Highly sensitive thin-film organic phototransistors: Effect of wavelength of light source on device performance

Organic phototransistors (OPTs) were fabricated from pentacene and copper phthalocyanine (CuPC) based on the geometry of organic field-effect transistors (OFETs); and the effect of the wavelength of the incident light source on their performance was examined. High performance OFETs with pentacene and CuPC were fabricated and the characteristics of the OPTs were examined under UV and visible-light irradiations with top illumination. The CuPC and pentacene OPTs show a high responsivities of 0.5-2 and 10-50 A/W and maximum I-Ph/I-Dark of 3000 and 1.3x10(5), respectively, under 365 nm UV light. However, under visible light, at a wavelength of 650 nm, the pentacene OPTs had 100 times less responsivity, 0.15-0.45 A/W, and a I-Ph/I-Dark of 1000, even though an absorption coefficient three times larger was observed at this wavelength than at 365 nm. A strong correlation was found between the performance of the OPTs and the incident photon to current conversion efficiency spectra of an organic semiconductor. The strong dependence on the wavelength of incident light of the performance of the prepared OPTs can be explained by an internal filter effect in which light with a large absorption coefficient is filtered at the top surface and through the bulk of the film when light is directed onto the opposite side of the OFET gate electrode. Thus, light cannot efficiently contribute to the generation of charge carriers in the channel regions that were formed in the first two molecular layers adjacent to the dielectric interface. Consequently, the most efficient OPTs were produced when the following conditions of incident light were satisfied: The photon energies (or frequencies) should be (i) larger than the band gap and (ii) have a relatively small absorption coefficient, since the light can penetrate down to the channel layer more efficiently when it is near the dielectric interface without any loss in absorption through the film. (c) 2005 American Institute of Physics.