Article pubs.acs.org/cm
Effect of Fluorination on the Properties of a Donor−Acceptor Copolymer for Use in Photovoltaic Cells and Transistors Hugo Bronstein,*,† Jarvist M. Frost,† Afshin Hadipour,‡ Youngju Kim,† Christian B. Nielsen,† Raja Shahid Ashraf,† Barry P. Rand,‡ Scott Watkins,§ and Iain McCulloch† †
Department of Chemistry and Centre for Plastic Electronics, Imperial College London, London, SW7 2AZ, U.K. IMEC, Kapeldreef 75, B-3001, Leuven, Belgium § CSIRO Materials Science and Engineering, Melbourne, VIC 3169, Australia ‡
S Supporting Information *
ABSTRACT: Two novel indacenodithiophene (IDT) based donor−acceptor conjugated polymers for use in organic field effect transistors and photovoltaic devices are synthesized and characterized. The effect of inclusion of two fluorine atoms on the acceptor portion of the polymer is thoroughly investigated via a range of techniques. The inductively withdrawing and mesomerically donating properties of the fluorine atoms result in a decrease of the highest occupied molecular orbital (HOMO), with little effect on the lowest unoccupied molecular orbital (LUMO) as demonstrated through density functional theory (DFT) analysis. Inclusion of fluorine atoms also leads to a potentially more planar backbone through inter and intrachain interactions. Use of the novel materials in organic field effect transistor (OFET) and organic photovoltaic (OPV) devices leads to high mobilities around 0.1 cm2/(V s) and solar cell efficiencies around 4.5%. KEYWORDS: solar cells, conjugated polymer, OFET, fluorine, OPV
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INTRODUCTION For conjugated polymers to be used as components in next generation electronics, their performance must be complementary to their inorganic-based counterparts. This has stimulated a steady increase in organic solar cell power conversion efficiency (PCE), to values of approximately 8%, and organic field effect transistor (OFET) hole mobilities in excess of 1 cm2/(V s).1−6 The general strategy that has proved to be most successful in achieving both high performing organic photovoltaic (OPV) and OFET devices is the donor−acceptor (D-A) approach, which involves the copolymerization of an electron rich (donor) monomer with an electron poor (acceptor) monomer. This typically results in polymers with a low band gap through hybridization of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels.7−10 The fine-tuning of these DA polymers is very important to extract the highest possible performance for these systems by maintaining the highest possible open circuit voltage (VOC): the maximum amount of light absorption while retaining efficient charge separation. Several groups have recently reported the use of fluorination of the conjugated backbone in an attempt to improve the performance and fine-tune energy levels.11−20 Zhou et al. first reported the synthesis of a conjugated polymer with an incorporated fluorinated benzothiadiazole (BT) unit, and demonstrated its improved performance relative to the unsubstituted BT moiety in a solar cell.21 The increase in performance arose predominately from a deeper HOMO level © 2013 American Chemical Society
which increased the open circuit voltage of solar cell devices. Other groups have also subsequently reported the use of the fluorinated BT unit which in general results in an improved photovoltaic performance.22−24 However, the origin of the observed effects on the frontier orbitals, or other properties of a material, is not yet fully understood. Indacenodithiophene (IDT) is a commonly used donor monomer which has been incorporated in several polymers for use in high performance OPV devices with reported efficiencies in excess of 5% PCE, and extremely high hole mobilities in OFETs.22,25−34 As a result of its broad applicability, it is an excellent choice to investigate the origin and magnitude of effects of an incorporated fluorinated BT unit. The accepting repeat unit of the D-A polymer was chosen to be di-2-thienyl-2′,1′,3′benzothiadiazole (DTBT), as this is a widely used comonomer, and allowed facile introduction of the two fluorine atoms on the benzothiadiazole heterocycle.7 Because of the high solubility of the ethylhexyl-substituted IDT, it was deemed unnecessary to have alkyl chains on the thiophenes flanking the BT unit.
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RESULTS AND DISCUSSION Synthesis. Both polymers were synthesized (Figure 1) using microwave assisted Stille polymerizations, in chlorobenReceived: June 19, 2012 Revised: January 14, 2013 Published: January 15, 2013 277
dx.doi.org/10.1021/cm301910t | Chem. Mater. 2013, 25, 277−285
Chemistry of Materials
Article
Figure 1. Synthesis of novel nonfluorinated (P1) and fluorinated polymers (P2).
Table 1. Physical Properties of Novel Polymers polymer
Mn [kDa]a
Mw [kDa ]a
PDIa
λ max. soln. [nm]b
λ max. film [nm]c
energy gap (soln.) [eV]b
energy gap (film) [eV]c
HOMO [eV]d
LUMO [eV]e
P1 P2
29 25
57 51
2.0 2.0
618 618
634 641
1.80 1.84
1.75 1.78
−5.36 −5.46
−3.61 −3.68
a Determined by GPC(CB) against PS standards. bDetermined from solution UV−vis spectra in chlorobenzene. cDetermined from thin film UV−vis spectra (films spun from chlorobenzene, 5 mg/mL, 1000 rpm). dDetermined by photoelectron spectroscopy in air (PESA). eDetermined from absorption onset in thin film UV−vis spectra + HOMO.
Figure 2. Solution (chlorobenzene, taken at max. abs.
