Organic Semiconductor-based Plastic Solar Cells What s PVs

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Organic Semiconductor-based Plastic Solar Cells
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Whats PVs
Light energy (photons)
Electrical energy
When sunlight is absorbed by some materials, the
solar energy knocks electrons loose from their
atoms, allowing the electrons to flow through the
material to produce electricity. This process of
converting light (photons) to electricity
(voltage) is called the photovoltaic (PV) effect.
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Solar energy
At earths surface average solar energy is 4 x
1024 J / year Global energy consumption (2001)
was 4 x 1020 J / year (increasing 2 annually)
Source DOE (U.S. Department of Energy)
Source N. Lewis (Caltech)
In US, average power requirement is 3.3 TW. With
10 efficient cells we would need 1.7 of land
area devoted to PV ( area occupied by interstate
highways)
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History

• 1839 Finding of Photovoltaic effect with
  liquid (Edmond becquerel)
• 1876 Photovoltaic effect in a solid (Heinrich
  Hertz)
• 1883 Se solar cell (C. Fritts)
• 1930 Research of Cu2O/Cu solar cell  
• 1941 Patent of Si solar cell (R. Ohl)
• 1954 Crystalline Si solar cell (Bell Lab.) 4
  efficiency
• 1958 Using as assistant power in the spaceship
  (Vanguard I ) 5 mW
• 1973 oil crisis
• 1980 solar cell using CdTe, CuInSe2 ,TiO2 etc.
• 1997 world product 100MWp
• 2000 research of an advanced materials and
  structures
• (dye sensitized solar cell, organic
  solar cell)
• ? cheap process , flexible substrate

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Photovoltaic (PV) effect
In a conventional semiconductor, light absorption
generates an electric field that separates the
photo-induced charges. Ec and Ev are the energies
at the conduction and valence bands,
respectively.
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Cell efficiency
ISC Short-circuit current ? Current
value when V 0 VOC Open-circuit voltage
? Voltage value when I 0 P Power output of
the cell P IV F.F Fill
factor
lt Power conversion efficiency (?) gt
lt Incident-photon-to-current conversion
efficiency (IPCE) gt
Under AM 1.5G simulated solar illumination
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Air mass (AM)
?z
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Sunlight spectrum
Global condition (G) including the diffusion
component (indirect component owing to scattering
and reflection in the atmosphere and surrounding
landscape) Direct condition (D) without the
diffusion component
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Classification
Conventional inorganic p/n junction solar cell
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Progress of cell efficiencies
Under AM 1.5G simulated solar illumination
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PV systems installed in Korea
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PV systems installed in Korea
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Why OPVs?
Advantages of Organic PVs (OPVs) -Processed
easily over large area using -spin-coating
-doctor blade techniques (wet-processing)
-evaporation through a mask (dry processing)
-printing -Low cost -Low weight
-Mechanical flexibility and transparency -Band
gap of organic materials can be easily tuned
chemically by incorporation of different
functional group
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Why OPVs?
The concept of third-generation PV technologies,
originally developed by Martin Green of the
University of New South Wales
1 PV cell based on silicon wafers 2 thin-film
technology 3 high-efficiency thin-film
technology using concepts such as hot carriers,
multiple electronhole pair creation, and
thermophotonics
Cost-efficiency analysis for first-, second-, and
third-generation PV technologies ( labeled 1, 2,
and 3, respectively). Region 3-1 depicts
very-high-efficiency devices that require novel
mechanisms of device operation. Region 3-2 (the
region in which organic PV devices lie) depicts
devices with moderate efficiencies and very low
costs.
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Why OPVs?
Source Siemens AG
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Why OPVs? (Application)
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Why OPVs? (Application)
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Requirement of OPVs
Source Siemens AG
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History of OPVs
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PV effect in conjugated polymer

• Light is absorbed in the polymer layer
• Absorption creates a bound electron-hole pair
  (exciton)
• Exciton is split into separate charges which are
  collected at contacts

• Exciton must be seperated so that a photocurrent
  can be collected.
• Excitons dissociated by electron transfer to an
  acceptor material, or hole transfer to a donor.
• Simplest approach is to make a donor-acceptor
  heterojunction

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Exciton generation separation
Excitons are produced in a conducting polymer. An
incident photon produces bound electronhole
pairs called excitons, which transport charges in
photovoltaic polymers.
Excitons dissociate at interfaces between
materials having different ionization energies
and electron affinities
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Photo-induced charge transfer

