Search

Transition metal nitrides are a fascinating class of catalyst materials due to their superior catalytic activity, low electrical resistance, good corrosion resistance and earth abundance; however, their conventional synthesis relies on high-temperature nitridation processes in hazardous environments. Here, we report a direct synthesis of Ni3N/Ni enriched with N-vacancies using one-step magnetron sputtering. The surface state of Ni3N(001) with 75% N-vacancies is hydrogen-terminated and exhibits four inequivalent Ni3-hollow sites. This leads to stronger H* binding compared to Ni(111), and is affirmed as the most stable surface termination under the electrochemical working conditions (pH ≈ 13.8 and E = −0.1 V) from the Pourbaix diagram. The Ni3N/Ni catalyst shows low crystallinity and good wettability and exhibits a low overpotential of 89 mV vs. RHE at 10 mA cm−2 in 1.0 M KOH with excellent stability over 3 days. This performance closely matches that of the Pt catalyst synthesized under the same conditions and surpasses that of other reported earth-abundant catalysts on planar substrates. The application of Ni3N/Ni as a cocatalyst on Si photocathodes produces an excellent ABPE of 9.3% and over 50 h stability. Moreover, its feasibility for practical application was confirmed with excellent performance on porous substrates and robustness at high operating currents in zero-gap alkaline electrolysis cells. Our work demonstrates a general approach for the feasible synthesis of other transition metal nitride catalysts for electrochemical and photoelectrochemical energy conversion applications.

HfO2 has many favorable characteristics for use in energy conversion devices including high thermodynamic stability, good chemical stability in corrosive electrolytes, high refractive index, and wide bandgap. Here, we report surface passivation of a c-Si photocathode by ultrathin HfO2 prepared using atomic layer deposition as an effective approach for enhancing its photoelectrochemical (PEC) performance. The effect of the thickness of HfO2, deposition temperature, and annealing in forming gas on the passivation performance are systematically investigated. We demonstrate that the Si photocathode with a p+/n/n+ structure decorated with a Ni3N/Ni cocatalyst and an HfO2 interlayer follows a metal–insulator–semiconductor mechanism with thicker HfO2 films proving detrimental to the PEC performance. The Si photocathode passivated with a 1 nm HfO2 layer exhibits an enhancement in the onset potential by 100 mV, an applied-bias photon-to-current efficiency of 9%, and improved operational stability. This work provides insights into the application of HfO2 as a passivating layer for Si photoelectrodes for solar hydrogen production.

Photovoltaic (PV)‐based solar hydrogen generation is a promising pathway for the scalable production of renewable fuels. Understanding the limitations of solar‐to‐hydrogen (STH) conversion efficiencies is critical to identify performance limits and conceptualize practical device designs. Herein, the losses in PV‐based solar hydrogen generation systems are quantified and the potential of loss‐mitigation techniques to improve the STH efficiency is assessed. The analysis shows that the two largest losses in an ideal system are current and voltage mismatches due to suboptimal system configurations and energy lost as heat in the PV component. A temperature‐dependent model is developed to evaluate the relative potential of two techniques to mitigate these losses: decoupling the PV system to remove current and voltage matching requirements and thermal integration to use the heat losses from PV to increase the electrolyte temperature and improve the reaction dynamics for water splitting. It is shown that optimal system configuration strategies provide more than three times the STH efficiency increase of thermal integration at high operating temperatures. Combining both techniques results in predicted STH efficiencies approaching 20% for low‐cost perovskite–silicon tandem‐based systems with earth‐abundant catalysts at realistic working temperatures.

S.K.K. and H.S. contributed equally to this work. The financial support from the Australian government through the Australian Research Council (ARC) and Australian Renewable Energy Agency (ARENA) is gratefully acknowledged. Access to the facilities of the Australian National Fabrication Facility (ANFF) is also gratefully acknowledged.

