Peter Fath

Mechanical wafer engineering for high efficiency solar cells: an investigation of the induced surface damage

During mechanical structuring of crystalline silicon an electronically active surface damage layer is induced whose complete removal is a prerequisite for the preparation of highly efficient mechanically textured multicrystalline silicon solar cells. In order to evaluate the presently unknown damage layer thickness of mechanically textured silicon, electron microscopy studies and microwave reflection lifetime measurements in combination with a step etching procedure were performed using mono and multicrystalline silicon as base material. The influence of the diamond grain size and the lateral cutting speed of the beveled sawing blades on the surface damage was studied to obtain a better understanding of the mechanical structuring of silicon. In order to confirm the results obtained from lifetime measurements, screenprinted mechanically V-grooved solar cells were processed with different etching times during the sawing damage removal process step. It could be shown that the electronically active surface damage layer has a thickness of about 3 /spl mu/m when applying standard grooving parameters and a diamond grain size of 4-6 /spl mu/m within the abrasive.

Bow reducing factors for thin screenprinted MC-Si solar cells with Al BSF

Silicon solar cell costs could be strongly reduced by the use of thinner wafers. However, thin wafers are susceptible to bowing caused by the influence of the metallisations on the front and rear side. Excessively bowed wafers lead to unacceptable yield losses during module construction. Screen printed aluminium paste is the major contributor to bowing which increases with decreasing wafer thickness. One barrier to using thinner wafers is the lack of an aluminium paste that combines good doping conditions for the BSF, good ohmic contact properties with a composition that has low bowing tendencies. In this work, we studied the effect of Al print thickness on electrical and mechanical performance of new aluminium pastes, developed by DuPont, using 200 /spl mu/m thick 12.5/spl times/12.5 cm wafers. The influence of aluminium paste composition, firing conditions and silver front side was also studied.

Low-cost back contact silicon solar cells

Back-contacted solar cells offer multiple advantages in regard of reducing module assembling costs and avoiding grid shadowing losses. The investigated emitter-wrap-through (EWT) device design has an electrical connection of the front emitter and the rear emitter grid in form of small holes drilled into the crystalline silicon wafer. The obtained cell structure is especially suitable for low-cost base material with small minority carrier diffusion lengths. Different industrially applicable solar cell manufacturing processes for EWT devices are described and compared. The latest experimental results are presented and interpreted; the photocurrent is found to be distinctly increased. The relation between open circuit voltage and rear side passivation is discussed based on two-dimensional (2-D) computer simulations.

Industrial manufacturing of semitransparent crystalline silicon POWER solar cells

This paper gives an extract of the state of the art of the manufacturing of semitransparent crystalline silicon POWER solar cells in an industrial environment. A short introduction of the POWER devices concept (see Fig. 1) will be given followed by an insight in the applied production process. Finally, examples effecting the efficiency distribution in the cell production and their solutions are given. It is believed that the lessons we learned in optimising the manufacturing process and production line of transparent POWER solar cells can be helpful for the increasing activities in the direction of thin wafers as well as novel solar cell devices.

Theoretical and experimental behavior of monolithically integrated crystalline silicon solar cells

A concept for the fabrication of monolithically integrated silicon solar cells is presented. The concept is based on standard Si wafer technology and does not use thin-film approaches. A key feature is isolation trenches dividing the wafer into several unit solar cells. Due to the imperfect isolation between unit cells defined on the same conductive wafer, some device aspects deviating from an ordinary series connection of solar cells arise. For the theoretical description, a model proposed by Valco et al. [G. J. Valco, V. J. Kapoor, J. C. Evans, Jr., and A. T. Chin, in Proceedings of the 15th IEEE Photovoltaic Specialists Conference, Orlando, FL (1981), p. 187] has been generalized by using a two-diode concept for the unit cells and by weakening the assumption of identical unit cells. The model was used to simulate the cell performance in dependence on light intensity, isolation resistance, cell area, and number of unit cells. As a result, general design rules for these truly monolithically integrated so...

