John O'Sullivan

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>