Trap states dependence on morphology of zinc phthalocyanine thin films

The effect of grain size and morphology on the trap states of zinc phthalocyanine has been investigated. Thin films of zinc phthalocyanine are deposited on gold or platinum interdigitated electrodes in a vacuum of 10-6 mbar. Depositions are performed at different temperatures between 60°C and 200°C in order to vary the grain size and morphology of the deposited crystals. Photocurrent measurements are performed by illuminating the samples with a helium-neon laser with a wavelength of 632.7 nm in the absence of ambient light. A shutter controls the duration of the exposure to the laser. The photocurrent decays after the shutter is closed and the decay is quantitatively analyzed. For a single trap state, the Twarowski model predicts a bi-exponential decay curve. Within the simple framework of this model, the photocurrent decay can be fit to a bi-exponential curve, and from this the de-trapping rate can be calculated. The photocurrent measurement is performed at temperatures from 20°C to 50°C, and the trap state depth can be extracted from the data. A correlation between trap state depth and deposition temperature is observed; trap state depths range from 0.158 eV at a deposition temperature of 180°C to 0.282 eV at a deposition temperature of 60°C. At deposition temperatures between 60°C and 120°C, the trap depth exhibits a weak dependence on deposition temperature, while at higher temperatures, trap depth depends more strongly on deposition temperature, which changes the grain size. The decrease in trap state depth with increasing grain size may be attributed to the importance of grain boundaries and morphology of the organic thin film.