This document provides an overview of low band gap semiconducting polymers and their potential application in organic photovoltaic cells. It discusses the importance of low band gap polymers for absorbing longer wavelengths of light more efficiently. Various polymerization techniques are described, including oxidative and metal-catalyzed routes. Characterization techniques to analyze the synthesized polymers are also outlined. Specific electron-rich and electron-deficient monomer units with band gaps between 1.46-1.60 eV are identified as promising candidates. The author's next steps involve synthesizing N-tosyl pyrrole and 3-hexyl pyrrole monomers to ultimately obtain a low band gap semiconducting polymer for use in solar cells.

1. Low band gap semiconducting
polymers for possible application in
organic photovoltaic cell
By: BONIFACE YEBOAH ANTWI
(PhD chemistry student)

2. Over view
• INTRODUCTION
• BACKGROUND
• MONOMER UNITS
• POLYMERIZATION TECHNIQUES
• CHARACTERIZATION TECHNIQUES
• SEMICONDUCTING MONOMER
UNITS OF INTEREST
• MONOMERS UNDERSTUDY
• CONCLUSION

3. INTRODUCTION

4. 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

5. BACKGROUND

6. 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

7. Why Low band gap semiconducting polymers cont.
HIGH OPEN CIRCUIT VOLTAGE VOC
The VOC – energy difference between
HOMO of a donor and LUMO of an
acceptor.
Low band gap of donor VOC(1) combined
with 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.

8. 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

9. 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.

10. MONOMER UNITS

11. 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

12. Known Monomers 7-10
(Electron deficient monomers)

13. Known Low Band gap Polymers 7-10

15. POLYMERIZATION
TECHNIQUES

16. Types
1. Oxidative preparative routes
I. 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

17. Oxidative Preparative routes

18. 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

19. Metal-Catalysed routes

21. General Mechanism

22. 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

23. CHARACTERIZATION
TECHNIQUES

24. • 1H and 13C NMR were
• Infrared spectrometry
• Cyclic voltammety
• Gel Permeation Chromatography (GPC)
• Thermal gravimetric analyses
(TGA).
• UV-visible
• And many more

25. SEMICONDUCTING MONOMER
UNITS OF INTEREST

26. Band gap
(1.46 eV to 1.60 eV)
• Mimic

27. MONOMERS
UNDERSTUDY

28. SELECTED SEMICONDUCTING MONOMER (2.80 eV)

29. What Next?

30. Synthesis of N-tosyl pyrrole
Synthesis of 3-hexyl pyrrole
by Friedel-Craft acylation

32. CONCLUSION

33. Together we shall obtain a Low band gap semiconducting
polymers for solar cell application