Po-Jui Chen (Y)
Optimization of light extraction efficiency (LEE), is of crucial importance for applications ranging from LEDs to weakly emitting fluorescence based thin film chemical sensors. Refractive index differences result in a large proportion of the internal luminescence, whether electrically or optically generated, being trapped and subsequently reabsorbed within the device. As a result, considerable effort has been expended both experimentally and in simulation to increase LEE by breaking the two dimensional symmetry of devices through surface modification. Due to the commercial importance of solid-state lighting, the majority of the work has concentrated on highly emissive devices (i.e. GaN) where the light propagation is limited and trapping results in an up to 96% reduction in luminescence. In contrast, this paper focuses on the effects of breaking two-dimensional symmetry in weakly absorbing thin films where trapped light can propagate hundreds of microns before absorption. (Such films may find usage in thin film chemical sensors – when a change in photoluminescence (PL) intensity signifies the presence of a chemical – or in luminescent solar collectors where one seeks absorb and then propagate light to strategically placed high-efficiency photovoltaic detectors.) Monte Carlo based ray tracing was used to quantify the effects of a one-dimensional perturbation of film thickness on PL of a weakly absorbing fluorescent film (polystyrene doped with 1% MEH-PPV). The addition of narrow (width=2500nm) grooves at 0.03 mm periodicity, resulted in the LEE increasing from 27% to 44%, a ~60% increase relative to the flat film. The additional emission occurred primarily in the vicinity of the groove edges. The extracted light's angular dependence was highly non-Lambertian and could be tuned by changing the angle of the grooves. This suggests that by PL emission can be concentrated in one area of the film or optimized in the direction of a farfield detector.