Intramolecular Donor–Acceptor Regioregular Poly(3‐hexylthiophene)s Presenting Octylphenanthrenyl‐Imidazole Moieties Exhibit Enhanced Charge Transfer for Heterojunction Solar Cell Applications

Abstract

Intramolecular donor–acceptor structures prepared by covalently binding conjugated octylphenanthrenyl‐imidazole moieties onto the side chains of regioregular poly(3‐hexylthiophene)s exhibit lowered bandgaps and enhanced electron transfer compared to the parent polymer, e.g., conjugation of 90 mol% octylphenanthrenyl‐imidazole moieties onto poly(3‐hexylthiophene) chains reduces the optical bandgap from 1.91 to 1.80 eV, and the electron transfer probability is at least twice as high as that of pure poly(3‐hexylthiophene) when blended with [6,6]‐phenyl‐C61‐butyric acid methyl ester. The lowered bandgap and the fast charge transfer both contribute to much higher external quantum efficiencies, thus much higher short‐circuit current densities for copolymers presenting octylphenanthrenyl‐imidazole moieties, relative to those of pure poly(3‐hexylthiophene)s. The short‐circuit current density of a device prepared from a copolymer presenting 90 mol% octylphenanthrenyl‐imidazole moieties is 13.7 mA · cm−2 which is an increase of 65% compared to the 8.3 mA · cm−2 observable for a device containing pure poly(3‐hexylthiophene). The maximum power conversion efficiency of this particular copolymer is 3.45% which suggest that such copolymers are promising polymeric photovoltaic materials.

The effect of intramolecular donor–acceptor moieties with donor–π-bridge–acceptor structure on the solar photovoltaic performance

“…After photoexcitation, the generated excitons are stabilized and fur-ther separated because of the extended conjugation length, and the electrons are subsequently transferred from the donors to the acceptors very rapidly [6,7]. Furthermore, the charge separation and transfer process within the planar intramolecular donoracceptor structure is more effective than the one in the twisted intramolecular donor-acceptor structure, therefore, the charge transfer rate will be higher if the acceptor units are tethered on the polymeric main chains in a coplanar manner [5,6,8].…”

“…More recently, a new imidazole unit as the acceptor moiety has been synthesized by Neophytou et al [10] and this kind of copolymer exhibits fast exciton dissociation at the polymerfullerene heterointerfaces. Lee et al [11] also reported a kind of intramolecular donor-acceptor copolymer with electron-rich carbazole presenting in the backbone as the donor unit and electron-deficient 14H-benzo [4,5]isoquino[2,1-a]perimidin-14-one moiety as pendant side chains. In a recent report, we have presented a new kind of intramolecular donoracceptor copolymers containing side-chain-tethered perylenebis(dicarboximide) (PDI) moieties [12].…”

“…One of the effective ways to lower the band gap of conjugated polymers is to make -(A) m -(B) n -random type copolymers by adjusting the ratio of the two monomer unit [1][2][3]. On the other hand, it is found that the introduction of electron-acceptor units onto the side chain of conjugated polymers could extend the absorption breadth of sunlight and lower the energy gap to some extent [4,5]. In other words, conjugated polymers that contain side-chain-tethered acceptor moieties can not only exhibit multiple absorptions but also enhance charge transfer ability.…”

“…This type of conjugated polymers falls into the intramolecular donor-acceptor systems. This system typically consists of conjugated polymers as electron donating groups, another conjugated groups or long alkyl chains as the bridges, and electron acceptors which can increase the interior charge transfer rate of molecules and broaden the absorption spectrum [5,6]. For this type of conjugated polymers, the electron donating groups are present in the main chain, while the electron accepting units are pendent as side chains.…”

“…Recently, a series of intramolecular donor-acceptor conjugated copolymers of -(A) m -(B) n -random type have been synthesized by Wei's group and used as the donor materials in the active layers of PSCs [4,5,9]. For this kind of conjugated polymers, thiophene derivatives were used as the electron donating groups and presented in the main chain, and phenanthrenylimidazole (PI) moieties with the characteristic of coplanar were employed as the side-chain-tethered electron accepting units.…”