A. Schütt

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.

Ohmic loss analysis for lateral balancing currents by CELLO and photoluminescence measurements

Inhomogeneous local photocurrent generation, as typical for multicrystalline silicon (mc-Si) solar cells, leads to lateral balancing currents, occurring also under open-circuit conditions. In general, all currents passing emitter and grid of a solar cell lead to ohmic losses which increase with the distance the currents have to flow through grid and emitter. Therefore, for the ohmic losses related to lateral balancing currents, the distribution of sites with low photocurrent production plays a crucial role: a 2-D clustering leads to significantly larger losses than a 1-D arrangement (or even an isolated occurrence) of such sites. These ohmic losses can be made visible both in CELLO and luminescence series resistance measurements, which also show that the strength of the losses varies with the coupling of such clusters to the grid.

Solving the Code of Series Resistance on Large Area Solar Cells: Average and Local Power Losses of External and Lateral Balancing Currents

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.

Qualitative and quantitative evaluation of thin-film solar cells using solar cell local characterization

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.