Collection Efficiency Of Solar Cells Research Articles

Optical design and bandgap engineering in ultrathin multiple quantum well solar cell featuring photonic nanocavity

AbstractUltrathin solar cells are efficient and captivating devices with unique technological and scientific features in terms of minimal material consumption, fast fabrication processes, and good compatibility with semi‐transparent applications. Such photovoltaic (PV) technologies can enable effective synergy between optical and electronic confinements with large tuning capabilities of all the optoelectronic characteristics. In this work, the implications of the optical design and the bandgap engineering in ultrathin hydrogenated amorphous Si/Ge multiple quantum well (MQW) solar cells featuring photonic nanocavity are analyzed based on experimental measurements and optoelectronic modelling. By changing the period thicknesses and the positions of QWs inside the deep‐subwavelength nanophotonic resonator, the spatial and spectral distributions of the optical field and the local absorption are strongly affected. This leads to a modulation of the absorption resonance condition, the absorption edge and the resulting photocurrent outputs. Because of quantum confinement effect, the change of MQW configurations with different individual QW periods while keeping similar total thickness of about 20 nm alters both the bandgap energy and the band offset at the QW/barrier heterojunctions. This in turn controls the photovoltage as well as the carrier collection efficiency in solar cells. The highest open circuit voltage and fill factor values are achieved by employing MQW device configuration with 2.5 nm‐thin QWs. A record efficiency above 5.5% is reached for such emerging ultrathin Si/Ge MQW solar cell technology using thinner QWs with sufficient number, because of the optimum trade‐off between all the optoelectronic characteristic outputs. The presented design rules for opaque ultrathin solar cells with quantum‐confined nanostructures integrated in a photonic nanocavity can be generalized for the engineering of relevant multifunctional semitransparent PV devices.

Strontium Insertion in Methylammonium Lead Iodide: Long Charge Carrier Lifetime and High Fill-Factor Solar Cells.

The addition of Sr2+ in CH3 NH3 PbI3 perovskite films enhances the charge carrier collection efficiency of solar cells leading to very high fill factors, up to 85%. The charge carrier lifetime of Sr2+ -containing perovskites is in excess of 40 μs, longer than those reported for perovskite single crystals.

A simple model for voltage-dependent carrier collection efficiency in solar cells

Here, we present a simple, yet powerful model developed to calculate collection efficiencies for photogenerated charge carriers in semiconductor devices. In the model, we define a new collection length, which is calculated using transit times that depend on both drift and diffusion for charge collection. The model accounts for a non-uniform electric field determined by the depletion approximation. The voltage-dependence of the photocurrent can be calculated using appropriate given photogeneration rate, and our results were found to match very closely with those calculated using a finite difference solution of the full device equations. The transit time model reported here leads to fully analytical equations for the collection length. They allow one to quickly explore wide ranges of parameters including mobility, minority carrier lifetime, and device thickness. The simplicity of this model allows it to be applied to other devices architectures as well. In fact, the motivating factor to develop the model was to quickly illuminate the advantages and disadvantages of various device designs, from nanostructured photovoltaics to light trapping strategies, and plasmonic enhancements.

Focused microprobes of high energy ions — versatile analytical probes for surfaces, interfaces and devices

Today, more than 50 laboratories world-wide (including more than seven in Japan and three in Australia) apply focused microprobes of high-energy ions to a wide range of problems involving materials analysis. MeV ions penetrate deeply into matter and can therefore be used to probe and image surface and deeply buried structures. High sensitivity (ppm) trace analysis is possible from induced X-rays and backscattered particles can be used to measure stoichiometry and produce depth profiles down to about 10 μm below the specimen surface. The forward recoil of hydrogen displaced by heavier ions can be used to map the hydrogen distribution. Induced charge can be collected, so images of the charge collection efficiency of the specimen can be produced. This presentation reviews recent applications of these techniques to the study of a diverse range of materials and devices including synthetic diamond, filiform corrosion tracks on aluminium, trace element contamination and charge collection efficiency in solar cells and the hydrogen distribution in solar cell material.

