Limiting efficiencies for intermediate band solar cells with partial absorptivity: the case for a quantum ratchet

The intermediate band solar cell (IBSC) concept aims to improve upon the Shockley–Queisser limit for single bandgap solar cells by also making use of below bandgap photons through sequential absorption processes via an intermediate band (IB). Current proposals for IBSCs suffer from low absorptivity values for transitions into and out of the IB. We therefore devise and evaluate a general, implementation‐independent thermodynamic model for an absorptivity‐constrained limiting efficiency of an IBSC to study the impact of absorptivity limitations on IBSCs. We find that, due to radiative recombination via the IB, conventional IBSCs cannot surpass the Shockley–Queisser limit at an illumination of one Sun unless the absorptivity from the valence band to the IB and the IB to the conduction band exceeds ≈36%. In contrast, the introduction of a quantum ratchet into the IBSC to suppress radiative recombination can enhance the efficiency of an IBSC beyond the Shockley–Queisser limit for any value of the IB absorptivity. Thus, the quantum ratchet could be the vital next step to engineer IBSCs that are more efficient than conventional single‐gap solar cells. © 2016 The Authors. Progress in Photovoltaics: Research and Applications published by John Wiley & Sons Ltd.     

The role of intersubband optical transitions on the electrical properties of InGaAs/GaAs quantum dot solar cells

We report on an experimental study of the intersubband optical response of an In_0.5Ga_0.5As/GaAs quantum dot solar cell (QDSC). By calculating the quantum dot absorption cross section, the strength of the intersubband optical transitions is gauged, and their importance and influence on the electrical properties of the solar cell can be compared with those of other physical processes such as thermal intersubband and optical interband transitions. The temperature‐dependent photocurrent and dark current characteristics of the QDSC have also been analyzed in detail, leading to an understanding of the specific effects reducing the open‐circuit voltage. The deviation of QDSCs from idealized models is discussed, and some conditions required for an improved open‐circuit voltage are suggested. Copyright © 2012 John Wiley & Sons, Ltd.     

Determination of the optimum material parameters for intermediate band solar cells using diffusion model

In this study, the optimum material parameters capable of providing high efficiencies close to the detailed balance limit are determined for intermediate band solar cells. A diffusion model, including the overlap effect between absorption coefficients, is used during the calculations for the first time. It is obtained that to achieve high efficiencies close to the detailed balance limit; the effective density of state value, N_CV, should be higher than 10¹⁷ cm⁻³ and the carrier mobility should be larger than 200 cm²/Vs, where the light concentration should not be higher than nearly 1000 sun. Besides, it is found out that the optimum intermediate band level and the base width depend on the mobility and effective density of state values. So they need to be optimized according to the material parameters. The effect of overlap between absorption coefficients on the performance of intermediate band solar cells is also investigated. Copyright © 2012 John Wiley & Sons, Ltd.     

Drift‐diffusion model for intermediate band solar cells including photofilling effects

A novel drift‐diffusion model for intermediate band solar cells (IBSC) is presented. The model differs from previous drift‐diffusion models by allowing the carrier concentrations in all three bands to vary. It is developed for the idealized case where only radiative recombination occurs and where the IB has zero width. The model is used to compare the performance of IBSCs where the IB‐region is doped to get a partially filled IB (prefilled IBSC) to IBSCs where the IB‐region is not doped to partially fill the IB (photofilled IBSC). Numerical results show that a photofilled IBSC can achieve high efficiencies when operated under concentrated light. In fact, for some particular cases, a photofilled cell will perform better than a prefilled cell. The optimal degree of prefilling, i.e. the ratio of the concentration of doping atoms to the total number of IB‐states, is found for a particular example. It is also examined how the carrier concentrations in all three bands, the conduction, the intermediate and the valence bands, vary in prefilled and photofilled IBSCs. Finally, the band diagrams of a prefilled and photofilled IBSC are discussed. Copyright © 2010 John Wiley & Sons, Ltd.     

On the reported experimental evidence for the quasi‐Fermi level split in quantum‐dot intermediate‐band solar cells

The reported experimental evidence for the quasi‐Fermi level split in quantum‐dot intermediate‐band solar cells is carefully examined. It is shown that the separation of the quasi‐Fermi level of the intermediate band from that of the conduction band is not consistent with the experimental results of the quantum efficiency and the luminescence intensity of the InAs/GaAs cells. The fact that the electroluminescence spectrum is too wide, extending much further than we expect on the basis of the measured quantum efficiency in the direction of increasing photon energies, indicates that the temperature of the optically active regions of the cell during the electroluminescence measurements is considerably higher than room temperature. The best agreement with the experimental results is achieved with a temperature of about 525 K. This temperature rise is probably a result of the heating effect of the relatively high forward current used in the luminescence experiments. It is argued that the lack of a quasi‐Fermi level split in this case is associated with the absence of a gap in the emission/absorption spectrum of sub‐bandgap photons. Copyright © 2011 John Wiley & Sons, Ltd.     

