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