Front Surface Recombination Velocity Research Articles

An interdigitated back contact solar cell with high-current collection

Internal current-collection efficiencies greater than 90 percent and energy-conversion efficiencies of 18-percent at 30 suns have been measured on a laboratory version of the interdigitated back contact (IBC) solar cell. The quantum efficiency at 600 nm was greater than 90 percent which implies a minority carrier lifetime of greater than 350 µsec and a front surface recombination velocity of less than 30 cm/sec on the better devices. The processing steps on which the high-current collection in the IBC cell depends will be discussed.

The determination of transport parameters of minority carriers in n– p junctions by means of an electron microscope. Critique of recent developments

It has recently been shown that amplitude modulated electron beams provide a novel means for the determination of minority carrier lifetimes, diffusion lengths, etc. in n– p junctions. In this paper we show that (a) a recently published analysis based on a cylindrically symmetric configuration is incorrect, (b) the correct approach leads to a system of dual integral equations for which the formal solution is given, (c) in general, the short circuit current can only be determined by means of extensive computer calculations except in the case of large front surface recombination velocities, (d) the difficulties encountered with cylindrically symmetric configurations (circular ohmic contacts and the like) are completely avoided with a choice of a planar geometry since simple closed form expressions for the short circuit current are readily available in this case.

High-efficiency p+-n-n+ back-surface-field silicon solar cells

The design and fabrication of high-efficiency p+-n-n+ back-surface-field silicon solar cells are described. The fabrication process has been developed to yield maximum attainable carrier lifetimes (∼0.7 msec) in the base region of the cell, thereby allowing the back n-n+ junction to effectively enhance the cell performance. A surprising conclusion drawn from a study of the device physics supporting the experimental development of the cell is that the front-surface recombination velocity controls the recombination in the emitter. That is, the bulk p+ emitter is ’’transparent’’ to minority-carrier (electron) flow. The recognition of the significance of the front silicon surface has led to process modifications that result in improvements in both the short-circuit current density and the open-circuit voltage of the cell. With these improvements, the cells exhibit AMl conversion efficiencies of nearly 17%. The fabrication process is reliable and reproducible with exceptionally high yield.

Normal and Anomalous Dember Effect

Using Gärtner's equations, effect of sample thickness on the Cember potential (VD) for strong and weak absorption is discussed. It has been found that for strong absorption, as the thickness of the sample increases, VD tends to a saturation value which decreases as the front-surface recombination velocity increases. For weak absorption, initially the behavior of VD depends upon front and back surface recombination velocities, but for large thickness it becomes independent of them and increases linearly with thickness. When front-surface recombination velocity is large as compared to back surface recombination velocity, a sign reversal can take place. Expressions are obtained for VD(i) when surface recombination is dominant, or (ii) when bulk recombination is dominant, for weak and strong absorption in the case of thin and thick specimens. A physical explanation of anomalous Dember effect is given by considering the contribution of the concentration gradients set up by front, back, and bulk recombinations. It appears that simultaneous measurements of Hall angle and ratio of PME and Dember potential can be helpful in measuring electron-hole mobility ratio.

Spectral Distribution of the Photomagnetoelectric Effect in Ge: Experiment

The photomagnetoelectric effect has been studied in germanium as a function of the wavelength of incident radiation in the region from 0.5 to 2.0 microns. The dependence of both photoconductivity and photo-magnetoelectric (PME) response has been measured in various samples, using front and back surface recombination velocities and bulk recombination as parameters. It has been found that under certain conditions of bulk and surface recombination a reversal in sign for the PME response occurs over the frequency range studied. Sign reversals obtain at wavelength in the range from 1.55 to 1.85 microns, corresponding to partial optical transparency. The exact frequencies at which reversal occurs depend on the surface and bulk recombination rates, the condition being that the Dember field be zero corresponding to equal carrier concentrations on the front and back surfaces. Furthermore, the present observations are shown to be in good qualitative agreement with the theoretical work reported by G\"artner. Experimental procedures are described, and it is shown how this effect can be used to advantage in the study of surface recombination velocities in various environments.

Indium Antimonide Photoelectromagnetic Infrared Detector

The theory of operation, construction, and performance data of an infrared detector based upon the PEM effect in InSb are presented. Theoretical derivations of the spectral noise equivalent input, noise equivalent power, and responsivity are made which show the dependence of these figures of merit upon the electrical properties of the semiconductor, sample thickness, surface area, front and back surface recombination velocities, and magnetic induction. Performance of the detector operating at room temperature shows a spectral response extending to 7.5 microns and a response time of less than 1 microsecond. The noise equivalent input of a detector having a 0.71-mm2 sensitive area has a minimum value at 6.6 microns of 6.7×10−10 w.