Copolymers with similar comonomers: Tuning frontier orbital energies for application in organic solar cells

Abstract

Today, there is an intense search for new electron donor polymers for the active layers of organic solar cells. The synthesis of copolymers, in which there is a mix of electron donor monomers with others that are electron acceptors, is a currently employed approach that is used to obtain new polymers with more interesting electronic properties. Usually, the comonomers have a very different structure and the energies of the frontier electronic levels are unpredictable. We demonstrate in this work that the approach of using similar comonomers allows for the provision of copolymers with predictable energies of the frontier electronic levels. We explore the properties of copolymers consisting of monomers of poly(3-hexylthiophene), poly(3-hexyloxythiophene) and some poly(3-hexylthiophene) derivatives. The results indicate that the copolymers have intermediate frontier orbital energies when compared to the parent homopolymers and these energies are easy to predict. This approach is an alternative way of designing a copolymer with favorable tuning of the energies of the frontier electronic levels and is supported by experimental evidence. POLYM. ENG. SCI., 56:479-487, 2016. V C 2016 Society of Plastics Engineers INTRODUCTIONToday, there is a wide spectrum of important applications for organic conducting polymers (OCPs) in organic electronics [1][2][3][4][5]. Therefore, a control mechanism for the energies of the highest occupied and lowest unoccupied molecular orbitals (E HOMO and E LUMO ), the bandgap E LUMO -E HOMO (DE HL ) and optical properties is desirable in order to optimize the various types of devices [6,7]. These properties are crucial to the development of the promising technology of organic solar cells (OSC) [8,9].Currently, the scientific community has focused on two different ways of improving the efficiency of solar cells by controlling the electronic properties of OCPs [8][9][10]. The first adopted strategy has been to adjust the frontier electronic energy levels of the donor material to the levels of a given acceptor material. Normally, this is achieved through the design of OCPs with a lower E HOMO in order to increase the open-circuit voltage in the device (V OC ) [8][9][10][11][12][13]. The use of chemical substitutions is an effective way of implementing this change in the energies of the frontier orbitals, but with no major changes in the bandgap, as recently shown for poly(3-hexylthiophene) (P3HT) derivatives [14,15]. The second strategy is based on the design of new OCPs with a low bandgap in order to collect more photons of the solar spectrum and to increase the electric current in the device [8-10, 16, 17]. The donor-acceptor (DA) concept, which is the most frequently employed approach, is the synthesis of copolymers in which there is a mix of electron donor monomers with others that are electron acceptors [7,10]. Usually, the monomers have a very different structure and the resulting OCPs demonstrate a lower bandgap, but also a significant worsening in the other properties [18] as a decrease i...

Reorganization energy for hole and electron transfer of poly(3-hexylthiophene) derivatives

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Theoretical Approach Towards Rational Design and Characterization of Benzo[1,2-b:5-B’]dithiophene (BDT)-Based (A-D-A) Small Molecules of Relevance for High Performance Solar Cells

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Theoretical evaluation of chemical substitutions along the main chain of poly(3‐hexylthienylene‐vinylene) for solar cell applications

“…However, BHJ OSCs have achieved laboratory efficiencies of around 10% in the conversion of solar to electrical energy, below that reached by inorganic solar cells (some around 40%). To improve the efficiency of BHJ OSCs, one of the options is to study/produce new organic materials for application in the active layer that have better intrinsic properties than those that are already used , –. Current works have focused on the search for new electron donor polymers,, since the derivatives of fullerenes used as electron acceptors are already well established in this area …”

“…To improve the efficiency of BHJ OSCs, one of the options is to study/produce new organic materials for application in the active layer that have better intrinsic properties than those that are already used. 2,[7][8][9][10] Current works have focused on the search for new electron donor polymers, 11,12 since the derivatives of fullerenes used as electron acceptors are already well established in this area. 3 The use of chemical substitutions in existing organic materials has been a very useful tool when it is desired to modify electronic properties.…”