Yimao Wan

Characterisation and optimisation of PECVD SiNx as an antireflection coating and passivation layer for silicon solar cells

This work was supported by an Australian Research Council Linkage between The Australian National University and Braggone Oy under Grant LP0989593.

Interface passivation using ultrathin polymer–fullerene films for high-efficiency perovskite solar cells with negligible hysteresis

Interfacial carrier recombination is one of the dominant loss mechanisms in high efficiency perovskite solar cells, and has also been linked to hysteresis and slow transient responses in these cells. Here we demonstrate an ultrathin passivation layer consisting of a PMMA:PCBM mixture that can effectively passivate defects at or near to the perovskite/TiO2 interface, significantly suppressing interfacial recombination. The passivation layer increases the open circuit voltage of mixed-cation perovskite cells by as much as 80 mV, with champion cells achieving Voc ∼ 1.18 V. As a result, we obtain efficient and stable perovskite solar cells with a steady-state PCE of 20.4% and negligible hysteresis over a large range of scan rates. In addition, we show that the passivated cells exhibit very fast current and voltage response times of less than 3 s under cyclic illumination. This new passivation approach addresses one of the key limitations of current perovskite cells, and paves the way to further efficiency gains through interface engineering.

Magnesium Fluoride Electron-Selective Contacts for Crystalline Silicon Solar Cells

In this study, we present a novel application of thin magnesium fluoride films to form electron-selective contacts to n-type crystalline silicon (c-Si). This allows the demonstration of a 20.1%-efficient c-Si solar cell. The electron-selective contact is composed of deposited layers of amorphous silicon (∼6.5 nm), magnesium fluoride (∼1 nm), and aluminum (∼300 nm). X-ray photoelectron spectroscopy reveals a work function of 3.5 eV at the MgF2/Al interface, significantly lower than that of aluminum itself (∼4.2 eV), enabling an Ohmic contact between the aluminum electrode and n-type c-Si. The optimized contact structure exhibits a contact resistivity of ∼76 mΩ·cm(2), sufficiently low for a full-area contact to solar cells, together with a very low contact recombination current density of ∼10 fA/cm(2). We demonstrate that electrodes functionalized with thin magnesium fluoride films significantly improve the performance of silicon solar cells. The novel contacts can potentially be implemented also in organic optoelectronic devices, including photovoltaics, thin film transistors, or light emitting diodes.

Low Surface Recombination Velocity by Low-Absorption Silicon Nitride on c-Si

We demonstrate that nearly stoichiometric amorphous silicon nitride (SiN<inf>x</inf>) can exhibit excellent surface passivation on both p- and n-type c-Si as well as low absorption at short wavelengths. The key process to obtain such a SiN<inf>x</inf> is the optimized deposition pressure. The effective carrier lifetimes of these samples exceed the commonly accepted intrinsic upper limit over a wide range of excess carrier densities. We achieve a low S<inf>eff,UL</inf> of 1.6 cm/s on 0.85-Ω·cm p-type and immeasurably low S<inf>eff,UL</inf> on 0.47-Ω·cm n-type silicon passivated by the SiN<inf>x</inf> deposited at 290 °C. Capacitance-voltage measurements reveal this SiN<inf>x</inf> has a density of interface states of 3.0 × 10¹¹ eV⁻¹cm⁻² at midgap and an insulator charge of 5.6 × 10¹¹ cm⁻². By comparing the measured injection-dependent S<inf>eff,UL</inf> to calculated S<inf>eff,UL</inf> by an extended Shockley-Read-Hall model, we conclude that either Defect A or B (or both) observed by Schmidt et al. is likely to dominate the surface recombination at our Si-SiN<inf>x</inf> interface. In addition to the outstanding surface passivation, this SiN<inf>x</inf> has a low absorption coefficient at short wavelengths. Compared to Si-rich SiN<inf>x</inf> of an equivalent passivation, the optimized SiN<inf>x</inf> would enhance the photo-generated current density by more than 0.66 mA/cm² or 1.40 mA/cm² for solar cells encapsulated in glass/EVA or operating in air, respectively. The SiN<inf>x</inf> described here is ideally suited for high-efficiency solar cells, which require good surface passivation and low absorption from their front surface coatings.

