Evaluation of optoelectronic performance of four organic photo detectors structures

Organic photosensor made of poly [N-9′′-heptadecanyl-2,7-carbazole-alt-5,5-(4′, 7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) are promising candidates for bio-sensing applications. This paper investigates the optoelectronic characteristics of 4 different structures through simulation, utilizing PCDTBT as the active absorption layer. The scheme 1 is formed by placing the PCDTBT layer on top of a SiO2 layer, and then interdigitated electrodes made of aluminium are placed onto PCDTBT. As to the scheme 2, the semiconductor layer is placed between an aluminium layer (bottom) and glass (top) layer coated with thick transparent interdigitated electrodes made of indium tin oxide (ITO). Regarding to scheme 3, layers from bottom to top are SiO₂, cathode, PCDTBT and anode. Cathode has the same area as SiO₂ and PCDTBT layers, but anode covers only partial of the semiconductor. Finally, in the scheme 4, the semiconductor layer is also placed over SiO₂ layer but here the anode and cathode are limiting the PCDTBT layer sides, having the same area for both sides. All schemes have same volume of semiconductor. The simulations have been realized in dark conditions and under light intensities 100 mW/cm² in the wavelength range of 400-550 nm. The best results were obtained for scheme 2, organic photoconductor with Metal-Semiconductor-Metal structure. For in this scheme which is under the conditions of 2 V bias, 500 nm wavelength and 100 mW/cm² illumination, the photocurrent, the internal and external quantum efficiency obtained were 8.53 μA, 88% and 45% respectively. As a conclusion, the scheme 2 Glass/PCDTBT/Aluminium with transparent electrodes has reached high performance desirable for bio-sensing.