Delivered by Prof. Alan Dunbar, University of Sheffield, as part of CDT-PV core-level training, Jan 2016

1. ORGANIC PHOTOVOLTAIC
DEVICES
Alan D. F. Dunbar
Department of Chemical and Biological
Engineering
University of Sheffield
a.dunbar@sheffield.ac.uk

2. OVERVIEW
1. Basic cell design and structure.
2. What properties are needed in the different layers?
3. Interactions with light
4. What is a conjugated polymer?
5. Structure of a polymer-fullerene blend (crystalline vs amorphous materials)
6. Device optimisation (Jsc & Voc)
7. What are the common OPV polymers (e.g. P3HT, PCDTBT, PTB7)
8. Where is the state of the art now?
9. Where next? Scale up - slot dye, spray-coating, IJP etc.

3. TYPICAL
PHOTOVOLTAIC
DEVICE
STRUCTURE
Light
Active Layer
Electron Transport Layer
Cathode
Hole Transport Layer
Transparent Conducting Anode
Transparent Substrate

4. Layer Required Properties Examples
Encapsulation Blocks H2O and O2 Epoxy & glass,
Cathode Layer Metallic conductor with suitable energy level to receive
electrons from BHJ
Calcium &
Aluminium, Silver
Electron transporting layer Conducts electrons well, blocks holes, has suitable energy
levels to make good contact to the cathode & active layer
PCBM, Not
always present
BHJ Active layer Semiconductor materials that absorb visible light, BHJ
structure with correct energy levels to promote excitons
separation, and interconnectivity to transport electrons to
the cathode and holes to the anode
P3HT:PCBM,
PCDTBT:PCBM
Hole transporting layer Conducts holes well, blocks electrons, has suitable energy
levels to make good contact to the anode & active layer.
Transparent.
PEDOT:PSS
Transparent Conducting
Electrode (Anode)
Transparent, conducts electricity and can be patterned to
define pixels
ITO, FTO,
Graphene
Substrate Transparent, may be flexible blocks H2O and O2 Glass, PET

5. 5
When atoms come together to form molecules or crystals
this results in a splitting of the energy levels into several
discrete levels
When many atoms come together to form a crystal there are
so many energy levels close together they are considered to
be a continuous energy band
Filled energy levels
Empty energy levels
UNDERSTANDING MATERIALS

6. 6
If the filled and empty bands overlap the crystal will be metallic –
car park analogy. If the bands do not overlap the material will be
either an insulator or a semiconductor (depending upon the size
of the energy gap)
Metal Semiconductor or Insulator
METALS, SEMICONDUCTORS &
INSULATORS

7. 7
Material
Type
Approx. Band
Gap Range
Properties
Metal None
Readily conducts electricity and heat
since many electrons are available for
conduction
Semimetal 0 eV to 0.5 eV
Reasonably high conductivity since at
room temperature some electrons
are available for conduction
Semiconductor 0.5eV to 3 eV
Small conductivity in the dark at
room temperature since only a very
small number of electrons are
available for conduction. This
increase if illuminated or heated
Insulator > 3 eV
Negligible conductivity at room
temperature since essentially no
electrons are available for conduction

8. EXCITATIONS
A photon (a particle of light) carries energy E = h.
If the photon energy > band gap energy, then the photon can be absorbed and
thereby ‘excite’ an electron from the valance-band to the conduction band.
This leaves a positively charged hole in the valance-band.
The energy of the incoming photon is “stored” in the energy-difference between the
electron and hole. In semiconductors, conductivity occurs by the flow (transport) of
electrons or by positively charged "holes" in the electron structure of the material.
8
Metal Semiconductor or Insulator
Very fast
~fs
Very fast
~fs
Slower
~µs

9. WHY ARE SEMICONDUCTORS
NEEDED IN A SOLAR CELL?
It comes down to the probability of being able to capture the energy
transferred from the photons before the generated excitons recombine.
In a metal the excited electron will rapidly relax because there is a complete
continuum of allowed energy states.
In a semiconductor the energy gap slows down the recombination
sufficiently that it is possible to separate the electron from the hole.
If a semiconductor does not have a suitable asymmetry to separate the
electrons and holes they will eventually recombine by emitting a photon
(not good for power generation).
If there is a suitable asymmetry in the device the excited electrons and holes
can be separated and collected.
In a conventional PV device this is achieved by utilizing a built in electric
field (at the p-n junction) which ‘sweeps’ out the negative and positively
charged electrons and holes before they recombine.
In an OPV this asymmetry is built in because of the structure of the bulk
heterojunction layer.

