• Energy Research

The beneficial results of the exponential expansion of photovoltaic installations in Germany and Italy are discussed. Remarkable falls in the peak price of electricity have been observed in both countries. The reasons are discussed in the light of the data from the Kombikraftwerk project. This has demonstrated, in a scaled, real-time experiment, how the demand on the German grid can be met by photovoltaics and wind with back-up from biogas and (pumped hydro) storage. We discuss the implications of the fall in price of photovoltaic cells particularly for 3rd generation technology. Using the specific example of the UK, we demonstrate the advantages of the complementary nature of wind and photovoltaic resources. We demonstrate that the wind and photovoltaic capacity targets for an all renewably powered UK are likely to be significantly lower than in Germany. We conclude by summarising the evidence in favour of a moratorium on all new electricity generation other than by the renewables.

We report on the development of an unconventional method for heating a Mo-coated substrate during the deposition of a Cu(In,Ga)Se2 (CIGS) layer by the pulsed electron deposition technique, to be used as absorber in thin film solar cells. This method is based on the application of a DC electrical power directly through the Mo back contact of the cell, converting electrical energy into heat by Joule effect. Since the current flows only on the superficial metal-coated region of the substrate, a localized heating of the surface can be achieved, thus limiting the heat losses. Due to the very efficient heat transfer to the thin Mo layer, a very little electrical power density (few W/cm2) is enough to achieve the required deposition temperature on the Mo surface, much lower compared to the traditional resistor- or lamp-based external heaters. The morphological and electrical properties of Joule-heated samples have been compared to those of CIGS films heated by a conventional external heater. As far as the structure concerns, a remarkable difference is revealed by Scanning Electron Microscopy analysis, indicating a significant enlargement of the CIGS grains size on Joule-heated samples. On the contrary, Capacitance-Voltage and Current-Voltage measurements evidence similar electrical features: both types of heated samples have a net free carrier concentration ≈5 × 1015 cm−3, resulting in a similar photovoltaic conversion efficiency (≈15%). The main recombination path, deduced from the dependence of VOC on the temperature, results to be the Shockley-Read-Hall mechanism in both types of the absorber layer. These results indicate that the Joule effect could be adopted as a feasible, low cost alternative heating method for growing high quality CIGS layers.

A quantum well solar cell is a special multiple-band gap device with intermediate properties between heterojunction cells (sum of the currents generated in the different materials but voltage controlled by the lowest of the two band gaps) and tandem cells (sum of the voltages but current determined by the worst of the two sub-cells). Strain-balanced GaAsP/InGaAs multi-quantum wells move the absorption edge of GaAs solar cells closer to the optimum value for single junction cells with no need for any partially relaxed buffer layer to accommodate lattice mismatch between the absorbing layers and the substrate. Covering a large spectral range in a single-junction cell has the benefit that the cell remains close to optimal efficiency in the varying spectral conditions of a typical terrestrial concentrator. Though monolithic multi-junction cells have significantly higher efficiency, the series-current constraint means that some of this advantage is lost as the illuminating spectra and the cell temperature change from the values at which the tandem was optimised. The good material quality which can be achieved with these structures makes the cell dark current at the typical operating conditions expected under moderate sunlight concentration (similar to 200x), increasingly dominated by radiative processes the deeper the quantum wells. We will report on high concentration measurements of strain-balanced quantum well solar cells with and without Bragg-stack reflectors and discuss the "additivity" between the short-circuit current and the dark-current. We discuss a 50 shallow well cell with measured AM1.5d efficiency of (26 +/- 1)% at around 200 x concentration. This is approximately 2% higher than a comparable p-n cell with comparable material quality. The good material quality is also responsible for another effect previously observed in single quantum wells becoming measurable in structures with 5 and 10 wells, that is the suppression of carrier recombination in quantum wells with respect to expectations assuming that the quasi-Fermi level separation in the depletion region is equal to the cell output voltage throughout the active region. The latest results are presented together with possible explanations for this effect both in the dark and under illumination. Finally a brief discussion about the potential applications of quantum well solar cells completes the paper.