Perovskite solar cells are notorious for exhibiting transient behaviour not seen in conventional inorganic semiconductor devices. Significant inroads have been made into understanding this fact in terms of rapid ion migration, now a well-established property of the prototype photovoltaic perovskite MAPbI$_3$ and strongly implicated in the newer mixed compositions. Here we study the manifestations of ion migration in frequency-domain small-signal measurements, focusing on the popular technique of Electrical Impedance Spectroscopy (EIS). We provide new interpretations for a variety of previously puzzling features, including giant photo-induced low-frequency capacitance and negative capacitance in a variety of forms. We show that these apparently strange measurements can be rationalized by the splitting of AC current into two components, one associated with charge-storage, and the other with the quasi-steady-state recombination current of electrons and holes. The latter contribution to the capacitance can take either a positive or a negative sign, and is potentially very large when slow, voltage-sensitive processes such as ion migration are at play. Using numerical drift-diffusion semiconductor models, we show that giant photo-induced capacitance, inductive loop features, and low-frequency negative capacitance all emerge naturally as consequences of ion migration via its coupling to quasi-steady-state electron and hole currents. In doing so, we unify the understanding of EIS measurements with the comparably well-developed theory of rate dependent current-voltage (I-V) measurements in perovskite cells. Comparing the two techniques, we argue that EIS is more suitable for quantifying I-V hysteresis than conventional methods based on I-V sweeps, and demonstrate this application on a variety of cell types., Fixed typos and amended the axes on Figure 3 for clarity

The needs of thin-film solar cells are a significant driver in transparent conductor research, making it vital that these requirements are properly understood. Here we demonstrate that the oft-quoted need for sheet resistances less than 10 Ω sq−1 arises only when the addition of a metal grid is unfeasible, and for cells of a particular size. In addition we show that the performance of a highly transparent layer with a metal grid is generally superior to that of a single layer fulfilling the 10 Ω sq−1 requirement without a grid. In order to clarify these issues we introduce simple measures of electrode performance which correspond directly to cell efficiency. These specialized figures of merit can be applied to electrodes with or without a metal grid, and also to those embedded in a tandem cell where good electrode performance is often imperative. By comparison we show that the ratio of DC to optical conductivity, the most widely used figure of merit for transparent conductors, is a bad predictor of performance in a solar cell. Our work jointly motivates the development of scalable techniques for incorporating thin metal wires into front-side electrodes, and research into transparent conductors which prioritize transparency over sheet resistance.

Optoelectronic devices based on colloidal quantum dots (CQD) are ideal candidates to benefit from plasmonic light trapping, due to fact that they have thin active layers and favorable material constants for absorbing light in the near-field of plasmonic resonances. We demonstrate a simple technique for designing gratings to couple incident light to surface plasmon polariton modes propagating on the metal–semiconductor interface of homojunction PbS CQD photodiodes, providing targeted photocurrent enhancement for solar harnessing or photodetection applications. As a result, an external quantum efficiency of ∼45% is achieved at the exciton peak for a photodiode with an ultrathin PbS CQD absorber of ∼100 nm.

A simple approach is demonstrated to combine a light trapping scheme and a conductive substrate for solution processed solar cells. By means of soft lithography, a new light-trapping architecture can be integrated as the bottom electrode for emerging thin-film solar-cell technologies without added costs, fully compatible with low-temperature processes, and yielding an enhancement in the photocurrent without altering the rest of the electrical performance of the device.

The issue of hysteresis in perovskite solar cells has now been convincingly linked to the presence of mobile ions within the perovskite layer. Here we test the limits of the ionic theory by attempting to account for a number of exotic characterization results using a detailed numerical device model that incorporates ionic charge accumulation at the perovskite interfaces. Our experimental observations include a temporary enhancement in open-circuit voltage following prolonged periods of negative bias, dramatically S-shaped current–voltage sweeps, decreased current extraction following positive biasing or “inverted hysteresis”, and non-monotonic transient behaviours in the dark and the light. Each one of these phenomena can be reproduced and ultimately explained by our models, providing further evidence for the ionic theory of hysteresis as well as valuable physical insight into the factors that coincide to bring these phenomena about. In particular we find that both interfacial recombination and carrier injection from the selective contacts are heavily affected by ionic accumulation, and are essential to explaining the non-monotonic voltage transients and S-shaped J–V curves. Inverted hysteresis is attributed to the occurrence of “positive” ionic accumulation, which may also be responsible for enhancing the stabilized open-circuit voltage in some perovskite cells.

