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Low band gap semiconducting polymers for possible application in
organic photovoltaic cell
By: BONIFACE YEBOAH ANTWI
(PhD chemistry student)
Over view
• INTRODUCTION
• BACKGROUND
• MONOMER UNITS
• POLYMERIZATION TECHNIQUES
• CHARACTERIZATION TECHNIQUES
• SEMICONDUCTING MONOMER UNITS OF INTEREST
• MONOMERS UNDERSTUDY
• CONCLUSION
INTRODUCTION
Introduction Band gap and Low band gap
Band- energy level
Band gap – the difference between bands of energy or difference between valence band and conduction band of atoms, molecules (HOMO-LUMO), etc.
Measured in eV (UV-vis spectroscopy etc.)
Low band gap polymers – band gap<2eV (absorbs light with longer wavelength, i.e. λ > 620nm) 1.
Semiconductor Small band gap. And conduct only at temperatures below its melting but not at
absolute zero (−273.15°C) 2 . Eg. B, Si, Ge, As, etc.
Polymer Material with repeating small molecular units that are covalently bond together. These
repeating unit is called a monomer 3. Eg. Polyethylene, polyvinylchloride, carbohydrates etc.
Organic photovoltaic cell Devices that utilize polymers and other carbon compounds in converting solar energy
to electrical energy 4.
1. Unlu H., (1992). "A Thermodynamic Model for Determining Pressure and Temperature Effects on the Bandgap Energies and other Properties of some Semiconductors". Solid state electronics 35 (9): 1343-1352.
2. http://chemwiki.ucdavis.edu/Physical_Chemistry/Quantum_Mechanics/Electronic_Structure/Band_Theory_of_Semiconductors; 03/07/2014: 17:15.
3. Allcock, Harry R.; Lampe, Frederick W.; Mark, James E. (2003). Contemporary Polymer Chemistry (3 ed.). Pearson Education. p. 21. ISBN 0-13-065056-0.
4. http://www.sigmaaldrich.com/content/dam/sigma-aldrich/materials-science/organic-electronics/opv-device.jpg; 03/07/2014; 18:15
Figure 1: Scheme showing
Energy levels and band gap 1
Figure 2: Polyethylene
monomer unit 2
Figure 3: Scheme of
organic photovoltaic cell 4
BACKGROUND
Figure 4: Sun
intensity
spectrum 5
Why Low band gap semiconducting polymers ?
More photons are generated at longer wavelength (Fig. 4).
But lower energy at longer wavelengths.
And so, narrower band gap for easy excitation to conduction band.
Larger photon absorption leads to higher current density 5.
Figure 5: Current
density plot of
benzothiadiazole-
thiophene
copolymers 5
5. Bundgaard E., Krebs F. C., 2007. Low band gap polymers for organic photovoltaics, Journal of Solar Energy Materials & Solar Cells; 91:954–985.
CURRENT DENSITY
Why Low band gap semiconducting polymers cont.
HIGH OPEN CIRCUIT VOLTAGE VOC
The VOC – energy difference betweenHOMO of a donor and LUMO of anacceptor.
Low band gap of donor VOC(1) combinedwith a higher LUMO of acceptor VOC(2)
VOC
photovoltaic of cell, hence its efficiency 5.
Figure 6: Increasing open circuit voltage by tuning the energy
levels in a bulk heterojunction OPV device.
5. Bundgaard E., Krebs F. C., 2007. Low band gap polymers for organic photovoltaics, Journal of Solar Energy Materials & Solar Cells; 91:954–985.
Factors to consider when forming low band gap polymers
This include;
intra-chain charge transfer
substituent effect
π-conjugation length etc. 5
To acheive this, copolymers are formed.
– polymers of two different monomer units
Electron Donor (D)-Electron Acceptor(A) molecules.
5. Bundgaard E., Krebs F. C., (2007). Low band gap polymers for organic photovoltaics, Journal of Solar Energy Materials & Solar Cells; 91:954–985.
Figure 7: copolymers based on thiophene and benzothiadiazole
Factors to consider when forming low band gap polymers cont.
Copolymer
Computational studies – longer π-conjugation
length reduces band gap 6.
Reduced bond-length alternation lowers the HOMO-
LUMO gap (Aromaticity *) 6
Intra-molecular charge transfer lowers band gap
of the copolymer due to new hydride orbitals 6.
EWG (electron withdrawing groups)
EDG (electron donating groups)
EDG increases HOMO of hybrid orbital while EWG
decreases LUMO of hybrid orbital.
