Six not so easy pieces in intermediate band solar cell research

The concept of "intermediate band solar cell" (IBSC) is, apparently, simple to grasp. However, since the idea was proposed, our understanding has improved and we feel now that we can explain better some concepts than we initially introduced. Clarifying these concepts is important, even if they are well-known for the advanced researcher, so that efforts can be driven in the right direction from start. The six pieces of this work are: Does a miniband need to be formed when the IBSC is implemented with quantum dots?; What are the problems of each of the main practical approaches that exist today? What are the simplest experimental techniques to demonstrate whether an IBSC is working as such or not? What is the issue with the absorption coefficient overlap? and Mott's transition? What the best system would be, if any?

A puzzling solar cell structure: an exercise to get insight on intermediate band solar cells

We introduce one trivial but puzzling solar cell structure. It consists of a high bandgap pn junction (top cell) grown on a substrate of lower bandgap. Let us assume, for example, that the bandgap of the top cell is 1.85 eV (Al 0.3Ga 0.7As) and the bandgap of the substrate is 1.42 eV (GaAs). Is the open-circuit of the top cell limited to 1.42 V or to 1.85 V? If the answer is ldquo1.85 Vrdquo we could then make the mind experiment in which we illuminate the cell with 1.5 eV photons (notice these photons would only be absorbed in the substrate). If we admit that these photons can generate photocurrent, then because we have also admitted that the voltage is limited to 1.85 V, it might be possible that the electron-hole pairs generated by these photons were extracted at 1.6 V for example. However, if we do so, the principles of thermodynamics could be violated because we would be extracting more energy from the photon than the energy it initially had. How can we then solve this puzzle?

Two-photon photocurrent and voltage up-conversion in a quantum dot intermediate band solar cell

It has been proposed that the use of self-assembled quantum dot (QD) arrays can break the Shockley-Queisser efficiency limit by extending the absorption of solar cells into the low-energy photon range while preserving their output voltage. This would be possible if the infrared photons are absorbed in the two sub-bandgap QD transitions simultaneously and the energy of two photons is added up to produce one single electron-hole pair, as described by the intermediate band model. Here, we present an InAs/Al 0.25Ga 0.75As QD solar cell that exhibits such electrical up-conversion of low-energy photons. When the device is monochromatically illuminated with 1.32 eV photons, open-circuit voltages as high as 1.58 V are measured (for a total gap of 1.8 eV). Moreover, the photocurrent produced by illumination with photons exciting the valence band to intermediate band (VB-IB) and the intermediate band to conduction band (IB-CB) transitions can be both spectrally resolved. The first corresponds to the QD inter-band transition and is observable for photons of energy mayor que 1 eV, and the later corresponds to the QD intra-band transition and peaks around 0.5 eV. The voltage up-conversion process reported here for the first time is the key to the use of the low-energy end of the solar spectrum to increase the conversion efficiency, and not only the photocurrent, of single-junction photovoltaic devices. In spite of the low absorption threshold measured in our devices - 0.25 eV - we report open-circuit voltages at room temperature as high as 1.12 V under concentrated broadband illumination.

Spectral study and classification of worldwide locations considering several multijunction solar cell technologies

Multi-junction solar cells are widely used in high-concentration photovoltaic systems (HCPV) attaining the highest efficiencies in photovoltaic energy generation. This technology is more dependent on the spectral variations of the impinging Direct Normal Irradiance (DNI) than conventional photovoltaics based on silicon solar cells and consequently demands a deeper knowledge of the solar resource characteristics. This article explores the capabilities of spectral indexes, namely, spectral matching ratios (SMR), to spectrally characterize the annual irradiation reaching a particular location on the Earth and to provide the necessary information for the spectral optimization of a MJ solar cell in that location as a starting point for CPV module spectral tuning. Additionally, the relationship between such indexes and the atmosphere parameters, such as the aerosol optical depth (AOD), precipitable water (PW), and air mass (AM), is discussed using radiative transfer models such as SMARTS to generate the spectrally-resolved DNI. The network of ground-based sun and sky-scanning radiometers AERONET (AErosol RObotic NETwork) is exploited to obtain the atmosphere parameters for a selected bunch of 34 sites worldwide. Finally, the SMR indexes are obtained for every location, and a comparative analysis is carried out for four architectures of triple junction solar cells, covering both lattice match and metamorphic technologies. The differences found among cell technologies are much less significant than among locations.