10. Filled states
(electrons in p-bonds)
Empty states
(antibonding p-bonds)
Vacuum level
Energy gap
Ethene
Carbon atoms have 4 electrons that can be used for bonding. In a single covalent
bond each atom must contribute one electron so that two electrons are shared. In
ethene the there is a double bond between the two central carbons. This results in
delocalized electrons are called p-electrons shared between the two carbon atoms.
These delocalized electrons can be excited to higher lying empty states (called anti-
bonding states) following the absorption of a photon. This gives us a system that
has semiconducting properties.
HOW CAN WE MAKE AN
ORGANIC SEMICONDUCTOR?
Energy

11. In PPV each carbon atom
makes 3 bonds (i.e. two
single and one double bond).
This results in a conjugated
structure which can be
represented by alternating
double and single covalent
bonds along the polymer
backbone. The p - electrons
are no longer associated
with any particular atom, and
are ‘delocalised’ along the
whole polymer molecule.
This gives rise to electronic
conductivity.
p -BONDS RESPONSIBLE FOR
ELECTRON DELOCALIZATION
poly(phenylene vinylene)
One of the simplest organic semiconductors is
the polymer poly(phenylene vinylene) or PPV.

12. EXCITONS
When we excite a p-electron to an excited state,
we also leave a ‘hole’ in the ground state. This
hole is positively charged.
Positive and negative charges attract each other.
This attraction forms a stable quasi-particle called
an exciton (a coulombically bound electron and
hole).
This, as we will see is a problem… the energy
associated with this binding (~0.5 eV) is stronger
than the thermal energy (0.025 eV). For this
reason, the most likely process to happen will be
for the electron to drop back down to its ground
state ‘destroying’ the exciton. This process is
called RECOMBINATION.
Recombination is not good for charge extraction
in a solar cell
Filled states
Empty states
Vacuum level
Energy gap
+
-
Energy

13. HOW CAN WE ‘STOP’ THE EXCITON
DECAYING?
Use a ‘blend’ of different semiconducting materials.
Very often, a molecule called PCBM (a derivative of the
Buckminster Fullerene) is used as this is one of the strongest
electron acceptors yet discovered. The excited state electron
on the semiconductor quickly ‘jumps’ from one molecule to
another BEFORE it has a chance to return to its ground state
and ‘destroy’ the exciton. The use of DONOR-ACCEPTOR
blends is critical to the operation of organic photovoltaic
devices.

14. WHY DOES THIS ENERGY TRANSFER
HAPPEN?
Optical excitation
2.7 eV
3.8 eV
6.1 eV4.8 eV
P3HT
Donor
PCBM
Acceptor
h
Vacuum
The empty level of the PCBM acceptor (called the LUMO level) lies below the P3HT LUMO
level, whilst the filled states of P3HT (called HOMO level) are above that of he PCBM HOMO.
This energy difference is bigger than the exciting binding energy thus exciton dissociation into
separated charge carriers is energetically favourable so it allows the system to rapidly loose
energy. We can therefore create stable, separated charges (electrons and holes).
LUMO= Lowest unoccupied molecular orbital. HOMO = Highest occupied molecular orbital
hole
electron
Electron-transfer
P3HT
PCBM
~100 fs
Vacuum
Energy

15. PLANAR DEVICES, BULK HETEROJUNCTIONS &
CONTROLLED ARCHITECTURES
INCREASING THE INTERFACE
Donor
Acceptor
Acceptor
Planar devices are simple to
make but do not have large areas
of interface where exciton
dissociation can occur
Bulk heterojunction devices rely
on self organised phase
separation of the two materials to
occur at the right lengthscale for
effective exciton dissociation
Controlled architectures could
provide the highest efficiency due
to the very large interfacial areas
possible but they are very difficult to
produce at the lenghtscale needed
for effective exciton dissociation

16. 2.0 1.5 1.0 0.5 0.0
0.0
0.5
1.0
1.5
2.0
qz
(A
-1
)
qx
(A
-1
)
SPECTROSCOPIC
ELLIPSOMETRY
 Film thickness (transparent
region)
 Full Spectrum Optical
Characterisation (n & k)
 Structural order at the nanoscale
 Crystallite size / orientation
X-RAY & NEUTRON
SCATTERING
CHARACTERISING FILM STRUCTURES
MICROSCOPY
(AFM, SEM, TEM)
 Crystallite size and shape
 Phase separation

