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Hydrogen produced using fossil fuel feedstocks causes greenhouse gas (GHG) emissions, even when carbon capture and storage (CCS) is used. By contrast, hydrogen produced using electrolysis and zero-emissions electricity does not create GHG emissions. Several countries advocating the use of ‘clean’ hydrogen put both technologies in the same category. Recent studies and strategies have compared these technologies, typically assuming high carbon capture rates, but have not assessed the impact of fugitive emissions and lower capture rates on total emissions and costs. We find that emissions from gas or coal based hydrogen production systems could be substantial even with CCS, and the cost of CCS is higher than often assumed. Carbon avoidance costs for high capture rates are notable. Carbon prices of $22–46/tCO2e would be required to make hydrogen from fossil fuels with CCS competitive with hydrogen produced from fossil fuels without CCS. At the same time there are indications that electrolysis with renewable energy could become cheaper than fossil fuel with CCS options, possibly in the near-term future. Establishing hydrogen supply chains on the basis of fossil fuels, as many national strategies foresee, may be incompatible with decarbonisation objectives and raise the risk of stranded assets.

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.

Hydrogen as a fuel is clean burning, but production can cause substantial greenhouse emissions. Some buyers will prefer to pay a higher price to ensure purchase of low-embedded emissions hydrogen, but it is impossible to determine embedded emissions by examining the end product. Certification of embedded emissions will thus play a key role in the future of hydrogen as a low-emission energy carrier. The boundaries of the supply-chain elements covered in the emissions accounting of certification schemes will have substantial implications for emission-reduction incentives and international tradability. We review the boundary definitions of existing and emerging hydrogen certification schemes. Further, we provide an evidence-based assessment of the magnitude of emissions likely to occur within each boundary of the supply chain. We find varying approaches to boundary definitions in the surveyed schemes. The exclusion of feedstock or transport elements risks ignoring major fractions of supply-chain emissions. In order to balance tradability and emissions-reduction incentives, we recommend that hydrogen certification schemes be designed to follow a modular approach. This type of modular approach would place those with decision-making power over the relevant piece of the supply chain in the position of certifying the emissions within that supply-chain boundary.

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

Organometal halide perovskite-based solar cells have rapidly achieved high efficiency in recent years. However, many fundamental recombination mechanisms underlying the excellent performance are still not well understood. Here we apply confocal photoluminescence microscopy to investigate the time and spatial characteristics of light-induced trap de-activation in CH3NH3PbI3 perovskite films. Trap de-activation is characterized by a dramatic increase in PL emission during continuous laser illumination accompanied by a lateral expansion of the PL enhancement far beyond the laser spot. These observations are attributed to an oxygen-assisted trap de-activation process associated with carrier diffusion. To model this effect, we add a trap de-activation term to the standard semiconductor carrier recombination and diffusion models. With this approach we are able to reproduce the observed temporal and spatial dependence of laser induced PL enhancement using realistic physical parameters. Furthermore, we experimentally investigate the role of trap diffusion in this process, and demonstrate that the trap de-activation is not permanent, with the traps appearing again once the illumination is turned off. This study provides new insights into recombination and trap dynamics in perovskite films that could offer a better understanding of perovskite solar cell performance.

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.

A novel method, snow globe coating, is found to show significant enhancement of the short circuit current JSC (35%) when applied as a scattering back reflector for polycrystalline silicon thin-film solar cells. The coating is formed from high refractive index titania particles without containing binder and gives close to 100% reflectance for wavelengths above 400 nm. Snow globe coating is a physicochemical coating method executable in pH neutral media. The mild conditions of this process make this method applicable to many different types of solar 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 experimental results for photocurrent enhancements in thin c-Si solar cells due to light-trapping by self-assembled, random Ag nanoparticle arrays. The experimental geometry is chosen to maximise the enhancement provided by employing previously reported design considerations for plasmonic light-trapping. The particles are located on the rear of the cells, decoupling light-trapping and anti-reflection effects, and the scattering resonances of the particles are red-shifted to target spectral regions which are poorly absorbed in Si, by over-coating with TiO2. We report a relative increase in photocurrent of 10% for 22 µm Si cells due to light-trapping. Incorporation of a detached mirror behind the nanoparticles increases the photocurrent enhancement to 13% and improves the external quantum efficiency by a factor of 5.6 for weakly absorbed light.

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.

We report the development of a joystick controlled gripper for the real-time manipulation of micron-sized objects, driven using holographic optical tweezers (HOTs). The gripper consists of an arrangement of four silica beads, located in optical traps, which can be positioned and scaled in order to trap an object indirectly. The joystick can be used to grasp, move (lateral or axial), and change the orientation of the target object. The ability to trap objects indirectly allows us to demonstrate the manipulation of a strongly scattering micron-sized metallic particle.

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.

An analytical approach based on the scalar diffraction theory is presented for design of blazed diffraction gratings for maximizing the light trapping in solar cells. The model provides a conceptual insight into the behaviour of blazed structures. The predictions of the analytical model are checked against numerical results obtained using the commercial FDTD modelling software. Within the limits of the scalar diffraction theory, this analytical approach can be used to design gratings with arbitrary shapes.

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.