In recent years, there has been rapid development in the field of nanoscale light trapping for solar cells. This has been driven by the decrease in thickness of solar cells in order to reduce materials costs, as well as advances in fabrication technology and computer power for simulating nanoscale structures. Nanoscale light trapping offers the possibility of enhancing absorption beyond the limits achievable with geometrical optics for certain structures. It also allows the optical design to be separated from the electrical design, as for example in plasmonic solar cells. Most importantly, thin-film cell designs will need to incorporate nanophotonic light trapping in order to reach their ultimate efficiency limits. In this article, we review the major types of nanophotonic light trapping, including plasmonic, diffraction gratings, and random scattering surfaces and describe the major advantages and disadvantages of each. In addition, we describe the most important related fabrication and characterization technologies and provide an outlook on future directions in this field.

We present a simple conceptual model describing the absorption enhancement provided by diffraction gratings due to resonant coupling to guided modes in a multi-layered structure. In doing so, we provide insight into why certain guided modes are more strongly excited than others and demonstrate that the spatial overlap of the mode profile with the grating is important. The model is verified by comparison to optical simulations and experimental measurements. We fabricate metal nanoparticle gratings integrated as back contacts in solution-processed PbS colloidal quantum dot photodiodes. The measured photocurrent at the target wavelength is enhanced by 250%, with reference to planar devices, due to resonant coupling to guided modes with strong spatial overlap with the gratings. In comparison, resonant coupling to weakly overlapping modes results in a 25% increase at the same wavelength.

We report broadband responsivity enhancement in PbScolloidalquantum dot (CQDs) photoconductive photodetectors due to absorption increase offered by a plasmonicscattering layer of Ag metal nanoparticles. Responsivity enhancements are observed in the near infrared with a maximum 2.4-fold increase near the absorption band edge of ∼1 μm for ∼400 nm thick devices. Additionally, we study the effect of the mode structure on the efficiency of light trapping provided by random nanoparticlescattering in CQD films and provide insights for plasmonicscattering enhancement in CQD thin films.

We provide a new physical interpretation of scattering from plasmonic nanoparticles on high-index substrates. We demonstrate the excitation of different types of resonant modes on disk-shaped, Ag nanoparticles. At short wavelengths, the resonances are localised at the top of the particle, while at longer wavelengths they are localised at the Ag/substrate interface. We attribute the long wavelength resonances to geometric resonances of surface plasmon polaritons (SPPs) at the Ag/substrate interface. We show that particles that support resonant SPP modes have enhanced scattering cross-sections when placed directly on a high-index substrate; up to 7.5 times larger than that of a dipole scatterer with an equivalent free-space resonance. This has implications for designing scattering nanostructures for light trapping solar cells.

Disk-shaped metal nanoparticles on high-index substrates can support resonant surface plasmon polariton (SPP) modes at the interface between the particle and the substrate. We demonstrate that this new conceptual model of nanoparticle scattering allows clear predictive abilities, beyond the dipole model. As would be expected from the nature of the mode, the SPP resonance is very sensitive to the area in contact with the substrate, and insensitive to particle height. We can employ this new understanding to minimise mode out-coupling and Ohmic losses in the particles. Taking into account optical losses due to parasitic absorption and outcoupling of scattered light, we estimate that an optimal array of nanoparticles on a 2 μm Si substrate can provide up to 71% of the enhancement in absorption achievable with an ideal Lambertian rear-reflector. This result compares to an estimate of 67% for conventional pyramid-type light trapping schemes.

In this paper we quantify the constraints and opportunities for radial junction nanowire solar cells, compared to single junction and multijunction solar cells, when light trapping is included. Both nanowire and multijunction designs are reliant on a very low level of traps in the junction region, and without this, single junction designs are optimal. If low trap density at the junction can be achieved, multijunction cells lead to higher efficiencies than nanowire cells for a given diffusion length, except in the case of submicron diffusion lengths. Thus the radial junction structure is not in itself an advantage in general, though if nanowires allow faster deposition or better light trapping than other structures they could still prove advantageous.

