Jeremy J. Intemann

Efficient all polymer solar cells from layer-evolved processing of a bilayer inverted structure

A new electron acceptor poly(indacenodiselenophene-difluorobenzothiadiazole) was used to fabricate bilayer all-polymer solar cells with a high power conversion efficiency (2.5%). By using the layer-evolved bilayer structure with interfacial modification, it alleviates unbalanced charge carrier mobility and reduces bimolecular recombination within the BHJ device.

Fluorescent polymer guest:small molecule host solution-processed OLEDs

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 solution-processed fluorescent OLEDs have not been investigated. In this work guest:host systems consisting of the small molecule 4,4′-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′-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 (ηL,max) of 3.4 cd A−1 (external quantum efficiency ηext = 2.4%), while the PVK-based device exhibits ηL,max = 1.7 cd A−1 (ηext = 1.2%). A green emissive BBO:CBP OLED exhibits ηL,max = 5.7 cd A−1 (ηext = 2.1%), while that in the PVK host is 3.1 cd A−1 (ηext = 1.1%). For MEH-PPV:CBP these values are 3.7 cd A−1 (ηext = 1.4%), compared to 2.9 cd A−1 (ηext = 1.0%) for MEH-PPV:PVK and 0.7 cd A−1 (ηext = 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.

Efficient synthesis of benzobisazole terpolymers containing thiophene and fluorene

We report the synthesis and lumuminescence properties of three novel polymers composed of 9,9-dioctylfluorene and a donor–acceptor-donor (D–A-D) triad of a benzobisazole moiety sandwiched between two octylthiophenes. The requiste monomers, 2,6-bis(5-bromo-3-octylthiophen-2-yl)-benzobisazoles were obtained efficiently via the Lewis acid catalyzed cyclization of 2-bromo-3-octyl-5-(triethoxymethyl)thiophene and the corresponding diamino diols or dithiols. The polymers were synthesized in excellent yield by the Suzuki coupling reaction between the D–A-D benzobisazole monomers and 9,9-dioctylfluorene bisboronic acid. Alkyl side chains provided the polymers with solubility in common organic solvents, enabling characterization using gel permeation chromatography, 1H NMR, UV-Vis and fluorescence spectroscopy. The polymers have optical bandgaps of 2.43–2.63 eV and HOMO levels at −5.54 to −5.65 eV relative to vacuum as determined by UV visible and photoelectron spectroscopy respectively. Light-emitting diodes using blends of the copolymers in a poly(N-vinyl carbazole) matrix yielded blue-green emission with luminous efficiencies of 0.86 Cd/A at ∼505 nm. This efficient and high-yielding route is a promising approach for the synthesis of polymers containing benzobisazole moieties.

Spontaneous thermal crosslinking of a sydnone-containing side-chain polymer with maleimides through a convergent [3 + 2] dual cycloaddition/cycloreversion process for electro-optics

A convergent [3 + 2] dual cycloaddition/cycloreversion process between sydnone and multi-functional maleimides is reported as an efficient protocol for thermal crosslinking of polymers. To alleviate the commonly observed pre-crosslinking problem, the reactivity of 1,3-dipolar sydnone moieties is tuned by increasing its steric effect through a sydnone-containing side-chain polymer (SCP) that is synthesized using a Mitsunobu post-functionalization method. Thin films and amorphous solid composites derived from SCP and a passive bis-maleimide cross-linker (BMI-1) are used to conduct a model study of the crosslinking process. The results from spectroscopic and thermal analyses show that the SCP/BMI-1 composites react efficiently at a modest curing temperature of ∼100 °C and can be converted into a highly cross-linked polymer network with excellent thermal stability. The spontaneity and high conversion efficiency of thermal cross-linking for SCP/BMI-1 suggest that an efficient three-step cascade reaction with minimal side reactions can be achieved in the solid-state. This lattice hardening process was implemented in a new cross-linkable electro-optic (EO) polymer SCP/BMI-2/TMI possessing a dipolar polyene chromophore crosslinker BMI-2. Through poling and in situ cross-linking, the poled films of SCP/BMI-2/TMI exhibit both large EO coefficient (r33 of 117 pm V−1 at 1.31 μm) and excellent long-term alignment stability at 85 °C.