In the quest for ultra-high-efficiency silicon solar cells, optimising surface passivation has emerged as a critical pathway to minimise losses and enhance device performance. Recent breakthroughs in aluminium oxide (AlOx) passivation show an interface to Si with low interface defect density and high negative charge density after activation annealing at 400-450 degrees C, enabling low surface recombination velocities. The formation of an interfacial SiOx layer has been recognised as a key factor. In this study, we present an in-depth investigation of a SiOx/AlOx/SiNx nanolayer stack interface with Si, where the SiOx is wet chemically grown. By varying the AlOx deposition from 5 to 40 ALD cycles, we observed a reduction in interface defect density, indicating the presence of negatively charged hydrogen in the AlOx layer. We reveal a distinctly different interface between Si and nanolayer stacks with or without AlOx. Activation annealing significantly reduced recombination losses for stacks with AlOx, attributed to increased charge density and decreased carrier capture velocity at the valence band-tail. We find lower electron capture rates in nanolayer stacks containing AlOx, suggesting effective passivation of donor states by negatively charged hydrogen. Additionally, the formation of new acceptor states was detected by an increase in hole capture velocity at the interface after annealing. Electron energy loss spectroscopy (EELS) identified an Al:SiOxNy layer of similar to 2.5 nm thick with excess oxygen content and a mixture of tetrahedral and octahedral coordinated Al, likely contributing to the formation of acceptor defects and suggest an intrinsic link between the chemical and field-effect passivation.

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/AlO x stacks deposited with ALD, with a J 0 < 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.

A highly efficient hole-selective passivating contact remains the crucial step required to increase the efficiency of polysilicon-based Si solar cells. The future development of solar modules depends on a device structure that can complement the electron-selective tunnel oxide passivating contact with an equivalent hole-selective contact. We investigate plasma enhanced chemical vapor deposited (PECVD) SiN x and atomic layer deposited AlO x as alternative nanolayers for the passivation layer in polysilicon tunnel contacts. We have fabricated p+ poly-Si contacts with resistivities below 100 m Omegacm2 using these alternative metal oxide and nitride nanolayers. Initial passivation tests yielded low levels of passivation; however, a detailed understanding of the nanolayers elucidated the strategies to improve passivation significantly, achieving an implied open-circuit voltage (iV OC) of 698 mV and dark saturation current density (J 0) of 34 fA/cm2 for a p+ poly-Si contact using a PECVD SiN x interlayer. These are among the best reported for nitride-based nanolayer tunneling contacts, with research into nitride-based tunneling contacts being still in its infancy.

Minimising charge losses at silicon interfaces is a major development area for highly efficient solar cells. Here we report on the interface improvements achieved by establishing a surface electric field during low-temperature firing of dielectric thin films. By inducing a corona electric field on the surface of a thin film stack, we observe significant modifications to the silicon-dielectric interface upon annealing, which correlate with the characteristics of interface defects. The passivation properties of the interfaces strongly depend on the polarity and strength of the electric field during firing, as well as the dielectric materials in the layer stack. We show that the surface electric fields not only influence surface carrier population but also affect the resulting chemical interface properties post-annealing. It is postulated that hydrogen migration plays a role in these observed effects. Leveraging the corona-induced electric field enables fine-tuning of both the chemical and field-effect passivation in thin film surface dielectrics, resulting in recombination current densities as low as 2.8 fA/cm2 in research-grade float zone silicon, and 14 fA/cm2 in industrial-grade textured silicon. The simplicity and versatility of the thin film electric polarisation enable a new strategy for controlling and exploiting the chemical enhancement of interfaces in solar cell devices, from current TOPCon and PERC devices to future multijunction silicon-based cells.

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 SiO2 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 Omega/square could enable efficiencies up to 44%, presenting a promising alternative to indium-based TCEs.

The unambiguous detection of hydrogen in solar cell contact structures is critical to understanding passivation and degradation phenomena. Deuterium is often used to depict the distribution of hydrogen more clearly. However, experimental noise and artifacts can hinder the clear identification of species. This work provides a report of time-of-flight elastic recoil detection (ToF-ERD) analysis to identify H/D contents in a thin poly-Si/SiOx passivating contact. The structure contained a 1.3 nm interfacial SiOx and an n+ doped poly-Si layer with a partly deuterated SiNx coating. The samples were annealed to release H/D, and ToF-ERD was used to detect H/D in monatomic, singly charged forms, without the detection artifacts associated with conventional secondary ion mass spectroscopy. Chlorine ions were used to recoil surface species, which were analyzed to clearly and unambiguously resolve H and D. Depth profiles for the recoiled Si, N, O, D, and H atoms were calculated from the energy and velocity information registered after scattering events, which enabled the analysis of the structure of the multilayer stack. Even though the surface roughness and experimental limitations cause visible broadening of the profiles, which can hinder clear detection at the interfacial oxide, the ability to resolve hydrogen-related species makes ToF-ERD a significant and promising tool for studying the role of hydrogen in the performance and degradation of solar cell passivating contacts.

The recombination of photogenerated charge carriers at metal-semiconductor interfaces remains a major source of efficiency loss in photovoltaic cells. Here, we present SiN x and AlO x nanolayers as promising interface dielectrics to enable high efficiency hole selective passivating contacts. It is demonstrated that SiN x deposited via direct plasma enhanced chemical vapour deposition can be grown controllably at thicknesses of 2 nm. The valence band offsets between crystalline silicon and ultrathin AlO x and SiN x nanolayers are measured as 3.5 and 1.4 eV, respectively. This predicts a larger tunnelling current for holes, compared to SiO x used typically. Resistivity measurements show that SiN x and AlO x nanolayers have lower contact resistivities compared to SiO x , with values as low as 100 mΩ·cm 2 . Analysis of the current transport mechanisms confirmed that tunnelling dominates the conduction through SiN x , while a mixture of tunnelling and pinholes are present in the AlO x structure. Lifetime measurements gave initial indications of the passivation quality of the films, with just 10 cycles of AlO x achieving 260 μ s after annealing and 1.9 ms with extrinsic field effect passivation added. Finally, the intrinsic built-in charge in the dielectrics was determined using surface photovoltage measurements and simulations are used to estimate the influence of nanolayer built-in charge in both poly-Si and dopant-free passivating contacts to enable future high efficiency solar cells.

Abstract The power conversion efficiency of solar cells is strongly impacted by an unwanted loss of charge carriers occurring at semiconductor surfaces and interfaces. Here the use of ion‐charged oxide nanolayers to enhance the passivation of silicon surfaces via the field effect mechanism is reported. The first report of enhanced passivation from rubidium and cesium ion‐charged oxide nanolayers is provided. The charge state and formation energy of ion‐charged silicon dioxide are calculated from first principles. Ion embedding is demonstrated and exploited to control the interface population of carriers and minimize electron‐hole pair recombination. The passivation quality directly improves with charge concentration, yet excess ions can produce detrimental interface states. An optimal ionic charge concentration of ≈1.5 × 1012 q cm−2 is deduced, and a recombination velocity and current density as low as 2.8 cm s−1 and 7.8 fA cm−2 are achieved at the Si‐SiO2 interface. Maximized charge is shown to provide efficiency improvements as high as 0.7% absolute. This work provides a unique route to enhance passivation without compromising the film synthesis, thus retaining the antireflection and hydrogenation film properties. As such, ion‐charged dielectrics provide complementary paths for surface and interface optimization in future single‐junction and tandem solar cells.