Performance Enhancement of Electronic and Optoelectronic Devices by Interfacial Layer Engineering

Abstract

Selection of an appropriate interfacial layer is a requirement to achieve high-performing electronic and optoelectronic devices. In particular, the performance of these devices is closely related to the transport of charge carriers, and thus can be improved by controlling the electrical property of interfacial layers with hole or electron doping. In recent years, organic materials and graphene have attracted great research interest for their potential applications in the next-generation devices such as flexible and transparent electronics. However, they can be easily damaged by established processes of doping which typically involves high energy impact, due to their small dissociation energies, and, especially, organic materials are vulnerable to chemicals. Therefore, experimental methods for controlling their electrical properties through doping to enhance device performance are limited. This dissertation aims to improve performance of devices based on organic materials and graphene, such as an organic light-emitting diode (OLED), a graphene field-effect transistor (FET), and a graphene perovskite solar cell by interfacial layer engineering. First, gold nanoparticles (Au NPs) have been precisely positioned in a specific plane in a fluorescent Alq3-based OLED, and showed that there exist an optimum distance between the NP layer and the emitting layer for the maximum external quantum efficiency of the device. Au NPs are positioned in the hole transport layer (HTL) using a dry, room temperature aerosol technique. By controlling the position and the density of the Au NPs, the external quantum efficiency of the Au-NP-embedded OLEDs can be optimized, presenting a 38% enhanced maximum efficiency compared to that of the control device without Au NPs. In contrast to typically used methods for incorporating metal NPs in an organic layer, such as vacuum thermal evaporation or spin coating, the aerosol-deposited Au NPs do not penetrate into the underlying organic layer, not only allowing for precise control of their vertical position, but also minimizing damage to the hole transport organic material. Optical and electrical characterizations show that the existence of the optimum distance results from the competition between the metal induced quenching and the increased electron‒hole recombination probability arising from electrostatic effects of holes trapped in the Au NPs. Secondly, reliable control of doping type and doping level of graphene has been realized by controlled deposition of aerosol-derived metal NPs on the channel of a graphene FET. Ag or Pt NPs with a fairly narrow size distribution are deposited on graphene channels using an aerosol technique that is capable of controlling size, shape, and deposited density of NPs independently. The transfer characteristics of the aerosol NP-decorated graphene FETs show that deposition of the Ag NPs and the Pt NPs induces a shift of the Dirac point in the negative and positive direction, respectively, indicating n-type and p-type doping of graphene, respectively. The change in doping level and doping type of graphene is attributed to the electron transfer between the metal NPs and graphene owing to the work function difference. Due to the consistent size and shape of the aerosol-derived metal NPs, doping level shows a monotonic change with increasing the surface coverage of the NPs, which has been hardly achieved with typically used methods such as thermal evaporation and spin coating due to morphology change and random aggregation of metal NPs. The minimum conductance at the Dirac point also shows no appreciable change with deposition of the aerosol NPs on graphene, implying that the aerosol NPs delivered on graphene by weak electrostatic force do not degrade the graphene sp2 hybridization during their deposition. Thirdly, highly efficient transparent conductive oxide (TCO)-free inverted perovskite solar cells have been fabricated by employing graphene as a transparent anode. The interface between the graphene electrode and the HTL, poly (3, 4-ethylenedioxythiophene):poly (styrenesulfonate) (PEDOT:PSS), has been engineered by introducing a few nanometer thick molybdenum trioxide (MoO3) layer to improve hydrophilicity and increase the work function of graphene. With a 2 nm-thick MoO3 layer, the graphene-based perovskite solar cells show the best PCE of 17.1% and the average PCE of 16.1% under AM 1.5G one sun illumination (100 mW cm‒2), which is so far the highest efficiency for TCO-free solar cells. Interestingly, the interface engineering using MoO3 also enhances the performance of the ITO-based devices, resulting in the best PCE of 18.8% and the average PCE of 18.2%, which is also the highest efficiency for inverted perovskite solar cells.