TiO2 assisted sensitivity enhancement in photosensitive nanocrystal skins

Abstract

Solution-processable semiconductor nanocrystals (NCs) have been widely used to create novel devices for the photovoltaic, light-emission, light-detection and biosensing applications. They are good candidates especially to develope more efficient and novel optoelectronic devices owing to the high absorption cross-section, spectral tunability, deposition easiness and low cost properties. In recent years, NC integrated photodetectors have been developed to be used in large-area light-sensing applications [1]. These NC-based photodetectors have the ability to convert an optical signal to an electrical signal using the NCs as the optical absorbers. These low-cost devices were initially operated on the basis of charge collection, where an electric field imposed on the detector dissociates the photogenerated excitons into electrons and holes, in which an electric current is produced [2]. On the other hand, as an alternative device structure, we have recently developed the light-sensitive nanocrystal skin (LS-NS) [3]. These LS-NS platforms, which were fabricated over areas up to 48 cm2, are operated on the basis of photogenerated potential buildup, as opposed to conventional charge collection. In operation, close interaction of the monolayer NCs of the LS-NS with the top interfacing contact, while the bottom one is isolated using a high dielectric spacing layer, results in highly sensitive photosensing in the absence of external bias application. Furthermore, NC monolayer of the LS-NS makes the device semi-transparent with sufficient absorption, while reducing the noise generation and dark current. In our other recent work, we also reported that, by using a thick photoactive NC layer, a much lower photovoltage buildup was observed in the LS-NSs and it was attributed to the self-absorption effect [4]. In addition, we demonstrated the sensitivity increase in the LS-NSs via the absorption enhancement of NC film with the integration of plasmonic nanoparticles [5]. However, the localized plasmonic resonance band strongly limits the observed enhancement factor and the resultant operating wavelength range. Furthermore, in the absence of an external bias in the LS-NSs, each exciton tends to remain in the NC layer, where it was created, and recombine with the photogenerated holes that accumulate at the top interfacing contact, which causes also lower voltage buildup in the device. To overcome all these problems, in this study, we propose a thin TiO2 layer as the electron-accepting material and demonstrate the first account of electron transfer in NC-based light-sensitive skins, which leads to significant broadband sensitivity enhancement in the active device architecture. Here, we prove that favorable conduction band offset aids in transferring photogenerated electrons from a monolayer of NCs to an electron-accepting layer, which is ultimately useful for photosensing platforms and the next generation of light-sensing NC devices. © 2014 IEEE.