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