R.J. Schwartz

Contactless nondestructive measurement of bulk and surface recombination using frequency-modulated free carrier absorption

Abstract A measurement procedure is described which allows the contactless measurement of bulk lifetime and surface recombination. The procedure uses the the free-carrier absorption of a long-wavelength laser beam by a modulated free-carrier wave to measure and separate the bulk recombination from the surface recombination. The dependence of the absorption on the modulation frequency is used to accomplish the separation. Limitations of the technique are also discussed.

Acoustic surface wave interaction charge‐coupled device

The observation of the synchronous drag of minority carriers by the electric field associated with an acoustic surface wave is reported. This phenomenon is exploited in a new charge‐coupled device (CCD) which uses a traveling piezoelectric surface acoustic wave in place of the gate structure of a conventional CCD. The surface wave interaction CCD (SWICC) should have several potential advantages over normal CCD’s, including increased information density, higher speed, and lower power consumption.

Efficiency of multijunction photovoltaic systems

In this paper, an expression for the PV system efficiency is derived that can be used in conjunction with measured device performance and detailed numerical modeling to analyze PV system performance. Such an analysis will help identify design trade-offs and also help to identify which system and cell design changes will be of greatest benefit to the enhancement of PV system performance.

Design of a GaInP/GaAs tandem solar cell for maximum daily, monthly, and yearly energy output

Solar concentrator cells are typically designed for maximum efficiency under the AM1.5d standard spectrum. While this methodology does allow for a direct comparison of cells produced by various laboratories, it does not guarantee maximum daily, monthly, or yearly energy production, as the relative distribution of spectral energy changes throughout the day and year. It has been suggested that achieving this goal requires designing under a nonstandard spectrum. In this work, a GaInP/GaAs tandem solar cell is designed for maximum energy production by optimizing for a set of geographically-dependent solar spectra using detailed numerical models. The optimization procedure focuses on finding the best combination of GaInP bandgap and GaInP and GaAs sub-cell absorber layer thicknesses. It is shown that optimizing for the AM1.5d standard spectrum produces nearly maximum yearly energy. This result simplifies the design of a dual-junction device considerably, is independent of the optical concentration up to at least 500 suns, and holds for a wide range of geographic locations. The simulation results are compared to those obtained using a more traditional, ideal-diode model.

Peak efficiency of multijunction photovoltaic systems

The maximum efficiency of multijunction photovoltaic systems is determined for independently connected solar cells that are optically in series. The maximum efficiency is computed as a function of solar concentration using both the Shockley-Queisser detailed balance radiative limit for the reverse saturation current density and a simple empirical expression for the reverse saturation current density obtained from published “state-of-the-art” solar cells performance characteristics. It is shown that there are an optimal number of junctions for peak efficiency after which the system efficiency will decrease as the number of junctions increases.

Photovoltaic power generation

It is noted that the wide acceptance and utilization of the photovoltaic generation of electrical power largely hinges on the ability to decrease the cost of production of solar cells and panels while at the same time increasing their conversion efficiency. A short tutorial is given on solar cells that are presently being investigated for cost reduction and efficiency improvement. The many types of cells are under investigation as well as the wide range of materials are examined. Efficiency improvements that have been achieved for many types of cells that are potentially low cost in large-volume production are described. >

The measurement of bulk and surface recombination by means of modulated free carrier absorption

A measurement technique is described which makes it possible to monitor the lifetime and surface recombination velocity of the starting wafer as well as a partially or completely processed wafer in the Si solar-cell fabrication process. This technique uses an infrared laser to monitor the carrier concentration via free carrier absorption while periodically exciting free carriers by means of a visible laser. The excited laser is sinusoidally modulated with an electro-optical modulator at frequencies of 100 Hz to 100 kHz. The free carriers generated by the exciter beam attenuate the probe beam, and the resultant output is detected with a phase-sensitive lock-in amplifier. The quantities measured are the amplitude and the phase of the detected signal relative to the exciter beam. The measured data are then fitted to theoretical expressions, and the bulk lifetime and surface recombination velocities are determined. The amplitude and phase are independent quantities, and the computed values from the two sets of data provide a self-consistency test.<<ETX>>

Estimating Saturation Current Based on Junction Temperature and Bandgap

This paper describes a method for estimating a solar cells reverse saturation current density, JO(T,EG(300 K)), based upon the bandgap energy at 300 K and the junction operating temperature. In an easy to use functional form, the solar cell performance can be calculated without knowing material specific parameters. This method can be used to optimize the bandgaps for highest conversion efficiency, during the initial design phase of a multijunction system.

The effects of high level injection on the performance of high intensity, high efficiency silicon solar cells

SCAP2D, a two-dimensional numerical code for silicon solar cells, was used to explore the mechanisms controlling performance in high-efficiency cells, focusing on the point contact cell for which record efficiencies have been reported. It is demonstrated that accurate predictions of the intensity dependence of the short circuit current and open-circuit voltage of these cells can be obtained by the inclusion of free-carrier bandgap narrowing in the base and the reduction of the minority-carrier mobility in the base to approximately one-half the conventional value of D.M. Caughy and R.E. Thomas (1977). In addition, the value for the ambipolar Auger coefficient obtained by J. Dziewior and W. Schmid (1977) is retained, despite measurements which suggest it may be a factor of four larger. Bandgap narrowing due to high-injection conditions in the base may be another important limiting factor in silicon solar cells.<<ETX>>

Numerical modeling of loss mechanisms resulting from the distributed emitter effect in concentrator solar cells

Power loss associated with lateral current flow in solar cell emitter layers is known to cause fill-factor degradation. In this work, a unique approach to modeling the lateral current flow in solar cell emitter layers is developed using MATLAB™ and current-generation models based upon one-dimensional detailed numerical cell models. An example is employed to illustrate the high degree of accuracy of this approach and that in addition to joule losses in the emitter layer and conducting electrodes, a third loss mechanism, which is the result of the potential gradient across the cell surface, also plays an important role. The relationship between these loss mechanisms is explored over a wide range of concentrations under both uniform and non-uniform illumination for the example cell.