Background information on organic solar cells with limitations and ways to improve efficiency

1. Developments
in
Akinola Oyedele
MSE 556 – Materials for Energy
Organic Solar Cells

2. Outline
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Background
Evolution
Limitations
Future Considerations
Conclusion
Konarka Technologies Inc
Cambridge University

3. Background

4. Conducting Polymers
• In 1977, discovery of electrical conductivity in doped
polyacetylene
• Nobel prize in chemistry in 2000 to Alan Heeger, Alan
McDiarmid and Hideki Shirakawa
• 1986, Organic photovoltaic cell OPV (Ching W Tang, Kodak)
• 1986, Orgaic field-effect transistor OFET (H Koezuka, Mitsubishi)
• 1987, Organic light-emitting diode OLED (Ching W Tang, Kodak)
Photo credit: NobelPrize.org

5. Chemical structures of conducting
polymers
Daniel J.Burke Energy Environ. Sci., 2013, 6, 2053

6. Advantages
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Cheap, low-temperature deposition techniques (e.g roll-to-roll, printing)
Environmental-friendly materials; Abundant and Cheap
Can be semitransparent or aesthetically pleasing
Ultra-flexible and even stretchable,
Lightweight
Low-light condition
Color-tunable

7. Companies Involved
2001 (bankrupted 2012) USA, Austria
2010, Cambridge, UK
2006, Dresden, Germany
2006, El Monte, California
http://www.youtube.com/watch?v=MirozECd8S8

8. Evolution of the active layer
Single-layer OSC
Efficiency = 0.1 %
Bi-layer OSC
Efficiency = 1 %
Bulk heterojunction OSC
Efficiency = 10 %
http://en.wikipedia.org/wiki/Organic_solar_cell

9. Construction of the OPV Devices
• Transparent electrode
1. As a transparent widow layer
2. Collect holes (anode)
• Hole Transporting Layer
1. Protect the active layer
2. As an electron-blocking layer
3. Assist hole transport
4. Smoothen the rough surfaces of the TCO
D. Ginley, Fundamentals of materials for Energy
• LiF as a cathode buffer layer
and Environmental Sustainability, page 232
1. To prevent diffusion of cathode elements to the active layer
2. To act as an electron-transport, Hole-blocking layer.
The main challenge is they require high deposition temperature which can potentially
damage the active layer

10. Energy-level band diagram
Energy-level band diagram of a typical P3HT:PCBM Organic Solar Cell
D. Ginley, Fundamentals of materials for Energy
and Environmental Sustainability, page 233

11. Progress in Organic Solar Cells
M. Gratzel, Nature 2012

12. Solar cells characteristics
Diode model of a solar cell
Current-voltage response of a solar cell
Omar A. AbdulRazzaq, Organic Solar Cells: A review of Materials, Limitations and Possibilities for Improvements, 2013; Pg 428

13. HOMO and LUMO energy levels
Energy levels in inorganic and organic semiconductors
Illustration of HOMO and LUMO energy levels
Tom J. Savenije, Organic Solar Cells Delft University

14. Limitations of Photocurrent in OSC
• Carrier transport mechanism in OSC
1.
2.
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Light absorption;
Diffusion of exciton to interface;
Charge separation;
Charge Transport
Charge Collection
Omar A. AbdulRazzaq, Organic Solar Cells: A review of Materials, Limitations and Possibilities for Improvements, 2013; Pg 431

15. Limitations of Photocurrent in OSC (2)
• Exciton
• Charge
Diffusion
Separation
Bulk-heterojunction solar cell
Low dielectric constant
Formation of exciton (tightly-bound)
Frenkel excitons

16. Considerations
• Collect a high number of photo-generated carriers
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Use small band-gap polymers
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Increase electrical conductivity by improving the crystal structure
Improve crystallinity by thermal
annealing of the solution-based
mixture
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Large donor-acceptor interface to
promote the dissociation of more
excitons
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Brabec and Durrant, Cambridge University (2008)

17. Absorb more light
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Tandem organic solar cells
Behaves like cells in series
Minimize thermalization losses
Same-current limitation
Coupling processing techniques
M. Gratzel, Materials interface engineering for solution-processed photovoltaics, Nature 306, vol 488, 2012

18. Ternary Organic Solar Cells
Sensitizers can be dyes, polymers or nano-particles
Eliminates the challenges of multi-junction solar cells
Improve the photon harvesting in thickness limited photoactive layers
Limitation: Lower Voc
Tayebeh Ameri Adv Mater. 2013, 25, 4243-4266

19. Cascade Charge Transfer
Schematic representation of the cascade
charge transfer in ternary solar cell
Illustration of an optimal microstructure of the
ternary blends
Tayebeh Ameri Adv Mater. 2013, 25, 4243-4266

20. Parallel-like Charge Transfer
Schematic representation of the parallel-like
charge transfer in a ternary solar cell
Tayebeh Ameri Adv Mater. 2013, 25, 4243-4266

21. Plasmonics in Organic Solar Cells
• Enhance light-trapping (increase in optical path length)
• First developed by Goetzberger et al. 1981
• Enable the use of ultra-thin layers (semi-transparency)
Creates a strong E-field
Grated back-contact
Light-trapping techniques used in thin-film solar cells
Atwater, H.A., and Polman, A. (2010). Plasmonics for improved photovoltaic devices, Nature Materials 9; 205-213

22. Plasmonics in OSC
• The shape and size of the nano-particles greatly affect the
angular spread
Sensitivity of plasmon light scattering to nanoparticles’ shape and size
Atwater, H.A., and Polman, A. (2010). Plasmonics for improved photovoltaic devices, Nature Materials 9; 205-213

23. Inverted OSC
Efficient Inverted Polymer Solar Cells. Applied Physics Letter 88 (2006)

24. Inverted OSC (2)
Hongbin Wu
South China University of Technology,
Guangzhou, 2012
PCE= 9.2 %
current density of 17.2 mA/cm2,
15.4 mA/cm2 for the regular device.

25. Tayebeh Ameri Adv Mater. 2013, 25, 4243-4266

26. Conclusion
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The expected high-efficiency per unit cost ratio
The simplicity in fabrication and processing
The mechanical flexibility of these materials
The short diffusion length
Low absorption of the active layer
Tandem architectures incorporated with plasmons
Organic cells made up of polymer nanocomposites

27. Let’s drive tomorrow today!
Thank you for your attention.

28. References
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D. Burke, et al (2013). Green chemistry for organic solar cells. Energy
Environ. Sci, 6: 2053
M. Graetzel, et al (2012). Materials interface engineering for solutionprocessed photovoltaics. Nature Review article 488: 304-312.
O. Abdulrazzaq, et al (2013). Organic Solar Cells: A review of materials,
limitations, possibilities for improvement. Particulate Sci and Tech, 31:
427-442
T. Ameri, et al (2013). Organic Ternary Solar Cells: A review. Advanced
Materials, 25: 4245-4266
M. Liu, et al (2013). Efficient planar heterojunction perovskite solar cells
by vapour deposition. Nature 501: 395-402
M. Green (2005). Silicon Photovoltaic Modules: A brief History of the first
50 years. Prog. Photovolt: Res. Appl. 13: 447-455