Papers by Jan-Martin Wagner
SiliconPV 2021, The 11th International Conference on Crystalline Silicon Photovoltaics
In this paper the CELLO technique (solar CELl LOcal characterization) is used for the local chara... more In this paper the CELLO technique (solar CELl LOcal characterization) is used for the local characterization of a-Si/µ-Si tandem solar cells. Applying a blue and an infrared laser in a confocal beam with different frequencies for the modulation of the intensities, the cells can be analyzed with respect to local current matching. Combining these data with e.g. the 2 nd harmonic response, i.e. the local non-linear interaction of the shortcircuit photocurrents generated by both lasers, additional local properties like the diode characteristics (leakage currents) of both sub-cells, or local series resistances can be analyzed. Several examples for the identification of local defects by combining CELLO maps will be discussed.
Chinese Physics Letters, 2006
ABSTRACT We investigate hexagonal BC2N in graphite unit cells using the first-principles method a... more ABSTRACT We investigate hexagonal BC2N in graphite unit cells using the first-principles method and calculate the total energies, lattice parameters, and electronic band structures after full relaxation. It is shown that stable hexagonal BC2N should be stacked sequentially with one graphite layer and one h-BN layer. The density of states indicates that this structure should have metallicity.
Peltier effect: "heat current" (not: "flow"!) accompanying an electric current, not directly obse... more Peltier effect: "heat current" (not: "flow"!) accompanying an electric current, not directly observable (in contrast to temperature-gradient-driven heat flow) General theory: thermoelectricity basics 3a Retreat 2009
Vibrational properties of bulk crystalline barium halides BaCl 2 , BaBr 2 , and BaI 2 are theoret... more Vibrational properties of bulk crystalline barium halides BaCl 2 , BaBr 2 , and BaI 2 are theoretically investigated for orthorhombic and hexagonal symmetry with ab initio methods in density functional theory. It is demonstrated that the used method is capable of predicting frequencies of vibrational modes, their symmetry types, and corresponding Raman intensities in reasonable agreement with experimental data for orthorhombic and hexagonal BaCl 2 and BaBr 2 nanocrystals embedded in fluorozirconate glasses. For orthorhombic BaCl 2 , a Raman-active phonon mode could be predicted theoretically that was not observed in measurements before.
Lateral balancing currents are an immanent feature of solar cells with an inhomogeneous distribut... more Lateral balancing currents are an immanent feature of solar cells with an inhomogeneous distribution of photocurrent Iph(x,y) and/or diode current j01(x,y) leading to additional power losses and thus to a significant increase of difficulty to analyze local efficiencies of solar cells. Due to fundamental physical restrictions like charge conservation and having a potential distribution across a 2D grid network, astonishingly only the histogram information (no local information!) of images of recombination strength and series resistance is needed to calculate all relevant average information. This is completely analogous to the technique of frequency analysis used for code breaking used for more than 1000 years where just a histogram analysis allows to identify the meaning and the position of all (most) characters within a text. In this contribution, the theory and quantitative results for several inhomogeneous solar cells with different kinds of lateral balancing currents are presented.
Renewable energy research is currently a major challenge. In recent years, the worldwide producti... more Renewable energy research is currently a major challenge. In recent years, the worldwide production of crystalline silicon solar cells has increased by approximately 30%.[1] New concepts for solar cells and new silicon feedstock are generated in the labs and on the solar cell market. Hence, the need for quality control and failure analysis of solar cells in research and industry has increased rapidly. This article demonstrates the possibilities of lock-in thermography (LIT) techniques for detailed failure analysis of solar cells. Lock-in thermography is well established for failure analysis application in integrated circuits (ICs). Special LIT techniques allow the evaluation of failure positions in ICs with good spatial resolution.[2,3] However, LIT is also a unique tool for characterization of solar cells. Currently, LIT techniques for solar cell research are fully developed and provide a wide range of applications for qualitative and quantitative analysis of solar cell parameters....
24Th European Photovoltaic Solar Energy Conference 21 25 September 2009 Hamburg Germany, Nov 18, 2009
We report on studies of the emission of light from industrial multicrystalline silicon solar cell... more We report on studies of the emission of light from industrial multicrystalline silicon solar cells under forward and reverse bias. Camera-based luminescence and dark lock-in thermography imaging techniques are used to study the spatial distribution of and energy dissipation at pre-breakdown sites. The pre-breakdown is associated with an increase in temperature and the emission of visible light. Three different breakdown types are distinguished by investigating the local luminescence and dark lock-in thermography signal as a function of the reverse bias. Moreover, we show that specific areas characterized by recombination active fine dendritic lines also give rise to sub-band-gap luminescence. This luminescence has a broad wavelength distribution with a maximum at around 1550 nm. Studies of multicrystalline silicon solar cells with different interstitial oxygen concentrations and electron microscopic analysis suggest that this light emission is directly related to clusters of oxygen.
An actuator is a mechanical device for moving or controlling a mechanism or system. Traditional a... more An actuator is a mechanical device for moving or controlling a mechanism or system. Traditional actuators include pneumatic actuators, electronic actuators, motors, and hydraulic cylinders. The weight, limited size, complex transmission, and restrictive shape of such actuators have led researchers to investigate alternative technologies for various applications. Electroactive polymers (EAPs) are polymers that respond to electrical stimulation with a significant size or shape change and are emerging as a new class of ...