17. 400 450 500 550 600 650
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
k
Wavelength/nm
0.25 0.30 0.35 0.40 0.45 0.50
1
2
3
4
5
Intensity
Q(A
-1
)
0s
15s
Stage I: Solvent loss Stage II: Fast crystallization Stage III: Slow crystallization
Wang, T. et al. The development of nanoscale morphology in polymer: fullerene photovoltaic blends during solvent
casting. Soft Matter 6, 4128-4134, (2010).
10 15 20 25 30 35
0
250
500
750
23
=90%
IIIII
Thickness/nm
Time/s
I
12
=50%
Thickness v time
Absorption coefficient
at various times
P3HT crystal x-ray
scattering at various
times
CHARACTERISING THE FILM AS IT DRIES FROM
SOLUTION TO FORM A BULK HETEROJUNCTION

18. SOME MATERIALS FORM A MORE
COMPLICATED MULTI-PHASE SYSTEM
Figure 3. Tapping mode AFM phase images of the films for the pristine film (left) and the thermally
annealed layer (right).
AFM image of as cast P3HT:PCBM blend and after thermal annealing
P3HT:PCBM blends annealed under optimal conditions consist of a mixture of
crystalline PCBM, crystalline P3HT and amorphous mixtures of the two
materials
as cast P3HT:PCBM blend Annealed P3HT:PCBM
Macromolecules 2011, 44,6503–6508

19. Hole
Electron
OPERATION OF A BULK
HETEROJUNCTION DEVICE
+
–
Upon absorption of a
photon an exciton is
created. This can
then diffuse through
the active layer. If it
then meets a suitable
interface it can
dissociate. Then if
there is a continuous
extraction path for
both electrons and
holes, they will be
extracted.
Polymer donor C60-based
acceptor
+
–

20. ROLE OF NANOSTRUCTURE - BLEND
RATIOS
Polymer donor
C60-based
acceptor
For efficient charge extraction
excitons must diffuse through
the polymer and find an
interface. Typically the can
diffuse ~ 10nm before
recombination occurs and it
returns to its ground state.
If there isn’t a sufficient density
of acceptors, and the average
distance between acceptor
domains > 10 nm then the
exciton will most probably never
reaches a ‘dissociation’
interface.
Low density of acceptors
+
–

21. ROLE OF NANOSTRUCTURE - PHASE
SEPARATION
Polymer donor
C60-based
acceptor
If the phase separation between
acceptors and donors is too
course this also results in the
average distance between
acceptor domains > 10 nm, so
again the excitons will most
probably never reaches a
‘dissociation’ interface.
Course phase separation
resulting in large domain sizes
+
–

22. COMPLICATED PROBLEM!
Clearly engineering exactly the right
nanostructure is a complex
multidimensional materials-science
problem!

23. COMMON DEVICES (P3HT:PCBM)
Average
PCE 4.2%
JSC = 11.7 mA /cm2
VOC = 0.61V
FF = 59%

24. ENERGY LEVELS WITHIN A P3HT:PCBM OPV
Light
P3HTPCBM
Contact
ITOContact
Ca & Al
Absorption → exciton generation
Charge separation → free carriers
Carrier transport → useful electricity
PEDOT-PSS
2.7 eV
3.8 eV
6.1 eV
4.8 eV
h

25. SPECTRAL COVERAGE
In order to collect as much of the solar energy as possible it is necessary to match
the absorption in a device to the solar spectrum. The solar emission peaks at ~ 500-
600 nm (green light), but there is still lots of power at wavelengths > 700 nm.
To improve device efficiency, need to be able to absorb long wavelength (low
energy) photons. This is not easy, as requires synthesis of organic materials having
very low energy-gaps (<1.6 eV).
Absorption of
P3HT
Solar irradiance (AM1.5)
Solar Spectrum

26. ACCEPTOR MATERIALS
PC60BM
Some OPV devices have been developed using polymeric electron acceptors
but in most cases the acceptor of choice is a fullerene derivative, typically either
PC60BM or PC70BM. Fullerenes are excellent electron acceptors and can be
chemically modified to improve solubility in organic solvents.

