Design and Synthesis of Donor-Acceptor Architectures for Novel Optical Materials
Organic photovoltaics (OPV) based on polymer-fullerene bulk-heterojunction (BHJ) cells have attracted increasing attention in the scientific community during the last few years for its promise of a low-cost power source.1 The BHJ cell is typically composed by an intimate blend between a p-type donor polymeric semiconductor and an n-type acceptor fullerene derivative such as [6, 6]-phenyl-C61-butyric acid methyl ester (PCBM). The fundamental mechanism of a OPV cell is a 4-step process:
- absorption of light by the absorbing polymeric semiconductor and generation of excitons (bound electron-hole pairs);
- diffusion of excitons towards the polymer-fullerene interface;
- dissociation of excitons at the interface due to the high electron affinity of the fullerene component and generation of free charge carriers;
- transport and collection of charge carriers to their respective electrodes.
Since only fullerene derivatives have been reported so far to give highly efficient OPV cells, the route for more performing devices goes through the optimization of the donor semiconducting polymer. The strategies to improve OPV power conversion efficiencies (PCEs) imply higher short-circuit current densities (Jsc), mainly by reducing the bandgap of polymers so as to harvest more sunlight, and higher open-circuit voltages (Voc). Being Voc proportional to the difference LUMOPCBM – HOMOpolymer, this value can be increased by lowering the HOMO of the polymers. Accordingly, the efficiency limitations of OPV cells have been recently discussed in terms of HOMO/LUMO and energy gap Eg energies. Namely, material-design rules involve:
- a LUMOpolymer energy higher than that of the LUMOPCBM, with an ideal minimal offset of 0.3 eV, which corresponds to a value of -4.0 eV (or higher) assuming -4.3 eV for LUMOPCBM;
- a bandgap in the range of 1.2 – 1.7 eV for efficient light harvesting extention to the low-energy portion of the solar emission;
- accordingly, a HOMOpolymer energy of ca. –5.2 to –5.7 eV. Recently, record efficiencies higher than 7% were achieved by using a donor low-bandgap polymer (PBDTTT–CF or PTB7 ) made of alternating electron-rich benzodithiophene and electron-poor thienothiophene fragments.
The vinylene (V)-spaced D-A low-bandgap copolymers attracted my attention because of their very limited investigation in comparison to their counterparts with direct D-A aromatic ring linkage. Previously a report was published on vinylene-linked D-A polymers in which Liming Ding and coworkers pointed out the fact that similar class of polymers are still largely unexplored because of the synthetic challenges.7 Indeed, I believe that vinylene-linked conjugated polymers might carry a series of relevant advantages over the more conventional systems, including planarization between adjacent aromatic units and ethenylic spacers, extended -conjugation, efficient delocalization, and, accordingly, reduced bandgaps and enhanced optical properties in the low-energy spectral region.
NOTE: This project was sanctioned and funded by Department of Science and Technology (DST), Govt. of India; Science and Engineering board under the scheme of Start-Up Research Grant for Young Scientist 2014.