Initial theoretical study of solar cells with an intermediate band of non‐zero width and a thermalized electron population

A model based on detailed balance principles is developed to study how the thermalized nature of the electrons in the intermediate band (IB) affects the efficiency of intermediate band solar cells. Published work on intermediate band solar cells with finite IB width has focused on the fundamental case when the absorptivity is assumed to be high for all photon energies above the smallest band gap. In this work, an attempt is made to incorporate variations in the absorptivity due to the thermal distribution of the IB electrons. In a wide IB with a thermalized electron population, there will be a low density of electrons close to the upper band edge. The density of unoccupied electron states close to the lower band edge will also be low. As a consequence, the absorption coefficients for photon energies where the only energetically allowed transitions involve exciting electrons from or to, respectively, such states can be expected to be low. The presented model incorporates the effect of the thermalized electron population in an idealized way. In some cases, the calculated efficiency is well above the limit for single band gap cells, whereas in other cases it is not. It is concluded that absorption coefficients rising rapidly from very low values to higher values are advantageous, that overlap between the absorption coefficients can be beneficial when the IB becomes sufficiently wide, and finally, that a case‐by‐case study probably is required to evaluate whether a particular IB material can give cells with high efficiency. Copyright © 2011 John Wiley & Sons, Ltd.     

Light absorption and many‐particle electron–phonon interactions in intermediate band quantum well solar cells

The carrier dynamics under solar excitation in the third generation AlAs‐Ga_0.27In_0.73As quantum well solar cell on an InP substrate is theoretically investigated. The quantum well shows a significantly higher carrier occupation compared with that of a pure bulk sample. Furthermore, the influence of carrier scattering via the interaction with a bath of longitudinal optical phonons stemming from the substrate is taken into account, showing strong intraband and interband relaxation of the carrier distribution toward the lowest subband states. Copyright © 2014 John Wiley & Sons, Ltd.     

Progress toward an advanced four‐subcell inverted metamorphic multi‐junction (IMM) solar cell

We report progress on the development of an advanced four‐subcell IMM CPV solar cell that is designed for extremely high conversion efficiency under realistic concentrator operating conditions. Practical considerations allowing the design to mitigate problems related to Al‐containing alloys, lattice mismatch, non‐ideal short‐wavelength response, and reflection losses are described. Performance modeling is used to guide the choice of optimal subcell band gaps for the new IMM cell. Early experimental efforts to develop and implement the new design are described and discussed. Copyright © 2015 John Wiley & Sons, Ltd.     

100‐period, 1.23‐eV bandgap InGaAs/GaAsP quantum wells for high‐efficiency GaAs solar cells: toward current‐matched Ge‐based tandem cells

Major challenges for InGaAs/GaAsP multiple quantum well (MQW) solar cells include both the difficulty in designing suitable structures and, because of the strain‐balancing requirement, growing high‐quality crystals. The present paper proposes a comprehensive design principle for MQWs that overcomes the trade‐off between light absorption and carrier transport that is based, in particular, on a systematical investigation of GaAsP barrier effects on carrier dynamics that occur for various barrier widths and heights. The fundamental strategies related to structure optimization are as follows: (i) acknowledging that InGaAs wells should be thinner and deeper for a given bandgap to achieve both a higher absorption coefficient for 1e‐1hh transitions and a lower compressive strain accumulation; (ii) understanding that GaAs interlayers with thicknesses of just a few nanometers effectively extend the absorption edge without additional compressive strain and suppress lattice relaxation during growth; and (iii) understanding that GaAsP barriers should be thinner than 3 nm to facilitate tunneling transport and that their phosphorus content should be minimized while avoiding detrimental lattice relaxation. After structural optimization of 1.23‐eV bandgap quantum wells, a cell with 100‐period In_0.30GaAs(3.5 nm)/GaAs(2.7 nm)/GaAsP_0.40(3.0 nm) MQWs exhibited significantly improved performance, showing 16.2% AM 1.5 efficiency without an anti‐reflection coating, and a 70% internal quantum efficiency beyond the GaAs band edge. When compared with the GaAs control cell, the optimized cell showed an absolute enhancement in AM 1.5 efficiency, and 1.22 times higher efficiency with 38% current enhancement with an AM 1.5 cut‐off using a 665‐nm long‐pass filter, thus indicating the strong potential of MQW cells in Ge‐based 3‐J tandem devices. Copyright © 2013 John Wiley & Sons, Ltd.     

