Synthesis of Polythiophene⁻Fullerene Hybrid Additives as Potential Compatibilizers of BHJ Active Layers

Tackling Performance Challenges in Organic Photovoltaics: An Overview about Compatibilizers

Organic Photovoltaics (OPVs) based on Bulk Heterojunction (BHJ) blends are a mature technology. Having started their intensive development two decades ago, their low cost, processability and flexibility rapidly funneled the interest of the scientific community, searching for new solutions to expand solar photovoltaics market and promote sustainable development. However, their robust implementation is hampered by some issues, concerning the choice of the donor/acceptor materials, the device thermal/photo-stability, and, last but not least, their morphology. Indeed, the morphological profile of BHJs has a strong impact over charge generation, collection, and recombination processes; control over nano/microstructural morphology would be desirable, aiming at finely tuning the device performance and overcoming those previously mentioned critical issues. The employ of compatibilizers has emerged as a promising, economically sustainable, and widely applicable approach for the donor/acceptor interface (D/A-I) optimization. Thus, improvements in the global performance of the devices can be achieved without making use of more complex architectures. Even though several materials have been deeply documented and reported as effective compatibilizing agents, scientific reports are quite fragmentary. Here we would like to offer a panoramic overview of the literature on compatibilizers, focusing on the progression documented in the last decade.

Copolymers and Hybrids Based on Carbazole Derivatives and Their Nanomorphology Investigation

Oligomers of the low-band-gap PCDTBT polymer, based on either 3,6 or 2,7 carbazole units, were modified with vinyl ω-chain end functionalities. The vinyl-functionalized oligomers were used as comonomers in free radical polymerizations with quinoline-based monomers such as 6-vinylphenyl-(2-pyridinyl)-4-phenyl-quinoline (vinyl-QPy), and 6-vinylphenyl-(2-perfluorophenyl)-4-phenyl quinoline (vinyl-5FQ). The co-polymeric materials bearing the vinyl-QPy moiety were developed as potential compatibilizers in polymer electron donor⁻fullerene acceptor blends for non-covalent interactions with the fullerene part. The co-polymeric materials bearing the vinyl-5FQ moiety were developed for the covalent attachment of carbon nanostructures; specifically, PC61BM. Both copolymers and hybrids, after thorough purification, were characterized in terms of their spectroscopic and optical properties as well as their ability to form nanophased separated films as such, or as additives at various percentages into PCDTBT: PC71BM blends.

Microstructure Formation of Functional Polymers by Evaporative Self-Assembly under Flexible Geometric Confinement

Polymer microstructures are widely used in optics, flexible electronics, and so forth. We demonstrate a cost-effective bottom-up manner for patterning polymer microstructures by evaporative self-assembly under a flexible geometric confinement at a high temperature. Two-parallel-plates confinement would become curve-to-flat shape geometric confinement as the polydimethylsiloxane (PDMS) cover plate deformed during solvent swelling. We found that a flexible cover plate would be favorable for the formation of gradient microstructures, with various periodicities and widths obtained at varied heights of clearance. After thermal annealing, the edge of the PMMA (Poly-methylmethacrylate) microstructures would become smooth, while the RR-P3HT (regioregular-poly(3-hexylthiophene)) might generate nanocrystals. The morphologies of RR-P3HT structures included thick films, straight lines, hierarchical stripes, incomplete stripes, and regular dots. Finally, a simple field-effect transistor (FET) device was demonstrated with the RR-P3HT micropattern as an active layer.

Long Lived Photoexcitation Dynamics in π-Conjugated Polymer/PbS Quantum Dot Blended Films for Photovoltaic Application

We used continuous wave photoinduced absorption (PIA) spectroscopy to investigate long-lived polarons in a blend of PbS quantum dot and regio-regular poly (3-hexylthiophene) (RR-P3HT). The charge transfer from RR-P3HT to PbS as well as from PbS to RR-P3HT were observed after changing the capping ligand of PbS from a long chain molecular to a short one. Therefore, PbS could be used to extend the working spectral range in hybrid solar cells with a proper capping ligand. However, we found that the recombination mechanism in the millisecond time region is dominated by the trap/defects in blended films, while it improves to a bimolecular recombination partially after ligand exchange. Our results suggest that passivating traps of nanocrystals by improving surface ligands will be crucial for relevant solar cell applications.