The power output of dual-junction mechanically stacked solar cells comprising different sub-cell materials in a terrestrial concentrating photovoltaic module has been evaluated. The ideal bandgap combination of both cells in a stack was found using EtaOpt. A combination of 1.4 eV and 0.7 eV has been found to produce the highest photovoltaic conversion efficiency under the AM1.5 Direct Solar Spectrum with x500 concentration. As EtaOpt does not consider the absorption profile of solar cell materials; the practical power output per unit area of a dual junction mechanically stacked solar cell has been modelled considering the optical absorption co-efficients and thicknesses of the individual solar cells. The model considered a GaAs top cell and a Ge, GaSb, Ga0.47In0.53As or Si bottom cell. It was found that GaSb gives the highest power contribution as a bottom cell in a dual junction configuration followed by Ge and GaInAs. While the additional power provided by a Si bottom cell is less than these it remains a suitable candidate for a bottom cell owing to its lower cost.

A newly designed room-temperature, sine-wave gated, InGaAs/InP SPAD based photon counting system with dead-time mitigation is described in this work. In this system, an InGaAs/InP SPAD was developed and used as the detector. To mitigate SPAD's dead time and enable room temperature operation, a sine-wave gated quenching circuit is employed for rapid quenching and post-pulse suppression. Furthermore, phase modulation schemes are utilized to mitigate dead-time that enables SPAD to detect incident photons with unknown arrival times when applied in Optical Time Domain Reflectometry (OTDR). Results show that the system developed can operate under room temperature with a dark count rate of less than 8 kcounts/s at bias voltage of 67 V. At this bias voltage, the photon efficiency reaches 9.5% at 1400 nm. A narrow time jitter of 146 ps was achieved and the afterpulsing possibility measured was around 10% at bias voltage of 65 V. To evaluate the system's performance, a photon counting optical time domain reflectometer system was built and tested using FC/PC connectors. It has been proved that the system is able to detect reflected signals from all connectors and accurately measure their position, eliminating the problem of dead-time in the sine wave gated quenching circuit.

Solar energy is an alternative to conventional fossil fuels, and maximising the energy collected through solar cell design improvements is worthwhile. Genetic algorithms (GAs) have been historically useful in pursuing design improvements in solar cell architecture and manufacturing, particularly in combination with cell modelling software. This study demonstrates solar cell structural optimisation using PC3D software in combination with a genetic algorithm (GA) to maximise solar cell power conversion efficiency. PC3D is an Excel-based tool for modelling solar cells. The cell models examined here are: Passivated-Emitter Rear Contact (PERC), Interdigitated Back Contact (IBC), Aluminium-Back Surface Field (Al-BSF), and Passivated-Emitter Rear Locally Diffused (PERL). These are all silicon PV cells with varying structural features that significantly alter the performance of each cell. Absolute efficiency improvements of 2% for PERC, 1.6% for Al-BSF, 0.9% for IBC, and 0.5% for PERL cells are achieved. The main parameters impacting cell efficiency in this model are cell thickness and minority charge carrier lifetime, which feature a necessary trade-off between the higher absorption resulting from a thicker cell and the increased likelihood of charge collection arising from longer carrier lifetimes. These parameters are managed through other related values such as cell doping.