Characterization of novel mono- and bifacially active semi-transparent crystalline silicon solar cells

This paper presents the latest cell results for semi-transparent mono- as well as bifacially active POWER (Polycrystalline Wafer Engineering Result) solar cells of different cell sizes on Cz and multicrystalline silicon substrates. Top efficiencies of 10.4% for monofacial and 12.9% for bifacial cells are reported. Attention has been paid to apply a fully industrially compatible production process. It uses dicing saw based mechanical texturization of the front and rear side of the silicon wafer and screen printing metallization. In the POWER solar cell concept, perpendicular grooves on the front and rear side create holes with a variable diameter at their crossing points. This results in a partial optical transparency of the solar cell. In this study, holes of 200 /spl mu/m diameter lead to a transparency of 16-18% on average for the total cell area. The cell characteristics for the different cell types are compared by means of illuminated and dark current-voltage (I-V), spectral response, and Laser Beam Induced Current (LBIC) measurements. While bifacial POWER cells need a more elaborate production process, they reveal better I-V characteristics and a higher efficiency as compared to monofacial cells. This is mainly explained by a better surface passivation due to an active emitter and a passivating silicon nitride ARC both on the front and rear surface.

Scanning IQE-measurement for accurate current determination on very large area solar cells

We developed a setup to measure the quantum efficiency and integral reflectance for large area solar cells where the cell is scanned underneath a 2/spl times/2 cm/sup 2/ illuminated area defined by a mask. Measurements with 90 wavelengths between 300 and 1200 nm on 12.5 /spl times/ 12.5 cm/sup 2/ solar cells are obtained in less than 15 minutes with very low noise due to the good signal-to-bias ratio. A self-consistent scaling procedure based on the analysis of the internal quantum efficiency is used to account for scaling errors due to the electrical measurement and stray light. This analysis also provides data which enable the calculation of the current loss in the emitter of the solar cell. A method is introduced to identify the bias light level at which the small signal quantum efficiency coincides with the integral large signal response eliminating the need to take the complete quantum efficiency for many bias levels.

ACE Designs: the beauty of rear contact solar cells

Presents an outline of the work done in the EC co-funded project ACE Designs. The objective of this project was to develop rear contact solar cell designs and to demonstrate their applicability as an alternative crystalline silicon technology for industrial module production. An overview of the results is given with links to the most relevant, publications for further details. The most important result of this project was that rear contact solar cells are a feasible, attractive and cost effective alternative to the well-known front contacted solar cell.

Effects of pn-junctions bordering on surfaces investigated by means of 2D-modeling

Several new solar cell designs, among them the emitter wrap through (EWT) and the POWER solar cell, suffer from reduced fill factors. These cells have interdigitated p- and n-type regions. At the margin of these regions, the p-n junction borders on the surface causing additional recombination. We investigate by means of two-dimensional modeling the recombination mechanisms occurring in such device regions, and we give an experimental example. It is shown that a poor quality of the surface passivation near to where the pn-junction borders, is mainly responsible for the observed losses in fill factor and open-circuit voltages.

Two- and three-dimensional optical carrier generation determination in crystalline silicon solar cells

Two- and three-dimensional analyses of the distribution of optically generated charge carriers in textured crystalline silicon solar cells of arbitrary geometry have been performed. The simulation algorithm, developed for that purpose, is based on geometrical optics and ray tracing. It determines the dominant contributions to the optical generation within textured silicon exactly. The contribution of weakly absorbed long-wavelength photons is calculated using a Monte-Carlo simulation. The presented algorithm is fast and accurate and can also be used to calculate reflectance and transmittance spectra in excellent agreement with measurements. Two- and three-dimensional generation profiles in single- and double-sided textured solar cells are presented and discussed in detail. Examples for applications are given. Finally, the presented algorithm is compared with a pure Monte-Carlo algorithm.