Evaluation Of Diffusion Length Research Articles

Thin Film Poly-Si Solar Cell on Glass Substrate Fabricated at Low Temperature

ABSTRACTThe performances of thin film poly-Si solar cells with a thickness of less than 5 μm on a glass substrate have been investigated. The cell of glass / back reflector / n-i-p type Si / ITO is well characterized by the structure of naturally _surface texture and enhanced absorption with a back reflector (STAR), where the active i-type poly-Si layer was fabricated by plasma chemical vapor deposition (CVD) at low temperature. The cell with a thickness of 2.0 μm demonstrated an intrinsic efficiency of 10.7% (aperture 10.1%), the open circuit voltage of 0.539 V and the short current density of 25.8 mA/cm2 as independently confirmed by Japan Quality Assurance. The optical confinement effect explains the excellent spectral response at long wavelength for our cells through the PCID analysis. The higher sensitivity at long-wavelength of our cell appeared in quantum efficiency curves is well correlated to the result of reflectance measurement. The efficiency of 9.3% cell with a thickness of 1.5 pm was proved to be entirely stable with respect to the lightsoaking. Based on the result of various evaluation of diffusion length, it is postulated that the low temperature poly-Si prepared by plasma CVD gives a device quality of poly-Si film.

Evaluation of diffusion length from a planar-collector-geometry electron-beam-induced current profile

The subject of this article is the determination of the minority carrier diffusion length L from a planar-collector-geometry electron-beam-induced current (EBIC) profile. Among extant techniques based on the analysis of a ln[IEBIC(x1)xα1] vs x1 plot, (x1 is beam-to-collector distance, and α is a constant), we find two major problems that demand immediate attention. First, the most widely used technique is found to be based on an invalid assumption, which results in the intrusion of large systematic errors into the values of L and surface recombination velocity sT. Therefore, this technique in its present form is no longer usable. Second, all these techniques are asymptotic (x1≳2L), a matter of great concern to experimentalists because large x1 means small signal-to-noise ratio and the proximity of adjacent active regions. We devise a nonasymptotic technique, based also on the analysis of a ln [IEBIC(x1)xα1] vs x1 plot, to evaluate L from a region close to the collector, as close as one-half, but no farther than two diffusion lengths away. This technique is applied to four separate sets of experimental EBIC profile data derived from three independent publications. In each case, L evaluated is either the same or nearly the same as the one obtained by curve fitting. We also show how this technique can be used to simultaneously evaluate L and η (=sTL/D,D=diffusion coefficient).

Evaluation of diffusion length at different excess carrier concentrations

The charge collection technique of scanning electron microscopy is widely used to measure the minority carrier diffusion length L in semiconductors. In order to obtain L, the continuity equation for the excess carrier concentration Δp must be solved with suitable boundary conditions. L is supposed to be constant as a function of the excess carrier concentration. This paper investigates the diffusion length in Si and GaAs in low and high injection regimes, where L is independent of the injection conditions. The results obtained are discussed on the basis of the Shockley-Read-Hall recombination model (W. Shockley and W. Read, Phys. Rev., 87 (1952) 835). The evaluation of the injection dose is also discussed. Indeed, this is very different for the semiconductors under investigation, due to their different electronic properties.

Evaluation of diffusion length and surface recombination velocity in semiconductor devices by the method of moments

The diffusion length and surface recombination velocity of the minority carriers are determined from electron beam induced current (EBIC) profiles on a semiconductor containing a barrier perpendicular to the scanned surface. The evaluation of both the parameters has been obtained by the procedure called ‘‘of the first moments,’’ due to Donolato [C. Donolato, Appl. Phys. Lett. 43, 120 (1983)], which is based on the calculation of the first moment about the origin of two induced current profiles. This analysis, based on an exact integral property of the EBIC scans, allows evaluation of the diffusion length and the surface recombination without fitting the experimental profiles. In addition, it is easy to handle and can also be readily applied to real devices.

Evaluation of diffusion length and surface-recombination velocity from a planar-collector-geometry electron-beam-induced current scan

For performing electron-beam-induced current (EBIC) measurements on sufficiently large samples, the use of a ‘‘planar-collector geometry’’ (i.e., with the collector covering part of the irradiated surface itself) is very attractive. However, the pertinent theoretical EBIC curves for finite surface-recombination velocities s have so far been lacking. This paper presents the complete theoretical expressions for arbitrary values of s and diffusion length L. Simple asymptotic solutions are given for point- and finite-size generation sources. Easy methods are developed to facilitate the application of these solutions in the practical evaluation of L and s from experimental EBIC curves. These methods are applied to experimental data available through the literature.