Multijunction solar cell model for translating I–V characteristics as a function of irradiance,spectrum, and cell temperature

We describe here a lumped diode model for concentrator multijunction solar cells, in which the temperature, irradiance and spectrum dependences are explicitly included. Moreover an experimental method based on it for the prediction of the I‐V curve under any irradiance‐spectrum‐temperature conditions from a single input measurement is proposed and applied to a set of commercial triple‐junction solar cells in order to demonstrate its validity. Component ‘isotype’ cells are used as reference cells for intensity and spectrum, sparing the measurement of light spectrum and cell spectral response. Finally, a mean RMS prediction error of 0.85% over a range of 100X‐25°C to 700X‐75°C is reported for the whole set when the model parameters inherent in the cell are assumed to be the same for every sample. If optimum parameters are extracted for every cell, the RMS error is reduced to 0.53%. Copyright © 2010 John Wiley & Sons, Ltd.     

Explanation for the dark I–V curve of III–V concentrator solar cells

The measurement of the dark I–V curve is one of the most straightforward methods for characterizing solar cells. Consequently, an accurate knowledge of its meaning is of high relevance for the comprehension and technological feedback of these devices. In this paper, an explanation of the dark I–V curve for concentrator III–V solar cells is presented using a 3D (three‐dimensional) model in order to provide a proper data fit that provides meaningful physical parameters that are also compatible and coherent with a data fit from illumination curves. The influence on the dark I–V curve of the most significant series resistance components of concentrator solar cells is also analysed concluding that only the vertical component as well as the front contact‐specific resistance can be assessable by means of this characterization method while both emitter and metal sheet resistances cannot be detected. For comparison purposes, the same experimental data have been fitted by means of a traditional two‐diode model showing that, although an accurate dark I–V curve fitting can be achieved, the extracted parameters are unable to reproduce illumination data since lumped models assume the same ohmic losses distribution for both dark and illumination conditions. Copyright © 2007 John Wiley & Sons, Ltd.     

InAs量子点尺寸和间距对GaInP/GaAs/GaInAs/Ge多结叠层电池的影响

A metamorphic GaInP/GaAs/GaInAs/Ge multi-junction solar cell with InAs quantum dots is investigated, and the analytical expression of the energy conversion efficiency on the multi-junction tandem solar cell is derived using the detailed balance principle and the Kronig-Penney model.The influences of interdot distance, quantum-dot size and the intermediate band location on the energy conversion efficiency are studied.This shows that the maximum efficiency,as a function of quantum-dot size and distance,is about 60.15%under the maximum concentration for only one InAs/GaAs subcell,and is even up to 39.69%for the overall cell.In addition,other efficiency factors such as current mismatch,the formation of a quasicontinuum conduction band and concentrated light are examined.     

Resolving spectral overlap issue of intermediate band solar cells using non‐uniform sub‐bandgap state filling

Intermediate band solar cell provides novel alternative to multi‐junction solar cell, but its efficiency is significantly degraded when spectral overlap exists between different absorption bands. Here, a scheme using non‐uniform sub‐bandgap state filling together with intermediate band transport is proposed to resolve the spectral overlap issue. On the basis of detailed balance calculation, spectrally decoupled devices using low–high state filling is shown to achieve 52.8% conversion efficiency when 4 eV spectral overlap is present between absorption coefficients of different bands, compared with baseline efficiency equal to 35.1% for conventional half‐filled intermediate band devices. If a base material without intermediate band is added to the two section low–high state filling devices, the efficiency is further increased to 61.5%, which approaches efficiency of 63.2% for intermediate band devices with no spectral overlap and 63.8% for unconstrained triple‐junction tandem cells. The junction thermalization loss associated with proposed new structures is shown to be equal to conventional half‐filled intermediate band devices. Copyright © 2013 John Wiley & Sons, Ltd.     

Numerical simulation of tunnel diodes for multi‐junction solar cells

This work presents efforts to simulate numerically the current voltage (IV) curve of a III–V based Esaki tunnel diode. Using a tunneling model, which takes into account the full nonlocality of the barrier, a good agreement between measured and simulated IV curves of a GaAs tunnel diode was achieved. The influence of a series resistance effect as well as the importance of trap assisted tunneling (TAT) could be shown. In addition, we present a two‐dimensional model of a III–V multi‐junction solar cell including the numerical model of the investigated Esaki tunnel diode. Copyright © 2008 John Wiley & Sons, Ltd.     

