Martin Kasemann

Determination of local minority carrier diffusion lengths in crystalline silicon from luminescence images

In 2007 Wurfel et al. [J. Appl. Phys. 101, 123110 (2007)] introduced a method to determine spatially resolved minority carrier diffusion lengths in silicon solar cells from electroluminescence intensity ratios. The key feature of this method was the exploitation of reabsorption of luminescence within a solar cell through optical short pass filters. The first part of this work deals with some experimental challenges which we encountered with the practical application of this method. A procedure of removing an artifact due to typical lateral filter inhomogeneities is introduced. Moreover, temperature dependence of luminescence is discussed and incorporated into the underlying model. The second part of this work aims at a determination of spatially resolved carrier diffusion lengths from photoluminescence (PL) on silicon wafers. The shortcomings of a diffusion length determination from PL intensity ratios are discussed. A straightforward method to determine spatially resolved integral excess charge carrier d...

Diode breakdown related to recombination active defects in block-cast multicrystalline silicon solar cells

Solar cells in modules are reverse biased when they are shaded. This can lead to diode breakdown and eventually to the occurrence of hot spots, which may, in the extreme case, destroy the module by thermal degradation. We observed at least three different types of diode breakdown in multicrystalline silicon solar cells. One of them is found to be related to the recombination activity of defects. This type is indicated by a slow increase in the reverse current with reverse bias and a relatively low breakdown voltage around −10 V. The local breakdown voltage depends significantly on the level of contamination of the material. When the solar cell is reverse biased, the breakdown sites emit bright light which shows a broad spectral distribution in the visible range with a maximum at 700 nm.

Comparison of luminescence imaging and illuminated lock-in thermography on silicon solar cells

Spatially resolved electroluminescence (EL) and photoluminescence (PL) images of solar cells are compared to spatially resolved power loss images obtained by illuminated lock-in thermography (ILIT). A significant difference is shown for a solar cell with shunts, while series resistance and charge carrier recombination cause only minor differences in the images. The PL image of a solar cell with shunts appears highly blurred in the shunted region. The origin of this effect is discussed, and a circuit simulation with an appropriate solar cell model is performed. The authors conclude that the blurring of shunted regions is inherent in the method of EL/PL imaging and that ILIT is advantageous for localizing shunts.

Spatially resolved determination of the dark saturation current of silicon solar cells from electroluminescence images

We present a novel method to determine spatially resolved the dark saturation current of standard silicon solar cells. For this two electroluminescence images are taken at two different voltages. From these two images, first the spatial voltage distribution can be calculated. Second by applying the Laplacian to the voltage image from Ohm’s law and the continuity equation, the current through the device at a certain position can be determined. Knowing the local current through the device, the local voltage, and the emitter sheet resistance allows to determine the local dark saturation current. The clue of this method is to cope with the noise by using an appropriate noise reduction algorithm. By simulating electroluminescence images with realistic noise and known dark saturation current we demonstrate the applicability of the method with our noise reduction algorithm. Experimentally we compare our method with spectral response light beam induced current on multicrystalline solar cell.

Spatially resolved silicon solar cell characterization using infrared imaging methods

We present a comprehensive overview over infrared imaging techniques for (electrical) silicon solar cell characterization. Recent method development in local series resistance imaging is reviewed in more detail and new results in local breakdown investigations on multicrystalline (mc) silicon solar cells are reported. We observe local junction breakdown sites on industrial mc-cells at reverse voltages as low as −7V and breakdown in great areas of the cell at voltages around −14V. As these breakdown sites (as well as local shunts) can cause hot spots which can damage the cell and the module, we also present an ultra-fast, simple and quantitative method for hot-spot detection. Typical measurement times in the order of 10 milliseconds are achieved.

Physical mechanisms of breakdown in multicrystalline silicon solar cells

We have identified at least five different local breakdown mechanisms according to the temperature coefficient (TC) and slope of their characteristics and electroluminescence (EL) under reverse bias. These are (1) early pre-breakdown (strongly negative TC, low slope), (2) edge breakdown (positive TC, low slope, no EL), (3) weak defect-induced breakdown (zero or weakly negative TC, moderate slope, 1550 nm defect luminescence), (4) strong defect-induced breakdown (zero or weakly negative TC, moderate slope, no or weak defect luminescence), and (5) avalanche breakdown at dislocation-induced etch pits (negative TC, high slope). The latter mechanism usually dominates at high reverse bias. In addition to the local breakdown sites there is evidence of an areal reverse current between the dominant breakdown sites showing a positive TC. Since defect-induced breakdown shows a zero or weakly negative TC and also leads to weak avalanche multiplication, we propose defect level-induced avalanche instead of trap-assisted tunneling to be responsible for this breakdown mechanism.

