A. Holt

Gettering of transition metal impurities during phosphorus emitter diffusion in multicrystalline silicon solar cell processing

We have investigated the gettering of transition metals in multicrystalline silicon wafers during a phosphorus emitter diffusion for solar cell processing. The results show that mainly regions of high initial recombination lifetime exhibit a significant lifetime enhancement upon phosphorus diffusion gettering. Nevertheless, transition metal profiles extracted by secondary ion mass spectrometry in a region of low initial lifetime reveal significant gradients in Cr, Fe, and Cu concentrations towards the surface after the emitter diffusion, without exhibiting a significant enhancement in the lifetime. In a region of higher initial lifetime, however, diminutive concentration gradients of the transition metal impurities are revealed, indicating a significantly lower initial concentration in these regions. From spatial maps of the dislocation density in the wafers, we find that lifetime enhancements mainly occur in regions of low dislocation density. Thus, it is believed that a generally higher concentration of...

High concentration in-diffusion of phosphorus in Si from a spray-on source

High concentration in-diffusion of phosphorus in both Czochralski grown and solar grade multicrystalline Si from a spray-on liquid source has been studied by secondary ion mass spectrometry and electrochemical capacitance-voltage profiling. By extraction of the concentration dependent effective diffusivity employing the Boltzmann-Matano analysis, we adapt an integrated diffusion model based on a previous work by Uematsu [J. Appl. Phys. 82, 2228 (1997)], in order to gain insight into the mechanisms governing such in-diffusions. We find that in the tail region of the profiles, diffusion is mediated by interaction with Si self-interstitials, whereas a vacancy mechanism via doubly negative vacancies dominates in the higher concentration region towards the surface, in correspondence with a previous analysis by Fair and Tsai [J. Electrochem. Soc. 124, 1107 (1977)]. Moreover, we find that both the vacancy and interstitial mechanisms can be described by an Arrhenius behavior, exhibiting apparent activation energi...

Double layer anti-reflective coatings for silicon solar cells

In this paper simulated single and double layer anti-reflective coatings based on the refractive index limits of silicon nitride (SiN) and silicon oxide (SiO/sub 2/) are presented. The best structure combines SiN and SiO/sub 2/, resulting in a reflectance of 0.044 based on the AM1.5 photon flux from 300-1150 nm. PC1D solar cell simulations show that an increase in short circuit current density of 6.4% was possible by replacing an optimised single SiN layer with the above mentioned double layer.

Transparent yttrium hydride thin films prepared by reactive sputtering

Abstract Metal hydrides have earlier been suggested for utilization in solar cells. With this as a motivation we have prepared thin films of yttrium hydride by reactive magnetron sputter deposition. The resulting films are metallic for low partial pressure of hydrogen during the deposition, and black or yellow-transparent for higher partial pressure of hydrogen. Both metallic and semiconducting transparent YH x films have been prepared directly in situ without the need of capping layers and post-deposition hydrogenation. Optically the films are similar to what is found for YH x films prepared by other techniques, but the crystal structure of the transparent films differ from the well-known YH 3− η phase, as they have an fcc lattice instead of hcp.

Hydrogen complexes in Zn deficient ZnO

This work presents a study of H complexes in Zn deficient ZnO by density functional calculations. It is found that the location of two H atoms at the Zn vacancy (VZn) is energetically more preferable than the distant H and VZn. Each of the two H atoms saturates one dangling O bond by forming an O–H bond. The resulting defect can be considered to be a 2(O–H)-VZn complex. Since there are four O dangling bonds at the VZn, the site can host four H atoms. It is shown that VZn with four H atoms is energetically more preferable than distant H and VZn. Depending on the number of H atoms at the VZn, the influence of the H-VZn system on the electronic structure of Zn deficient ZnO can be qualitatively different. VZn without a H is a double acceptor and VZn containing one H, i.e., 1(O–H)-VZn complex, is a single acceptor. In ZnO with 2(O–H)-VZn complexes the Fermi level is located at the top of the valence band. A donor level is formed by the 3(O–H)-VZn and 4(O–H)-VZn complexes. Two of the H atoms in 2(O–H)-VZn, 3(O...