We investigate the influence of nanoparticle height on light trapping in thin-film solar cells covered with metal nanoparticles. We show that in taller nanoparticles the scattering cross-section is enhanced by resonant excitation of plasmonic standing waves. Tall nanoparticles have higher coupling efficiency when placed on the illuminated surface of the cell than on the rear of the cell due to their forward scattering nature. One of the major factors affecting the coupling efficiency of these particles is the phase shift of surface plasmon polaritons propagating along the nanoparticle due to reflection from the Ag/Si or Ag/air interface. The high scattering cross-sections of tall nanoparticles on the illuminated surface of the cell could be exploited for efficient light trapping by modifying the coupling efficiency of nanoparticles by engineering this phase shift. We demonstrate that the path length enhancement (with a nanoparticle of height 500 nm) at an incident wavelength of 700 nm can be increased from ∼6 to ∼16 by modifying the phase shift at the Ag/air interface by coating the surface of the nanoparticle with a layer of Si.

K.R.C. acknowledges the support of an Australian Research Council fellowship and the EU FP7 PRIMA project.

Significant photocurrent enhancement has been achieved for evaporated solid-phase-crystallized polycrystalline silicon thin-film solar cells on glass, due to light trapping provided by Ag nanoparticles located on the rear silicon surface of the cells. This configuration takes advantage of the high scattering cross-section and coupling efficiency of rear-located particles formed directly on the optically dense silicon layer. We report short-circuit current enhancement of 29% due to Ag nanoparticles, increasing to 38% when combined with a detached back surface reflector. Compared to conventional light trapping schemes for these cells, this method achieves 1/3 higher short-circuit current.

We show experimentally that there is asymmetry in photocurrent enhancement by Ag nanoparticle arrays located on the front or on the rear of solar cells. The scattering cross-section calculated for front- and rear-located nanoparticles can differ by up to a factor of 3.7, but the coupling efficiency remains the same. We attribute this to differences in the electric field strength and show that the normalized scattering cross-section of a front-located nanoparticle varies from two to eight depending on the intensity of the driving field. In addition, the scattering cross-section of rear-located particles can be increased fourfold using ultrathin spacer layers.

The authors acknowledge the A. R. C. and NOW for research conducted at the FOM as a part of the Joint Solar Programme for financial support.

Effective light management is imperative in maintaining high efficiencies as photovoltaic devices become thinner. We demonstrate a simple and effective method of enhancing light trapping in solar cells with thin absorber layers by tuning localized surface plasmons in arrays of Ag nanoparticles. By redshifting the surface plasmon resonances by up to 200 nm, through the modification of the local dielectric environment of the particles, we can increase the optical absorption in an underlying Si wafer fivefold at a wavelength of 1100 nm and enhance the external quantum efficiency of thin Si solar cells by a factor of 2.3 at this wavelength where transmission losses are prevalent. Additionally, by locating the nanoparticles on the rear of the solar cells, we can avoid absorption losses below the resonance wavelength due to interference effects, while still allowing long wavelength light to be coupled into the cell. Results from numerical simulations support the experimental findings and show that the fraction ...

Surface plasmons in metallic nanoparticle arrays have been shown to increase the absorption of an underlying silicon substrate. This has wide ranging applications, not least in the photovoltaic industry. Incident light excites localised surface plasmons in the silver nanoparticles and is coupled into the silicon in trapped modes. The radiative behaviour of the nanoparticle film is changed by the proximity of a high refractive index surface, causing radiation to be directed into the silicon and providing a light-trapping layer. We investigate a simple and effective method of tuning the surface plasmon resonance frequency, and hence the spectral region at which the absorption enhancement is seen, by varying the underlying dielectric. The particle geometry and distribution are modified by the surface conditions provided by the dielectric layer, and both this and the change in refractive index alter the resonance position. Three common dielectrics used in the photovoltaic industry were investigated as surfaces on which to form arrays of self-assembled silver nanoparticles atmospheric pressure chemical vapour deposited titanium dioxide (APCVD TiO2), low pressure chemical vapour deposited silicon nitride (LPCVD Si3N4) and thermally grown silicon dioxide (SiO2). We show, by optical and electrical measurements, that the red-shifted resonances produced by nanoparticle films on APCVD TiO2, and LPCVD Si3N4 with relatively high refractive indices, correspond to an increase in optical absorption and external quantum efficiency in thin, crystalline solar cells at longer wavelengths.

We review recent progress and report new results on the use of particle plasmons to enhance the efficiency of solar cells. We describe the basic mechanisms at work, and provide an outlook on future prospects.