Molecular-level architectural design using benzothiadiazole-based polymers for photovoltaic applications

Molecular-level architectural design using benzothiadiazolebased polymers for photovoltaic applications Vinila N. Viswanathan1, Arun D. Rao1, Upendra K. Pandey2, Arul Varman Kesavan1 and Praveen C. Ramamurthy*1,2,§ Full Research Paper Address: 1Department of Materials Engineering, Indian Institute of Science, Bangalore, Karnataka, India and 2Interdisciplinary Centre for Energy Research, Indian Institute of Science, Bangalore, Karnataka, India Email: Praveen C. Ramamurthy* - Open Access Beilstein J. Org. Chem. 2017, 13, 863–873. doi:10.3762/bjoc.13.87 Received: 31 December 2016 Accepted: 06 April 2017 Published: 10 May 2017 Associate Editor: H. Ritter * Corresponding author § Fax: +91-80-2360-0472; Tel: +91-80-2293-2627 © 2017 Viswanathan et al.; licensee Beilstein-Institut. License and terms: see end of document. Keywords: bulk heterojunction; donor–acceptor–donor polymer; low band gap polymer; organic photovoltaics Abstract A series of low band gap, planar conjugated polymers, P1 (PFDTBT), P2 (PFDTDFBT) and P3 (PFDTTBT), based on fluorene and benzothiadiazole, was synthesized. The effect of fluorine substitution and fused aromatic spacers on the optoelectronic and photovoltaic performance was studied. The polymer, derived from dithienylated benzothiodiazole and fluorene, P1, exhibited a highest occupied molecular orbital (HOMO) energy level at −5.48 eV. Density functional theory (DFT) studies as well as experimental measurements suggested that upon substitution of the acceptor with fluorine, both the HOMO and lowest unoccupied molecular orbital (LUMO) energy levels of the resulting polymer, P2, were lowered, leading to a higher open circuit voltage and short circuit current with an overall improvement of more than 110% for the photovoltaic devices. Moreover, a decrease in the torsion angle between the units was also observed for the fluorinated polymer P2 due to the enhanced electrostatic interaction between the fluorine substituents and sulfur atoms, leading to a high hole mobility. The use of a fused π-bridge in polymer P3 for the enhancement of the planarity as compared to the P1 backbone was also studied. This enhanced planarity led to the highest observed mobility among the reported three polymers as well as to an improvement in the device efficiency by more than 40% for P3. Introduction The great interest in organic photovoltaic (OPV) devices is motivated by their ease of low-temperature solution processing, light weight, flexibility and potential to produce large area devices [1]. The introduction of an interpenetrating donor and acceptor architecture in the active layer of the OPV devices led to a new type of device, the so-called bulk heterojunction (BHJ) solar cells, with improved power-conversion efficiency (PCE) [2-4]. A large number of polymer semiconducting materials of 863 Beilstein J. Org. Chem. 2017, 13, 863–873. donor–acceptor–donor (D–A–D) architecture have been synthesized and used in OPV devices recently reaching remarkable PCEs of up to 11.7% [5-7]. However, the diversity of monomeric units and the numerous available reports on the structural complexity of D–A–D-conjugated p-type polymers indicate that there is still need for new materials which can further improve the performance of OPV devices based on D–A–D polymers [8-13]. The properties of D–A–D-type materials such as band gap, structural planarity, charge carrier transport, etc., can be easily tailored by careful selection, combination, and position of the donor and acceptor moieties. For OPV systems, it is desirable that p-type polymers should have a low band gap for a broad absorption area of the solar spectrum to harvest a maximum number of photons [14]. Simultaneously, these compounds should also be soluble in common organic solvents and have good film forming properties. However, these are not the only parameters to consider for the design of a new donor polymer system. In OPV devices, a bicontinuous layer of a donor and an acceptor material is sandwiched between two electrodes. After the absorption of light, excitons are generated which dissociate towards the interface of the donor–acceptor layer and are separated into free carriers. These free charge carriers are then collected at the electrode for current generation [15]. The driving force for this charge separation originates from the energy offset between the frontier molecular energy levels of the donor and acceptor material, broadly known as the binding energy [15]. While reducing the band gap by adjusting the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels, a downhill driving force for exciton dissociation should be maintained for optimum performance of the OPV. If not, the total exciton dissociation will decrease, and hence, the overall device efficiency too. Moreover, for efficient OPV systems, a moderately high charge carrier mobility is required, which is attainable by increasing the crystallinity of polymers with firmly packed polymer chains. However, an increase in polymer crystallinity will simultaneously decrease their processability in solution. This will result in the reduced formation of the desired bicontinuous morphology with the acceptor [16]. Hence, when designing new molecules for OPV applications, a subtle balance between lowering the band gap, crystallinity, and solubility should be maintained. Extensive studies have been reported for the tuning of optoelectronic and photovoltaic properties by architectural design at the molecular level [17-19], such as quinoidation of the polymer backbone [20], alternate D–A–D architectures [21,22], and substitution with electron-withdrawing or electrondonating groups [23,24]. Substitutions can be used to tune the band gap, energy levels, solubility, packing of material and morphology [8]. Among them, the introduction of fluorine has attained great interest because of its small size and strong electron-withdrawing nature, and fluorine substitution will amend both the HOMO and LUMO energy levels. In addition, substitution along the backbone persuades more inter- and intramolecular interactions [25-29]. Furthermore, the modification of π-bridges between the donor and acceptor unit of p-type molecules plays a significant role in increasing the efficiency for OPVs [30,31]. However, fused π-bridges (such as thienothiophene) having a larger molecular structure and higher degree of conjugation are less explored with respect to thiophene and furan spacers. Thienothiophene ensures a highly delocalized electron system and higher charge carrier mobility due to its rigid and coplanar fused structure. Also, some thienothiophenebased polymers show a noticeable bathochromic shift in their absorption spectra in comparison with thiophene-substituted polymers [32-34]. Herein, keeping all these criteria in mind, we endeavored to obtain a series of low band gap polymers, P1, P2, and P3, with matching HOMO–LUMO energy levels wi (...truncated)