27. POLYMER DONORS Much effort world-wide being made
to synthesize better polymers for
OPV these need to harvest more of
sun’s power.
This can be achieved by using a
lower energy gap polymer. These
materials also need to have high
charge-carrier mobility (drift-velocity
in applied field), as well as the need
to have large ionisation potential to
limit losses when electrons are
transferred to the fullerene.
They must dissolve in suitable
solvents and phase separate to form
a BHJ with the right lengthscale
when blended with a suitable
acceptor.
PCDTBT
P3HT
PTB7
PDTG–TPD

28. CHAMPION DEVICES
1. Heliatek (http://www.heliatek.com/)holds the current record for OPV device efficiency at
12% using vacuum deposited oligomeric materials. The details of these materials have
not been released and are under patent.
2. Mitusbishi have also reported high efficiency devices at 11.7%
(http://www.mitsubishichem-hd.co.jp/english/discover_kaiteki/kaiteki_value/detail01.html)
3. Blends of PffBT4T-2OD:TC71BM have been used to produce devices with efficiencies of
10.8% (Nature Communications 5, Article number: 5293)
4. Other high efficiency OPV material systems include PTB7:PC70BM with an efficiency of
9.2% (Nat. Photon., 6 (2012), p. 591)
5. When polyDTG-TPD:PC70BM blends are utilized in inverted bulk heterojunction solar
cells, the cells display average power conversion efficiencies of 7.3% (J. Am. Chem.
Soc., 2011, 133 (26), pp 10062–10065)
6. PCDTBT devices have been made with efficiencies ~7% (J. Mater. Chem. A, 2013, 1,
11097 )
7. P3HT devices have been reported with an efficiency of 4.4% (nature materials VOL 4
NOVEMBER 2005 pp 864-868)

29. WHAT ARE THE BEST DEVICE
EFFICIENCIES?
NREL chart
References for best of the common polymers

30. 3 x 1.5 mm
Very small devices are
produced by spin casting
in a glovebox. This is an
inherently batch process
and therefore difficult to
scale up to an industrial
process
LABORATORY DEVICE FABRICATION
Spin Coating

31. ROUTES TO OPV COMMERCIALIZATION
ROLL TO ROLL PROCESSING
Konarka ‘Powerplastic’
Screen Printing
Ink Jet Printing
Doctor blading
Spray coating
Slot die printing
Gravure printing
Solar Energy Materials and Solar Cells
Volume 93, Issue 4, April 2009, Pages 394–412
Organic solar film
HeliaFilm®

32. web
ink
Doctor Blading
ROUTES TO OPV COMMERCIALIZATION
ROLL TO ROLL PROCESSING
Konarka ‘Powerplastic’
Screen Printing
Ink Jet Printing
Doctor blading
Spray coating
Slot die printing
Gravure printing
Solar Energy Materials and Solar Cells
Volume 93, Issue 4, April 2009, Pages 394–412
Organic solar film
HeliaFilm®

33. web
ink
Doctor Blading
ROUTES TO OPV COMMERCIALIZATION
ROLL TO ROLL PROCESSING
Konarka ‘Powerplastic’
Screen Printing
Ink Jet Printing
Doctor blading
Spray coating
Slot die printing
Gravure printing
Solar Energy Materials and Solar Cells
Volume 93, Issue 4, April 2009, Pages 394–412
web
ink
Slot die printing
Organic solar film
HeliaFilm®

34. web
ink
Doctor Blading
ROUTES TO OPV COMMERCIALIZATION
ROLL TO ROLL PROCESSING
Konarka ‘Powerplastic’
Screen Printing
Ink Jet Printing
Doctor blading
Spray coating
Slot die printing
Gravure printing
Solar Energy Materials and Solar Cells
Volume 93, Issue 4, April 2009, Pages 394–412
web
ink
Slot die printing
web
ink
Gravure printing
(patterned roller)
Organic solar film
HeliaFilm®

35. SUMMARY
 OPVs offer the possibility of low cost, light weight, flexible
solar cells
 Research is focused on controlling and optimising the
materials and nanostructures used
 Their power conversion efficiencies are currently <12%
 Upon photon absorption excitons are produced which then
must dissociate (unlike conventional solar cells where free
charges are produced)
 Exciton dissociation is heavily dependant upon the energy
levels and nanostructure within the devices
 They are solution processable so can be made using high
throughput manufacture techniques