The displacement of fluoride by cyanide offers a simple route to strongly electron accepting cyano substituted benzothiadiazole monomers. Copolymerisation with electron rich donors leads to low band gap polymers which exhibit electron transporting behaviour.

We report a systematic study into the effects of cyano substitution on the electron accepting ability of the common acceptor 4,7-bis(thiophen-2-yl)-2,1,3-benzothiadiazole (DTBT). We describe the synthesis of DTBT monomers with either zero, 1 or 2 cyano groups on the BT unit, and their corresponding co-polymers with the electron rich donor dithienogermole (DTG). The presence of the cyano group is found to have a strong influence on the optoelectronic properties of the resulting donor-acceptor polymers, with the optical band gap red-shifting by approximately 0.15 eV per cyano substituent. We find that the polymer electron affinity is significantly increased by ca. 0.25 eV upon addition of each cyano group, whilst the ionization potential is less strongly affected, increasing by less than 0.1 eV per cyano substituent. In organic photovoltaic (OPV) devices power conversion efficiencies (PCE) are almost doubled from around 3.5% for the unsubstituted BT polymer to over 6.5% for the mono-cyano substituted BT polymer However, the PCE drops to less than 1% for the di-cyano substituted BT polymer. These differences are mainly related to differences in the photocurrent, which varies by one order of magnitude between the best (1CN) and worst devices (2CN). The origin of this variation in photocurrent was investigated by studying the charge generation properties of the photoactive polymer:fullerene blends using fluorescence and transient absorption spectroscopic techniques. These measurements revealed that the improved photocurrent of 1CN in comparison to 0CN was due to improved light harvesting properties whilst maintaining a high exciton dissociation yield. The addition of one cyano group to the BT unit optimized the position of the polymer LUMO level closer to that of the electron acceptor PC71BM, such that the polymer's light harvesting properties were improved without sacrificing either exciton dissociation yield or device V OC . We also identify that the drop in performance for the 2CN polymer is caused by very limited yields of electron transfer from the polymer to the fullerene LUMO levels, likely caused by poor orbital energy level alignment with the fullerene acceptor (PC71BM). This work highlights the impact that small changes in chemical structure can have on the optoelectronic and device properties of semiconducting polymer. In particular this work highlights the effect of LUMO-LUMO offset on the excited state dynamics of polymer:fullerene blends.

Copolymers of carbazole and 4,7-bis(2-thienyl)-2,1,3-benzothiadiazole (dTBT) incorporating thioalkyl (−SR) and alkoxy (−OR) solubilizing groups on the 2,1,3-benzothiazdiazole (BT) unit are synthesized and compared. The introduction of −SR and −OR groups onto the BT unit of the polymer was found to have different effects on the electronic properties of the polymers as well as the conformation of the polymer backbone. Large conformational changes between the ground state (GS) and excited state (ES) geometries of the polymers with −SR groups led to very large Stokes shifts of up to 224 nm. The polymer with −OR groups was found to have approximately double the photovoltaic efficiency at ∼4% compared to the polymers with −SR groups (PCE ∼ 2%). However, polymers with −SR groups were found to give very high open circuit voltages (V OC) of over 1 V. Changing the −SR chain length from ethyl to dodecyl was found to have little influence on the solar cell performance of the polymer or the magnitude of the Stokes shift.

Backbone functionalisation of conjugated polymers is crucial to their performance in many applications, from electronic displays to nanoparticle biosensors, yet there are limited approaches to introduce functionality. To address this challenge we have developed a method for the direct modification of the aromatic backbone of a conjugated polymer, post-polymerisation. This is achieved via a quantitative nucleophilic aromatic substitution (SNAr) reaction on a range of fluorinated electron-deficient comonomers. The method allows for facile tuning of the physical and optoelectronic properties within a batch of consistent molecular weight and dispersity. It also enables the introduction of multiple different functional groups onto the polymer backbone in a controlled manner. To demonstrate the versatility of this reaction, we designed and synthesised a range of emissive poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT)-based polymers for the creation of mono and multifunctional semiconducting polymer nanoparticles (SPNs) capable of two orthogonal bioconjugation reactions on the same surface.

A simple route to the preparation of alkylamine, thioalkyl, and alkoxy monofunctionalized 4,7-di(thiophen-2yl)-2,1,3-benzothiadiazole) based monomers is reported from a common fluorinated precursor. Copolymerization with a carbazole comonomer under Suzuki conditions yielded a series of analogous donor−acceptor copolymers in which the only difference was the nature of the heteroatom (N, O, or S) on the benzothiadiazole core. This was shown to have a significant impact on the wavelength and intensity of the intramolecular charge transfer (ICT) absorption peak due to a combination of electronic and steric factors. Substantial differences were also observed in the solar cell performance of blends with PC 71 BM, with the octylamino substituted polymer exhibiting significantly lower performance than the other two polymers. This polymer also exhibited a reversible change in the optical spectra upon exposure to acid, suggesting potential as a sensing material.