Current Transport Efficiency Research Articles

Current transport efficiency analysis of multijunction solar cells by luminescence imaging

AbstractMultijunction solar cells are currently leading in efficiency among all types of solar cells. However, the efficiency of those cells under concentrated illumination is limited by the trade‐off between shadowing by the contact grid and series resistances. In this paper, we provide a method using luminescence to image the current transport efficiency for a precise and rapid measurement of the resistance effect. Light beam induced current (LBIC) measurement is used for the first time to calculate current transport efficiency in comparison with that of the luminescence method. Profiles of transport efficiency in both means are in good agreement. Furthermore, a quasi‐3‐D simulation using SPICE is used to explain the transport efficiency image. We find that the carrier transport efficiency distribution is primarily influenced by the perimeter recombination of the middle cell.

Spatially Resolved Identification of Shunt Defects in Thin Film Solar Cells via Current Transport Efficiency Imaging Combined with 3D Finite Element Modeling

Inhomogeneously distributed shunt defects significantly reduce the performance of thin film solar cells. The existing uses of simple equivalent circuit models and lumped cell‐level parameters are insufficient to understand the behaviors of different shunting types. This study demonstrates how the reciprocity theorem, which bridges the relationship between the terminal differential changes and local responses of a solar cell, for the current transport efficiency can be used to spatially identify ohmic and nonohmic shunt defects exemplified in CdTe solar cells. Differential electroluminescence imaging and three‐dimensional finite element modeling are used to determine the current transport efficiency images. Due to the specific local differential conductance behaviors, the application of current transport efficiency imaging for the detailed analysis of different shunt defects is successfully verified in both experimental and simulation methods. In addition, the influences of two types of shunt defects on the electrical potentials and current flow of a cell are discussed. The modeling results indicate that a nonohmic shunt defect with a larger junction voltage dip and leakage current is more detrimental than an ohmic shunt defect within the cell.

Determination of Current Transport Efficiency Map by Optoelectronic Reciprocity Relation in CdTe Solar Cells

Transport efficiency (f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> ) maps of photon-generated current from the junction to the terminal are demonstrated in CdTe solar cells, based on luminescence images and a recent reciprocity theorem. Current transport efficiency maps directly reveal the local electrical losses distribution at different working points, which help to understand the main detrimental mechanisms in CdTe cells. Electroluminescence and photoluminescence imaging measurements are used to distinguish and discuss the localized defects as well. The validity of this luminescence-derived method is successfully verified by comparison with simulation and standard electrical measurements. The diode-like shunts, which rob currents from nearby areas, cause significant decreases of local f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> , while the effect of series resistance depends on the terminal current and dominates the additional applied voltage when operating over VOC.

Experimental Demonstration of Optically Determined Solar Cell Current Transport Efficiency Map

A recently suggested reciprocity relation states that the current transport efficiency from the junction to the cell terminal can be determined by differentiating luminescence images with respect to the terminal voltage. The validity of this relation is shown experimentally in this paper, by comparison with simultaneously measured electrical currents and simulations. Moreover, we verify that the method is applicable under various light concentrations and applied voltages, which allows us to investigate the cell in relevant conditions. Results evidence several kinds of series resistances affecting the current transport efficiencies. We show that the relative contribution of those different resistances to the loss in current collection is a function of the illumination intensity.

From junction to terminal: Extended reciprocity relations in solar cell operation

The electronic and optoelectronic reciprocity relations for solar cells in their differential forms are extended to account for series-resistance effects. They are expressed in terms of the terminal current and voltage at any device-operating point. Three new reciprocity relations are derived for the carrier collection efficiency density, current transport efficiency, and the current conversion efficiency, whose definition and use are discussed. The potential usefulness of these relations in electroluminescence imaging, spectral response measurements, and solar cell modeling is outlined.

Magnetic Bending of Laser Guided Electron Beams

Experiments have been performed to study magnetic turning of laser-guided electron beams. This technique is of interest for beam transport in circular high-current electron beam accelerators such as Sandia's recirculating linac. A 1-MeV, 2-kA, 50-ns, electron beam was turned through a 45° angle with > 90% current transport efficiency. The 45° bend was accomplished by switching the beam between two laser ionized guide channels which intersected in the center of a 680 Gauss turning magnet. The beam radius was observed to increase as a result of turning by the uniform field in agreement with single particle simulations. These simulations predict much smaller emittance growth for optimized sector magnet bending.

Radial oscillations of a relativistic electron beam in an accelerating gap

Radial oscillations of hollow, relativistic electron beams have been observed on Sandia’s RADLAC I, a multistage high-current linear induction accelerator. These oscillations, which arise because the presence of the accelerating gaps prevents radial force balance, can be responsible for poor beam quality and low current-transport efficiency. The method of magnetic field shaping to suppress oscillations is investigated. Two analytical models are formulated to derive nonuniform fields necessary to maintain force balance in a single-gap geometry. Particle code simulations have been made to test beam propagation in contoured fields. Field shaping does suppress oscillations and is not sensitive to fluctuations in beam and gap parameters.

Multistage linear electron acceleration using pulsed transmission lines

A four-stage linear electron accelerator is described which uses pulsed radial transmission lines as the basic accelerating units. An annular electron beam produced by a foilless diode is guided through the accelerator by a strong axial magnetic field. Synchronous firing of the injector and the acccelerating modules is accomplished with self-breaking oil switches. The device has accelerated beam currents of 25 kA to kinetic energies of 9 MV, with 90% current transport efficiency. The average accelerating gradient is 3 MV/m.