High photothermal conversion efficiency for semiconducting polymer/fullerene nanoparticles and its correlation with photoluminescence quenching

Abstract

Semiconducting polymer nanoparticles (SPNs) have potential applications in a variety of fields due to their remarkable capacity to convert light into other forms of energy. This conversion can occur through photoluminescence (PL), photovoltaic (PV), and/or photothermal (PT) effects, and therefore SPNs can be employed in organic solar cells (OSCs) or nano agents for photothermal therapy (PTT), for instance. Here we explore those properties by studying the correlation between PT efficiency (η) enhancement and SPN PL quenching with increasing fullerene (PC₇₁BM) doping. This correlation was observed for nanoparticles of three different conjugated polymers so that the highest PT efficiency was found for SPNs of MDMO-PPV/PC₇₁BM (η ≈ 74.8%). Using a combination of a kinetic model with quantum chemistry calculations, it is suggested that those phenomena derive from the fast non-radiative (NR) decay of charge transfer (CT) excitons at the polymer/fullerene interface. Our analysis revealed key physical properties that mostly influence the PT effect and enabled us to determine NR rates of the systems under study. The improvement of η with doping essentially occurs because the NR deactivation of CT states is faster than the NR recombination of the Frenkel excitons in the donor. Since we observed that the CT exciton NR rate tends to follow an energy gap law, NR recombination improvement may originate from the stronger vibrational overlap between CT state and ground state. Finally, our findings are in agreement with the open circuit voltage average loss observed in bulk heterojunction OSC using the same polymers as donors. The synergetic experimental characterization of PT and PL effects together with the proper theoretical modeling can thus provide valuable information on the factors that enhance a particular kind of energy conversion in SPNs. This methodology can be a valuable tool to prospect new materials aiming at theranostic applications and/or clean energy generation.