Figure 8: Interaction of energy level of a donor (D) and acceptor (A)
leading to a narrower HLG 6
6. Qian G. and Wang Z. Y. , (2010). Near-Infrared Organic Compounds and Emerging Applications. Chem. Asian J.;5:1006 – 1029.
MONOMER UNITS
2-(2,5-di(pyrrol-2-yl)thiophen-3-yl)ethyl 2-
bromopropanoate)
(PyThon)
Known Monomers 7-10
(Electron rich monomers)
7. Xu T. and Yu L., (2014). How to design low band gap polymers for highly efficient organic solar cells. Materials Today; 17:1:1-5.
8. Dai Liming, (1999). Advanced Syntheses and Microfabrications of Conjugated Polymers, C60-containing Polymers and Carbon Nanotubes for Optoelectronic Application. Polym. Adv. Technol. 10, 357-420.
9. Strover L. T., Malmström J. , Laita O., Reynisson J., Aydemir N., Nieuwoudt M. K., Williams D. E., Dunbar R. P., Brimble M. A., Travas-Sejdic J., (2013). A new precursor for conducting polymer-based brush interfaces with
electroactivity in aqueous solution. J. Polymer, 54; 1305-1317.
3,4-dihydro-3,3-dialkyl-6,8-
bis(trimethylstannyl)-2H-
thieno[3,4-b][1,4]dioxepines
10. Mishra S. P., Palai A. K., Patri, (2010). Synthesis and characterization of soluble narrow band gap conducting polymers based on diketopyrrolopyrrole and propylenedioxythiophenes. J. Synthetic Metals, 160, 2422–2429.
poly(styrenesulfonate) anion (PSS−) bis-thienylpyrrole
Known Monomers 7-10
(Electron deficient monomers)
Known Low Band gap Polymers 7-10
POLYMERIZATION TECHNIQUES
Types
1. Oxidative preparative routesI. Electrochemical polymerization
II. Chemical oxidative polymerization
2. Metal-Catalysed routes
I. Kumada cross coupling
II. Suzuki cross coupling
III. Stille coupling
IV. Yamamoto cross coupling
Oxidative Preparative routes
Oxidative Preparative routes
Electrochemical Polymerization (EP)Electrochemical cell utilised
Monomer is oxidized by electrolyte
Polymer deposited on anode
Eg. Pyrrole11, thiophene12, etc.
Similar to EP
But utilizes chemical oxidant, such as FeCl3 for polyaniline 13.
(Regioselectivity, intractability problems)
Chemical oxidative polymerization
11. A. F. Diaz and K. Keiji Kanazawj, (1979). Electrochemical Polymerization of Pyrrole. J.C.S. Chem. Comm.; 1-2
12. Albery W.J., Li F., Mount A.R., (1991). Electrochemical polymerization of poly(thiophene-3-acetic acid),
poly(thiophene-co-thiophene-3-acetic acid) and determination of their molar mass. Prog. Polym. Sci.; 301: 1-2:
239–253.13. Gospodinova N. , Terlemezyan l., (1998). Conducting polymers prepared by Oxidative polymerization: polyaniline. Polym. Sci.; 23, 1443–1484.
Figure 9: Electrochemical cell for
polymerization
Metal-Catalysed routes
General Mechanism
Types and differences
Cross coupling Methods Reacting species Catalyst Comments
Kumada
Nickel
or
Palladium• Limited functional group tolerant
(Grignard reagent-high reactivity)
Negishi Zinc
• Cross coupling at lower
temperatures
• Tolerant of other functional
groups
• Toxic catalyst
Suzuki Palladium• Mild reaction conditions
• availability of boric acid
• Less toxic
Stille Tin
• Polymerize at higher
temperatures
• Difficulties in purification
• Toxic catalyst
CHARACTERIZATION TECHNIQUES
• 1H and 13C NMR were
• Infrared spectrometry
• Cyclic voltammety
• Gel Permeation Chromatography (GPC)
• Thermal gravimetric analyses(TGA).
• UV-visible
• And many more
SEMICONDUCTING MONOMER UNITS OF INTEREST
Band gap
(1.46 eV to 1.60 eV)
• Mimic
MONOMERS UNDERSTUDY
SELECTED SEMICONDUCTING MONOMER (2.80 eV)
What Next?
Synthesis of N-tosyl pyrroleSynthesis of 3-hexyl pyrrole
by Friedel-Craft acylation
CONCLUSION
Together we shall obtain a Low band gap semiconducting polymers for solar cell application