Why can't I measure the external quantum efficiency of the Ge subcell of my multijunction solar cell?

The measurement of the external quantum efficiency (EQE) of low bandgap subcells in a multijunction solar cell can be sometimes problematic. In particular, this paper describes a set of cases where the EQE of a Ge subcell in a conventional GaInP/GaInAs/Ge triple-junction solar cell cannot be fully measured. We describe the way to identify each case by tracing the I-V curve under the same light-bias conditions applied for the EQE measurement, together with the strategies that could be implemented to attain the best possible measurement of the EQE of the Ge subcell.

A numerical study of Bi-periodic binary diffraction gratings for solar cell applications

In this paper, a numerical study is made of simple bi-periodic binary diffraction gratings for solar cell applications. The gratings consist of hexagonal arrays of elliptical towers and wells etched directly into the solar cell substrate. The gratings are applied to two distinct solar cell technologies: a quantum dot intermediate band solar cell (QD-IBSC) and a crystalline silicon solar cell (SSC). In each case, the expected photocurrent increase due to the presence of the grating is calculated assuming AM1.5D illumination. For each technology, the grating period, well/tower depth and well/tower radii are optimised to maximise the photocurrent. The optimum parameters are presented. Results are presented for QD-IBSCs with a range of quantum dot layers and for SSCs with a range of thicknesses. For the QD-IBSC, it is found that the optimised grating leads to an absorption enhancement above that calculated for an ideally Lambertian scatterer for cells with less than 70 quantum dot layers. In a QD-IBSC with 50 quantum dot layers equipped with the optimum grating, the weak intermediate band to conduction band transition absorbs roughly half the photons in the corresponding sub-range of the AM1.5D spectrum. For the SSC, it is found that the optimised grating leads to an absorption enhancement above that calculated for an ideally Lambertian scatterer for cells with thicknesses of 10 ?m or greater. A 20um thick SSC equipped with the optimised grating leads to an absorption enhancement above that of a 200um thick SSC equipped with a planar back reflector.

Low-injection behaviour of a solar cell with a metallic grating back-reflector

Recent studies have dealt with the possibility of increasing light absorption by using the so-called electric field enhancement taking place within the grooves of metallic gratings. In order to evaluate the potential improvements derived from the absorption increase, we employ a simplified model to analyze the low-injection behaviour of a solar cell with a metallic grating back-reflector.

Enhanced iron gettering by short, optimized low-temperature annealing after phosphorus emitter diffusion for industrial silicon solar cell processing

The introduction of a low-temperature (LT) tail after P emitter diffusion was shown to lead to considerable improvements in electron lifetime and solar cell performance by different researchers. So far, the drawback of the investigated extended gettering treatments has been the lack of knowledge about optimum annealing times and temperatures and the important increase in processing time. In this manuscript, we calculate optimum annealing temperatures of Fe-contaminated Si wafers for different annealing durations. Subsequently, it is shown theoretically and experimentally that a relatively short LT tail of 15 min can lead to a significant reduction of interstitial Fe and an increase in electron lifetime. Finally, we calculate the potential improvement of solar cell efficiency when such a short-tail extended P diffusion gettering is included in an industrial fabrication process.

Mono-crystalline silicon wafers manufactured by casting methods: Optoelectronic, structural and solar cell study

1) Introduction 2) The Quasi-mono, pseudo-mono, mono-like ERA. 3) Manufacturing mono-cast ingots: COST (seed recycling) 4) Summary and findings 5) Current status at DCWafers

TCAD for PV: a fast method for accurately modelling metal impurity evolution during solar cell processing

Coupled device and process silumation tools, collectively known as technology computer-aided design (TCAD), have been used in the integrated circuit industry for over 30 years. These tools allow researchers to quickly converge on optimized devide designs and manufacturing processes with minimal experimental expenditures. The PV industry has been slower to adopt these tools, but is quickly developing competency in using them. This paper introduces a predictive defect engineering paradigm and simulation tool, while demonstrating its effectiveness at increasing the performance and throughput of current industrial processes. the impurity-to-efficiency (I2E) simulator is a coupled process and device simulation tool that links wafer material purity, processing parameters and cell desigh to device performance. The tool has been validated with experimental data and used successfully with partners in industry. The simulator has also been deployed in a free web-accessible applet, which is available for use by the industrial and academic communities.

