Enhanced Performance of Organic Solar Cells with Increased End Group Dipole Moment in Indacenodithieno[3,2‐b]thiophene‐Based Molecules

Abstract

wileyonlinelibrary.comPresently, the performance of the top molecular donors depends heavily on their degree of semicrystallinity and requires solvent additives and postprocessing techniques, such as annealing, in order to control bulk heterojunction (BHJ) morphology. [ 5 ] This causes an increase in device fabrication complexity and reduces batch-to-batch cell reproducibility.Molecular donors that are insensitive to processing conditions and that are less dependent on crystallinity for effi cient charge transport are highly desirable and may be better suited for roll-to-roll device fabrication. Among polymeric donors, one strategy that has proven successful for producing both high device effi ciencies and an insensitivity to processing conditions, is the use of fused ring ladder-type donor moieties, as the coplanar ladder-type structure provides high hole mobilities in BHJs, even in amporphous fi lms. [ 6 ] To this end, attempts have been made to incorporate the ladder-type indacenodithiophene (IDT) donor moiety into the structure of novel molecular donors, with the resulting materials indeed showing a general insensitivity to processing conditions. [ 7 ] However, these materials have exhibited very low fi ll factors (FF) in devices (0.35-0.48), likely resulting from poor morphology and charge transport in BHJs. Due to the low FF, IDT-based molecules have only achieved PCEs up to 4.7%, far lower than the 7.5% achieved by IDT-based polymers. [ 8 ] Indacenodithieno[3,2-b]thiophene (IDTT) is an alternate ladder-type donor moiety that has performed exceptionally well in polymeric semiconductors. [ 6b,d , 9 ] Its longer fused ring structure produces higher hole mobilities and extinction coeffi cients for improved light harvesting, compared to IDT in polymeric materials, while still maintaining a reasonably low sensitivity to processing conditions. We believed these features could be used to make IDTT-based molecular donors for high effi ciency OPVs.Herein, we report four new IDTT-based molecules utilizing a D1-A-D2-A-D1 type structure, where D1 is an alkyl oligiothiophene-based donor, A is a benzothiadiazole (BT) acceptor, and D2 is IDTT ( Scheme 1 ). Because the out-of-plane side chains of IDTT disrupt π-stacking, we hypothesized that π-stacking would occur primarily at the ends of the molecule, and therefore, the nature of the D1-A portion (hereafter referred to as the end group) would have a substantial effect on the morphology

Conjugated Side Chain Tuning Effect of Indacenodithieno[3,2‐b]thiophene and Fluoro‐Benzothiadiazole‐Based Regioregular Copolymers for High‐Performance Organic Field‐Effect Transistors

“…However, no obvious diffraction pattern was observed for both polymer films, as shown in Figure S9 (Supporting Information). This is consistent with previous reports for the amorphous nature of the IDTT‐based polymers . Varying side chains have been observed to have great influence on the molecular packing property of polymers in the solid state, as a result of the structural alteration of the conjugated backbone due to variation in the bulkiness and mobility of side chains .…”

Donor End-Capped Hexafluorinated Oligomers for Organic Solar Cells with 9.3% Efficiency by Engineering the Position of π-Bridge and Sequence of Two-Step Annealing

“…This should be related to chemical structure, molecular packing, and morphology and carrier transporting factors. Recent reports provide evidence that the increase of end-group ground dipole moment has a positive impact on molecular packing, the FF, and mobility of devices . Due to the symmetrical structure and the same central core of our isomers, we postulate that difference of molecular packing interaction of isomers would be mainly contributed from the ground dipole moment of the rest half of the molecules.…”

Near-IR Absorbing D–A–D Zn-Porphyrin-Based Small-Molecule Donors for Organic Solar Cells with Low-Voltage Loss

“…Moreover, the higher dipole moment value may also lead to high charge carrier mobility, as confirmed from the mobility results. The larger dipole moment resulted in better self-assembly of the molecules, and this led to the formation of longer supramolecular chains and corroborated the improved percolation pathways for charge carriers . Moreover, the high dipole moment and dielectric constant for VC5 may also reduce the nonradiative recombination and lead to a reduction in E loss …”