Papers by Author: Otwin Breitenstein

Abstract: In this contribution the basic principles of DLIT (dark lock-in thermography) are introduced and typical application examples are shown. These results are compared with that of other solar cell characterization techniques like electroluminescence (EL) and photoluminescence (PL) imaging, which are also very popular in Germany. It will be shown that these techniques are largely complementary to each other. Luminescence techniques are most sensitive for the detection of local recombination centers in the bulk and of series resistance problems of the cells, whereas DLIT is most effective for investigating all problems being connected with the dark current of the cells. A new DLIT technique is introduced which allows a separate imaging of the so-called diffusion current and the recombination current. These two contributions of the dark current are based on different physical mechanisms (recombination in the bulk and in the depletion region, respectively), and their spatial distributions differ significantly. Such investigations are impossible by applying luminescence-based imaging techniques.

Abstract: This work introduces two different approaches to explain the growth of silicon carbide (SiC) filaments, found in the bulk material and in grain boundaries of solar cells made from multicrystalline (mc) silicon. These filaments are responsible for ohmic shunts. The first model proposes that the SiC filaments grow at the solid-liquid interface of the mc-Si ingot, whereas the second model proposes a growth due to solid state diffusion of carbon atoms in the solid fraction of the ingot during the block-casting process. The melt interface model can explain quantitatively the observed morphologies, diameters and mean distances of SiC filaments. The modeling of the temperature- and time-dependent carbon diffusion to a grain boundary in the cooling ingot shows that solid state diffusion based on literature data is not sufficient to transport the required amount of approximately 3.4  1017 carbon atoms per cm2 to form typical SiC filaments found in grain boundaries of mc-Si for solar cells. However, possible mechanisms are discussed to explain an enhanced diffusion of carbon to the grain boundaries.

Abstract: The current-voltage (I-V) characteristics of most industrial silicon solar cells deviate rather strongly from the exponential behavior expected from textbook knowledge. Thus, the recombination current may be orders of magnitude larger than expected for the given material quality and often shows an ideality factor larger than 2 in a wide bias-range, which cannot be explained by classical theory either. Sometimes, the cells contain ohmic shunts although the cell’s edges have been perfectly insolated. Even in the absence of such shunts, the characteristics are linear or super-linear under reverse bias, while a saturation would be classically expected. Especially in multicrystalline cells the breakdown does not tend to occur at -50 V reverse bias, as expected, but already at about -15 V or even below. These deviations are typically caused by extended defects in the cells. This paper reviews the present knowledge of the origin of such non-ideal I-V characteristics of silicon solar cells and introduces new results on recombination involving coupled defect levels.