With the global increase in the deployment of photovoltaic (PV) modules in recent years, the need to explore and understand their reported failure mechanisms has become crucial. Despite PV modules being considered reliable devices, failures and extreme degradations often occur. Some degradations and failures within the normal range may be minor and not cause significant harm. Others may initially be mild but can rapidly deteriorate, leading to catastrophic accidents, particularly in harsh environments. This paper conducts a state-of-the-art literature review to examine PV failures, their types, and their root causes based on the components of PV modules (from protective glass to junction box). It outlines the hazardous consequences arising from PV module failures and describes the potential damage they can bring to the PV system. The literature reveals that each component is susceptible to specific types of failure, with some components deteriorating on their own and others impacting additional PV components, leading to more severe failures. Finally, this review briefly summarises PV failure detection techniques, emphasising the significance of electrical characterisation techniques and underlining the importance of considering more electrical parameters. Most importantly, this review identifies the most prevalent degradation processes, laying the foundation for further investigation by the PV research community through modelling and experimental studies. This allows for early detection by comparing PV performance when failures or degradation occur to prevent serious progression. It is worth noting that most of the studies included in this review primarily focus on detailing failures and degradation observed in PV operations, which can be attributed to various factors, including the manufacturing process and other external influences. Hence, they provide explanations of these failure mechanisms and causes but do not extensively explore corrective actions or propose solutions based on either laboratory experiments or real-world experience. Although, within this field of study, there are corresponding studies that have designed experiments to suggest preventive measures and potential solutions, an in-depth review of those studies is beyond the scope of this paper. However, this paper, in turn, serves as a valuable resource for scholars by confining PV failures to critically evaluate available studies for preventative measures and corrective actions.

It has been shown that a reduction in the shunt resistance can lead to solar module degradation over time, resulting ultimately in module failure. This paper reports how the effects of reduced shunt resistance on the current–voltage (I-V) characteristics of a PV cell can be used to identify degradation before it becomes critical. Five commercial polycrystalline solar cell samples had their shunt resistance artificially lowered before measuring their I-V characteristics. Analyses of the effect of lowered shunt resistance on maximum power output (Pmax), short-circuit current (ISC), open-circuit voltage (VOC) and fill factor were conducted. Reduction in shunt resistance was correlated with the cells' electrical parameters to determine the critical shunt resistance where degradation becomes catastrophic. Linear models were developed relating reduction in shunt resistance to the solar cell's Pmax and VOC. These relationships are proposed as strong predictors and observers of shunt resistance degradation and are suitable for implementation in online monitoring systems for operational PV modules.

A photon counting system using covered single-photon avalanche diode (SPAD) based on a standard integrated circuit (IC) process ( $0.18~\mu \text{m}$ ) is designed and analyzed in this work. The SPAD is formed using the medium voltage (MV) doping layers of the process. To reduce the dark count rate (DCR) in the SPAD, a shaded SPAD with the same structure is fabricated on the same chip which is covered by a metal layer and only providing DCR for the DCR correction. This DCR provided by the shaded SPAD can also be used for the real time on chip monitoring of some other parameters, such as temperature, breakdown voltage, and afterpulsing probability. Experimental results show that the SPAD developed is able to detect the visible light from 450 to 850 nm with a 35% peak photon detection probability (PDP) achieved at around 550 nm with a bias voltage of 16 V (excess voltage of 3 V). A timing jitter of 176 ps is measured with an excess voltage of 3 V. The DCR in the SPAD tested is about 1.38 cps/ $\mu \text{m}^{2}$ with an excess bias voltage of 1 V and 14.62 cps/ $\mu \text{m}^{2}$ with the excess bias voltage of 3 V without the DCR correction. Results also show that a reduction of more than 85% in the DCR (background noise) can be achieved when the DCR correction is applied resulting in a DCR of 1.68 cps/ $\mu \text{m}^{2}$ with an excess bias voltage of 3 V. By monitoring the DCR of the shaded SPAD, the breakdown voltage and temperature of other on chip SPAD can be measured. The potential usefulness of the afterpulsing probability monitoring using the shaded SPAD and the crosstalk probability (CTP) between SPADs on the chip are analyzed. In addition, the effects of process variations on the SPAD performance are investigated by testing ten chips with the same SPADs fabricated, and a potential method is proposed for alleviate the process variations in the SPAD arrays.

