The dark I-V characteristics of crystalline silicon solar cells usually deviate from that expected by classical diode theory by an unusually high ideality factor and magnitude at biases smaller than about 0.5 V. It had been shown that the recombination current is flowing preferentially in certain local extended defect positions like the edge or local shunts. There, the local density of recombination centers in the pn-junction is higher than in the bulk by orders of magnitude. In this work, we go beyond the SRH theory to explain the recombination effects occurring in such heavily damaged pn-junction regions. Firstly, we apply the coupled defect recombination via two shallow (or one shallow and one deep) level, which explains the observed high ideality factors due to trap-assisted tunneling. Secondly, we apply the coupling of two defects to recombination via deep donor-acceptor pairs, which cause the high ideality factors due to saturation of the recombination rate between the two defects. Thirdly, the local extension of the recombination region across the edge of the cell (due to electrostatic charging) is an other explanation of very high recombination currents.

No abstract

Solar cells made from multi- or mono-crystalline silicon wafers are the base of today’s photovoltaics industry. These devices are essentially large-area semiconductor p-n junctions. Technically, solar cells have a relatively simple structure, and the theory of p-n junctions was established already decades ago. The generally accepted model for describing them is the so-called two-diode model. However, the current-voltage characteristics of industrial solar cells, particularly of that made from multi-crystalline silicon material, show significant deviations from established diode theory. These deviations regard the forward and the reverse dark characteristics as well as the relation between the illuminated characteristics to the dark ones. In the recent years it has been found that the characteristics of industrial solar cells can only be understood by taking into account local inhomogeneities of the dark current flow. Such inhomogeneities can be investigated by applying lock-in thermography techniques. Based on these and other investigations, meanwhile the basic properties of industrial silicon solar cells are well understood. This contribution reviews the most important experimental results leading to the present state of physical understanding of the dark and illuminated characteristics of multi-crystalline industrial solar cells. This analysis should be helpful for the continuing process of optimizing such cells for further increasing their energy conversion efficiency.

A local electrothermal simulation of a model solar cell is presented. A rigorous discussion of the heat dissipation mechanisms in a solar cell is performed, showing that the total dissipated heat splits into heating terms (thermalization, recombination, and Joule heat) and different Peltier cooling terms. Such simulations are important for interpreting lock-in thermography images of real solar cells. The simulated model cell consists of a circular noncontacted region surrounded by a grid line and a nonlinear edge shunt. Based on this simulation, a special lock-in thermography operation mode is proposed, which enables noncontacted regions in real solar cells to be imaged. Experimental results confirm the theoretical predictions.

No abstract