Degradation study of III–V solar cells for concentrator applications

AlGaAs/GaAs heteroface solar cells with a high aluminium content tend to degrade. The degradation mechanism has been examined and appropriate accelerated ageing procedures have been established. They effectively test the ruggedness of the device against oxidation. Changing the window layer material to (Al_xGa_1−x)_0.51In_0.49P with x = 0, 0.5 or 1 leads to stable devices. In addition, III–V tandem solar cells for concentrator applications were subjected to accelerated ageing tests. They proved to be robust against oxidation. The potential degradation due to the high current density involved in concentrator solar cells was assessed in preliminary experiments. Copyright © 2005 John Wiley & Sons, Ltd.     

Spectral response measurements of monolithic GaInP/Ga(In)As/Ge triple‐junction solar cells: Measurement artifacts and their explanation

Procedures for measuring the spectral response of multi‐junction cells in general require variation of the bias spectrum and voltage biasing. It is shown that a refined procedure including optimization of bias spectrum and voltage is necessary to minimize a measurement artifact, which appears if the subcell under test has non‐ideal properties, such as a low shunt resistance or a low reverse breakdown voltage. This measurement artifact is often observed on measuring the spectral response of the Ge bottom cell of GaInP/Ga(In)As/Ge triple‐junction cells. The main aspect of the measurement artifact is that the response of another subcell is simultaneously measured, while at the same time the signal of the Ge subcell is too low. Additionally, the shape of the spectral response curve is influenced under certain measurement conditions. In this paper the measurement artifact is thoroughly discussed by measurement results and simulation. Based on this analysis, a detailed procedure for the spectral response measurement of multi‐junction cells is developed, specially designed to minimize such measurement artifacts. Copyright © 2003 John Wiley & Sons, Ltd.     

Feasibility study of two‐terminal tandem solar cells integrated with smart stack,areal current matching,and low concentration

The “SMAC module” is a low‐cost, high‐efficiency photovoltaic module that integrates three techniques: a “SM art stack,” “A real current matching,” and “solar C oncentration.” This paper presents the result of a proof‐of‐concept study of the SMAC module conducted using device simulations and indoor experiments. The simulation results show that an SMAC module with a two‐terminal GaAs/Si tandem solar cell can achieve an efficiency of approximately 30% and superior electricity generation per unit top cell area. The performance of the GaAs/Si solar cell developed in this study is similar to that of a GaAs/InGaAsP solar cell under concentrated artificial sunlight and is consistent with the simulation results. © 2016 The Authors. Progress in Photovoltaics: Research and Applications published by John Wiley & Sons Ltd.     

Moth‐eye antireflection coating fabricated by nanoimprint lithography on 1 eV dilute nitride solar cell

We report on the performance of biomimicked antireflection coating applied to dilute nitride solar cell. The coating consists of nanostructures replicating the moth‐eye geometry and has been fabricated by nanoimprint lithography directly within the window layer covering the dilute nitride absorbing junction. The mean reflectivity within the spectral range of 320–1800 nm remains under 5% for incident angles up to 45°. The effect of the coating on the cell performance was assessed by measuring the current–voltage characteristics under simulated solar illumination. A clear performance increase was identified when comparing a solar cell with the moth‐eye coating with a solar cell having a standard SiN_x/SiO₂ coating. Copyright © 2012 John Wiley & Sons, Ltd.     

Characterization of tandem organic solar cells comprising subcells of identical absorber material

Recently organic tandem solar cells with record efficiency had been shown comprising identical absorber materials in both subcells. Such structures pose new challenges for characterization. The standard test methods for measuring spectral response of tandem solar cells can not be applied. The standard procedures demand for different bias illumination during measuring spectral response allowing to select the subcell being current limiting. With subcells comprising identical absorber materials, thus having identical absorption spectra, such a selection is not trivial. In this paper, we show that with the help of detailed optical simulations of such tandem organic solar cells, their characterization is possible, and we apply the proposed method to a sample structure. Copyright © 2014 John Wiley & Sons, Ltd.     

Adhesive bonding for mechanically stacked solar cells

Mechanically stacked solar cells formed using adhesive bonding are proposed as a route to high‐efficiency devices as they enable the combination of a wide range of materials and bandgaps. The concept involves adhesive bonding of subcells using polymeric materials widely used in semiconductor processing and outlines how the absolute efficiency can be maximised by optimisation of the adhesive layer thickness and optical matching of the adhesive layer with both the subcells and their anti‐reflection coatings. A dual‐junction, GaAs‐InGaAs, mechanically stacked solar cell is demonstrated using a benzocyclobutene adhesive layer with a measured PV conversion efficiency of 25.2% under 1‐sun AM1.5G conditions. Copyright © 2014 John Wiley & Sons, Ltd.