• Energy Research

Abstract We investigated the ground state of approximants consisting of ⩽ 165 Si atoms ( d QD ⩽ 18.5 A ) with full termination of the Si interface with F, OH, NH2, CH3 and H groups simulating Si QDs embedded in an ionic environment, SiO2, Si3N4, SiC matrix and a co-valent environment, respectively, with ab initio methods. As the polarity of the Si/matrix interface increases the optical band-gap becomes increasingly dominated by charge transfer at the interface rather than by quantum confinement. For Si QDs with d QD = 7.3 –37 A, the interface determines the electronic structure in competition with quantum confinement for H and CH3 terminations and only as a secondary effect for strong polar interfaces (NH2, OH). We present an estimate of band gaps of different QD materials with the same interface and interpret the ab initio results in conventional quantum mechanics.

AbstractDouble barrier Quantum Dot structures are potential candidates for Energy Selective Contacts for Hot Carrier Solar Cells. Silicon QD in SiO2 matrix is studied for its energy selectivity using a two dimensional model. The impact of the disorders- configurational, morphological, thickness variations of the SiO2 barriers- on conductance and selectivity of the structure is studied. Application of an external electric field modifies the confinement potential and thus the resonant properties of the structure. The impact of the applied electric field on the properties of the energy selective contact is also studied in this paper.

Abstract Double barrier resonant tunnelling structures consisting of silicon quantum dots (QDs) in silicon dioxide ( SiO 2 ) matrix have been studied for Energy Selective Contacts for Hot Carrier solar cell. A single layer of silicon QDs has been fabricated by high temperature annealing of SiO 2 / Si-rich oxide (SRO)/ SiO 2 layers deposited by RF magnetron sputtering. Compositional analysis of SRO films obtained with different sputtering target has been accurately measured with Rutherford backscattering spectroscopy. Size-controlled growth of Si QDs has been studied with photoluminescence measurements which demonstrate that QD sizes can be controlled with SRO layer thickness. In addition, resonant tunnelling behaviour of SiO 2 / Si QD/ SiO 2 structures has been investigated.

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.

The photovoltaics industry is growing at a remarkable rate, more than 35% per year for the past 8 years. This reflects both the recent enlightened favourable rebates and tariff policies in several European countries and Japan but also the growing awareness of energy security and greenhouse abatement imperatives. This rapid growth is mirrored by an intense research effort which critically depends on materials research, with several complex ternary, quarternary inorganic materials being used and a raft of new organic and molecular materials as well as a growing importance of quantum confined nanostructures, which allow extra degrees of freedom in tailoring materials properties.

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.

AbstractThe deposition of sub-stoichiometric silicon rich oxide (SRO) is the first step to obtain well ordered silicon Quantum Dots (QDs) in a dielectric matrix. This structure is used also for third generation photovoltaic devices operating in a tandem architecture. A precise control and assessment of the stoichiometry of these films is crucial to tune the electrical and optical properties of the device. In this paper we discuss two optical techniques to assess the composition of such films and we compare their results.

Abstract The hot carrier solar cell is a promising concept for high efficiency photovoltaics. Obtaining the dynamic temperature information for the hot carriers in an absorber is of crucial importance to this concept. In this paper, we present a dynamical hot carrier theory that extracts the carrier temperature and the instantaneous carrier occupation from transient absorption spectra. We have applied this model to two transition metal nitride materials, HfN and ZrN. The analysis showed that for HfN the initial carrier temperature rose to 360 K immediately after a femtosecond pulse excitation and then dropped rapidly to 330 K within 0.13 ps. After this initial process, the carrier temperature still remained relatively elevated at above 320 K for as long as 2 ns. The initial fast process was attributed to the strong carrier–carrier scattering in the material while the slow decay of temperature might be due to unusually weak electron–phonon interactions. A shorter cooling time and overall lower hot carrier temperatures were observed in ZrN.