Graphene-Porphyrin Supramolecular Assemblies and Their Consequences in Organic Photovoltaic Cells

Abstract

Organic photovoltaic cells based on porphyrins, regardless they are small molecules or placed within a conjugated polymer backbone, have become increasingly more performants over the past few years (Solar RRL, 2017, 1, 1-26; Solar RRL, 2018, 2, 1-9; ACS Applied Materials & Interfaces, 2018, 10, 992-1004; ChemPlusChem, 2017, 82, 625-630; ACS Applied Materials & Interfaces, 2019, 11, 28078-28087). In the bulk heterojunction structure, the porphyrin chromophores act as both antennas for exciton energy migration and energy transfer, and as electron donors. At the interfaces of the electron donor and acceptor phases, supramolecular contacts between the porphyrin pigments and electron acceptors occurs. Among the electron acceptors, carbon-based semi-conducting materials such as fullerene- and graphene-type species represent the largest group. Although these photophysical processes at the are deduced from the macroscopic scale behaviors of the solar cells and the use of molecular models where the donor and acceptors of energy and electron are covalently bonded together (Phys. Chem. Chem. Phys. 2017, 19, 24018-24028; Dalton Trans. 2017, 46, 6278-6290; Phys. Chem. Chem. Phys. 2017, 19, 2926-2939), the use of supramolecular assemblies as models is rather limited to a few examples and suffer from the unavoidable weak binding constant of the p-stacked donor-acceptor dyads in solution (Inorg. Chem. 2016, 55, 9230-9239). This weak donor-acceptor binding results in an undesired mixture of assembled and free species in solution rendering the analysis somewhat difficult and often unreliable. The design of more realistic models requires robust anchoring molecular devices using moieties that do not interfere with the photophysical behaviors of the assemblies. Graphene is certainly a good electron acceptor, but the preparation of rigorously reduced graphene is tedious. Concurrently, the alternative is to use reduced graphene oxide but suffers from the poor solubility, and perhaps the presence of oxidized fragments on the graphene sheet. To overcome these inconveniences, the use of nanoribbons of graphene, where soluble groups flank the sides of the ribbons and the graphene segment is free of oxidized portions, appears ideal. The selected anchoring groups are two pyrenes to secure stable assemblies of porphyrin dyes and allowing a coverage of 30% of the available surface. This presentation focusses on the syntheses and characterization of the nanoribbons and their assemblies with zincporphyrin pigments. These assemblies form charge transfer excited states (CT states) and a detailed investigation of their photophysical properties addressing the various processes, namely excitation energy migration, energy (including nanoribbon to zincporphyrin CT state) and electron transfers, is made. These events can be ultrafast often smaller than 1 ps, but also sometime occurring well within the laser pulse, which can be within the 100-150 fs. At first glance these ultrafast events suggest that the photoconversion efficiency, PCE, can not limited by the time scale of these events, but the occurrence of competitive excited state relaxations and back electron transfers also being ultrafast (several hundreds of ps), can contribute in reducing the production of charge separated Frenkel species at the donor-electron interfaces. In these soluble models, it is interesting to discover that the graphene nanoribbons can also act as antennas permitting more photons to be harvested, which subsequently promote energy transfer to the CT assemblies, thus increasing the relative amount of excited CT states at the interfaces (J. Phys. Chem. C 2020, 124, 16248-16260). The creation of these charge transfer assemblies turns out to play a dominant role in their resulting photophysical traits and the enhancement of such CT states in terms of relative amount and spectral position in the red region, can be modulated upon using the push-pull conjugated polymers. The synthesis of such type of conjugated polymers is tricky (J. Org. Chem. 2019, 84, 3590-3594; Macromolecules 2015, 48, 7024-7038; Chem. Commun. 2011, 47, 10942-10944) and will be briefly discussed. Figure 1