Moustafa Y. Ghannam

Latest efficiency results with the screenprinting technology and comparison with the buried contact structure

A lot of progress has been made in the pilot-production of mono- and multicrystalline silicon solar cells by the screenprinting technology. For example, a screenprinting process using new metal pastes, fine linewidth printing, the incorporation of surface passivation, Al gettering, BSF and a selective emitter structure yields efficiencies over 17% on 100 cm/sup 2/ Cz-Si material. These results ask for a detailed comparison concerning the relative performance of state-of-the-art screenprinted cells compared to the state-of-the-art buried contact structure. This is done by means of 2D-simulations using the simulation program SIMUL. The efficiency difference between a buried contact technology and a screenprinting technology, both with selective emitter and good surface passivation, is only 0.5-0.6% absolute. The efficiency difference between screenprinted cells with a homogeneous emitter and buried contact cells with a selective emitter turns out to be 1% absolute, provided a good surface passivation is applied. Looking at the above-mentioned difference in efficiency, only detailed cost calculations for an industrial production scenario can reveal which process leads to the lowest cost per Wp.

Direct calculation of two‐dimensional collection probability in pn junction solar cells, and study of grain‐boundary recombination in polycrystalline silicon cells

A new method for the direct calculation of the two‐dimensional collection probability in pn junction solar cells is presented. Based on its reciprocity properties, the inhomogeneous continuity equation for excess carriers is transformed to a homogeneous partial differential equation (PDE) for the probability of carriers being collected in the external circuit. The new PDE is easier to solve and directly gives the short‐circuit current. The method is applied to study the impact of grain‐boundary recombination on the performance of polycrystalline silicon solar cells. A critical grain width of four times the carrier diffusion length in the base is found to be the limiting boundary between polycrystalline behavior and monocrystalline behavior of the cell. The sensitivity of short‐circuit AM1.5 collection efficiency to the grain width Wg, the grain‐boundary recombination velocity Sg, minority‐carrier diffusion lengths, and surface recombination velocities, is quantified for a variety of cell types and recombi...

Recent improvements in the screenprinting technology and comparison with the buried contact technology by 2D-simulation

Abstract Recently, a lot of progress has been made in the pilot-production of mono- and multicrystalline silicon solar cells by the screenprinting technology. For example, a process based on screenprinting combined with oxide passivation and a plasma-nitride ARC yields cell efficiencies approaching 17% on 100 cm 2 Cz Si Material. These results ask for a detailed comparison concerning the relative performance of state-of-the-art screenprinted cells compared to the state-of-the-art buried contact structure. This is done by means of 213-simulations using the simulation program SIMUL. It comes out that the efficiency difference between a buried contact technology and a screenprinting technology, both with selective emitter and good surface passivation, is only 0.5% absolute. The efficiency difference between screenprinted cells with a homogeneous emitter and buried contact cells with a selective emitter turns out to be between 1 and 1.5% absolute, depending on the surface passivation quality. Looking at the above-mentioned difference in efficiency, only detailed cost calculations for an industrial production scenario can reveal which process leads to the lowest cost per Wp.

Interpretation of macropore shape transformation in crystalline silicon upon high temperature processing

A physical interpretation is proposed for the sequence of transformations that macropores embedded in crystalline silicon undergo during high temperature treatments. First, cylindrical pores spheroidize by surface diffusion at constant volume. In the presence of stress and due to perturbation in the spherical symmetry stored elastic energy competes with cavity surface energy transforming large cavities by surface diffusion into an oblate shape with a major radius that continuously expands. At a critical condition close to the Griffith fracture criterion, the cavity collapses catastrophically into an ultrathin uniform slit that splits one or more thin crystalline film off the original substrate. On the other hand, if the stress is not high enough or the major radius of the cavity is not large enough the cavity does not collapse and maintains a rounded shape. Annealing in an ambient gas with a high partial pressure enhances the surface reaction which accelerates cavity growth and wall smoothening. The propo...

Influence of the Recombination Parameters at the Si/SiO 2 Interface on the Ideality of the Dark Current of High Efficiency Silicon Solar Cells

Analytical study of surface recombination at the Si/SiO 2 interface is carried out in order to set the optimum surface conditions that result in minimum dark base current and maximum open circuit voltage in silicon solar cells. Recombination centers are assumed to form a continuum rather than to be at a single energy level in the energy gap. It is shown that the presence of a hump in the dark I-V characteristics of high efficiency PERL cells is due to the dark current transition from a high surface recombination regime at low voltage to a low surface recombination regime at high voltage. Successful fitting of reported dark I-V characteristics of a typical PERL cell is obtained with several possible combinations of surface parameters including equal electron and hole capture cross sections.