Multi-quantum well photovoltaic cells offer a number of advantages over conventional "single-gap" cells for thermophotovoltaic applications, first of all because they can reach a higher open circuit voltage under the same radiation source and with the same absorption edge. Material quality issues and the constraints imposed by the commercial available substrates indicate that InxGa1-xAs/InyGa1-yAs/InP strain-balanced heterostructures are suitable to obtain good quality multi-quantum wells with an absorption edge just below 2.0 mum. Structural stability in the presence of a high density of elastic energy such as in the case of a strain-balanced multi-layer is a very important issue to be addressed by optimising key parameters like composition, thickness of wells and barriers and number of periods. In this paper we present and discuss the mechanisms of plastic relaxation of these structures with a particular attention to the impact of the extended defects generated by the local breakdown of the crystal lattice to the electrical properties of the devices. Then, after the presentation of the optimum structure with an absorption edge at 1.96 mum, we discuss the issue of a further extension of the absorption edge through the use of a so-called virtual substrate, that is a buffer structure between the substrate and the device designed to relax to a given extent with a minimum number of dislocations propagating towards the active region. On the basis of a recipe based on the experimental results on InGaAs single and multi-layers grown on GaAs, we have designed a series of step-graded buffer structures providing good virtual substrates with a lattice parameter larger than GaAs. Strain-balanced multi-quantum wells have been grown on InxGa1-xAs virtual substrates with 0.14 < x < 0.35 with a residual density of threading dislocations of about 10(5) cm(-2). Work is in progress to remove the residual morphological undulation (cross hatch) induced by the misfit dislocations confined in the buffer structure and to extend this approach to InP.

In this paper we report on the single stage deposition of CuInxGa1-xSe2 (CIGS)-based bifacial solar cells on glass coated with Fluorine-doped Tin Oxide (FTO) or Indium Tin Oxide (ITO) by single-stage low-temperature (250 °C) pulsed electron deposition (LTPED). We show that the mechanism of Sodium incorporation during the low-temperature deposition of CIGS on both FTO and ITO leads to the formation of a stable n+/p+ ohmic tunnel junction and photovoltaic efficiencies exceeding 14% can be obtained without any intentional bandgap grading of CIGS. The significant degradation of the cell fill factor with decreasing CIGS thickness is found to be related to the presence of craters left behind by micro-fragments of CIGS target, which are weakly incorporated in the film during the LTPED growth and removed during the subsequent process steps. Evidence is also presented that the low-temperature deposition of CIGS on ITO leads to the formation of a Ga-rich CIGS layer at the interface and to an unintentional compositional grading propagating towards the active region of the solar cells. The defects associated with this grading may be responsible for the loss in FF and Voc with respect to the cells deposited on FTO and Mo back contacts.

Abstract Strain-balanced quantum well solar cells (SB-QWSC) extend the photon absorption edge beyond that of bulk GaAs by incorporation of quantum wells in the i-region of a p–i–n device. The addition of a distributed Bragg reflector (DBR) can substantially increase the photocurrent with little or no detriment to the dark-current. Experimental results are presented that show improvements of DBR cell efficiencies over SB-QWSC's without DBR's. In addition, at high dark-current levels appropriate to high concentration, we observe that the dark-currents of the SB-QWSC's exhibit ideal diode behaviour. We present evidence that the ideality n = 1 dark-current is reduced in the DBR cells and discuss the possible efficiency improvements if the dark-current is radiatively dominant.

Quantum well solar cells are the result of the application of nanotechnology to enhance of the efficiency of concentrator solar cells. The band gap of a quantum well (QW) solar cell can be adapted to the incident spectral conditions by tailoring the QW depth. The single-junction strain-balanced quantum well solar cell (SB-QWSC) has achieved an efficiency of 28.3%. The dominant loss mechanism at the high concentrator cell operating bias is due to radiative recombination, so a major route to further efficiency improvement requires a restriction of the optical losses. It has been found that (100) biaxial compressive strain suppresses a mode of radiative recombination in the plane of the QWs. As biaxial strain can only be engineered into a solar cell on the nanoscale, SB-QWSCs are seen to have a fundamental efficiency advantage over equivalent bulk cells. Strain-balanced quantum wells in multi-junction solar cells can current match the sub-cells without the introduction of dislocations. Quantum-well (QW) technology was proved to boos the efficiency of triple-junction cells by around 3 percent in absolute terms. In fact 40 percent median efficiency has been achieved across production wafers.

QuantaSol Ltd, a spin-off of Imperial College London, was formed in 2006 and is based on the research of Professor Keith Barnham, Emeritus Professor of Physics and Senior Research Investigator in the Department of Physics at Imperial College London; Dr. Massimo Mazzer, Senior Researcher of the National Research Council of Italy; and Dr. John Roberts, Senior Research Scientist at The University of Sheffield. The company also has a strong and experienced management team in place which includes CEO Kevin Arthur who has twenty-three years of international business development and operational experience in semiconductor manufacturing companies and technology start-ups. In July 2011 QuantaSol Ltd. was acquired by the US multinational JDSU (http://www.jdsu.com/go/quantasol/Pages/default.aspx)