Three new donor–acceptor conjugated polymers were synthesized by combining electron-donating 3-octylthiophenes with various electron-accepting benzobisazoles. The influence of the structural differences of the three benzobisazoles on the electrochemical, optical, and photovoltaic properties of the polymers composed from them was investigated. According to our results, changing the arrangement of the oxygen atoms of the benzobisoxzoles from the cis to trans orientation slightly stabilized both the HOMO and LUMO levels, whereas replacing the oxygen atoms in trans-BBO with sulfur atoms only stabilized the HOMO level. Bulk heterojunction photovoltaic devices were fabricated by using the copolymers as electron donors and PC71BM ([6,6]-phenyl C71-butyric acid methyl ester). It was found that open-circuit voltages (V
ocs) as high as 0.86 V, and power conversion efficiencies (PCEs) up to 1.14%, were obtained under simulated AM 1.5 solar irradiation of 100 mW/cm2. Field-effect transistors based on these polymers exhibited hole mobilities as high as of 4.9 × 10–3 cm2/(V s) with the trans-BBO polymer giving the best performance in both devices.
Benzobisoxazoles (BBOs) are known to increase the electron affinities and improve the electron transporting properties of materials containing them. However, BBO copolymers generally do not perform well as emissive guests in guest−host PLEDs due to inefficient Forster resonance energy transfer (FRET) between host and guest. The incomplete FRET results in a large amount of host emission and limits the potential efficiencies of the devices. In all previously reported BBO copolymers, the conjugation pathway was through the oxazole rings. Herein we report six new BBO copolymers with backbone connectivity directly on the central benzene ring, resulting in a conjugation pathway for the polymers that is perpendicular to the previously reported pathway. Guest−host PLEDs made using these polymers show that the new conjugation pathway improves FRET between the poly(N-vinylcarbazole) host and the BBO-containing polymer guest. Because of highly efficient FRET, no host emission is observed even at lower guest concentrations. The improved energy transfer results in devices with luminous efficiencies up to 3.1 Cd/A, a 3-fold improvement over previously reported BBO-based PLEDs. These results indicate that the conjugation pathway plays a critical role in designing emissive materials for guest−host PLEDs.
Benzobisoxazole polymers possessing a conjugation pathway directly through the central benzene ring possess reduced optical band gaps and more efficient electroluminescence in polymer light-emitting diodes.
Solution-processed OLEDs with polymer hosts and polymer or small-molecule guests have been studied extensively. More recently, efficient solution-processed OLEDs with small molecule hosts and small molecule guests were also reported. However, small molecule hosts of polymer guests in solutionprocessed fluorescent OLEDs have not been investigated. In this work guest:host systems consisting of the small molecule 4,4 0 -bis(9-carbazolyl)-biphenyl (CBP) as host to polymer guests such as novel benzobisoxazole (BBO)-containing copolymers and well-known poly(2-methoxy-5-(2 0 -ethyl-hexyloxy)-1,4-phenylene vinylene) (MEH-PPV) are compared to those with poly(N-vinyl carbazole) (PVK) host, which previously yielded highly efficient phosphorescent OLEDs. In the case of MEH-PPV, guest:host OLEDs are also compared to those with a neat MEH-PPV emitting layer. It is found that replacing the polymer host PVK with the small molecule host CBP improves efficiencies by up to 100%. A blue emissive BBO-polymer:CBP device reaches a luminous efficiency (h L,max ) of 3.4 cd A À1 (external quantum efficiency h ext ¼ 2.4%), while the PVK-based device exhibits h L,max ¼ 1.7 cd A À1 (h ext ¼ 1.2%). A green emissive BBO:CBP OLED exhibits h L,max ¼ 5.7 cd A À1 (h ext ¼ 2.1%), while that in the PVK host is 3.1 cd A À1 (h ext ¼ 1.1%). For MEH-PPV:CBP these values are 3.7 cd A À1 (h ext ¼ 1.4%), compared to 2.9 cd A À1 (h ext ¼ 1.0%) for MEH-PPV:PVK and 0.7 cd A À1 (h ext ¼ 0.4%) for the neat MEH-PPV device. Possible origins of the improvement are discussed, including increased charge mobility, smoother film morphology, and the potential effect of multiple non-coiling host small molecules (in contrast to the likely coiled PVK) surrounding a polymer guest.
3,7-Diiodo-2,6-di(thiophen-2-yl)benzo[1,2-b:4,5-b']difurans are efficiently prepared by an iodine-promoted double cyclization. This new heterocyclic core is readily modified by the attachment of alkyl chains for improved solubility. The use of these compounds for the synthesis of new conjugated polymers is also reported.
ABSTRACT:In an effort to design efficient low-cost polymers for use in organic photovoltaic cells the easily prepared donoracceptor-donor triad of a either cis-benzobisoxazole, transbenzobisoxazole or trans-benzobisthiazole flanked by two thiophene rings was combined with the electron-rich 4,8-The electrochemical, optical, morphological, charge transport, and photovoltaic properties of the resulting terpolymers were investigated. Although the polymers differed in the arrangement and/or nature of the chalcogens, they all had similar highest occupied molecular orbital energy levels (25.2 to 25.3 eV) and optical band gaps (2.1-2.2 eV). However, the lowest unoccupied molecular orbital energy levels ranged from 23.1 to 23.5 eV. When the polymers were used as electron donors in bulk heterojunction photovoltaic devices with PC 71 BM ([6,6]-phenyl C 71 -butyric acid methyl ester) as the acceptor, the transbenzobisoxazole polymer had the best performance with a power conversion efficiency of 2.8%.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.