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ORGANIC PHOTOVOLTAIC
DEVICES
Alan D. F. Dunbar
Department of Chemical and Biological
Engineering
University of Sheffield
a...
OVERVIEW
1. Basic cell design and structure.
2. What properties are needed in the different layers?
3. Interactions with l...
TYPICAL
PHOTOVOLTAIC
DEVICE
STRUCTURE
Light
Active Layer
Electron Transport Layer
Cathode
Hole Transport Layer
Transparent...
Layer Required Properties Examples
Encapsulation Blocks H2O and O2 Epoxy & glass,
Cathode Layer Metallic conductor with su...
5
When atoms come together to form molecules or crystals
this results in a splitting of the energy levels into several
dis...
6
If the filled and empty bands overlap the crystal will be metallic –
car park analogy. If the bands do not overlap the m...
7
Material
Type
Approx. Band
Gap Range
Properties
Metal None
Readily conducts electricity and heat
since many electrons ar...
EXCITATIONS
A photon (a particle of light) carries energy E = h.
If the photon energy > band gap energy, then the photon ...
WHY ARE SEMICONDUCTORS
NEEDED IN A SOLAR CELL?
It comes down to the probability of being able to capture the energy
transf...
Filled states
(electrons in p-bonds)
Empty states
(antibonding p-bonds)
Vacuum level
Energy gap
Ethene
Carbon atoms have 4...
In PPV each carbon atom
makes 3 bonds (i.e. two
single and one double bond).
This results in a conjugated
structure which ...
EXCITONS
When we excite a p-electron to an excited state,
we also leave a ‘hole’ in the ground state. This
hole is positiv...
HOW CAN WE ‘STOP’ THE EXCITON
DECAYING?
Use a ‘blend’ of different semiconducting materials.
Very often, a molecule called...
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...
PLANAR DEVICES, BULK HETEROJUNCTIONS &
CONTROLLED ARCHITECTURES
INCREASING THE INTERFACE
Donor
Acceptor
Acceptor
Planar de...
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
reg...
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
Intens...
SOME MATERIALS FORM A MORE
COMPLICATED MULTI-PHASE SYSTEM
Figure 3. Tapping mode AFM phase images of the films for the pri...
Hole
Electron
OPERATION OF A BULK
HETEROJUNCTION DEVICE
+
–
Upon absorption of a
photon an exciton is
created. This can
th...
ROLE OF NANOSTRUCTURE - BLEND
RATIOS
Polymer donor
C60-based
acceptor
For efficient charge extraction
excitons must diffus...
ROLE OF NANOSTRUCTURE - PHASE
SEPARATION
Polymer donor
C60-based
acceptor
If the phase separation between
acceptors and do...
COMPLICATED PROBLEM!
Clearly engineering exactly the right
nanostructure is a complex
multidimensional materials-science
p...
COMMON DEVICES (P3HT:PCBM)
Average
PCE 4.2%
JSC = 11.7 mA /cm2
VOC = 0.61V
FF = 59%
ENERGY LEVELS WITHIN A P3HT:PCBM OPV
Light
P3HTPCBM
Contact
ITOContact
Ca & Al
Absorption → exciton generation
Charge se...
SPECTRAL COVERAGE
In order to collect as much of the solar energy as possible it is necessary to match
the absorption in a...
ACCEPTOR MATERIALS
PC60BM
Some OPV devices have been developed using polymeric electron acceptors
but in most cases the ac...
POLYMER DONORS Much effort world-wide being made
to synthesize better polymers for
OPV these need to harvest more of
sun’s...
CHAMPION DEVICES
1. Heliatek (http://www.heliatek.com/)holds the current record for OPV device efficiency at
12% using vac...
WHAT ARE THE BEST DEVICE
EFFICIENCIES?
NREL chart
References for best of the common polymers
3 x 1.5 mm
Very small devices are
produced by spin casting
in a glovebox. This is an
inherently batch process
and therefor...
ROUTES TO OPV COMMERCIALIZATION
ROLL TO ROLL PROCESSING
Konarka ‘Powerplastic’
Screen Printing
Ink Jet Printing
Doctor bla...
web
ink
Doctor Blading
ROUTES TO OPV COMMERCIALIZATION
ROLL TO ROLL PROCESSING
Konarka ‘Powerplastic’
Screen Printing
Ink ...
web
ink
Doctor Blading
ROUTES TO OPV COMMERCIALIZATION
ROLL TO ROLL PROCESSING
Konarka ‘Powerplastic’
Screen Printing
Ink ...
web
ink
Doctor Blading
ROUTES TO OPV COMMERCIALIZATION
ROLL TO ROLL PROCESSING
Konarka ‘Powerplastic’
Screen Printing
Ink ...
SUMMARY
 OPVs offer the possibility of low cost, light weight, flexible
solar cells
 Research is focused on controlling ...
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Organic Photovoltaic Devices

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

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Organic Photovoltaic Devices

  1. 1. ORGANIC PHOTOVOLTAIC DEVICES Alan D. F. Dunbar Department of Chemical and Biological Engineering University of Sheffield a.dunbar@sheffield.ac.uk
  2. 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. 3. TYPICAL PHOTOVOLTAIC DEVICE STRUCTURE Light Active Layer Electron Transport Layer Cathode Hole Transport Layer Transparent Conducting Anode Transparent Substrate
  4. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 22. COMPLICATED PROBLEM! Clearly engineering exactly the right nanostructure is a complex multidimensional materials-science problem!
  23. 23. COMMON DEVICES (P3HT:PCBM) Average PCE 4.2% JSC = 11.7 mA /cm2 VOC = 0.61V FF = 59%
  24. 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. 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. 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. 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. 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. 29. WHAT ARE THE BEST DEVICE EFFICIENCIES? NREL chart References for best of the common polymers
  30. 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. 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. 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. 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. 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. 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

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