• Photon absorption (?A)
• Exciton generation by absorption of light
• Exciton diffusion (?ED)
• Exciton diffusion over LD (20 nm)
• Charge-transfer reaction (?CT)
• Exciton dissociation by rapid and efficient
  charge transfer
• Collection of the carriers (?CC)
• Charge extraction by the internal electric field

?CT
?CC
?A
?ED
cathode
anode
donor
acceptor
?EQE external quantum efficiency ?IQE internal
quantum efficiency
?EQE ?A?IQE ?A?ED?CT?CC
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Device architecture
Top electrode

• Single-layer
• Bilayer
• Bulk heterojunction

Active layer
ITO glass
Single-layer PV cell
Bilayer PV cell
Bulk heterojunction PV cell
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Organic or polymer single-layer PVs
Disadvantage
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Single-layer PVs
Organic single crystals
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Limitation of single-layer PVs
High exciton binding energy
Low bipolar mobility in one molecule
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Organic or polymer bilayer PVs

• Charge transfer can occur between two
  semiconductors with offset energy levels.
• Excitons can diffuse approximately 10 nm to an
  interface. (less than 20 nm)
• A film thickness of approximately 100 nm is
  needed to absorb most of the light.
• Polymer bilayer cell showed 1.9 energy
  conversion efficiency.
• Small molecule bilayer cell showed 3.6 power
  conversion efficiency with 3 layers.

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Bilayer PVs
Small molecular organic bilayer PV cell
Improving molecular PV cell
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Limitation of organic or polymer bilayer PVs
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• Bulk heterojunction (BHJ) PVs

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Device geometries
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Working principle of BHJ device
1. Incoming photons are absorbed ? Creation of
excitons on the Donor / Acceptor 2. Exciton is
separated at the donor / acceptor interface ?
Creation of charge carriers 3. Charge carriers
within drift distance reach electrodes ? Creation
of short circuit current ISC 1. The
photodoping leads to splitting of Fermi levels
? Creation of open circuit voltage VOC 2. Charge
transport properties, module geometry ? Fill
factor FF
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Photoinduced charge generation
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Charge transfer
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Charge recombination
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3-D percolation
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3-D percolation - PL quenching
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3-D percolation - morphology and transport
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3-D percolation morphology and transport
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Efficiencies
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Production
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Film preparation
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Film preparation
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Optimization for high efficiency
1. Conjugated polymer with low band gap Maximum
photon flux of sun 700 nm Eg 1.24 / 0.7
1.77 eV Maximum absorption of photon of sun
Isc tuning of the transport property (mobility)
Optimization of cell geometry in dependence of
the cell thickness Voc tuning of the electronic
energy level of the donor-acceptor system Voc of
2 V observed in polymeric donor- acceptor
system F.F tuning of the contacts and
morphology lowering of serial resistance
2. Bulk heterojunction morphology exciton
diffusion length of conjugated polymer below
20 nm
3. High carrier mobility electron and hole
mobility of conjugated polymer
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Materials for BHJ organic solar cell
PCBM
MDMO-PPV
F8BT
P3HT
CuPc pentacene
TiO2 or ZnO nanoparticles
C60
PFB
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• Polymer/PCBM interpenetrating system