The detailed balance principle and the reciprocity theorem between photocarrier collection and dark carrier distribution in solar cells

It is shown that a recently described general relationship between the local collection efficiency of solar cells and the dark carrier concentration (reciprocity theorem) directly follows from the principle of detailed balance. We derive the relationship for situations where transport of charge carriers occurs between discrete states as well as for the situation where electronic transport is described in terms of continuous functions. Combining both situations allows to extend the range of applicability of the reciprocity theorem to all types of solar cells, including, e.g., metal-insulator-semiconductor-type, electrochemical solar cells, as well as the inclusion of the impurity photovoltaic effect. We generalize the theorem further to situations where the occupation probability of electronic states is governed by Fermi–Dirac statistics instead of Boltzmann statistics as underlying preceding work. In such a situation the reciprocity theorem is restricted to small departures from equilibrium.

Solar-cell collection efficiency and its variation with voltage

Internal collection efficiency of solar cells is calculated as a function of depletion width, minority-carrier diffusion length, solar spectrum, and absorption coefficient. Particular attention is given to the variation of depletion width with voltage, which may reduce collection efficiency by a few percent at operating voltages and may be misinterpreted as a shunting effect. Calculations of collection-efficiency magnitude and voltage dependence, plus comparisons with experiment, are presented for several types of solar cells. Primary emphasis is placed on polycrystalline thin-film cells with relatively short diffusion lengths and hence potentially more pronounced collection efficiency effects.  

Preparation, morphology and physical properties of microrelief InP surfaces

The morphology of microrelief InP surfaces, prepared by anisotropic chemical etching without masking and the kinetics of this etching were studied. A statistical analysis of surface profiles shows that the surface studied has a Gaussian roughness distribution and the ratio of root-mean-square surface roughness δ to autocorrelation length σ is small and amounts to (2–5) × 10 −2 . For such a surface a reduction in specular reflection and increase in photocurrent of barrier structures have been observed. Such microrelief structures may be used to increase the collection efficiency of solar cells and the sensitivity of photodetectors.

Influence of diffusion of hot carriers on collection efficiency of solar cells:a-Si:H

The author has considered the effects of the optically generated hot carriers to explain the decrease in collection efficiency of solar cells at short wavelengths. The diffusion of these hot carriers to a contact or interface before thermalisation is the major factor for reduction of collection efficiency.

Hot carrier effects in the collection efficiency of solar cells: a-Si:H

Recent models for the falloff in spectral response at short wavelengths in low mobility solar cell materials have been based upon the diffusion of thermalized carriers against the electric field to high recombination contacts or interfaces. These models neglect the excess energy imparted to the carriers by short wavelength light, that enable them to easily move against the field while thermalizing. Calculations of the distance the carriers diffuse before losing their excess energy by phonon emission indicate that in the high absorption coefficient region of a-Si:H up to 50% of the carriers can reach a contact or interface before thermalizing.

Graded band-gap pAlxGa1–xAs-nGaAs heterojunction solar cells prepared by molecular beam epitaxy

pAlxGa1–xAs-nGaAs graded band-gap haterojunction solar cells have been prepared by molecular beam epitaxy (MBE). The graded band-gap structure effectively reduces the surface recombination and improves the collection efficiency of solar cells. MBE is the most adequate crystal-growth technique to fabricate this structure. It is easy to realize the profile for the most suitable electric-field distribution. For the alloy obtained by varying the temperatures of Al and Ga effusion cells linearly with time, the profile measured by Auger electron spectroscopy with ion-sputter etching has been obtained as an ideal one to drive carriers generated near the surface towards the junction. The measured spectral response of graded band-gap cells indicated the remarkable improvement of collection efficiency especially in the region of shorter wave-length in comparison with abrupt heterojunction cells with uniform compositions.

Considerations on the Solar Cell

The collection efficiency in solar cells is trealed by a new method in which all the effects of the solar spectrum and the absorption curve are contained in a single funetion readily obtained by numerical integration. The method is illustrated by a detailed study of the effects of surface recombination, body recombination and junction depth in silicon cells. The method is also generalized to include built-in electric fields, and calc Illations are given for silicon. Sufficiently strong fields 10 improve the collection efficiency markedly can be produced in some compollnd semiconductors from a gradient in the energy gap. A disclLssion is given of the dependence of the collection efficiency on the absorption curve of the semiconductor. It is shown that silicon has a very favorable absorption curve in camparison with GaAs or InP. Finally, a treatment is given of the minority carrier collection in a two-Junction cell, and calculations arc presented for silicon. It is concluded that this structure may be important for cells with high energy gaps and short lifetimes.