Photovoltaic Properties of Monolayer 2D Transition Metal Dichalcogenides




Tebyetekerwa, Mike

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This thesis focuses on understanding photovoltaic (PV) properties and the performance of two-dimensional (2D) semiconducting monolayer transition metal dichalcogenides (TMDs). The work is broadly divided into two major parts: (1) monolayers and (2) heterostructures. I first show an approach to predict the maximum possible open-circuit voltage (Voc) from various TMDs by using the Planck generalised law of emission. A contactless and non-destructive method that utilises micro-photoluminescence (PL) and absorption spectroscopy is employed. The results show that Voc values of up to 1.40 V can potentially be achieved from PV devices fabricated from monolayer TMDs under 1-sun illumination. Moreover, this value of Voc is inhomogeneous across these thin layers due to the increased defects in the materials. These results are the foundations for quantifying the maximum theoretical efficiency of TMD-based solar cells. Second, I engineer the defects in monolayer TMDs by introducing fluorescent organic materials, called aggregation-induced emission (AIE) molecules beneath the TMDs, forming vertical organic-inorganic (O-I) heterostructures. Compared to their pristine materials, the O-I heterostructures show a uniform PL emission and enhancement of up to 950% in the TMDs. The strong PL enhancement is mainly attributed to the efficient photogenerated carrier process in the AIE (courtesy of its restricted rotor intermolecular motions in the solid state) and the charge-transfer process in the created Type-I O-I heterostructures. Moreover, the PV properties of the TMDs in the heterostructures are improved due to defect engineering brought about by the AIE molecules. The extra material layer introduced above manipulates well the light absorption and emission of the monolayer TMDs, making it possible to realise unique applications and devices. However, it is challenging to obtain improved electronic properties in such Type-I heterostructures where emission and absorption are high. To overcome this challenge, in the next part of the thesis, I introduce chromium germanium telluride (Cr2Ge2Te6, CGT) quantum dots (QDs) beneath the TMDs, which presents complex light-matter interactions with the atomically thin layered materials from the heterostructures, depending on the QDs thickness. I demonstrate controllability of light emissions in various CGT QDs-TMDs heterostructures, and all with improved electronic properties. Furthermore, in this thesis, I reveal that a relative twist between two TMD monolayers is paramount in modulating the optoelectronic properties of 2D TMD-TMD vertical heterobilayer structures. Understanding their stacking nature is key to realising new optoelectronic devices. As an example, MoS2/WS2 heterobilayers are studied. I show that these materials have wide freedom of emission amplitude (the energy difference between the lowest and the highest emission energies) arising from their indirect interlayer exciton emission modulation, realised by simply changing the twist angle between monolayers MoS2 and WS2 in the heterobilayer. Finally, I demonstrate the effects of relative twists between the monolayers in MoS2 /WS2 heterobilayers on their PV properties, including absorption, minority carrier lifetime and light emission. The detailed experimental analysis shows that the interlayer twist in MoS2/WS2 heterobilayers slightly affects the absorption in the heterobilayers, but significantly the carrier lifetime and diffusion. From a broader perspective, the efforts in the last part of the thesis could facilitate the research and development of high-performance monolayer TMD-based PV devices.






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