Microscopic reversibility demands lower open circuit voltage in multiple exciton generation solar cells

Multiple exciton generation (MEG) increases the short circuit current of solar cells and is, therefore, often cited as a candidate scheme for surpassing the efficiency limit of single junction solar cells. Conventionally, limiting efficiencies for MEG solar cells have been calculated using quasi-equilibrium models that implicitly assume an effective separation of timescales between different processes. We show here that this separation of timescales is not possible for MEG solar cells, with Auger recombination, the inverse process to multi-exciton generation, needing to be considered explicitly in the efficiency limits of an MEG solar cell. We assess the impact of Auger recombination using a non-equilibrium model of a quantum dot solar cell that satisfies microscopic reversibility and can approximate experimental external quantum efficiency (EQE) curves of MEG solar cells. Recombination - both Auger and radiative - is treated in a quasi-equilibrium approach, which can be justified with a clear model for the separation of timescales. A key insight of this model is that the achievable voltage of the device, and hence the solar energy conversion efficiency, depends on the absolute values of the impact ionization rate and the rate at which electrons lose energy through phonon scattering. By contrast, the EQE profile at short circuit depends only on the ratio of these two rates. This shows that the potential of certain MEG solar cell approaches cannot be assessed from EQE improvements alone, which highlights the importance of considering non-equilibrium processes in models of solar energy conversion devices

A coupled thermal and electrical model of a sheet-and-tube hybrid photovoltaic/thermal (PV/T) collector

Papers presented to the 11th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 20-23 July 2015.The goal of this paper is to obtain the (pair of) efficiency curves of a hybrid PV/T collector with a sheet-and-tube design and to evaluate the effect of a non-uniform temperature distribution on the surface of the solar cell on its electrical power output. A 3-dimensional numerical model is developed to estimate the performance of such a collector. The model allows various design parameters of the PV/T to be varied so that the influence of each of these parameters can be studied on the overall system performance both at steady-state and at varying atmospheric conditions. The main parameters considered are the number of glass covers, ranging from an unglazed collector configuration to a double-glazed collector configuration, and the width-to-pipe diameter (W/D) ratio. The results show that, while the thermal efficiency increases with the additional glazing, the electrical efficiency deteriorates due to the higher temperature of the fluid and due to increased optical losses, as expected. The dynamic performance of the PV/T collector and system are also investigated. Results from the dynamic model and also from a simplified quasi-steady state model are reported. The results show that in the case of highly fluctuating incident radiation, e.g. from clouds, the quasi-steady solution can deviate by up to 20% from the dynamic solution in the evaluation of the thermal energy output in the case of low incident radiation with large fluctuations.am201

Photovoltaic characterisation of GaAsBi/GaAs multiple quantum well devices

A series of strained GaAsBi/GaAs multiple quantum well diodes are characterised to assess the potential of GaAsBi for photovoltaic applications. The devices are compared with strained and strain-balanced InGaAs based devices. The dark currents of the GaAsBi based devices are around 20 times higher than those of the InGaAs based devices. The GaAsBi devices that have undergone significant strain relaxation have dark currents that are a further 10–20 times higher. Quantum efficiency measurements show the GaAsBi devices have a lower energy absorption edge and stronger absorption than the strained InGaAs devices. These measurements also indicate incomplete carrier extraction from the GaAsBi based devices at short circuit, despite the devices having a relatively low background doping. This is attributed to hole trapping within the quantum wells, due to the large valence band offset of GaAsBi

Photoluminescence upconversion at interfaces driven by a sequential two-photon absorption mechanism

This paper reports on the results of an investigation into the nature of photoluminescence upconversion at GaAs/InGaP2 interfaces. Using a dual-beam excitation experiment, we demonstrate that the upconversion in our sample proceeds via a sequential two-photon optical absorption mechanism. Measurements of photoluminescence and upconversion photoluminescence revealed evidence of the spatial localization of carriers in the InGaP2 material, arising from partial ordering of the InGaP2. We also observed the excitation of a two-dimensional electron gas at the GaAs/InGaP2 heterojunction that manifests as a high-energy shoulder in the GaAs photoluminescence spectrum. Furthermore, the results of upconversion photoluminescence excitation spectroscopy demonstrate that the photon energy onset of upconversion luminescence coincides with the energy of the two-dimensional electron gas at the GaAs/InGaP2 interface, suggesting that charge accumulation at the interface can play a crucial role in the upconversion process

Unveiling microscopic carrier loss mechanisms in 12 efficient Cu2ZnSnSe4 solar cells

Understanding carrier loss mechanisms at microscopic regions is imperative for the development of high performance polycrystalline inorganic thin film solar cells. Despite the progress achieved for kesterite, a promising environmentally benign and earth abundant thin film photovoltaic material, the microscopic carrier loss mechanisms and their impact on device performance remain largely unknown. Herein, we unveil these mechanisms in state of the art Cu2ZnSnSe4 CZTSe solar cells using a framework that integrates multiple microscopic and macroscopic characterizations with three dimensional device simulations. The results indicate the CZTSe films have a relatively long intragrain electron lifetime of 10 30 amp; 8201;ns and small recombination losses through bandgap and or electrostatic potential fluctuations. We identify that the effective minority carrier lifetime of CZTSe is dominated by a large grain boundary recombination velocity 104 amp; 8201;cm amp; 8201;s amp; 8722;1 , which is the major limiting factor of present device performance. These findings and the framework can greatly advance the research of kesterite and other emerging photovoltaic material

