Balance between light trapping and charge carrier collection : electro-photonic optimization of organic photovoltaics with ridge-patterned back electrodes

Light trapping strategies are frequently suggested to improve organic photovoltaic (OPV) cell efficiencies. However, one cannot overlook the side-effects to charge carrier collection which are introduced when seeking optical enhancements. A comprehensive electro-photonic model is utilized to study the optical and electrical effects of patterning poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] and poly(3-hexylthiophene) based solar cells with simple optical grating structures. It is found that for the most part, optical absorption improvements are attenuated by enhanced electrical losses. Optimized device structures that overcome this tradeoff are proposed and their detailed electro-optical characteristics are discussed. When the hole mobility is smaller than the electron mobility, the results suggest that in general, an inverted structure has a better chance to outperform a flat active layer than a conventional architecture in an OPV cell with the ridge-patterned back electrode

Dynamic Optical Gratings Accessed by Reversible Shape Memory

Shape memory polymers (SMPs) have been shown to accurately replicate photonic structures that produce tunable optical responses, but in practice, these responses are limited by the irreversibility of conventional shape memory processes. Here, we report the intensity modulation of a diffraction grating utilizing two-way reversible shape changes. Reversible shifting of the grating height was accomplished through partial melting and recrystallization of semicrystalline poly­(octylene adipate). The concurrent variations of the grating shape and diffraction intensity were monitored via atomic force microscopy and first order diffraction measurements, respectively. A maximum reversibility of the diffraction intensity of 36% was repeatable over multiple cycles. To that end, the reversible shape memory process is shown to broaden the functionality of SMP-based optical devices