Helmut Mackel

A contactless photoconductance technique to evaluate the quantum efficiency of solar cell emitters

A new technique, the spectral response of the steady-state photoconductance, is proposed for solar cell characterisation in research and development. The emphasis of this paper is on the evaluation of the carrier collection efficiency of the emitter region based on a simple, two-wavelength approach. We show that in addition to the well-established determination of the wafer recombination properties that results from a long-wavelength photoconductance measurement, detailed emitter quantum-efficiency information can be obtained by performing a second measurement with short-wavelength light. The method is experimentally demonstrated with silicon solar cell precursors having emitters with markedly different levels of surface and bulk recombination losses. The main advantages of the spectral photoconductance technique are that it is fast, contactless, and can be used immediately after junction formation before metallisation; these properties make it very appropriate for routine monitoring of the emitter region, including in-line process control.

Effect of gettered iron on recombination in diffused regions of crystalline silicon wafers

Crystalline silicon wafers, intentionally precontaminated with iron, were diffused with phosphorus and boron, and the recombination properties of the bulk and diffused regions extracted from injection-dependent carrier lifetime measurements. While the phosphorus diffusions were found to getter more than 99% of the iron from the bulk, the boron diffusions only extracted 65% in the best case. The presence of this gettered iron caused significant additional recombination in the boron diffused layers, while it had no measurable impact on the phosphorus diffused regions. This may be a consequence of the small capture cross section for holes of interstitial iron.

Simple Data Acquisition of the Current-Voltage and Illumination-Voltage Curves of Solar Cells1

Three electronic circuits that facilitate the measurement of I-V characteristics of solar cells are described and analyzed. The first circuit enables the measurement of the one-sun illumination and dark I-V curve in a steady state fashion. The second circuit is based on a large capacitor, which, when discharged, forces the cell to progress in a quasi-static (QSS) transition through different voltage and current states. The circuit can also be used to set transient conditions and characterize the cells open circuit voltage decay (OCVD) and series resistance. The third circuit is a logarithmic converter, which allows, combined with a photodiode, for the detection of light intensity over eight decades of magnitude. It facilitates the measurement of wide ranges of QSS Suns-Voc in a very inexpensive way. The accuracy of these circuits and measurement methods has been checked on silicon solar cells, leading to an excellent agreement with established techniques

FTIR Analysis of Microwave-Excited PECVD Silicon Nitride Layers

This paper presents infrared absorption (FTIR) measurements of SiN layers and correlates them to their ability to passivate silicon wafer surfaces. The best passivation was obtained for films having a nitrogen to silicon atomic composition in the proximity of N/Si=1.2, together with a high concentration of Si-N bonds (approximately 1times1023 cm-3) and a refractive index in the vicinity of n=2. The total hydrogen concentration in these films remained practically unchanged after a high temperature firing cycle, which indicates a good thermal stability. In contrast, silicon rich layers (higher refractive index and lower Si-N bond density) suffered a large reduction in the total hydrogen content. These results support the suggestion by ECN researchers that the Si-N bond concentration can be a good indicator of the ultimate electronic impact of the SiN layers

Generalized models of the spectral response of the voltage for the extraction of recombination parameters in silicon devices

Analytical models of the spectral response of the voltage of silicon devices have been generalized using the concept of the internal quantum efficiency of the semiconductor. This allows the extension of models used in the analysis of the internal quantum efficiency to the spectral response of the voltage. Existing models for the spectral response of the voltage that are largely employed in the surface photovoltage technique are shown to be special cases that approximate the internal quantum efficiency. A more sophisticated model of the internal quantum efficiency, the model of Isenberg, and a model that allows the analysis of the internal quantum efficiency of rear-illuminated devices have been adapted to the spectral response of the voltage. This paves the way to analyze solar cells, Schottky devices, or chemically treated silicon wafers independently of the light intensity and using front or rear illumination. The models have been validated by comparing the analysis of the spectral response of the short...

Unveiling the Injection-Dependence of the Diffusion Length Via the Spectral Response of the Voltage of Silicon Solar Cells

The minority carrier diffusion length as a function of carrier density has been extracted from the spectral response of the open-circuit voltage of silicon solar cells, which has been measured with the quasi-steady-state spectral photovoltage technique, QSSVoc -lambda. The method can determine a relative internal quantum efficiency for a broad range of voltages, and therefore, of carrier injection levels. From the internal quantum efficiency, the diffusion length is derived using standard procedures. High-efficiency solar cells fabricated on p-type multicrystalline silicon of four different resistivities have been used in this investigation. While the analysis of the spectral response of the short-circuit current did not show any injection dependence of the diffusion length, the analysis of the spectral response of the voltage, on the contrary, unveiled that the diffusion length can have a strong dependence on carrier density, even at very low injection levels

Recombination in n- and p-Type Silicon Emitters Contaminated with Iron

Crystalline silicon wafers containing deliberately introduced Fe were subject to phosphorus and boron diffusions, in order to examine the effect of gettered Fe on the recombination properties of the diffused emitter regions. For the case of boron diffusions, the presence of gettered Fe caused increased recombination in the emitter region, while for phosphorus diffusions there was no noticeable effect. This occurred despite the fact that the boron diffusions were much less effective at gettering Fe from the wafer. The results can apparently be explained by the reduced recombination activity of the Fe-related centres in n-type silicon compared to p-type

Open-circuit voltage quantum efficiency technique for defect spectroscopy in semiconductors

The temperature-dependent quantum efficiency of the open-circuit voltage is introduced for defect characterization in semiconductors. This technique measures the spectral response of the open-circuit voltage of a diode at different temperatures. The diffusion length is extracted from the spectral photovoltage and converted into carrier lifetime. This results in temperature-dependent lifetime curves that can be analyzed with the Shockley–Read–Hall model. The method allows defect analysis to be performed as soon as a junction is formed in the device and is also applicable to solar cells and Schottky diodes. The determination of the lifetime via the spectral response avoids trapping effects that commonly hamper other lifetime spectroscopy techniques. Examples of the application of the technique are given, showing good agreement with the temperature-dependent quantum efficiency of the short-circuit current. The results are consistent with temperature-dependent lifetime spectroscopy reported in the literature.