Voc 0.82 V Jsc 5.25 mA/cm2 FF 0.61 ? AM1.5G
2.5 (under 80 mW/cm2)
PCBM
MDMO-PPV
lt S. E. Shaheen, et al., Appl. Phys. Lett. 1998,
395, 257 gt
A
D
Metal electrode
LiF
Active layer
PEDOTPSS
ITO
glass
Donor/Acceptor composite solution
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Materials issue - matching the solar emission
The flexibility in chemical tailoring is
necessary for matching the absorption of the PV
material to the solar emission spectrum.
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Bandgap engineering
lt The parameters determining the bandgap of
conjugated polymers gt
E?r the energy contribution from bond length
alternation RE the resonance energy ET the
energy caused by the inter ring torsion angle
ESUB the influence of the substituents.
EG E?r RE E? Esub
1. Aromatic form shows higher stabilization
energy and therefore the higher bandgap. 2.
Resonance energy leads to an energy stabilization
and so to an increased splitting of the HOMO-LUMO
energy. 3. Torsion between the ring plain
interrupts the conjugation and therefore
increases the bandgap. 4. Electron donating
groups raise the HOMO level and electron
withdrawing groups lower the LUMO. 5. In the
solid phase, additional intermolecular effects
between the chains have to be taken into account,
which generally leads to broader bands and a
lower bandgap.
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Synthetic strategy
1. Introduction of side groups increase or
decrease the electron density 2. Push-pull
polymers The bandgap of copolymers with
alternating of electron rich and electron poor
compounds can decrease significantly. (The bond
length alternation is reduced and so the Peierls
stabilisation.) 3. Introduction of methine
groups between the ring systems The quinoid form
minimize the inter annular rotation by the double
bond character of the bridge bonds as well as the
bond length alternation . (The structure becomes
more flat and the resonance between the rings is
increased.)
lt Potential diagram vs. bond length alternation
for (a) trans-polyacethylene as conjugated
polymer with degenerate ground state (b) for
polyphenylene as conjugated polymer with
nondegenerate ground stategt
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Low bandgap polymer - PTPTB
Poly-N-dodecyl-2,5-bis(2-thienyl)pyrrole-2,1,3-be
nzothiadiazole PTPTB
the push-pull concept by altering electron rich
N-dodecyl-2,5-bis(2-thienyl)pyrrole and electron
deficient 2,1,3-benzothiadiazole groups
Voc 720 mV ISC 3 mA/cm2 FF 0.38 ? 1
Enhancement of absorption area
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Origin of open-circuit voltage (Voc)
Which is the Voc? Is it in the electrodes? (MIM
picture) ? Voc Is it in the bulk-heterojunction?
(p/n-like picture) ? Voc
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Variation of Voc

• Voc in plastic solar cell is directly related to
  the acceptor strength of the fullerene.
• The variation of negative electrode work function
  influences the Voc in only a minor way.

S1 gt S2
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Optimization of contacts
LiF layers forms ohmic contact for electrons at
the Al electrode Contact resistivity limits FF.
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Optimization of morphology
Voc 810 mV ISC 5.2 mA/cm2 FF 0.62 ? 3
IPCEmax 50 EQE 8090
The efficiency is strongly enhanced by using
cholorobenzene in MDMO-PPV /PCBM mixture.
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P3HTPCBM post production treatment
P3HT post production treatment increased
absorption strength in the red Higher
conversion efficiency Diode characteristics is
improved
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Percolation problem in composites
Both donor and acceptor phases have to be
percolated.
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Double Cable polymers
Isc 0.42 mA/cm2 Voc 0.83 V FF 0.29 (white
light 88mW/cm2)
Both donor and acceptor phases will be percolated
at very low filling into a host polymer
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• Hybrid polymer/nanoporous TiO2 system

lt Ideal device structure - ordered bulk
heterojunctions gt
TiO2 can be easily patterned into a continuous
network for electron transport.
For efficient photoinduced charge generation
Easy to model Semiconductors can be
changed without changing the geometry.
Almost all excitons can be split No
deadends Polymer chains can be aligned
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Photoinduced charge generation - infiltration
ltInfiltrating polymers into mesoporous TiO2 filmgt
ltPL quenching measurementgt
Charge geneation
e-
Radiative decay (recombination)
Because excitons in conjugated polymer typically
travel less than 20 nm before recombination,
electron donor and acceptor must form
interpenetration of 3-D network for efficient
photoinduced charge generation using a
infiltration step.
h
TiO2
Polymer
Good infiltration Effective charge generation Low
PL efficiency
Poor infiltration charge recombination High PL
efficiency
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P3HT/nanoporous TiO2 PV cell
lt Melt infiltration step gt
lt Mesoporous titania films gt
33 of the volume of the film can be filled in
several min.
61

• Polymer/CdSe nanocrystal system

Quantum dot CdSe ? Well defined
photosensitivity large
excition bohr radius quantum
confinement
7 nm by 7 nm 7 nm by 30 nm 7 nm by
60 nm
W.U. Huynh, J. Dittmer, A.P. Alivisatos Science,
295 (2002) 2425
Power conversion efficiency 1.7
62