The intermediate band approach in the third solar cell generation context

Hydrogenated amorphous silicon thin films were deposited using a high pressure sputtering (HPS) system. In this work, we have studied the composition and optical properties of the films (band-gap, absorption coefficient), and their dependence with the deposition parameters. For films deposited at high pressure (1 mbar), composition measurements show a critical dependence of the purity of the films with the RF power. Films manufactured with RF-power above 80W exhibit good properties for future application, similar to the films deposited by CVD (Chemical Vapor Deposition) for hydrogenated amorphous silicon.

Intermediate Band Solar Cell with Extreme Broadband Spectrum Quantum Efficiency

We report, for the first time, about an intermediate band solar cell implemented with InAs/AlGaAs quantum dots whose photoresponse expands from 250 to ~ 6000  nm. To our knowledge, this is the broadest quantum efficiency reported to date for a solar cell and demonstrates that the intermediate band solar cell is capable of producing photocurrent when illuminated with photons whose energy equals the energy of the lowest band gap. We show experimental evidence indicating that this result is in agreement with the theory of the intermediate band solar cell, according to which the generation recombination between the intermediate band and the valence band makes this photocurrent detectable. © 2015 American Physical Society

Building intuition of iron evolution during solar cell processing through analysis of different process models

An important aspect of Process Simulators for photovoltaics is prediction of defect evolution during device fabrication. Over the last twenty years, these tools have accelerated process optimization, and several Process Simulators for iron, a ubiquitous and deleterious impurity in silicon, have been developed. The diversity of these tools can make it difficult to build intuition about the physics governing iron behavior during processing. Thus, in one unified software environment and using self-consistent terminology, we combine and describe three of these Simulators. We vary structural defect distribution and iron precipitation equations to create eight distinct Models, which we then use to simulate different stages of processing. We find that the structural defect distribution influences the final interstitial iron concentration ([Fe-i]) more strongly than the iron precipitation equations. We identify two regimes of iron behavior: (1) diffusivity-limited, in which iron evolution is kinetically limited and bulk [Fe-i] predictions can vary by an order of magnitude or more, and (2) solubility-limited, in which iron evolution is near thermodynamic equilibrium and the Models yield similar results. This rigorous analysis provides new intuition that can inform Process Simulation, material, and process development, and it enables scientists and engineers to choose an appropriate level of Model complexity based on wafer type and quality, processing conditions, and available computation time.

Radiative thermal escape in intermediate band solar cells

To achieve high efficiency, the intermediate band (IB) solar cell must generate photocurrent from sub-bandgap photons at a voltage higher than that of a single contributing sub-bandgap photon. To achieve the latter, it is necessary that the IB levels be properly isolated from the valence and conduction bands. We prove that this is not the case for IB cells formed with the confined levels of InAs quantum dots (QDs) in GaAs grown so far due to the strong density of internal thermal photons at the transition energies involved. To counteract this, the QD must be smaller.

Impact of MOVPE Environment on Silicon Substrates for III-V-on-Si Multijunction Solar Cells

With the final goal of integrating III-V materials to silicon for tandem solar cells, the influence of the metal-organic vapor phase epitaxy (MOVPE) environment on the minority carrier properties of silicon wafers has been evaluated. These properties will essentially determine the photovoltaic performance of the bottom cell in a III-V-on-Si tandem solar cell device. A comparison of the base minority carrier lifetimes obtained for different thermal processes carried out in a MOVPE reactor on Czochralski silicon wafers has been carried out. The effect of the formation of the emitter by phosphorus diffusion has also been evaluated.