N-type multicrystalline silicon wafers and rear junction solar cells

Abstract

N -type silicon presents several advantages compared to p-type material, among them, the most important is the small capture cross sections of metallic impurities, which are neatly smaller. As a consequence lifetime and also diffusion length of minority carriers should be neatly higher in n-type than in p-type, for a given impurity concentration. This is of a paramount interest for multicrystalline silicon wafers, in which the impurity-extended crystallographic defects interaction governs the recombination strength of minority carriers. It is experimentally verified that in 1.2 Ω cm raw wafers lifetimes about 200 µs and diffusion lengths around 220 µm are measured. These values increase strongly after gettering treatments like phosphorus diffusion or Al-Si alloying. Scan maps reveal that extended defects are poorly active, although in regions where the density of dislocations is higher than 10 6 cm −2 . Abrupt p + n junctions are obtained by Al-Si alloying after annealing between 850 and 900 • C, which could be used for rear junction cells. Such cells can be processed by means of similar processing steps used to make conventional p-type base cells.

Photoelectrochemical etching of cone‐shaped macropores in N‐type Si for solar cell texturization

“…N‐type Si is an alternative source of material for Si solar cells with some potential advantages over P‐type, in particular higher diffusion lengths at a given impurity concentration 1. At present it is mostly used for very high efficiency monocrystalline Si (c‐Si) solar cells 2.…”

n‐Type multicrystalline silicon wafers prepared from plasma torch refined upgraded metallurgical feedstock

“…After the thicknesses of the wafers have been reduced to 130 mm by chemical etching, rear junction n-type cells were prepared by means of the conventional processing steps used to make solar cells with ptype silicon wafers, as described previously. 9 The rear junction was obtained during the Al-Si alloy formation; the n þ front side phosphorus diffused region was used to passivate the front surface. A TiO 2 antireflection coating layer completes the cell structure described in Figure 2.…”

“…Although very high conversion effi-ciencies have been obtained with single crystal cells, 4,5 p-type silicon is the most frequently used and n-type mc-Si is not involved in the industrial large scale production of solar cells. Though n-type silicon is of a great interest because it possesses some remarkable advantages: the absence of boron-oxygen complexes, a low variation of minority carrier lifetimes with the injection level and, most importantly, the fact that minority carrier capture cross sections s of several metallic impurities like Fe, Mo, Ni, Ti and Co are markedly smaller for holes than for electrons, [6][7][8][9][10][11] except for Cr and Au atoms. Although the diffusion coefficient of electrons is three times higher than that of holes, it is expected that minority carrier diffusion lengths-the most important property for the base of solar cells-will be markedly higher in n-type than in p-type silicon.…”

Enhancement of minority carrier lifetime in low quality silicon by ion implantation of arsenic and antimony

“…The primary differences are that the implanted ions served as the N-layer rather than phosphorous and the devices had only aluminum for the back contact as opposed to the P+ layer created during the boron doping stage. Aluminum can be used in place of boron as a P-type dopant provided that the annealing temperature is high enough (T > 550 °C) [12].…”