electroluminescent conjugated polymers. As an example, Figure 3 shows the anisotropic optical constants of a film of F8BT. The ordinary absorption was again in good agreement with the spectrum obtained from normal incidence transmission measurements. Although not as anisotropic as OC1C10ยฑ PPV, F8BT shows considerable anisotropy, with a birefringence of 0.19 at 633 nm. As with OC1C10ยฑPPV there is a slight blue shift in the absorption peaks for the extraordinary direction with respect to the ordinary direction, again indicating a decreased conjugation length for chains lying out of the plane of the film.
In conclusion, we have shown that a combination of reflection and transmission ellipsometry can be reliably employed to determine the ordinary and extraordinary optical constants in conjugated polymer films in both the absorbing and transparent regions. We have presented optical constants for two commonly used electroluminescent polymers, which will be useful in optical modeling of LEDs and photovoltaic devices. The technique provides structural information about the degree of chain alignment, and will in the future be applied to study the effects of annealing on optical properties, and to investigate possible variation of anisotropy with film thickness in thinner films than were studied here.
Experimental
OC1C10ยฑPPV and F8BT were spin-coated on 1.2 mm Spectrosil-B substrates. Spectrosil-B has well-known optical constants, with negligible refractive index dispersion and no absorption over the wavelength range of interest. The OC1C10ยฑPPV film was spin-coated from a chloroform solution at a concentration of 5 g/L and the F8BT film from a xylene solution at 15 g/L. The samples were annealed for 14 h at 115 C in vacuum.
Measurements were performed using a J. A. Woollam M-2000 diode-array rotating-compensator ellipsometer, with a xenon lamp source. The rotating compensator system does not suffer from regions that are insensitive to D, unlike rotating-polarizer or rotating-analyzer systems, which are insensitive around D = 0 and D = 180. This meant that D t , which was always close to 0 in this experiment, was not affected by excessive noise. Reflection and transmission ellipsometry measurements were performed in air over a wavelength range of 245 nmยฑ900 nm. The angles of incidence used were between 55 and 70 for reflection ellipsometry and between 40 and 60 for transmission ellipsometry, in steps of 5. No evidence for biaxial anisotropy within the plane of the film was observed either by rotating the sample in the plane of the film, or from non-zero off-diagonal elements in the Jones matrix describing the sample response.