Theory and simulation of quantum photovoltaic devices based on the non-equilibrium Green's function formalism

This article reviews the application of the non-equilibrium Green's function formalism to the simulation of novel photovoltaic devices utilizing quantum confinement effects in low dimensional absorber structures. It covers well-known aspects of the fundamental NEGF theory for a system of interacting electrons, photons and phonons with relevance for the simulation of optoelectronic devices and introduces at the same time new approaches to the theoretical description of the elementary processes of photovoltaic device operation, such as photogeneration via coherent excitonic absorption, phonon-mediated indirect optical transitions or non-radiative recombination via defect states. While the description of the theoretical framework is kept as general as possible, two specific prototypical quantum photovoltaic devices, a single quantum well photodiode and a silicon-oxide based superlattice absorber, are used to illustrated the kind of unique insight that numerical simulations based on the theory are able to provide.Comment: 20 pages, 10 figures; invited review pape

An investigation into the efficiency enhancement of strained and strain-balanced quantum well solar cells

The optical absorption of a p-i-n solar cell may be increased by incorporating quantum well layers in the intrinsic region, resulting in a larger short-circuit-current. Further, the additional quantum well absorption may be achieved without incurring a comparable loss in open-circuit-voltage; offering potential for enhanced solar power conversion efficiencies. The work reported in this thesis is concerned with the application of quantum well techniques to the GaAs cell, currently the highest efficiency single band-gap cell. In the absence of lattice matched lower band-gap materials, strained InGaAs was used as the quantum well material. An optical study of the carrier dynamics at the solar cell operating point indicates suppressed quasi-Fermi levels in the QW with respect to the bulk material, with advantageous implications for the open-circuit-voltage. However the strained quantum wells introduce misfit dislocations into the structure that dominate the recombination of the cell. The associated loss in open-circuit-voltage is shown to be too large to allow strained GaAs/InGaAs cells to realistically match GaAs in terms of power conversion efficiency. A strain-balance technique is presented, surmounting the limitations imposed by strain and is applied to a 5% InGaAs virtual substrate GaAs/InGaAs structure and a GaAs substrate GaAsP/InGaAs structure. An equivalent power conversion efficiency to GaAs is demonstrated for the GaAsP/InGaAs cell. The scope for further improvement of the GaAsP/InGaAs cell is discussed, with particular reference to light trapping. (author)Available from British Library Document Supply Centre-DSC:DXN035941 / BLDSC - British Library Document Supply CentreSIGLEGBUnited Kingdo

A feasibility study of solar PV-powered electric cars using an interdisciplinary modeling approach for the electricity balance, CO2 emissions, and economic aspects: the cases of The Netherlands, Norway, Brazil, and Australia

Electric vehicles (EVs) are becoming an increasingly attractive option to effectively and economically efficiently reduce global fossil fuel consumption as well as CO2 emissions associated with road transportation. In general, the grid provides the electricity required to charge an EV's battery. However, it could be worthwhile to consider EV charging by specific solar photovoltaic (PV) systems to further facilitate the use of renewable energy and to minimize CO2 emissions. Additional benefits could, for instance, be less overloaded local grids and additional grid flexibility. Because little information and experiences exist with so-called solar PV-powered EVs, this paper explores how well PV systems—with the possible combination of battery energy storage systems (BESSs)—might contribute to charging of EVs in four different countries, namely, The Netherlands, Norway, Brazil, and Australia. To this end, a model has been developed that calculates the interactions between PV-BESS systems, EVs, and the grid in each country to determine the electricity balance, financial consequences, and avoided CO2 emissions of PV-powered EVs, compared with EVs that are solely charged by the grid, as well as conventional passenger cars with an internal combustion engine (ICE-V). It is logically found that in countries with a high irradiation, the whole year through, such as Brazil and Australia, solar PV-powered EVs can be operated more effectively than in countries with a high variability of irradiation over the year such as The Netherlands and Norway. If the charging system's PV share is increased from 0% to 50%, the number of required grid charging events per year can be reduced from 104 to 34 in The Netherlands and from 123 to 55 in Norway. PV charging can also reduce CO2 emissions of EVs by 18% to 93% as compared with ICE-Vs depending on the location. From a financial perspective, PV-powered EVs are not yet financially feasible in all countries; however, in some nations, 100% PV charging is already a viable option. In general, it can be concluded that in contrast to driving an ICE-V, the further PV-powered EVs are driven, the more affordable they become—they might even generate financial revenues—and hence, the higher their positive environmental impact will be. On the basis of this study, it can therefore be concluded that solar PV-powered EVs are a technically feasible and increasingly financially attractive option for transport sector emission reductions in most countries when compared with regular grid charging of EVs and certainly as compared with ICE-Vs