Formation Of Boron-oxygen Complexes Research Articles

Impact of carbon co-doping on the performance of crystalline silicon solar cells

We have investigated the impact of carbon co-doping on the performance of boron–doped Czochralski-grown silicon solar cells. It is found that carbon co-doping will deteriorate the initial performance of Aluminium-back-surface-field solar cells before light-induced degradation (LID), owing to the enhancement effect on the formation of oxygen precipitation. However, carbon co-doping can effectively suppress the formation of boron-oxygen complexes in the solar cells, which becomes more significant with an increase of the carbon concentration. Therefore, the performance of carbon co-doped silicon solar cells is better than that of conventional silicon solar cells after LID. All these results are of great significance for the practical application of carbon co-doped silicon solar cells with low LID effect in photovoltaic industry.

Investigation of Self Interstitial Influences in Light and Dark Induced Degradation in p-type Compensated Silicon

In this study, based on p-type strongly compensated electronic grade monocrystalline Cz silicon, one tried to find if self interstitials have a direct or an indirect role in light and dark induced degradation (LID and DID respectively). Many studies have been carried out on LID phenomenon due to the formation of boron-oxygen complexes under light exposure. Some of them compare as-cut wafers and wafers which have been treated by phosphorus diffusion. They show that the LID phenomenon is reduced when samples have been phosphorus diffused. This effect has been explained by self interstitial injection in the bulk during the phosphorus diffusion, but the role of these self-interstitials on LID phenomenon is not yet well defined [1] [2] [3]. In this paper, investigations on the degradation kinetic are made in as-cut wafers, in phosphorus diffused wafers and in wafers which were annealed in the same conditions used for the phosphorus diffusion. Comparisons of the results allow us to pinpoint the role of self-interstitials on LID phenomenon. It seems that the presence of a high self interstitial concentration reduce the formation of BsO2i complexes responsible for the first faster degradation.

Light-induced degradation in compensated p- and n-type Czochralski silicon wafers

Light-induced degradation (LID) due to boron-oxygen complex formation seriously diminishes the minority carrier lifetime of p-type Czochralski-grown (Cz) wafers. Depending linearly on the boron concentration NA in uncompensated silicon, the boron-oxygen defect density was suggested to depend on the net doping concentration p0 = NA − ND in compensated p-type samples, containing similar amounts of boron and phosphorus [D. Macdonald, F. Rougieux, A. Cuevas, et al., Journal of Applied Physics 105, 093704 (2009)]. However, this dependency contradicts observations of LID in compensated n-type silicon wafers [T. Schutz-Kuchly, J. Veirman, S. Dubois, et al., Applied Physics Letters 96, 1 (2010)], which are confirmed in this study by investigating the boron-oxygen complex formation on a large variety of compensated p- and n-type samples. In spite of their high boron content, compensated n-type samples may show a less pronounced LID than p-type samples containing less boron. Our experiments indicate that in compensated silicon, the defect concentration is only a function of the compensation ratio RC = (NA + ND)/(NA – ND).

Comprehensive analytical model for locally contacted rear surface passivated solar cells

For optimum performance of solar cells featuring a locally contacted rear surface, the metallization fraction as well as the size and distribution of the local contacts are crucial, since Ohmic and recombination losses have to be balanced. In this work we present a set of equations which enable to calculate this trade off without the need of numerical simulations. Our model combines established analytical and empirical equations to predict the energy conversion efficiency of a locally contacted device. For experimental verification, we fabricate devices from float zone silicon wafers of different resistivity using the laser fired contact technology for forming the local rear contacts. The detailed characterization of test structures enables the determination of important physical parameters, such as the surface recombination velocity at the contacted area and the spreading resistance of the contacts. Our analytical model reproduces the experimental results very well and correctly predicts the optimum contact spacing without the use of free fitting parameters. We use our model to estimate the optimum bulk resistivity for locally contacted devices fabricated from conventional Czochralski-grown silicon material. These calculations use literature values for the stable minority carrier lifetime to account for the bulk recombination caused by the formation of boron-oxygen complexes under carrier injection.

Kinetics of the boron-oxygen related defect in theory and experiment

The formation of boron-oxygen complexes in boron-doped crystalline silicon can lead to a severe reduction in the minority charge carrier lifetime. This strongly influences, e.g., solar cell efficiencies if the material is used for photovoltaic application. Recent investigations have shown that a recovery of the carrier lifetime can be achieved by a subsequent thermally enhanced reaction induced by charge carriers. A model of the reaction dynamics of the boron-oxygen complex by means of rate equations is presented in this paper. Following a mathematical description of the reactions involved, the consequences based on the calculations are presented and allow a prediction of the observable electrical parameters. The fundamental agreement with measured data is proven experimentally for different phenomena.

Nature of the metastable boron–oxygen complex formation in crystalline silicon

Transient capacitance measurements reveal new physics of metastable defect formation in boron-doped oxygen-containing crystalline silicon solar cells. These measurements demonstrate that holes are deeply trapped during defect formation and removed during thermal annealing with activation energy of 1.3 eV. Previous theoretical models {Du et al., [Phys. Rev. Lett. 97, 256602 (2006)] and Adey et al., [Phys. Rev. Lett. 93, 055504 (2004)]} are supported by present findings that defect formation is a slow two-stage process with activation energies of 0.17 eV and 0.4 eV at high and low temperature, respectively. Repulsive hole capture by a positive oxygen-dimer determines the defect formation rate at low temperature {Du et al., [Phys. Rev. Lett. 97, 256602 (2006)]}. The high temperature process is governed by a structural conversion of the dimer {Du et al., [Phys. Rev. Lett. 97, 256602 (2006)] and Adey et al., [Phys. Rev. Lett. 93, 055504 (2004)]}. An abnormally low rate prefactor allows this low-enthalpy reaction to be observed at the higher temperature. This dimer conversion presents an excellent example of an “entropy barrier” that explains the low conversion rate. Disparate formation and annealing results published here and in other publications are related by the Meyer–Neldel rule with an isokinetic temperature of 410 K.

Slow down of the light-induced-degradation in compensated solar-grade multicrystalline silicon

This letter focuses on the kinetics of the light-induced-degradation in multicrystalline silicon, comparing electronic grade and strongly compensated solar-grade materials. In electronic grade material, the results fit well with the models developed for Czochralski grown single-crystals. In contrast, in solar grade material, the light-induced-degradation kinetics are much slower and cannot be described by the existing models. We discuss how the formation of boron-oxygen complexes may be altered by the effects of compensation.