Developments in Organic Solar Cells Akinola Oyedele MSE 556 – Materials for Energy
May 10, 2015
Developments
in
Organic Solar CellsAkinola OyedeleMSE 556 – Materials for Energy
Outline
• Background• Evolution• Limitations• Future Considerations• Conclusion
Konarka Technologies Inc
Cambridge University
Background
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
Chemical structures of conducting polymers
Daniel J.Burke Energy Environ. Sci., 2013, 6, 2053
Advantages• 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
Companies Involved
http://www.youtube.com/watch?v=MirozECd8S8
2010, Cambridge, UK2006, Dresden, Germany
2006, El Monte, California
2001 (bankrupted 2012) USA, Austria
Evolution of the active layer
Single-layer OSC Bi-layer OSC
Bulk heterojunction OSC
Efficiency = 0.1 %
Efficiency = 10 %
Efficiency = 1 %
http://en.wikipedia.org/wiki/Organic_solar_cell
Construction of the OPV Devices
• Transparent electrode1. As a transparent widow layer2. Collect holes (anode)
• Hole Transporting Layer1. Protect the active layer2. As an electron-blocking layer3. Assist hole transport4. Smoothen the rough surfaces of the TCO
• LiF as a cathode buffer layer1. To prevent diffusion of cathode elements to the active layer2. To act as an electron-transport, Hole-blocking layer.
The main challenge is they require high deposition temperature which can potentiallydamage the active layer
D. Ginley, Fundamentals of materials for Energy and Environmental Sustainability, page 232
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
Progress in Organic Solar Cells
M. Gratzel, Nature 2012
Solar cells characteristics
Current-voltage response of a solar cell
Diode model of a solar cell
Omar A. AbdulRazzaq, Organic Solar Cells: A review of Materials, Limitations and Possibilities for Improvements, 2013; Pg 428
HOMO and LUMO energy levels
Tom J. Savenije, Organic Solar Cells Delft University
Energy levels in inorganic and organic semiconductors
Illustration of HOMO and LUMO energy levels
Limitations of Photocurrent in OSC
• Carrier transport mechanism in OSC1. Light absorption;2. Diffusion of exciton to interface; 3. Charge separation;4. Charge Transport5. Charge Collection
Omar A. AbdulRazzaq, Organic Solar Cells: A review of Materials, Limitations and Possibilities for Improvements, 2013; Pg 431
Limitations of Photocurrent in OSC (2)
Bulk-heterojunction solar cell
• Exciton Diffusion
• Charge Separation
• Exciton Diffusion
• Charge Separation
Low dielectric constantFormation of exciton (tightly-bound)Frenkel excitons
Considerations
• Collect a high number of photo-generated carriers
Brabec and Durrant, Cambridge University (2008)
• Use small band-gap polymers
• Increase electrical conductivity by improving the crystal structure
• Large donor-acceptor interface to promote the dissociation of more excitons
• Improve crystallinity by thermal annealing of the solution-based mixture
Absorb more light
• Tandem organic solar cells
Behaves like cells in series
Same-current limitation
Coupling processing techniques
Minimize thermalization losses
M. Gratzel, Materials interface engineering for solution-processed photovoltaics, Nature 306, vol 488, 2012
Ternary Organic Solar Cells
Tayebeh Ameri Adv Mater. 2013, 25, 4243-4266
Improve the photon harvesting in thickness limited photoactive layers
Eliminates the challenges of multi-junction solar cells
Limitation: Lower Voc
Sensitizers can be dyes, polymers or nano-particles
Cascade Charge Transfer
Schematic representation of the cascade charge transfer in ternary solar cell
Tayebeh Ameri Adv Mater. 2013, 25, 4243-4266
Illustration of an optimal microstructure of the ternary blends
Parallel-like Charge Transfer
Tayebeh Ameri Adv Mater. 2013, 25, 4243-4266
Schematic representation of the parallel-like charge transfer in a ternary solar cell
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)
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
Grated back-contactCreates a strong E-field
Plasmonics in OSC
• The shape and size of the nano-particles greatly affect the angular spread
Atwater, H.A., and Polman, A. (2010). Plasmonics for improved photovoltaic devices, Nature Materials 9; 205-213
Sensitivity of plasmon light scattering to nanoparticles’ shape and size
Inverted OSC
Efficient Inverted Polymer Solar Cells. Applied Physics Letter 88 (2006)
Inverted OSC (2)
PCE= 9.2 %current density of 17.2 mA/cm2, 15.4 mA/cm2 for the regular device.
South China University of Technology, Guangzhou, 2012
Hongbin Wu
Tayebeh Ameri Adv Mater. 2013, 25, 4243-4266
Conclusion
• 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
Let’s drive tomorrow today!
Thank you for your attention.
References• 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 solution-
processed 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