Using Momentum Resolved Spectroscopies to Quantify Organic Semiconductor-Surface Plasmon Polariton Coupling in a Drastically Reorientable Small Molecule System

Momentum-resolved spectroscopies have proven to be extremely effective techniques for quantifying optical anisotropies intrinsic to organic semiconductor films. Such optical anisotropies profoundly impact light absorption and emission rates in photovoltaic and light-emitting devices. To date, a molecular system that can controllably adopt both “in-plane” (molecular backbones parallel to substrate interface) and “out-of-plane” molecular orientations has been elusive. Here, we use a combination of momentum-resolved spectroscopies and ellipsometry to demonstrate that small-molecule p-SIDT(FBTTh₂)₂ can controllably adopt both orientations when spin-cast from solution. We show, by comparing SPP dispersion measurements with theory, that this combination of techniques allows extraction of anisotropic optical constants with remarkable precision. Finally, we show that this molecular reorientation yields a two-fold increase in PL output for thin films on plasmonic substrates, and that this improvement agrees precisely with theory. These results highlight the importance of molecular orientation in the context of plasmonic device architectures.