UV photodetector based on self-affine Langmuir-Blodgett film of nickel octabutoxy phthalocyanine doped with stearic acid

Abstract

We report our investigations on Langmuir-Blodgett (LB) films of different thicknesses of Nickel (II)1,4,8, 11, 15, 18,22,25-octabutoxy-29H, 31H-phthalocyanine (NiPc(OBu)8) doped with stearic acid on silicon and quartz substrates. We have characterized these films using grazing incidence X-ray diffraction (GIXD), atomic force microscopy and UV–visible spectroscopy techniques. The crystalline nature of the thin film polymorph was ascertained using GIXD. Multilayers deposited at 25 mN/m exhibited a Bragg peak at 4.15° (d = 21.3 Å). The crystallite size is found to increase with thickness and saturates beyond 75 nm. Topographic images of these multilayers acquired using an atomic force microscope reveal the presence of granular morphology. In addition, the height difference correlation analysis of these multilayers show that they are of self-affine nature. The scaling exponents (α = 0.92 ± 0.05, β = 0.22 ± 0.1 and 1/z = 0.17 ± 0.01) signifies kinetic roughening. UV–visible absorbance studies on LB films of NiPc(OBu)8 doped with stearic acid show a blue shift of Q and B bands compared to the pristine film. Further, the electrical characterization of the film was carried out in dark as well as under UV illumination (365 nm). The transient photocurrent behavior of the device for different illumination cycles is measured at different power densities and bias voltages. A slower response (on the order of few tens of sec) is inferred from the response time of the rise and decay curves. The responsivity and external quantum efficiency of the device for different power densities is also measured.