Hot carrier solar cells have the promise to increase photovoltaic conversion efficiency beyond the Shockley-Quiesser limit and towards the thermodynamic maximum of 85%. The concept relies on the ability to extract photo-generated electrons from an absorber region faster than they can lose energy to the lattice in a process termed thermalisation. We have previously presented a realization of such a cell under limited operating conditions, in particular at low temperature, for narrowband illumination and with low total absorption of light. In this work we present the idea of a metallic absorber to address some of these limitations and show how such an absorber is a promising candidate to realize the hot carrier solar cell. In addition to a theoretical justification of the metallic hot carrier solar cell, we show device fabrication and experimental current-voltage characteristics of an initial cell, showing absorption of light in a thin-film metal region and a photo-current driven by this absorption.
In this work, we use an analytical drift-diffusion model, coupled with detailed carrier transport and minority carrier lifetime estimates, to make realistic predictions of the conversion efficiency of InP-based triple junction cells. We evaluate the possible strategies for overcoming the problematic top cell for the triple junction, and make comparisons of the more realistic charge transport model with incumbent technologies grown on Ge or GaAs substrates.
This work uses simulations to predict the performance of InAlAsSb solar cells for use as the top cell of triple
junction cells lattice matched to InP. The InP-based material system has the potential to achieve extremely high
efficiencies due the availability of lattice matched materials close to the ideal bandgaps for solar energy conversion.
The band-parameters, optical properties and minority carrier transport properties are modeled based on literature
data for the InAlAsSb quaternary, and an analytical drift-diffusion model is used to realistically predict the solar cell
The modeling of high efficiency, multijunction (MJ) solar cells away from the radiative limit is presented. In the model,
we quantify the effect of non-radiative recombination by using radiative efficiency as a figure of merit to extract realistic
values of performance under different spectral conditions. This approach represents a deviation from the traditional
detailed balance approximation, where losses in the device are assumed to occur purely through radiative recombination.
For lattice matched multijunction solar cells, the model predicts efficiency values of 37.1% for AM0 conditions and
52.8% under AM1.5D at 1 sun and 500X, respectively. In addition to the theoretical study, we present an experimental
approach to achieving these high efficiencies by implementing a lattice matched triple junction (TJ) solar cell grown on
InP substrates. The projected efficiencies of this approach are compared to results for the state of the art inverted-metamorphic
(IMM) technology. We account for the effect of metamorphic junctions, essential in IMM technology, by
employing reduced radiative efficiencies as derived from recent data. We show that high efficiencies, comparable to
current GaAs-based MJ technology, can be accomplished without any relaxed layers for growth on InP, and derive the
optimum energy gaps, material alloys, and quantum-well structures necessary to realize them.
We present single mode optically pumped lasing from a new resonator structure for polymer lasers employing nonperiodic
circular Bragg gratings. Our devices, using a polyfluorene derivative (BN-PFO) as gain medium, are the first
blue-emitting circular grating semiconductor lasers (either organic or inorganic). They exhibit feature sizes as small as
47 nm and emit azimuthally polarized beams with a spectral linewidth ≈ 0.2 nm. We find a minimum lasing threshold
energy density of 1.2 μJ/cm<sup>2</sup> (10 Hz, 8 ns, 355 nm Nd:YAG laser excitation). The quality factor of the resonator modal
fields is found to be at least 2200 for these devices.
Free space optical communications (FSO) requires receivers with a wide field of view, large collection area and high bandwidth, as well as good rejection of unwanted ambient illumination. At present most of the optoelectronic components used in these systems are designed for fibre-optic systems and as such are not optimal for this application.
Work at the Universities of Oxford, Cambridge, Huddersfield and Imperial College has produced receivers incorporating detectors and preamplifiers specifically optimised for FSO and these show performance beyond that available commercially available. In this paper we describe the design, fabrication and performance of these integrated components. Further, we describe how this performance might scale with further optimisation, and future directions for optical receiver design.
