Klaus Ramspeck

Series resistance imaging of solar cells by voltage dependent electroluminescence

This letter introduces a method based on electroluminescence imaging to determine mappings of the local series resistance of large area semiconductor devices such as solar cells. The method combines the local electroluminescence emission Φi(U) and its derivative Φi′(U) with respect to the applied voltage U. The combined analysis of these two quantities yields the local series resistance Rise and proves the physical validity of the used current transport model and thus the physical relevance of the determined Rise value. The method is verified on a monocrystalline silicon solar cell with local shunts and local series resistance problems.

Recombination current and series resistance imaging of solar cells by combined luminescence and lock-in thermography

We perform recombination current and series resistance imaging on large-area crystalline silicon solar cells using a combined analysis of camera-based dark lock-in thermography (DLIT) and electroluminescence (EL) imaging. The solar cells are imaged both by DLIT and EL under identical operating conditions. The quantitative analysis of the DLIT measurement produces an image of the local heating power and the EL picture results in an image of the local cell voltage. Combining the two images pixel by pixel allows us to calculate images of the local recombination current and the local series resistance of the solar cell.

Explanation of commonly observed shunt currents in c-Si solar cells by means of recombination statistics beyond the Shockley-Read-Hall approximation

The current-voltage (I–V) characteristics of industrially fabricated, crystalline silicon solar cells are often influenced by non-linear shunts that originate from localized, highly disturbed regions and cause ideality factors n > 2. We show that recombination within such locations needs model descriptions that go beyond the Shockley-Read-Hall (SRH) approximation, because the density of defects is so high that recombination does not occur via isolated, but coupled defect states. We use a variant of coupled defect level (CDL) recombination, the donor-acceptor-pair (DAP) recombination, but via deep levels (as opposed to shallow levels). With this model, we quantitatively reproduce the I–V curves of solar cells that we subjected to various degrees of cleaving, laser scribing or diamond scratching to form shunt locations in a controlled manner. The suggested model explains the transition from ideality factors n   2 when going from low to high defect densities. We also explain the non-saturating rever...

Dynamic carrier lifetime imaging of silicon wafers using an infrared-camera-based approach

We present a calibration-free dynamic infrared carrier lifetime mapping technique, yielding images of the carrier lifetime of multicrystalline silicon wafers within seconds. Images of the infrared emission of the sample under test are taken directly after switching on a monochromatic illumination source and after steady-state conditions have been established in the sample. Making use of the proportionality between the infrared emission and the free carrier density inside the sample, the carrier lifetime is calculated from the signal ratio of these two images by an analytical method. We achieve an excellent agreement when comparing our results with carrier lifetime mappings obtained by the microwave-detected photoconductance decay technique.

Luminescence emission from forward- and reverse-biased multicrystalline silicon solar cells

We study the emission of light from industrial multicrystalline silicon solar cells under forward and reverse biases. Camera-based luminescence imaging techniques and dark lock-in thermography are used to gain information about the spatial distribution and the energy dissipation at pre-breakdown sites frequently found in multicrystalline silicon solar cells. The pre-breakdown occurs at specific sites and is associated with an increase in temperature and the emission of visible light under reverse bias. Moreover, additional light emission is found in some regions in the subband-gap range between 1400 and 1700 nm under forward bias. Investigations of multicrystalline silicon solar cells with different interstitial oxygen concentrations and with an electron microscopic analysis suggest that the local light emission in these areas is directly related to clusters of oxygen.

Determination of the effective diffusion length of silicon solar cells from photoluminescence

We present a method to determine the effective diffusion length Leff of silicon solar cells from photoluminescence (PL) measurements carried out under two different operating conditions. Measuring the photoluminescence emission under open circuit condition (PL-oc), where the solar cell is not contacted at all, and short circuit condition (PL-sc), where the solar cell is held at zero voltage, Leff directly follows from the ratio of the PL-oc and the PL-sc signals. Detailed knowledge about the optical properties of the experimental setup is not necessary since the optical properties cancel out completely. We explain the theoretical background of our method and derive an analytical description for the PL-oc and the PL-sc luminescence emissions. The applicability of our method is demonstrated by the comparison of effective diffusion lengths from PL measurements with values determined from the analysis of internal quantum efficiency measurements.

Influence of Defects on Solar Cell Characteristics

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.

Combined dynamic and steady-state infrared camera based carrier lifetime imaging of silicon wafers

We report on a calibration-free dynamic carrier lifetime imaging technique yielding spatially resolved carrier lifetime maps of silicon wafers within data acquisition times of seconds. Our approach is based on infrared lifetime mapping (ILM), which exploits the proportionality between the measured infrared emission and the free carrier density. Dynamic ILM determines the lifetime analytically from the signal ratio of infrared camera images recorded directly after turning on an excitation source and after steady-state conditions are established within the sample. We investigate the applicability of dynamic infrared lifetime mapping on silicon wafers with rough surfaces, study the impact of injection dependencies, and examine the technical requirements for measuring low lifetime values in the range of microseconds. While the dynamic ILM approach is suitable for lifetimes exceeding 10μs, a combination with steady-state ILM is required to measure lifetime values in the range of 1μs. The injection dependence d...

Interpretation of the Commonly Observed I-V Characteristics of C-SI Cells Having Ideality Factor Larger Than Two

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

Imaging Techniques for the Analysis of Silicon Wafers and Solar Cells

For large area devices a spatial analysis of local device and material parameters is essentially important. Imaging techniques allowing a fast and contactless analysis with a high spatial resolution have become a versatile characterization tool during the last decade. We present a comprehensive overview over the existing imaging techniques for the analysis of silicon wafers and solar cells utilizing different spectral ranges of photon emission. Additionally, we report on recent studies of local junction breakdown and the emission of light from solar cells under forward and reverse bias using luminescence imaging and dark lock-in thermography. Finally we present a calibration-free dynamic infrared carrier lifetime mapping (dynamic-ILM) technique, yielding images of the carrier lifetime of multi-crystalline silicon wafers within seconds.