Manipulation of Intermolecular Interactions for Active Layer Morphology Optimization in Organic Photovoltaics

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Organic photovoltaics (OPVs) are an attractive solar energy technology for low-current applications. Herein is described the supramolecular design and methodology to manipulate intermolecular interactions in order to create an active layer in OPVs devices composed of crystalline and amorphous donor-acceptor domains, which has been proposed as the ideal morphology for high performance. To this end, a series of symmetric and asymmetric diketopyrrolopyrrole (DPP) derivatives containing either an amide (capable of hydrogen-bonding) or ester endgroups were synthesized. The symmetric designs faced problems with solubility, strong segregation and low performance, so asymmetric donors having one amide/ester were used. Upon addressing initial stability problems, analysis of the ester films with X-ray diffraction displayed greater crystallinity and π−π stacking. The amide formed short aggregates with smaller, less ordered domains, resulting from competition between hydrogen bonding and π−π stacking, which interestingly endowed devices with higher current and 50% increase in device efficiency over the ester. To better match solar emission, the DPP core was substituted by benzodithiophene (BDT). Amides again outperformed esters, but introduction of a benzothiadiazole π-spacer between the amide/ester endgroups led to electron traps and lowered performance; replacing it with phenyldithiophene reduced stacking ability. A recurring issue was the competition between noncovalent interactions, which motivated the use of barbituric acid endgroups, but solubility was compromised. After addressing each problem, a design having a BDT core with planar π-spacers, connected by an alkyl linker to the hydrogen-bonding endgroups is predicted to display optimized optoelectronic properties and cooperative noncovalent interaction. Next, a series of BDT-core molecules with DPP endgroups and alkyl tails resembling solvent additives (which improve donor-acceptor interaction but increase processing complexity) were synthesized. Preliminary molecules showed promising efficiency but lacked solubility. An asymmetric DPP group with less stacking ability was used, but exposed possible electron traps. The BDT core was then modified to be more electron-rich but led to lower performance. Consequently, an extended molecule with fully symmetric DPP endgroups was used, but the large number of alkyl tails caused segregation from the acceptor. Therefore, a BDT design with symmetric DPP termini and re-positioned alkyl tails is proposed to address electron traps, solubility and segregation problems.

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  • 02/27/2018
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