Real-Time Photoluminescence Studies of Structure Evolution in Organic Solar Cells

A method to study the structural evolution of organic bulk heterojunctions via real-time, in-situ, steady-state photoluminescence (PL) is presented. In-situ PL, in combination with real-time transmission and reflection measurements, allows us to quantitatively describe the progression of intimate mixing during blade coating of two OPV systems: the common model system poly(3-hexylthiophene-2,5-diyl)/phenyl-C61-butyric-acid-methyl ester (P3HT/PCBM), and the higher power conversion efficiency system 7,7′-(4,4bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′]dithiophene-2,6-diyl)bis(6-fluoro-5-(5′-hexyl-[2,2′-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazaole), p-DTS(FBTTh2)2/[70]PCBM. Evaluating the time dependence of the PL intensity during drying using a 3D-random-walk diffusion model allows for the quantitative determination of the ratio of characteristic domain size to exciton diffusion length during solidification in the presence of the processing additives 1-chloronaphtalene (CN), 1,8-octanedithiol (ODT), and 1,8-diiodooctane (DIO). In both cases, the obtained results are in good agreement with the typically observed fibril widths and grain sizes, for P3HT and p-DTS(FBTTh2)2, respectively.