A semiquantitative model of a porous silicon layer used as a light diffuser in a thin film solar cell

Abstract An optical model of a porous silicon layer used as a light diffuser in thin-film solar cells is proposed in the framework of Twerskys multiple scattering theory of electromagnetic waves in random media. A clear distinction is made between the coherent and diffuse components of the field. The optical properties of the coherent field are described in terms of a complex refractive index calculated using Foldy–Twerskys integral equation, while the diffuse component is estimated by means of the Boltzmann-like radiative transfer equation. A brief description of the experimental verification of the model is then given. The model gives very good results for a low-porosity porous silicon layer sandwiched between the two crystalline silicon layers, but will hopefully give a good qualitative picture in more general situations.

Limitations of ray tracing techniques in optical modeling of silicon solar cells and photodiodes

The physical optical effects which take place when the size of the surface textures of a solar cell or a photodiode is in the micron range are studied. Both the antireflective and the light trapping properties of linear periodic grooves of triangular cross sections are treated in the framework of an exact, coupled-wave, electromagnetic analysis. The most important physical optics effects are the diffraction of light at the groove edges, the interference of light reflected by the different surfaces of the grooves, and the tunneling across the grooved surface of light incident from the silicon side at an angle greater than the critical angle. There is also the effect of the interference of the light reflected by the front and back surfaces of the silicon layer which is important only for very thin layers. As the groove period increases above the wavelength, the exact results approach the geometrical optics predictions rather slowly in an oscillatory way. Simple physical optics arguments are presented for ob...

REFINED MODELING OF OPTICAL CONFINEMENT IN THIN SILICON LAYERS WITH A LAMBERTIAN BACK REFLECTOR FOR SOLAR CELL APPLICATIONS

An attempt is made to assess the accuracy of the simplifying assumption of total transmission (total loss) of the light inside the “loss” or “escape” cone which is made in many models of optical confinement in thin-layer silicon solar cells. A closed form expression is derived for the absorption enhancement factor as a function of the refractive index in the low-absorption limit for a thin layer with a Lambertian back-reflector and is compared with the expression derived previously on the basis of the “total loss” assumption. The generation profile, needed for the evaluation of solar cell performance parameters, is also obtained and studied. More general problems involving realistic light trapping schemes and/or actual material properties are treated numerically, sometimes by means of a suitably modified ray-tracing computer program. It turns out that the error in making the above-mentioned approximation is relatively minor if the layer has a Lambertian back-reflector, whether its front surface is texture...

Optimum sensitivity and two-dimensional modeling of microwave detected photoconductance decay carrier lifetime measurement

Optimization of the measurement sensitivity using closed form analytical expressions derived from an electrical equivalent circuit is carried out for microwave detected photoconductance decay system used for nondestructive carrier lifetime measurement in silicon wafers. The effect of transverse inhomogeneity in the sample conductivity on the microwave power reflection is discussed using the equivalent circuit model. The effect of lateral inhomogeneity in the sample conductivity due to local illumination by a source with a laterally varying intensity is studied using an elaborate analytical two-dimensional model. Finally, the effect of transient lateral carrier diffusion in a sample subjected to a pulsed illumination on the extracted value of the lifetime is investigated.

Optimum two-dimensional short circuit collection efficiency in thin multicrystalline silicon solar cells with optical confinement

The two-dimensional short-circuit AM1.5 collection efficiency is studied in thin multicrystalline silicon solar cells with optical confinement. The collection efficiency is calculated by linking an optical analytical generation profile with the two-dimensional collection probability in pn junction solar cells. The calculations are carried out for variable grain boundary recombination velocity, cell thickness, grain width, diffusion length, and back surface recombination velocity. The role of optical confinement leading to a strong dependence of the collection efficiency on the cell thickness in very thin cells is confirmed. The optimum cell thickness for maximum collection efficiency increases in cells with low back reflection or poor back surface passivation. Also, the optimum thickness in very thin cells increases significantly with increasing the diffusion length. It is also found that the effect of grain boundary recombination is predominant if the cell thickness is larger than the diffusion length and if the diffusion length is larger than half the grain width, especially, in cells with unpassivated grain boundaries. On the other hand, back surface recombination dominates the response in cells with unpassivated back surface if the thickness is smaller than or comparable to the diffusion length.