EAGER: Vertical-carrier-transport two-dimensional photo-harvesting devices with nanocavity enhancement

Abstract Nontechnical: Photodetector and photovoltaic devices are key components for converting light energy to electricity. To evaluate the performance of these devices, power conversion efficiency is one of the most important parameters, describing how much light energy is converted to electricity. Due to the advances of two-dimensional materials in recent years, it is believed that revolutionary ultra-thin optoelectronic devices are possible. However, because of the weak light absorption within these atomically thin layers, the power conversion efficiency of two-dimensional-material-based photodetector and photovoltaic devices is low. Therefore, it is essential to design and develop new strategies to improve the power conversion efficiency of these two-dimensional energy harvesting devices. This project will combine electrical (i.e. vertical carrier transport) and optical (i.e. Nano-cavity) architectures to overcome this grand challenge. (1) Since the transport distance of electrons and holes is only a few nanometers, the carrier transport efficiency is superior compared with their bulk counterparts. (2) The Nano-cavity structure significantly enhances the optical absorption within these two-dimensional layers due to strong optical interference resonances. Therefore, the power conversion efficiency can be increased drastically. This EAGER program will perform a combined fundamental and experimental investigation to validate this scientific hypothesis, which will pave the way towards the development of practically high-efficiency two-dimensional photo-harvesting devices. Technical: In most thin-film energy harvesting/conversion applications, there is a long-existing trade-off between optical absorption and thickness of semiconductor materials. It is particularly true in recently emerging two-dimensional-material-based optoelectronic photodetector and photovoltaic devices. Due to the atomically thin layers, their light-matter interactions (e.g., optical absorption and energy conversion enfficiency) are weak. Consequently, absorption enhancement strategies will introduce revolutionary advances to these two-dimensional-based light-harvesting devices. On the other hand, it is generally believed that minimizing the volume of active material can suppress the noise and thermal excess carriers in the diffusion-limited operating mode for photodetectors. It can also minimize the recombination of carriers in photovoltaic devices. However, this advantage is only viable in the vertical device architecture where carriers flow along the vertical direction and dominated by out-of-plane quantum tunneling. In this project, we will develop a fundemantal strategy to enhance the light-matter interaction of two-dimensional-materials based on strong interference effect in planar nanocavities, and overcome the limitation between the optical absorption and film thickness for energy harvesting devices. This principle is quite general and will be applied to explore the spectrally tunable absorption enhancement of various absorptive two-dimensional materials and will create new regimes of optical physics and energy applications. Combined with more efficient carrier transport properties along the vertical direction, the untapped potential of this mechanism for enhanced light-matter interaction will be demonstrated in Nano-cavity enhanced two-dimensional photodetector and photovoltaic devices with significantly improved power conversion efficiency.