A Combined Statistical and TCAD Model as a Method for Understanding and Reducing Variations in Multicrystalline Si Solar Cell Production

AbstractMonitoring the I-V parameters in mass production yields a distribution that cannot be understood in a simple manner. For example, if Voc varies greatly, it is not obvious whether this is mainly due to variations in the bulk lifetime or in the surface passivation or due to other sources.In this work, we develop a method where statistics is combined with numerical device modeling to obtain a physical interpretation of the observed variations. In the first part, we derive a multivariate statistical model to extract the main influences of fabrication fluctuations on the I-V parameters. This statistical model is based on cell parameters measured on a representative sample of solar cells from production. In the second part, we develop a computer-aided design (TCAD) device simulation model for multicrystalline Si solar cells. This TCAD model quantifies the I-V variations on a physically sound basis. However, the number of simulations is grossly reduced by feeding in solely the main influences obtained from the statistical model. In the third part, we verify this method by comparing the calculated distribution with production data.This model is used for optimization strategies for higher cell efficiency, smaller variations in cell parameters and improved yield in mass production. Furthermore, we will apply our methodology to advanced cell concepts. It will allow the early consideration of production fluctuation in device simulation of advanced cell concepts, and therefore a realistic assessment of such concepts

Advances in the Surface Passivation of Silicon Solar Cells

AbstractThe surface passivation properties of aluminium oxide (Al2O3) on crystalline Si are compared with the traditional passivation system of silicon nitride (SiNx). It is shown that Al2O3 has fundamental advantages over SiNx when applied to the rear of p-type silicon solar cells as well as to the p+ emitter of n-type silicon solar cells. Special emphasis is paid to the transfer of Al2O3 into industrial solar cell production. We compare different Al2O3 deposition techniques suitable for mass production such as ultrafast spatial atomic layer deposition, inline plasma-enhanced chemical vapour deposition and reactive sputtering. Finally, we review the most recent cell results with Al2O3 passivation and give a brief outlook on the future prospects of Al2O3 in silicon solar cell production

Physically sound parameterization of incomplete ionization in aluminum-doped silicon

Incomplete ionization is an important issue when modeling silicon devices featuring aluminum-doped p+ (Al-p+) regions. Aluminum has a rather deep state in the band gap compared to boron or phosphorus, causing strong incomplete ionization. In this paper, we considerably improve our recent parameterization [Steinkemper et al., J. Appl. Phys. 117, 074504 (2015)]. On the one hand, we found a fundamental criterion to further reduce the number of free parameters in our fitting procedure. And on the other hand, we address a mistake in the original publication of the incomplete ionization formalism in Altermatt et al., J. Appl. Phys. 100, 113715 (2006)

Band gap narrowing in p-type base regions of solar cells

This paper demonstrates that for an adequate simulation of solar cells it is very important to include band gap narrowing even for base resistivities as high as 0.5 omega cm. Numerical simulations were performed using the band gab narrowing model recently developed by A. Schenk (JAP 84, 3684 (1998)). The simulated open-circuit voltages are in excellent agreement with the V(ind OC) values measured on RP-PERC solar cells in the doping range between 2x10(exp 15) and 5.5x10(exp 17) cm-3. By comparing the measurements with a simulation performed without band gab narrowing it was possible to determine new apparent BGN data points which are in excellent agreement with the Schenk-model

Application of an improved band-gap narrowing model to the numerical simulation of recombination of phosphorus-doped silicon emitters

The commonly used band-gap narrowing (BGN) models for crystalline silicon do not describe heavily doped emitters with desirable precision. One of the reasons for this is that the applied BGN models were empirically derived from measurements assuming Boltzmann statistics. We apply a new BGN model derived by Schenk from quantum mechanical principles and demonstrate that carrier degeneracy and the new BGN model both substantially affect the electron-hole product within the emitter region. Simulated saturation current densities of heavily phosphorus-doped emitters, calculated with the new BGN model, are lower than results obtained with the widely used empirical BGN model of del Alamo

Injection intensity-dependent recombination at various grain boundary types in multicrystalline silicon solar cells

If the ratio of two open circuit photoluminescence (Voc-PL) images taken at two different light intensities is displayed, some grain boundaries (GBs) may show up as bright lines. This indicates that these special GBs show distinct injection intensity-dependent recombination properties. It will be shown here that this results in an apparent ideality factor of the light emission smaller than unity. The effect is reproduced with numerical device simulations using a usual distribution of defects in the band gap along grain boundaries. Quantitative imaging of this apparent luminescence ideality factor by PL imaging is complicated by the lateral horizontal balancing currents flowing at open circuit. The local voltage response of an inhomogeneous solar cell at different injection levels under open circuit is modelled by Griddler simulations, based on PL investigations of this cell. The evaluation of Voc-PL images at different illumination intensities allows us to conclude that the apparent luminescence ideality factor at the special GBs is about 0.89, whereas in the other regions it is between 0.94 and 0.95. Reverse bias electroluminescence showed no pre-breakdown sites, and hyperspectral PL imaging showed only in one of the investigated GBs particular defect luminescence. TEM investigations of two GBs, one showing distinct injection intensity-dependent recombination and the other one showing none, revealed that the investigated special GB is a large-angle GB whereas the GB not showing this effect is a small-angle GB

Excellent passivation of highly doped p-type Si surfaces by the negative-charge-dielectric Al2O3

From lifetime measurements, including a direct experimental comparison with thermal SiO2, a-Si:H, and as-deposited a-SiNx:H, it is demonstrated that Al2O3 provides an excellent level of surface passivation on highly B-doped c-Si with doping concentrations around 10(19) cm(-3). The Al2O3 films, synthesized by plasma-assisted atomic layer deposition and with a high fixed negative charge density, limit the emitter saturation current density of B-diffused p(+)-emitters to similar to 10 and similar to 30 fA/cm(2) on >100 and 54 Omega/sq sheet resistance p(+)-emitters, respectively. These results demonstrate that highly doped p-type Si surfaces can be passivated as effectively as highly doped n-type surface