The design and implementation of a real-time breakdown voltage and on-chip temperature monitoring system for single photon avalanche diodes (SPADs) is described in this work. In the system, an on-chip shaded (active area of the detector covered by a metal layer) SPAD is used to provide a dark count rate for the breakdown voltage and temperature calculation. A bias circuit was designed to provide a bias voltage scanning for the shaded SPAD. A microcontroller records the pulses from the anode of the shaded SPAD and calculates its real-time dark count rate. An algorithm was developed for the microcontroller to calculate the SPAD’s breakdown voltage and the on-chip temperature in real time. Experimental results show that the system is capable of measuring the SPAD’s breakdown voltage with a mismatch of less than 1.2%. Results also show that the system can provide real-time on-chip temperature monitoring for the range of −10 to 50 °C with errors of less than 1.7 °C. The system proposed can be used for the real-time SPAD’s breakdown voltage and temperature estimation for dual-SPADs or SPAD arrays chip where identical detectors are fabricated on the same chip and one or more dummy SPADs are shaded. With the breakdown voltage and the on-chip temperature monitoring, intelligent control logic can be developed to optimize the performance of the SPAD-based photon counting system by adjusting the parameters such as excess bias voltage and dead-time. This is particularly useful for SPAD photon counting systems used in complex working environments such as the applications in 3D LIDAR imaging for geodesy, geology, geomorphology, forestry, atmospheric physics and autonomous vehicles.

In recent years, emphasis has been placed on the design and implementation of sustainable energy system solutions to combat the adverse environmental impact of emissions from the power and transportation sectors. This study applies a systems elimination method using numerical simulation to validate and optimise recently-reported results demonstrating the benefits of photovoltaic (PV)–diesel — battery hybrid integrated power systems (IPS) for commercial centres, with Abuja in Nigeria used for the case study. An optimal IPS was identified from 20,200 candidate solutions analysed by assessment against environmental (1st priority) and economic (2nd priority) metrics. Although environmental conditions were prioritised, the optimal system was economically viable. The environmentally optimal system emitted 33% less greenhouse gas (GHG) emissions (CO2 tonnes/yr.) than the economically optimised solution (PV–diesel) over their operational lifetimes ($/20 years), and was 4% costlier than same. The results demonstrate that carbon taxation or outright bans on independent fossil fuel systems (IFFSs) in emerging economies might not be effective policies in mitigating the impact of climate change on our environment. This study contributes to the body of knowledge on energising unserved and underserved communities in sub-Saharan Africa, considering the case study country of Nigeria. It decries the common practice of prioritising economic factors over environmental factors in optimising the operations of grid defected power system solutions as continental and regional electrification efforts are being ramped up. This is particularly of importance (an environmental responsibility), as immediate economic gains could have far-reaching environmental and social implications that elicits the limitations of economically prioritised power development projects in the offing.

One of the common failures in photovoltaic modules is the degradation of the ethylene-vinyl acetate (EVA) encapsulant due to prolonged ultraviolet exposure and other environmental stress factors, such as temperature and humidity. Experimental studies have shown that significant reduction in the optical transmission due to EVA degradation leads to loss in the available power by more than 50%. In this article, a novel approach to predict the early degradation of EVA encapsulant is proposed by correlating EVA degradation with short-circuit current (I SC ). An electrical circuit simulator, simulation program with integrated circuit emphasis (SPICE), is used to evaluate the short-circuit current obtained under varying optical transmission caused by EVA discoloration. The simulation follows three steps: simulation of the transmitted solar spectrum; simulation of the spectral short-circuit current density; and simulation of the current-voltage (I-V) curve to obtain short-circuit current (I SC ), maximum power output (P max ), open-circuit voltage (V OC ) and fill factor. Results show that the reduction in short-circuit current due to EVA degradation differs from the reductions expected due to a spectrally-uniform reduction of intensity of the solar irradiance. Both types of variation are linear, however, the slope due to EVA degradation is larger than the slope obtained for normal intensity variations in the solar irradiance. This model, when applied in conjunction with solar irradiance measurements, can predict early onset of EVA encapsulant failure, thereby enabling preventative measures to be taken.

