• Energy Research

AbstractHydrogen production using water splitting, based on a photoelectrochemical (PEC) process, has attracted considerable attention since the introduction of TiO2 photo-electrodes. A tandem PEC cell, with photo-voltages added from two photo-electrodes, is a possible solution to generate voltage high enough to split water while absorbing more light from a greater part of the solar spectrum.This paper studies some of the potential semiconductor materials that may be suitable for photo-cathode for the tandem PEC cells. The effect of electrolyte pH in changing the photo-cathode material electrochemical properties is analyzed and the effect of semiconductor corrosion property is discussed in this paper. The paper concludes by presenting the results of several semiconductors that have been tested.

Abstract Hot carrier generation in silicon (Si) quantum dots (QDs) is studied with power dependent continuous wave photoluminescence (CWPL) spectroscopy. By taking sub-band gap absorption into account, a modified Maxwell-Boltzmann-form equation was employed to achieve accurate theoretical fitting to the CWPL spectra of the Si QDs. As a fitting parameter, the excited carrier temperature was calculated. A steady-state carrier population was revealed with a temperature 500 K above room temperature under illumination equivalent to one standard sun (100 mW/cm2). In addition, since the carrier temperature increased with the power of illumination, a state filling effect is proposed as a reasonable cause for the elevated carrier temperature by comparative study of the CWPL spectra of Si QDs with three different sizes. These Si QDs show great potential for one of the steps towards a practical hot carrier solar cell (HCSC) device as high carrier temperatures can be achieved by state filling under mild illumination.

Abstract Application of fluorescent materials has proven to be an effective method for urban overheating mitigation due to their unique ability to re-emit a portion of the absorbed solar irradiation through photoluminescence (PL) effect. Herein, we introduced the idea of using quantum dots (QDs) as tunable fluorescent materials with potentially higher cooling capacity than their bulk counterparts and proposed a novel algorithm to model their thermo-optic behaviour under the sunlight. Our proposed method represents a major step forward in the understanding of the heat-rejection mechanism through PL effect and optimization of QDs fluorescent properties for urban overheating mitigation. Since it’s complicated to distinguish surface temperature reduction caused by reflection from that of PL effect, our developed algorithm could be used as a reliable tool for precise estimation of the PL effect contribution to heat dissipation.

Quantum dot (QD) solids are an emerging platform for developing a range of optoelectronic devices. Thus, understanding exciton dynamics is essential towards developing and optimizing QD devices. Here, using transient absorption microscopy, we reveal the initial exciton dynamics in QDs with femtosecond timescales. We observe high exciton diffusivity (~10² cm² s¯¹) in lead chalcogenide QDs within the first few hundred femtoseconds after photoexcitation followed by a transition to a slower regime (~10¯¹–1 cm² s¯¹). QD solids with larger interdot distances exhibit higher initial diffusivity and a delayed transition to the slower regime, while higher QD packing density and heterogeneity accelerate this transition. The fast transport regime occurs only in materials with exciton Bohr radii much larger than the QD sizes, suggesting the transport of delocalized excitons in this regime and a transition to slower transport governed by exciton localization. These findings suggest routes to control the optoelectronic properties of QD solids.

AbstractThe concepts of third generation solar cells critically depend on the dynamics of ultrafast carrier relaxation and electron-phonon interactions on very short times scales. The hot carrier solar cell is a type of third generation cell which especially depends on the reduction in the energy relaxation rate in an absorber material. We investigated the ultrafast carrier dynamics in 1μm bulk In0.265GaN thin film grown by a new thin-film growth technique called energetic neutral atom-beam lithography/epitaxy (ENABLE) which was done by our collaborators in the Los Alamos National laboratory(LANL), US. Characterization including steady state photoluminescence (SSPL), time-resolved photoluminescence (TRPL) and transient absorption (TA) in the time scale of picoseconds have been measured and analysed. It indicates the overall relaxation time of our sample is about 22.42ps through the Maxwell-Boltzmann approximation. This indicates that the mechanisms are mediated by the Indium content. Moreover, the indium fluctuation introduced extrinsic energy states in the forbidden energy as observed through TRPL.

A spin-coating-free fabrication sequence has been developed for the fabrication of highly efficient organic-inorganic halide perovskite solar cells (PSCs). A novel blow-drying method is demonstrated to be successful in depositing high quality mesoporous TiO2 (mp-TiO2), methylammonium lead halide (CH3NH3PbI3) perovskite and spiro-MeOTAD layers. When combined with compact TiO2 (c-TiO2) deposited by spray pyrolysis which is also a spin-coating-free process, a stabilized power conversion efficiency exceeding 17% can be achieved for the glass/FTO/c-TiO2/mp-TiO2/ CH3NH3PbI3/spiro-MeOTAD/Au device. This is the highest efficiency for PSCs fabricated without the use of spin-coating to our knowledge. This method provides a pathway towards a scalable process for fabricating high-performance, large area and reproducible PSCs.

All-Si tandem solar cells based on Si quantum dots (QDs) are a promising approach to future high-performance, thin film solar cells using abundant, stable and non-toxic materials. An important prerequisite to achieve a high conversion efficiency in such cells is the ability to control the geometry of the Si QD network. This includes the ability to control both, the size and arrangement of Si QDs embedded in a higher bandgap matrix. Using plasmon tomography we show the size, shape and density of Si QDs, that form in Si rich oxide (SRO)/SiO2 multilayers upon annealing, can be controlled by varying the SRO stoichiometry. Smaller, more spherical QDs of higher densities are obtained at lower Si concentrations. In richer SRO layers ellipsoidal QDs tend to form. Using electronic structure calculations within the effective mass approximation we show that ellipsoidal QDs give rise to reduced inter-QD coupling in the layer. Efficient carrier transport via mini-bands is in this case more likely across the multilayers provided the SiO2 spacer layer is thin enough to allow coupling in the vertical direction.

Abstract Understanding and engineering exciton transport in quantum dot (QD) solids is both of fundamental interest and crucial to their broad applications in devices1-6. Till date, studies of exciton transport in QD solids on pico/nano-second timescales have led to the conclusion that closer packing of QDs enables faster exciton transport, while energetic/structural heterogeneity leads to reduction of exciton diffusivity over time7,8. Here we study PbS QD solids using transient absorption microscopy with 13 femtoseconds time resolution and 10 nm spatial precision. We find exciton diffusivities in the range of ~102 cm2 s-1 within the first few hundred femtoseconds after photoexcitation, followed by the transition to a slower transport regime with diffusivities in the range 10-1 to 1 cm2 s-1. Counterintuitively, the initial diffusivity is higher and the time before the transition to the slower transport phase is longer in QD solids with longer ligand lengths. This suggests a transition from early-time transport of delocalized excitons to later time hopping based transport of localized excitons, where QD packing density and heterogeneity accelerate the localization process. Our results reveal a new regime for exciton transport in QD solids and provide design rules to engineer desired transport properties in these systems on a range of timescales.