Most free-space line-of sight systems require tracking in order to keep links aligned, and in order to achieve this a number of disparate optical, optoelectronic and electronic components are required. A key factor in determining the performance of these systems is the ability to integrate these in a scalable compact fashion, and to optimise components for the somewhat distinct requirements of free-space links.
A number of UK universities have been involved in a consortium that has fabricated integrated transceivers that use fully custom components optimised for an indoor free-space link application. The transmitters use arrays of Resonant Cavity LED (RCLED) devices integrated with custom CMOS driver circuitry and the necessary beamshaping optics, so that operating a particular LED in the array transmits light at a particular angle. A similar approach is taken at the receiver; light from a particular angle illuminates one element of a PIN photodiode array. This is integrated with an array of custom CMOS receivers and the necessary optics, creating a compact receiver subsystem. In this paper the components and subsystems are detailed and their application to long-distance links discussed.
The widespread use of Optical LANs is dependent on the ability to fabricate low cost transceiver components. These are usually complex, and fabrication involves the integration of optoelectronic and electronic devices, as well as optical components. A consortium of four UK universities are currently involved in a project to demonstrate integrated optical wireless transceiver subsystems that can provide eye-safe line of sight in-building communication at 155Mbit/s and above.
In this paper we discuss the flip-chip integration of two-dimensional arrays of novel microcavity LEDs with custom CMOS integrated circuits in order to produce solid state tracking emitters. Design, fabrication and integration of these structures are detailed. The scaleability and future capability available given further optimisation and development of these systems is also discussed.
The widespread use of Optical LANs is dependent on the ability to fabricate low cost transceiver components. These are usually complex, and fabrication involves the integration of optoelectronic and electronic devices, as well as optical components. A number of UK universities are currently involved in a project to demonstrate integrated optical wireless transceiver subsystems that can provide eyesafe line of sight in-building communication at 155Mbit/s and above, using 1550nm eyesafe emitters. The system uses two-dimensional arrays of novel microcavity LED emitters, and arrays of detectors integrated with custom CMOS integrated circuits to implement tracking transceiver components. The project includes design and fabrication of the optoelectronic devices, transimpedance amplifiers and optical systems, as well as flip-chip bonding of the optoelectronic and CMOS integrated circuits to create components scaleable to the large numbers of sources and detectors required. In this paper we report initial results from the first seven channel demonstrator system. Performance of individual components, their limitations and future directions are detailed.
We present results on Resonant Cavity Light Emitting Diodes (RCLEDs) emitting at 650 nm, which have high efficiencies and low voltages. In particular, we report on the angular properties of these devices, and highlight the observation that overall spectral linewidth increases with collection angle. This unusual property of RCLEDs is largely a consequence of employing a microcavity in the design. An additional contributing factor is the relative distribution of gain amongst the cavity modes (i.e. the level of tuning or detuning of the underlying emission, defined with respect to the longitudinal cavity mode). We have used measurement techniques which spectrally resolve angular radiation profiles to determine the (de)tuning directly. Moreover, these profiles demonstrate how the overall spectral linewidth increases with collection angle. To this end, we have developed a semi- empirical method for determining the overall linewidth as a function of emission numerical aperture (NA). A 4 nm detuned device has been investigated and linewidths have been found to increase from 3.1 nm to 13.6 nm over a range of NA approximately equals 0 to NA equals 1, an increase by a factor of around 4. Obviously, a variable linewidth also implies a variable coherence length with NA. Consequently, the coherence length was found to decrease from 30 micrometer to 9 micrometer over the same range. Independent coherence length measurements were carried out by direct interferometric measurements, and confirmed the expected trends.
12 Maintaining high bandwidth indoor optical wireless channels under a wide range of operating conditions usually requires relatively complex transceiver components. Integrating optical, optoelectronic and optical components using techniques that are suitable for mass manufacture is an important step in the development of these systems. This paper describes work to develop low cost integrated tracking transmitter and receiver components for use in a cellular indoor optical wireless network. A seven channel demonstrator operating at 155 Mb/s is under construction, using arrays of Resonant Cavity LEDs, PIN detectors, Silicon CMOS driver circuits and associated optics. Development of components, design methodology and initial results are detailed.