The limited access to affordable, reliable and sustainable energy in sub-Saharan Africa could inhibit the region's realisation of the United Nations Sustainable Development Goals by 2030. The intermittency and unreliability of power supply in the region has led countries, especially in the eastern sub-region, to implement sustainable energy solutions for rural electrification, thereby improving electricity supply access to underserved and unserved communities. With this focus on rural electrification, a deficit in electricity supply to urban settlements could arise, owing to the economic feasibility of extending the power grid towards securing electricity access for a growing population and the increasing number of rural-urban migrators. This paper reviews existing literature on electrifying sub-Saharan Africa, highlighting the prescriptions for deploying energy solutions in the region. Consequently, a country-level case study on grid defection solutions for Nigerian commercial centres assessing 14 different designs of Integrated Power Systems' (IPS) operations against the three impact metrics of cost implication ($/lifetime), greenhouse gas (GHG) emissions (CO2 tonnes/yr.) and surplus energy (MWh/yr.), is presented. The systematic analysis demonstrates that an integrated hybrid-solar-photovoltaics (PV)-based system (IHSS) without battery storage, serving 56% of its load from solar-PV and 44% from fossil-fuelled generators provides the lowest cost power supply option. The modelled system generated 25 MWh/yr. in surplus energy and emitted 53% fewer GHG emissions than the largest emitter. A compelling case is made whereby augmenting existing infrastructure with an appropriately sized PV plant will significantly reduce costs and simultaneously have a significant impact on GHG emissions. The generation of surplus energy also presents an opportunity to augment urban electrification through custom-fit sustainable energy solutions and the formation of a transactive electricity market.

A compact single-photon counting module that can accurately control the bias voltage and hold-off time is developed in this work. The module is a microcontroller-based system which mainly consists of a microcontroller, a programmable negative voltage generator, a silicon-based single-photon avalanche diode, and an integrated active quench and reset circuit. The module is 3.8 cm × 3.6 cm × 2 cm in size and can communicate with the end user and be powered through a USB cable (5 V). In this module, the bias voltage of the single-photon avalanche diode (SPAD) is precisely controllable from −14 V ~ −38 V and the hold-off time (consequently the dead time) of the SPAD can be adjusted from a few nanoseconds to around 1.6 μs with a setting resolution of ∼6.5 ns. Experimental results show that the module achieves a minimum dead time of around 28.5 ns, giving a saturation counting rate of around 35 Mcounts/s. Results also show that at a controlled reverse bias voltage of 26.8 V, the dark count rate measured is about 300 counts/s and the timing jitter measured is about 158 ps. Photodetection probability measurements show that the module is suited for detection of visible light from 450 nm to 800 nm with a 40% peak photon detection efficiency achieved at around 600 nm.

In this work, a single photon avalanche diode (SPAD) based configurable photon counting system is developed. The photon counting system is able to operate in three working modes to enhance the system's different performance characteristics for various use cases (e.g., strong/weak incident light signal, high/low ambient light, long/short distance ranging/sensing). The working modes can be switched digitally by the user or signal processing circuit that makes it possible to add intelligent control logic for switching in real-time between working modes to achieve a stable and optimized performance when it is used under varying working conditions. Performance characteristics of the system operation in each working mode are measured and compared.

In this work, a high dynamic range APD-based photo-detection module is designed and developed. In the design, on-chip dual avalanche photodiodes (APDs) are fabricated with one biased to work in linear mode and the other one biased to work in single photon mode. The APD operating in linear mode is connected to a two-stage amplifier I-V conversion circuit and the APD operating in single photon mode is connected to a custom designed active quench and reset integrated circuit. The design enables the two on-chip APDs operate in different modes simultaneously without user intervention. This simplifies the system operation and a wide range of incident light intensities can be easily detected. Experimental results show that a high dynamic range of 164.2 dB is achieved by the module.

Rights © 2006 American Institute of Physics.This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. The following article appeared in Chróinín, J. N. and Morrison, A. P. (2006) 'Thermionic emission perpendicular to bulk and multiquantum AlxGa1−xInP barriers', Applied Physics Letters, 88(14), pp. 142110 and may be found at http://aip.scitation.org/doi/abs/10.1063/1.2181648