Journal of Physics: Condensed Matter, 1996
ABSTRACT The plane-wave pseudopotential approach to the density-functional theory (DFT) in the lo... more ABSTRACT The plane-wave pseudopotential approach to the density-functional theory (DFT) in the local-density approximation (LDA) is used for an ab initio calculation of pressure-dependent structural, lattice-dynamical, and dielectric properties of cubic silicon carbide (3C SiC). Whereas the influence of hydrostatic pressure on the structural and the underlying bonding properties is treated within the DFT-LDA, the linear-response theory is applied to describe the pressure dependence of the zone-centre phonon frequencies, the dielectric constant, and the Born effective charge of 3C SiC. Furthermore, the results for 3C SiC are compared with the corresponding ones for silicon and diamond.
Journal of Applied Physics, 2013
The light-beam-induced current-based CELLO measurement technique (solar CELl LOcal characterizati... more The light-beam-induced current-based CELLO measurement technique (solar CELl LOcal characterization), originally developed for wafer-based silicon solar cells, can successfully be applied to thin-film solar cells, provided that contacting of a single cell is possible. This is shown exemplarily for several crystalline silicon on glass samples, having varying quality with respect to photocurrent extraction, series resistance, and power losses. For the latter, a comparison with results obtained from dark lock-in thermography gives quantitative agreement, provided that the cells are not severely shunted. V
Applied Surface Science, 2008
Guided by the interpretation of CELLO open circuit voltage measurements a model for calculating l... more Guided by the interpretation of CELLO open circuit voltage measurements a model for calculating local serial resistance maps Rser(x, y) from luminescence images is introduced which uses the Taylor series in linear order of the extracted current starting from open circuit condition. This approach is in strong contrast to the model of independent diodes frequently used for obtaining Rser(x, y) maps from luminescence data. An optimal mode when just using two luminescence maps is introduced, together with two additional variants, one which just needs two pure electroluminescence maps and one which is completely contactless. Especially the contactless "shaded luminescence" mode has a high potential to be used as inline measurement technique. On several examples the results of the three modes are compared. Limitations and possible improvements in data evaluation for the two additional modes are discussed. For typical mc-Si solar cells all three modes give similar quantitative re...
International Journal of Modelling and Simulation
physica status solidi (a)
The standard equivalent circuit of a solar cell amounts to a lumped description by separate diode... more The standard equivalent circuit of a solar cell amounts to a lumped description by separate diode and resistor elements. As its application to a large-area silicon solar cell effectively implies averaging the emitter resistance which, however, is closely coupled to the p-n junction, it is not self-evident that it works more or less well. Using an analytically solvable distributed series resistance model and systematically treating the deviations from the ideal case of zero emitter resistance, the equivalent circuit is found in linear order in the sheet resistivity. In this linear order, the lumped voltage losses are fully compatible with the integrated Joule losses; this compatibility turns out to be a necessary and sufficient condition for modeling the local series resistance of a large-area silicon solar cell. In higher orders of the sheet resistivity, however, the lumped voltage losses are not compatible with the integrated Joule losses, which means that the equivalent circuit cannot describe these higher orders. The equivalent circuit resulting from the linear-order lumped series resistance accounts for the experimentally observed variation of the lumped series resistance along the current-voltage characteristic, which turns out to be fully described by a dependence on the dark diode current only.
15th International Conference on Concentrator Photovoltaic Systems (CPV-15)
From its definition based on lateral voltage drops, the local series resistance isn't an arithmet... more From its definition based on lateral voltage drops, the local series resistance isn't an arithmetically averageable quantity because voltage isn't extensive; however, only such quantities can be averaged. Still, empirically it was found that averaging series images arithmetically provides rather reasonable results. Since the emitter of a large-area silicon solar cell is nearly an equipotential layer, one can linearize the description of the lateral voltage distribution (with respect to the forward-bias current and the emitter resistivity), from which we obtain an averageable local series resistance for the H-type grid geometry; its averageability stems from averaging the parallel paths of vertical current flow. By construction, the average obtained from this linear response theory is identical to the lumped value coming from the illumination intensity variation method, and it also shows the experimentally observed dependency on the forward-bias diode current.
Luminescence imaging, both in electroluminescence and photoluminescence mode, is employed to test... more Luminescence imaging, both in electroluminescence and photoluminescence mode, is employed to test the widely accepted notion that the lumped series resistance, Rs, measured in the dark, Rs,dark, differs from its value measured under illumination, Rs,light. Using the illumination intensity variation method to determine the series resistance allows us to treat measurements in the dark and under illumination on equal footing. We find (i) the already-known variation of Rs along the current-voltage characteristic in dependence on the total dark current ID and (ii) that for standard operation conditions there is no difference between Rs,dark and Rs,light but only for external currents Iext exceeding the 1-sun photocurrent. Previously reported differences are traced back to systematic errors in the Rs evaluation schemes used in those works.
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Papers by Jan-Martin Wagner