Amorphous silicon passivated contacts for diffused junction silicon solar cells

Carrier recombination at the metal contacts is a major obstacle in the development of high-performance crystalline silicon homojunction solar cells. To address this issue, we insert thin intrinsic hydrogenated amorphous silicon [a-Si:H(i)] passivating films between the dopant-diffused silicon surface and aluminum contacts. We find that with increasing a-Si:H(i) interlayer thickness (from 0 to 16 nm) the recombination loss at metal-contacted phosphorus (n+) and boron (p+) diffused surfaces decreases by factors of ∼25 and ∼10, respectively. Conversely, the contact resistivity increases in both cases before saturating to still acceptable values of ∼ 50 mΩ cm2 for n+ and ∼100 mΩ cm2 for p+ surfaces. Carrier transport towards the contacts likely occurs by a combination of carrier tunneling and aluminum spiking through the a-Si:H(i) layer, as supported by scanning transmission electron microscopy–energy dispersive x-ray maps. We explain the superior contact selectivity obtained on n+ surfaces by more favorable band offsets and capture cross section ratios of recombination centers at the c-Si/a-Si:H(i) interface.

Grown-in defects limiting the bulk lifetime of p-type float-zone silicon wafers

This work has been supported by the Australian Renewable Energy Agency (ARENA) fellowships program and the Australian Research Council (ARC) Future Fellowships program.

Proof-of-concept p-type silicon solar cells with molybdenum oxide partial rear contacts

This paper explores the application of transparent MoOx (x<;3) films as hole-collecting contacts on the rear-side of crystalline silicon solar cells. 2D simulations, which consider experimental contact recombination J0c and resistivity ρc values, demonstrate that the benefits of the MoOx based contacts are best exploited by reducing the rear contact fraction. This concept is demonstrated experimentally using simple p-type cells featuring a ~5% rear fraction MoOx contact. These cells attain a conversion efficiency of 20.4%, a promising result, given the early stage of development for this technology.

Highly effective electronic passivation of silicon surfaces by atomic layer deposited hafnium oxide

This paper investigates the application of hafnium oxide (HfO2) thin films to crystalline silicon (c-Si) solar cells. Excellent passivation of both n- and p-type crystalline silicon surfaces has been achieved by the application of thin HfO2 films prepared by atomic layer deposition. Effective surface recombination velocities as low as 3.3 and 9.9 cm s−1 have been recorded with 15 nm thick films on n- and p-type 1 Ω cm c-Si, respectively. The surface passivation by HfO2 is activated at 350 °C by a forming gas anneal. Capacitance voltage measurement shows an interface state density of 3.6 × 1010 cm−2 eV−1 and a positive charge density of 5 × 1011 cm−2 on annealed p-type 1 Ω cm c-Si. X-ray diffraction unveils a positive correlation between surface recombination and crystallinity of the HfO2 and a dependence of the crystallinity on both annealing temperature and film thickness. In summary, HfO2 is demonstrated to be an excellent candidate for surface passivation of crystalline silicon solar cells.

Upgraded metallurgical-grade silicon solar cells with efficiency above 20%

We present solar cells fabricated with n-type Czochralski–silicon wafers grown with strongly compensated 100% upgraded metallurgical-grade feedstock, with efficiencies above 20%. The cells have a passivated boron-diffused front surface, and a rear locally phosphorus-diffused structure fabricated using an etch-back process. The local heavy phosphorus diffusion on the rear helps to maintain a high bulk lifetime in the substrates via phosphorus gettering, whilst also reducing recombination under the rear-side metal contacts. The independently measured results yield a peak efficiency of 20.9% for the best upgraded metallurgical-grade silicon cell and 21.9% for a control device made with electronic-grade float-zone silicon. The presence of boron-oxygen related defects in the cells is also investigated, and we confirm that these defects can be partially deactivated permanently by annealing under illumination.

Skin care for healthy silicon solar cells

Effective surface treatments suppress possible recombination losses and confine photogenerated electrons and holes within the bulk of the silicon wafer, thus maximizing their number and the electrochemical potential that they can deliver to a load. For that to happen, it is necessary to create regions with a high conductivity for one carrier and low for the other, which is the basis for their separation. There is a common thread joining surface passivation and carrier-selective contacts, and the same principles apply to both. One is the manipulation of the concentrations of electrons and holes, which can be achieved by doping or by depositing materials with an appropriate bandgap, work function and conductivity. The other method is to use hydrogen-rich semi-insulators that bond chemically to the silicon atoms. When used as part of a contact structure, they need to be sufficiently thin to permit current flow. Examples of such passivated contacts are dopant diffusions coated with thin insulators or a-Si:H(i), doped polysilicon/SiOx structures, and some transparent conductors.