Evaluating Crystalline Silicon Solar Cells at Low Light Intensities Using Intensity-Dependent Analysis of I–V Parameters

This paper discusses the influence of different solar cell loss mechanisms at low light intensities and presents a simple method for the analysis of solar cell performance under various illumination intensities below 1 sun. Suns-PL and Suns - Voc are used to measure the intensity-dependent pseudo I-V curves of symmetric test structures and of finished silicon solar cells in an intensity range between 1 sun and 10-3 suns. The solar cell parameters from the pseudo I-V curves are compared with the parameters evaluated by intensity-dependent measurements of the whole I-V curve. The pseudo efficiency and pseudo fill factor are found to be in good agreement with the real values at low intensities as the influence of the series resistance vanishes. Based on this finding, we compare the passivation quality of silicon dioxide and silicon nitride in combination with emitter windows on test structures. Above 0.1 suns, both passivation layers show similar performance. Below 0.1 suns, the pseudo fill factors and pseudo efficiencies of the silicon nitride passivated sample are strongly reduced compared with the sample with silicon dioxide. The open-circuit voltage starts differing below 0.01 suns.

Defect-Induced Breakdown in Multicrystalline Silicon Solar Cells

We have identified at least five different kinds of local breakdown according to the temperature coefficient (TC) and slope of their characteristics and electroluminescence (EL) under a reverse bias. These are 1) early prebreakdown (negative TC, low slope), 2) edge breakdown (positive TC, low slope, no EL), 3) weak defect-induced breakdown (zero or weakly negative TC, moderate slope, 1550-nm defect luminescence), 4) strong defect-induced breakdown (zero or weakly negative TC, moderate slope, no or weak defect luminescence), and 5) avalanche breakdown at dislocation-induced etch pits (negative TC, high slope). The latter mechanism usually dominates at a high reverse bias. The defects leading to the etch pits are investigated in detail. In addition to the local breakdown sites, there is evidence of an areal reverse current between the dominant breakdown sites showing a positive TC. Defect-induced breakdown shows a zero or weakly negative TC and also leads to weak avalanche multiplication. It has been found recently that it is caused by metal-containing precipitates lying in grain boundaries.

Emissivity-corrected power loss calibration for lock-in thermography measurements on silicon solar cells

This paper describes power loss calibration procedures with implemented emissivity correction. The determination of our emissivity correction matrix does neither rely on blackbody reference measurements nor on the knowledge of any sample temperatures. To describe the emissivity-corrected power calibration procedures in detail, we review the theory behind lock-in thermography and show experimentally that the lock-in signal is proportional to the power dissipation in the solar cell. Experiments show the successful application of our emissivity correction procedure, which significantly improves the informative value of lock-in thermography images and the reliability of the conclusions drawn from these images.

Ultrathin Titanium Dioxide Nanolayers by Atomic Layer Deposition for Surface Passivation of Crystalline Silicon

We demonstrate a low surface recombination velocity of 14 cm/s with only 1.5 nm thin titanium dioxide (TiO2) layers on undiffused 10 Ωcm p-type crystalline silicon. The TiO2 nanolayers were deposited by thermal atomic layer deposition at 150 °C and 200 °C substrate temperatures using tetrakis-dimethyl-amido titanium as the Ti precursor and water as the oxidant. The influence of a post-deposition anneal in forming gas at different temperatures was investigated. We have observed that a subsequent anneal in forming gas at 350 °C enhances the surface passivation quality of the TiO2 layers tremendously. Increasing the thickness of the TiO2 layers leads to a reduction of the surface passivation quality. Introducing a thin interfacial layer of silicon oxide (1.6 nm) grown by rapid thermal oxidation underneath the TiO2 layer improves the surface passivation of thicker TiO2 layers (5.5 and 15 nm). These results show that ultrathin TiO2 layers with a thickness of only 1.5 nm can be used to effectively passivate the c-Si surface.