Dual single-photon avalanche photodiodes (SPADs) integrated on the same chip enable the effective compensation of dark count rate (DCR) in the SPAD and also the real-time monitoring of the chip temperature. In the design, two identical SPADs are fabricated on the same chip, one operating normally and the other one covered by a metal layer to be kept in the dark. The two SPADs are identically biased and connected to identical active quench and reset integrated circuits. As both detectors are identical in structure, the dark count is expected to be similar for both. Experimental measurements show that the two SPADs exhibit similar DCR performance over a range of bias voltages and temperatures. By measuring the DCR from the covered SPAD, the DCR from the normally operated SPAD can be accounted for directly. This can be particularly useful for SPADs, where the DCR is high. Experiments under illumination show that the shaded SPAD is immune to illumination over a wide range of incident light power. This enables the real-time monitoring of the temperature on the sensor chip using the counting rate from the dark operated avalanche photodiode (APD).

In this work, a regulated high negative voltage generator for biasing single-photon avalanche photodiodes (SPAD) was developed. The circuit provides up to -70 V from a positive voltage source. This circuit allows users to control the negative output voltage using a positive voltage rail, thus eliminating the requirement of a negative voltage for the negative voltage control. This approach simplifies the setting of the output and facilitates integration in miniaturized photon counting systems. The testing on a fabricated PCB of this circuit show that the output voltage can be accurately controlled up to -70 V with ripples of less than 80 mV. A SPAD based experimental setup was also built and the experimental results show that the circuit is able to maintain a stable bias voltage for a planar SPAD at both low and high counting rates.

An 8×8 (64) Internet of Things (IoT) wind farm platform is built using miniaturized wind turbines with wireless connectivity. The farm is being deployed on the south east coast of Ireland to remotely collect data for offline evaluation of a data driven wind turbine power output model and study aerodynamic interactions between turbines within the farm. The cluster is build such that a range of different downwind distances (between the turbines) can be tested with minimum rearrangement effort. To remotely collect data for an offline evaluation of wind turbine power models, each turbine within the farm is equipped with an IoT platform. Each of the 64 turbines simultaneously will report its voltage output measured across a load resistor. An ultrasonic wind sensor equipped with the same IoT platform will measure the wind speed and wind direction necessary for the offline wind turbine power model evaluation. In this work we present an energy analysis for the IoT platform (node) based on lab power measurements conducted while running firmware that will be used on the deployment site. The work emphasizes the potential of using an inexpensive wireless, battery powered, IoT node for remote data collection in preference to a wired solution using a data-logger that has limited storage that cannot be remotely accessed.

Silicon-based single photon avalanche diodes (SPADs) are widely used as single photon detectors of visible and near infrared photons. There has, however, been a lack of models accurately interpreting the physics of impact ionization (the mechanism behind avalanche breakdown) for these devices. In this paper, we present a statistical simulation model for silicon SPADs that is capable of predicting breakdown probability, mean time to breakdown, and timing jitter. Our model inherently incorporates carriers’ dead space due to phonon scattering and allows for nonuniform electric fields. Model validation included avalanche gain, excess noise factor, breakdown voltage, breakdown probability, and timing statistics. Simulating an n-on-p and a p-on-n SPAD design using our model, we found that the n-on-p design offers significantly improved mean time to breakdown and timing jitter characteristics. For a breakdown probability of 0.5, mean time to breakdown and timing jitter from the n-on-p design were 3 and 4 times smaller compared to those from the p-on-n design. The data reported in this paper are available from the ORDA digital repository (DOI: 10.15131/shef.data.4823248).

A Geiger-mode avalanche photodiode (GM-APD) photon counting system is presented in this letter. The system provides a maximum counting rate of 35 Mcounts/s and is capable of directly displaying the counting rate and data logging to a PC. In this system, the detector can be easily changed to enhance its usefulness in different applications. A novel active quench and reset integrated circuit (AQR-IC) is designed for the system with adjustable hold-off time from several nanoseconds up to 1.6 μs with a setting resolution of ~6.5 ns. This facilitates optimal performance when using different types of APDs. The AQR-IC also registers each avalanche event as a TTL pulse that is processed by a microcontroller to calculate the photon-counting rate. The microcontroller can be interfaced with a PC over USB to record the measured data and to allow further processing. Software was also written to calculate the photon-counting rate, display the results and save the data to files.