John O'Sullivan

Abstract:

Doping two-dimensional (2D) semiconductors without direct chemical or structural modification of the channel remains a central challenge for device integration. Here we demonstrate an electrostatic doping strategy on monolayer MoS₂ based on embedding fixed charge in engineered dielectric stacks, enabling carrier modulation in the absence of volatile external bias. By comparing different dielectric architectures, we show that effective electrostatic doping is governed by the defect landscape of the capping dielectrics and their interface with the 2D channel. A self-consistent electrostatic model reveals that interface states control the partitioning of the dielectric embedded charge between carriers trapped in defects or free for conduction in the channel, posing limits to the effectiveness of electrostatic coupling. This work establishes electrostatic doping as a viable strategy for carrier modulation in 2D semiconductors and identifies dielectric defect engineering as central to its implementation.

Abstract:

The files in this deposit contain the data used to generate and support the results, tables and figures reported in the associated publication. The research data were created during the experimental, computational and analytical work undertaken for the study as detailed in the main manuscript. Where necessary, the original data were processed, converted and organised into tabulated or structured data files to facilitate reuse, verification and visualisation. Tabulated source data for the figures presented in the publication are included wherever applicable. The deposited files may include Microsoft Excel workbooks, comma-separated value files, MATLAB `.mat` files, NumPy data files, Origin project or worksheet files, and other standard tabulated or structured data formats. Data are generally stored as tables, arrays, structures or data frames, with column headings, variable names, units and relevant metadata included where available. Excel and CSV files can be opened using standard spreadsheet or data-analysis software. MATLAB `.mat` files may be opened using MATLAB or compatible software capable of reading MATLAB data formats. NumPy files may be opened using Python with NumPy or other compatible scientific-computing software. Origin files may require Origin or OriginPro, although equivalent tabulated data are provided where possible to support access using other software. The data may be replotted, analysed or visualised using any suitable spreadsheet, programming, scientific-computing or graphing software that supports the relevant file format. No bespoke software is required unless specifically stated in the accompanying file-level documentation.

Abstract:

Transparent conducting electrodes (TCEs) combine high optical transmittance and electrical conductivity, and are an essential component of tandem solar cells. Although tandem cells offer a pathway to power conversion efficiencies exceeding 40% at low cost, they introduce new challenges for TCE design. Achieving the required balance of optical, electrical and chemical properties has so far limited practical TCEs in tandems to a small set of high-cost, indium-based oxides. Recent advances in computational and experimental techniques have improved understanding of TCE solid-state physics, revealing promising alternative materials. In this Review, we examine the material properties essential for TCEs in perovskite–silicon tandems, evaluate current candidates, and highlight the key challenges and opportunities for next-generation TCE development. Our goal is to bridge the gap between materials science and device engineering, providing a roadmap to accelerate the integration of advanced TCEs in high-efficiency optoelectronic devices.

Abstract:

High-efficiency solar cell architectures, including silicon heterojunction (SHJ) and perovskite/silicon tandems, rely heavily on the unique properties of transparent conducting oxides (TCOs). The push towards terawatt-scale PV manufacturing means it is increasingly desirable to develop indium-free TCOs to facilitate the upscaled manufacturing of high-efficiency cell designs. Aluminium-doped ZnO (AZO) deposited by atomic layer deposition (ALD) has emerged as a promising candidate due to its combination of optical transparency and electrical conductivity. In addition, AZO has also been shown to passivate the c-Si surface. The ability for one material to provide all three properties without requiring any indium is advantageous in single junction and tandem solar devices. Herein, we demonstrate exceptional silicon surface passivation using AZO/AlOx stacks deposited with ALD, with a J0 < 1 fA cm−2 and corresponding implied open circuit voltage (iVOC) of 740 mV. We provide a comprehensive analysis of the role of ALD precursor dosing to achieve optimised performance. A broad range of characterisation approaches were used to probe the structural, compositional, and chemical properties of AZO films. These indicated that the passivation properties are governed by a delicate interplay between the Zn and Al concentrations in the film, highlighting the importance of precise process control. Optical modelling in a single junction SHJ architecture indicates these AZO films are close in performance to high-mobility indium-containing TCOs. The insights provided by this work may help to further the case of indium-free TCOs, which is critical for upscaled production of high-efficiency solar cells.

Abstract:

<jats:title>Abstract</jats:title><jats:p>Indium‐based transparent conducting electrodes (TCEs) are a major limiting factor in perovskite/silicon tandem cell scalability, while also limiting maximum cell efficiencies. In this work, we propose a novel TCE based on electrostatically doped graphene monolayers to circumvent these challenges. The electrode is enabled by a thin film dielectric that is charged and interfaced to a graphene film, optimally exploiting electrostatic doping. The field effect mechanism allows the modulation of charge carriers in monolayer graphene as a function of charge concentration in the dielectric thin film. Electrostatic charge was deposited on SiO<jats:sub>2</jats:sub> membranes, and graphene transferred onto them exhibited a reduction in sheet resistance because of the induced charge carriers. We show a reduction in sheet resistance of graphene by 60% in just 3 min of dielectric charging, without impacting the transmission of light through the film stack. Hall effect measurements indicated that the mobility of the films was not significantly degraded. The deposition of negative electrostatic charge reversed this effect, allowing for precise tunability of charge concentration from n‐ to p‐type. We develop a model to determine the required sheet resistance of a graphene TCE with 97% transmittance in a perovskite/silicon tandem cell. As the technique here reported does not impact transmittance, a graphene TCE with a sheet resistance below 50 Ω/□ could enable efficiencies up to 44%, presenting a promising alternative to indium‐based TCEs.</jats:p>