyakonov, Bolink. Sci Rep, 4:6071, 2014
M. A. Green, Prog. Photovolt: Res. Appl. 20:472 (2012)

Intermediate Conclusions on  
Radiative Efficiency in Perov’SC
Perovskite Hybrid Solar Cells
• steep absorption onset:  
high degree of order
• reciprocity fulﬁlled
• EL yield: promising
• voltage induced PL quenching:
dominantly free carriers
K. Tvingstedt, Malinkiewicz, Baumann, Deibel, Snaith, Dyakonov, Bolink. Sci Rep, 4:6071, 2014
www.nature.com/scientificre

Acknowledgments…
Andreas Zusan
Michael Heiber →
Kristofer Tvingstedt
Alexander Wagenpfahl →
Alexander Förtig
Jens Pﬂaum
Vladimir Dyakonov
Martin Heeney
Koen Vandewal
(→TUD)
Alberto Salleo
Juliane Kniepert
Thomas Brenner
Dieter Neher
Arved Hübler 
Bystrik Trnovec 
Tino Zillger et al.

Fundamental Processes in   Organic and Hybrid Solar Cells

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