Download ORGANIC Solar Cells - U of L Class Index

08/03/2016
ORGANIC SOLAR
CELLS
Solar Cells: How Do They Work?
Three Steps:
■ Photons absorbed by semiconducting material, creating an exciton
■ Electrons in semiconducting material are excited into conduction band and move to
fill in holes
■ Movement of electrons and holes to electrodes generates charge which is converted
to usable electricity
1
08/03/2016
Junctions
■ Boundary between n and p type semiconductors
■ For Silicon semiconductors: Phosphorous (n-type) and Boron (p-type) doping
■ Doping creates holes (p-type) or extra electrons (n-type); electrons can flow from
excess to holes generating charge
■ Require electrodes to prevent formation of equilibria
2
08/03/2016
Why Use Organic Solar Cells?
■ Cheap and efficient production of materials
■ Mass production
■ Low bio-footprint
■ Flexible
Photosynthesis: A Model Example
3
08/03/2016
Chlorophyll a
Chlorophyll b
The Basis of Organic Solar Cells
■ Modeled after p-n semiconductor junctions or Schottky junctions
■ Require n-type donor molecule and p-type acceptor molecule
■ Importance of HOMO-LUMO interactions and band gap differences
4
08/03/2016
Structure of Organic Solar Cells
■ Organic layer(s) sandwiched between metals
■ Cathode: Metal with low work function (eg Al, Mg, Ca, or Ag)
■ Anode: Metal with high work function (eg indium doped tin oxide (ITO))
■ Can be joined in tandem to form an array
Types of Organic Solar Cells
Single active-layer
■ Single layer of organic material
■ Electric field set up by electrodes forces
charge separation of excitons
■ Single layer does not absorb all light
■ Very low efficiency (<1%)
5
08/03/2016
Double active-layer
■ Two layers of organic material
■ Similar to p-n type junction
■ More efficient charge separation of excitons (15%)
Bulk heterojunction photovoltaic cell
■ Donor and acceptor materials mixed together
■ Greater surface area for increased band gap
interactions
■ Size of domains affect efficiency of charge
transport and charge generation
■ Most excitons generated are efficiently charge
separated
6
08/03/2016
a) Small domains with large interfaces
b) Large domains with small interface area
c) Intermediate domain size
Types of Molecules
Donor: polyphenylene vinylene analogue
Acceptor: Fullerene
7
08/03/2016
Donors
■ Pthalocyanines, Polythiophenes,
and derivatives
■ 18 pi-electron porphyrin
analogues
■ Extended conjugated system
allows for easily excited electrons
■ Huge variety in types of
molecules!
8
08/03/2016
Acceptors
■ Fullerene and derivatives
■ Low lying LUMO which can easily accept electrons from p-type materials
■ LUMO is triply degenerate – can accept up to 6 electrons
■ Extremely fast time scale of charge transfer from donor to fullerene and derivatives (
45 fs)
■ However, poor solubility and tendencies to crystallize provide challenges
9
08/03/2016
Tuning the HOMO-LUMO Gap
■ Conjugation decreases HOMO-LUMO gap
■ Electron donating groups – raise HOMO
■ Electron withdrawing groups – lower LUMO
■ Use of both EDG and EWG within the same conjugated polymer to form alternating
donor and acceptor moieties within the same chain – intramolecular charge transfer
■ Use of tethers between aromatic rings to force planar conformations to extend
conjugation
■ Band gap can be reduced by forcing polymers into quinoid form
10
08/03/2016
Other Factors
■ Efficiency = (VOC)(ISC)(FF)/Pin
VOC = voltage in open circuit conditions
ISC = voltage in short circuit conditions
FF = fill factor of device
Pin = incident light
■ VOC : limited by energy difference between HOMO of donor and LUMO of acceptor and
quality of contact at junction
■ ISC : maximized by choosing materials that absorb in near IR region of EM spectrum
■ FF: Important to minimize charge recombination and facilitate charge separation and
mobility
11
08/03/2016
A Long Way to Go
■ Efficiency of ~10% at the very high end
■ Current research focused on reducing recombination of excitons and facilitating
quick charge separation
■ Fine tuning donor and acceptor molecules
■ New research looking into inverted electrode model may be promising
References
■
1.
Chen, J.-T.; Hsu, C.-S., Conjugated polymer nanostructures for organic solar cell applications.
Polymer Chemistry 2011, 2 (12), 2707-2722.
■
2.
Cheng, Y.-J.; Yang, S.-H.; Hsu, C.-S., Synthesis of conjugated polymers for organic solar cell
applications. Chemical reviews 2009, 109 (11), 5868-5923.
■
3.
Etxebarria, I.; Ajuria, J.; Pacios, R., Solution-processable polymeric solar cells: A review on
materials, strategies and cell architectures to overcome 10%. Organic Electronics 2015, 19, 34-60.
■
4.
Hoppe, H.; Sariciftci, N. S., Organic solar cells: An overview. Journal of Materials Research
2004, 19 (07), 1924-1945.
■
5.
Kaur, N.; Singh, M.; Pathak, D.; Wagner, T.; Nunzi, J., Organic materials for photovoltaic
applications: Review and mechanism. Synthetic Metals 2014, 190, 20-26.
■
6.
Wöhrle, D.; Meissner, D., Organic solar cells. Advanced Materials 1991, 3 (3), 129-138.
12