A.B. Sproul

Aluminum-induced crystallization of amorphous silicon on glass substrates above and below the eutectic temperature

The achievement of high-quality continuous polycrystalline silicon (poly-Si) layers onto glass substrates by using aluminum-induced crystallization is reported. The crystallization behavior of dc sputtered amorphous silicon on glass induced by an Al interface layer has been investigated above and below the eutectic temperature of 577 °C. Secondary electron micrographs in combination with energy-dispersive x-ray microanalysis show that annealing below this temperature leads to the juxtaposed Al and Si layers exchanging places. The newly formed poly-Si layer is fully crystallized and of good crystalline quality, according to Raman spectroscopy and transmission electron microscopy investigations. At 500 °C, the time needed to crystallize a 500-nm-thick Si layer is as short as 30 min. By annealing above the eutectic temperatures, layer exchange is not as pronounced and the newly formed Al layer is found to contain a network of crystallized Si.

Improved value for the silicon intrinsic carrier concentration from 275 to 375 K

A recent review has suggested that the commonly cited value of 1.45×1010 cm−3 for the silicon intrinsic carrier concentration at 300 K is inconsistent with the best experimental data. An alternate value of 1.08×1010 cm−3 was proposed. From measurements of the current‐voltage characteristics of p‐n junction diodes, this paper reports a new and more accurate determination of this parameter over the 275–375 K temperature range which supports such lower values. The one‐standard‐deviation uncertainty in the measurement of the intrinsic carrier concentration is estimated to lie in the 3%–4% range, about three times smaller than previous measurements at these temperatures. Additionally, this technique provides information on the minority carrier electron diffusivity in silicon.

Injection‐level‐dependent recombination velocities at the Si‐SiO2 interface for various dopant concentrations

Measurements of the dependence of the effective surface recombination velocity Seff at the Si‐SiO2 interface on light‐induced carrier concentration are presented. Contactless lifetime measurements by modulated free‐carrier IR absorption with additional bias light and by photoconductance decay measurements of samples immersed in HF were used to extract Seff for varying illumination levels. For various dopant concentrations the measured surface recombination velocities are compared with theoretical predictions calculated using an extended Shockley–Read–Hall formalism for Si‐SiO2 interfaces. Good agreement was obtained. The consequences for solar cell performance and lifetime measurement techniques are discussed.

Accurate determination of minority carrier‐ and lattice scattering‐mobility in silicon from photoconductance decay

Accurate measurements of the minority carrier‐ and lattice scattering‐diffusion constant and mobility in float zone silicon have been determined using photoconductance decay. For the more lightly doped specimens our results indicate slightly higher mobility than published majority carrier values. This is attributed to purer samples which allow a more accurate measurement of the lattice scattering mobility. In the dopant range 1015–1017 cm−3 the results for both electrons and holes are, within experimental error, equal to the majority carrier values. Unlike other methods this technique is a very direct measurement of the diffusion constant as only the thickness and decay time of the wafer need to be determined. The method is estimated to have a one standard deviation uncertainty of 2%–4% which is comparable to the best accuracy previously obtained for majority carrier measurements.

Intrinsic carrier concentration and minority‐carrier mobility of silicon from 77 to 300 K

A considerable improvement in the accuracy of the measurement of the intrinsic carrier concentration in silicon near room temperature has recently been reported. This was achieved by the accurate analysis of minority‐carrier current flow in specially fabricated p‐n junction devices. In this paper this technique has been extended to measurements down to 77 K. A further improvement of the technique has been the simultaneous measurement of the minority‐carrier electron mobility utilizing open‐circuit voltage decay measurements.

Improved modeling of grain boundary recombination in bulk and p‐n junction regions of polycrystalline silicon solar cells

This article provides a theoretical investigation of recombination at grain boundaries in both bulk and p‐n junction regions of silicon solar cells. Previous models of grain boundaries and grain boundary properties are reviewed. A two dimensional numerical model of grain boundary recombination is presented. This numerical model is compared to existing analytical models of grain boundary recombination within both bulk and p‐n junction regions of silicon solar cells. This analysis shows that, under some conditions, existing models poorly predict the recombination current at grain boundaries. Within bulk regions of a device, the effective surface recombination velocity at grain boundaries is overestimated in cases where the region around the grain boundary is not fully depleted of majority carriers. For vertical grain boundaries (columnar grains), existing models are shown to underestimate the recombination current within p‐n junction depletion regions. This current has an ideality factor of about 1.8. An im...

18.7% efficient laser-doped solar cell on p-type Czochralski silicon

The use of laser doping in solar cell fabrication has received increased attention in recent years, especially due to its ability to form a selective emitter without subjecting the wafer to prolonged high-temperature processes. At the University of New South Wales, a laser doping method was developed that combines the formation of the selective emitter with a self-aligned metallization pattern. This letter reports 18.7% efficient laser-doped solar cells, fabricated on large area commercial-grade p-type Czochralski silicon, and analyzes the loss mechanisms.

Impedance analysis : A powerful method for the determination of the doping concentration and built-in potential of nonideal semiconductor p-n diodes

An impedance analysis method is introduced that enables the reliable determination of the doping concentration and the built-in potential of nonideal semiconductor p‐n diodes featuring poor values for the shunt resistance, the series resistance, and∕or the diode saturation current. The sample doping concentration on the lightly doped side of the p‐n junction and the built-in potential are determined using the classic 1∕C2 vs V representation. The small-signal capacitance C for each reverse bias voltage V is directly extracted from the measured frequency dependence of the sample’s impedance Z. A crucial feature of the method is the determination of the diode’s series resistance and shunt resistance for each reverse bias voltage used. The method is verified using high-quality p‐n junction diodes fabricated in silicon wafer substrates and its capabilities are demonstrated on nonideal p‐n junction diodes fabricated in polycrystalline silicon thin films on glass substrates.

Quantitative interpretation of electron-beam-induced current grain boundary contrast profiles with application to silicon

An improved method is described for extracting material parameters from an experimental electron-beam-induced current (EBIC) contrast profile across a vertical grain boundary by directly fitting an analytical expression. This allows the least-squares values of the grain boundary recombination velocity and the diffusion length in each grain to be determined without the need for the reduction of the experimental profile to a few integral parameters, as is required in a previously reported method. Greater accuracy of the extracted values is expected since none of the information contained in the experimental contrast data is discarded and a less extensive spatial range of measured data is required than in the commonly used method. Different models of the carrier generation volume are used in the fitting and the effect of the choice of generation model on extracted values is investigated. In common with other EBIC approaches, this method is insensitive to changes in the diffusion length when the collection ef...

16.4% efficient, thin active layer silicon solar cell grown by liquid phase epitaxy

An energy conversion efficiency of 16.4% is reported for a silicon solar cell of 4.11 cm2 total area with a thin active layer of 32 μm grown by liquid phase epitaxy (LPE). This is the highest ever total area efficiency for a cell of this type and is due to a number of improvements over earlier reported results. The thin active layer was grown by LPE on an inactive silicon substrate from an indium solution in a 20% hydrogen/argon forming gas mixture ambient rather than pure hydrogen. Higher current density and efficiency than previously reported for similar cell structures have been achieved by employing microgroove texturing of the front surface, a very shallow (0.25 μm) and high sheet resistivity (220 Ω□) top surface phosphorus diffusion, an optimized ZnS/MgF2 double layer antireflection coating on top of a 200A thick, high quality passivation SiO2 layer, a large aspect ratio (0.45) for the metal contacts, and a graded doping level within the 32 μm thick LPE active layer. The effect of the improved techniques on the cell performance and the properties of the thin active layers are discussed.