High-Speed Organic Photodetectors for Visible Light Communication

Abstract

Organic semiconductors possess a variety of characteristics that surpass conventional rigid electronics. In recent years, visible light communication (VLC) has attracted considerable interest for use in in-home and sensor networks. Organic optoelectronic devices such as organic light-emitting diodes (OLEDs) and organic photodetectors (OPDs) have great potential for VLC links because of low-cost fabrication and integration on flexible substrates. However, these are developed primarily for display applications and efficient photovoltaics, respectively, and there are few reports on their high-speed operation. Here, the research aims to achieve high-bandwidth OPDs for VLC links. The focus is on simulating optimised bandwidth performance and fabricating prototype devices based on organic semiconductors. In end-user contexts like LiFi and device-to-device communications and sensor networks like the Internet of Things (IoT), visible light communication (VLC) provides an energy-efficient and cost-effective connection. VLC uses existing lighting infrastructure, giving it a cost-effective and energy-efficient data transmission solution. Second, it improves security because light signals do not penetrate walls, lowering the chance of eavesdropping. VLC allows for speedier communication in areas where radio frequency-based technologies may encounter interference or saturation. Due to these advantages, VLC is a promising technique for various applications, including indoor location, smart lighting systems, and data communication in sensitive situations. Organic optoelectronic devices, more specifically organic light-emitting diodes (OLEDs) and organic photodetectors (OPDs), show significant promise for these kinds of systems because of their capacity to be manufactured at large scales utilising materials that can be solution-processed at a low cost and because they can be integrated on flexible substrates without any problems. On the other hand, OLEDs are largely tailored for display applications, emphasising high brightness, power efficiency, stability, and longevity, while OPDs, which are needed for energy harvesting (photovoltaics), are developed to attain high photon conversion efficiency. This is an important distinction to make, but it is often overlooked. Surprisingly, the bandwidth performance of these devices and the degree to which they have been optimised for high-speed operation have received only a limited amount of research. Consequently, this research focuses on computational and experimental efforts to attain OPDs for VLC systems with large bandwidths (~ MHz range), which are suitable for many sensor applications. The design for the OPD that has been presented consists of an ultra-thin multi-layered structure that incorporates PTCBI and CuPc organic semiconductors. This structure enables quick exciton dissociation and efficient charge collection at the appropriate electrodes. State-of-the-art unique modelling, simulation and experimental studies are conducted. The work extensively explores the fabrication equipment and structure of an organic photodiode (OPD), summarizing the fabrication process and current research trends on OPD structures. The research work highlights modelling and simulation findings of bilayer and multilayer devices. Fundamental modelling concepts for an organic photodiode, the impact of biasing regimes, and the introduction of the novel organic simulation software, OghmaNano, lead to the development of an ultra-thin bi-layer heterojunction-layered organic photodiode. Insightful results from modelling the bi-layer heterojunction thin-layered OPD device show remarkable consistency between experimental measurements and predicted outcomes. The device exhibits exceptional dark current performance, sensitivity to nanowatt-level incident visible light, and a noteworthy Linear Dynamic Range (LDR) of 168 dB. The research extends to the discussion of fabricated multiple-layered ultra-thin organic photodetectors. A simulation study assesses their performance, reporting excellent external quantum efficiency. Current voltage trends for dark and light curves are evaluated through simulation and validated with fabrication results, demonstrating decent linearity across various devices with varying layers. The temperature dependence of the dark current is explored, incorporating different activation energy values through the Arrhenius plot. Photocurrent dynamics are investigated, achieving a rapid 3-dB response around 6 MHz for a 4.5 mm2 area and 8 MHz for a 2 mm2 area, highlighting the significant potential for visible light communication and diverse sensing applications in these multi-layered photodetectors.