Winfried Wolke

Optical modelling of the front surface for honeycomb-textured silicon solar cells

In this study we assess the reproducibility of the honeycomb texturing process, and we compare experimental values to a simulation. Our results show that the honeycomb-texture can be simulated with the OPAL program. It calculates the losses of any angle of incidence, where the short computation time is achieved by decoupling the ray tracing from the Fresnel equation. We investigate various surface morphologies and their impact on reflection values. The samples were etched at different etching times, whereby the honeycomb geometries were changed. Using our optimal acid etching solution ratio, we are able to fabricate a HC-texture with a reflectivity of R = 19.4 % (ω = 85°). Between the simulation values and those of the measurement, we see a difference of R = 0.3 %abs. This small deviation causes the relatively inaccuracies of the simulation and the measured samples. The focus is to simulate the different honeycomb structures and compare the values with the experimental values.

Passivation of solar cell emitters using aluminum nitride

Layers of hydrogenated aluminum nitride have proven excellent passivation properties on lowly doped silicon. Effective surface recombination velocities below 8 cm/s have been reached due to a very low interface defect density. In this work, the passivation of highly doped silicon is studied by measuring the emitter saturation current of boron as well as phosphorous emitters. It is shown that hydrogenation is a prerequisite for reaching effective passivation. Emitter saturation current densities of around 100 fA/cm2 are presented for highly doped p+-type and n+-type silicon allowing maximal open circuit voltages of ~680 mV for silicon solar cells.

Investigation of methods for adhesion characterization of evaporated aluminum layers

This paper analyzes methods for measuring the adhesion quality of aluminum (Al) rear metallization deposited by means of physical vapour deposition (PVD) on rear passivation layers of passivated emitter and rear silicon solar cells (PERC). Since the standard test procedures for adhesion testing of solar cells cannot be applied, a peel-test and a direct-pull method are introduced and used for measuring the adhesion of test samples; criteria for adhesion evaluation are developed and applied. The results of adhesion tests are compared and are in good general agreement. The applicability of the two methods is discussed and it is found that the peel-test shows more reliable results whereas the direct-pull method is easier in preparation. Moreover, the results of the test samples show sufficiently good adhesion quality for the PVD rear metallization of PERC solar cells.