Hydrogen Release and Defect Formation During Heat Treatments of SiNx:H/a-Si: H Double Passivation Layer on C-SI Substrate

The quality and temperature stability of surface passivation of silicon by a double layer consisting of a hydrogenated amorphous silicon thin film capped by a silicon nitride anti-reflection coating are studied. It is established that the passivation effect of the double layer can be significantly enhanced after short annealing for temperatures up to about 500degC, whereas annealing at higher temperatures results in degradation of the passivation properties. It is found that the increased effective recombination lifetime after annealing at temperatures below 500degC results from hydrogen redistribution in the interface region. Furthermore, presence of interfacial structural defects formed due to hydrogen release at temperatures around 600degC, is believed to be the cause of the lifetime decrease after heat treatments at higher temperatures

Recombination lifetime and trap density variations in multicrystalline silicon wafers through the block

We have studied the variations in recombination lifetime and density of non-recombinative traps between wafers through the entire height of a multicrystalline silicon block. We find that the low lifetime regions in the very bottom and top part of an ingot are described by a relatively high density of non-recombinative traps. It is argued that in the bottom region in-diffused impurities from the crucible, as well as grain boundaries and dislocations, could be the major trap contributors. In the upper region, trapping is believed to be due to high concentrations of metallic impurities, caused by segregation during growth and solid-state diffusion during cooling of the ingot. Moreover, we have investigated the gettering response of wafers from different positions within the block from a 50/spl Omega//sq phosphorus emitter diffusion. The results show that the most notable lifetime enhancements occur in regions of the block where the trap density is high, indicating the relevance of mobile impurities as non-recombinative trapping centers.

Acidic texturing of multicrystalline silicon wafers

In this paper, results from investigations of acidic texturing are presented. As-cut, multicrystalline and polished, single crystal Si wafers have been etched in a range of acidic mixtures. The mixtures contained hydrofluoric and nitric acid, with de-ionized water, phosphoric acid or sulphuric acid added as diluents. In the initial phases of the etching process, surface cracks originating from the wafer sawing were transformed into deep and elongated pits. The reflectance of such textures was low. However, as the etching proceeded beyond the damaged surface region, more reflective, bubble-like textures were obtained.

Production of Silicon from SiH4 in a Fluidized Bed, Operation and Results

Abstract: For an installed silicon-based solar cell panel, about 40% of the energy needed for fabrication is consumed for production of the silicon feedstock. Reducing the energy consumption in this step is therefore crucial in order to minimize the energy payback time and cost of the technology. The most promising alternative to the conventional methods is to use fluidized bed reactors. In this article, we report the results from a novel reactor layout with a selectively cooled distribution arrangement. Important aspects of fludized bed monitoring and operation are described. Two different operation regimes are stated yielding hence porous and dense growth. Further, the method of encapsulating fines by means of more dense depositions is verified, and the nature of the scavenged material is characterized. Also, two different types of fines formation are identified and accounted for.

Correspondence between sheet resistance and emitter profile of phosphorus diffused emitters from a spray-on dopant

The evolution of the emitter sheet resistance upon phosphorus in-diffusion from a spray-on dopant has been studied in the temperature range 840-990 /spl deg/C. In complement with investigations of emitter diffusion profiles, by both electrochemical capacitance-voltage profiling and secondary ion mass spectrometry, we find that the sheet resistance is determined directly by the depth of the flat plateau near the surface in profiles of electrically active phosphorus. Thus, with respect to sheet resistance, an emitter can be regarded as a constant concentration abrupt box layer, with thickness and concentration specified by the flat plateau, irrespective of the actual profile at lower concentrations. Therefore, independent optimization of the emitter profile at lower concentrations is enabled.