Molecular functionalization of interfaces is a powerful strategy already pursued in various energy conversion schemes, including dye-sensitized solar cells, polymer-protected electrodes in batteries, polymer electrolyte fuel cells, and molecule-decorated electrodes in PEC cells.
In perspective, energy conversion schemes featuring molecular components as in organic photovoltaics (OPV) or organic LEDs (OLEDs), molecular photocatalysts or organic and hybrid batteries represent promising approaches to reduce the overall device and energy cost (and with it the energy payback time) and to implement scalable and more sustainable device elements. Molecularly-functionalized interfaces can offer supreme structural definition of atomic distances, spatial arrangements at the nanoscale and an enormous space of tunable physical properties. This capability first derives from the possibility to build molecular building blocks such as ligands, molecular catalysts, light-harvesting antennas or molecular linkers with full atomistic definition and well-defined properties, and second, the ability to assemble such functional building blocks in controlled spatial and orientational relationships that result in targeted physical interactions such as light harvesting, catalytic activity, or charge transfer.
The current challenges of organically modified and hybrid materials systems in the context of energy conversion include limited long-term stability and efficiency. Given that molecular systems often move through highly excited states during energy conversion processes, there are typically several relaxation channels, some of which may be destructive over time (such as photooxidation of photocatalysts). Efficiency limitations arise prominently from insufficient electronic coupling between molecular reaction partners and to inorganic device components, as well as a low volume density of the active molecular units. Inefficient coupling to inorganic components and low-density limitations apply also prominently to biological or biomimetic building blocks, which otherwise would represent a most appealing low-cost, sustainable and highly functionalized class of molecular components.