• Small molecular weight organic system

lt Small molecular organic semiconductor materials
gt
63
CuPc/PTCBI device
C. W. Tangs Heterojunction Solar Cell first
heterojunction for efficient charge generation
0.95 conversion efficiency nearly ideal IVs
(FF0.65) under full solar illumination (1 sun)
Photoluminescence (PL) probes the exciton
lifetime Exciton lifetime depends on proximity
of donor acceptor interface
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Double heterojunction
cathode metal diffusion deposition damage
exciton-plasmon interaction vanishing optical
field electrical shorts
Introduce Exciton Blocking Layer (EBL) to
confine excitons to active region act as a
damage-absorber
65
Exciton blocking layer
Exciton Blocking Layer (EBL) Improves thin cell
efficiency
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• Laminated polymeric system

Au/PEDOT/ POPTMEH-CN-PPV (191) laminated at 200
C onto the MEH-CN-PPVPOPT (191) /Ca Power
conversion efficiency around 4.8 at 480nm
irradiation. Calculated AM1.5 efficiency around
1.9 Large scale large area fabrication
potential M. Granström, K. Petritsch, A. Arias,
A. Lux, M. Andersson and R. H. Friend, Nature
395, 257 (1998)
MEH-CN-PPV rich
POPT rich
anealing
200 oC
D
A
laminated fabrication
Al or Ca
glass
ITO or PEDOT on gold
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• Dye-Sensitized Solar Cell (DSSC)

lt Cell reactions gt - S(adsorbed) h?
? S ?(adsorbed) - S ?(adsorbed) ? S
(adsorbed) e-(injected) - S
(adsorbed) A- ? S(adsorbed) A -
A(cathode) e- ? A-(cathode)
lt Structure Principle gt
(S/S?)
e-
cb
e-
VOC
e-
(A/A-)
electrolyte
e-
(S/S)

vb

e-
Adsorbed dye
lt Advantages gt Low cost Utilization
of visible range of light Simple
manufacturing process Environmental
compatibility Transparent solar cell
- Window Moderate efficiency 10
ITO
TiO2
Pt
e-
B. ORegan and M. Grätzel, Nature, 353, 737
(1991)
68
Key Components
1. Nanocrystalline SC ? large surface area, high
porosity, pore size distribution, light
scattering, electron percolation, Anatase (TiO2),
ZnO, SnO2, Nb2O5 2. Sensitizers (Dye) ?
distribution of the dyes on the semiconductor
surface, spectral properties, redox properties in
the ground and excited state, anchoring groups
(carboxylate or phosphonate), Polypyridyl,
Porphyrins, or Phthalocyanines complexes 3.
Electrolyte ? ionic conductivity, electron
barrier and hole conductor, redox potential,
mechanical separator, interfacial contact for
dye, TiO2 and counter electrode (I/I3) 4. Extra
? transparent conductive oxide (conductivity,
transmittance), sealing, metal grid, counter
electrode
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Dynamics

• Excitation of dye under illumination (ns)
• Electron injection (ps)
• Electron transport (ms)
• Regeneration of dye (10 ns)
• Recombination with oxidized redox (ms)
• Recombination with oxidized dye (s)

30 mM of I is enough to reduce the most of dye
cations
I. Montanari et al., J. Phys. Chem. B, 106, 12203
(2002)
70
Current issues
To increase performance of DSSC
Jsc Diffusion coefficient (Length) H or
Li cation on TiO2 TiCl4 acidic sol.
Treatment Increase in adsorbed dye
Power curve (I-V curve)
FF
Voc Electron lifetime (Recombination)
TBP, Ammonia in electrolyte
Secondary oxide layer
Jsc
Voc
FF Series and Shunt Resistance
(Recombination) Secondary oxide layer
Competition between Jsc and Voc High carrier ?
High probability of

recombination
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Summary technology challenges
Polymer PV devices are widely recognized to
have potential to provide flexible, low cost,
renewable energy for a wide range of
applications For these devices to be
commercially viable, three important areas must
be addressed
Source CDT
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Summary optimization
Materials Up to now, polymers for PVs have
largely been taken from the LEP program Work
underway at CDT to develop new polymers optimized
to absorb solar radiation Materials optimized
for electron or hole transport
Device Architecture Morphology of polymer blend
crucial to determining device performance
Morphology can be controlled through careful
processing, surface treatment and materials
design Many advances in LEP architecture are
applicable to PV device development
Source CDT
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Summary interdisciplinary R D
Source Linz Institute for Organic Solar Cells
(LIOS)