Entwicklung neuer Verfahren zur räumlich hochauflösenden Charakterisierung von Solarzellen

Today’s raising demand for energy relies to a degree of 85% on the consumption of fossil fuels. A change to regenerative forms of energy is an important and inevitable step in order to face the challenges of climate change and fading natural resources. Photovoltaic’s (PV) plays a special role within the various forms of renewable energy since it converts sunlight, our most important and virtually endless energy source, directly into electricity. However, currently available PV-systems are still very expensive and, in combination with their relatively low performance, can hardly or cannot compete with conventional sources of energy from an economical point of view. One possibility to overcome this problem is the combination of highly efficient multi junction solar cells with cost-efficient concentrator optics that focus the incident sunlight to a small spot. The material system (GaIn)(NAs) is envisioned to play an important role in a future generation of multi junction solar cells for concentrator applications being a further development of existing device concepts. However, especially the carrier diffusion lengths in (GaIn)(NAs)-based solar cell layers are currently to low for the fabrication of highly efficient PV-structures. In this work, two novel techniques for the characterization of solar cells are developed and evaluated by experiments on test structures and numerical simulations. Both are based on the measurement of laser-induced currents. Spatially-resolved photocurrent spectroscopy (SRPS) allows a spatially-resolved determination of locally induced photocurrents at a fixed bias voltage while spatially-resolved IV-characteristics (SRIV) are measurements of local I-V-characteristics at a certain position. It is found that SRPS and SRIV allow for a reliable and meaningful characterization of solar cell prototypes with a high spatial resolution. Especially the local p-n-parameters of the sample become accessible. These are the short circuit current, the saturation current, the ideality factor and the excitation-induced shunt resistance. Subsequently, SRPS and SRIV served as characterization techniques for the evaluation of annealing experiments on processed (GaIn)(NAs) solar cell layers for concentrator applications. The goal of these experiments was to improve the material quality of such structures by treating them with high electrical currents and strong lasers pulses. Such enhanced devices could be used in a future generation of PV-systems leading to a significantly increased efficiency. Concerning this some first achievements were made. The use of high electrical currents led to a significant increase of the short circuit current, especially if applied as reverse bias. However, this also caused a reduction of the shunt resistance which eventually dropped down to a few ohms. The application of intensive laser radiation led under certain conditions to a considerable increase of the carrier lifetime and photoluminescence but no improvement of the electrical properties of the devices could be found. A second project dealt with the effect of carrier-depletion on the evaluation of data obtained by the variable stripe length method which is a technique for measuring the optical gain of potential laser materials. Numerical simulations and accompanying experiments on a GaAs-quantum well structure showed that this approach is only valid within a small parameter range. Outside this range an unavoidable carrier depletion caused by stimulated emission processes leads to significant errors in the obtained results. The determination of the corresponding borders is very complicated and requires a detailed knowledge of the sample properties. As the variable stripe length method is typically used to get a first insight into the properties of a novel sample such knowledge is generally missing. Therefore the reliability of experimental results obtained by this technique is doubtful.