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[en] Recently, the modelling investigation on thin film chalcogenide solar cells has been focused on the front region of the devices as a possible source of fill factor losses through a light dependent barrier modulation. We present a model based on a modulated barrier photodiode in series with the main junction, which consistently explains apparent quantum efficiency measurements and, light and dark IV characteristics of CdTe devices. This bulk barrier forms in the front region of the device as a consequence of an increased compensation of donors in CdS, which can induce crossover and in extreme cases rollover in the IV curves. The internal resistance due to this majority carrier barrier, which depends on the irradiance level, is an important limiting factor for cell efficiency, inherent to the cell structure. Numerical simulation gives consistent results with the observed measurement characteristics
[en] Variable wavelength and voltage OBIC maps of different CdTe devices with intermediate lifetime (∼2 years) have evidenced the anomalous formation of negative photocurrent areas in forward bias conditions. These features have remarkable dimensions in the order of some millimetres, characteristic spectral distribution in different region of the spectrum and sub-bandgap generation tails. The maps were traced at wavelengths ranging from 350 to 1050 nm, with a spot size of approximately 300 μm, and a table resolution of 150 μm. An analysis of these phenomena is rather complicated, and only partially understood. However, the analysis of apparent quantum efficiency provides a qualitative picture for the formation of these reverse currents and deterioration of local and combined IV characteristics. There are two possible causes for this inversion, namely distributed series resistance and light-modulated barrier phenomena. In particular, the latter can be induced by diffusion of species within the device and related compensation effects, determining degradation and fundamental efficiency limits in CdTe solar cells
[en] By now, extensive experimental research is available on thin film solar cells based on CdTe and on CIGS, and their electrical and optical behaviour is characterised by a multitude of diverse characterisation techniques. At the same time, numerical simulation programmes have matured and are available to the research community to assist in interpreting these measurements consistently. Once multiple measurements are (more or less) quantitatively described, the numerical simulation can be used to explore the effect of a variation of materials parameter (e.g. the presence or absence of a property, or variation in a range of values) to the final solar cell characteristics. Examples of such analysis for CdTe solar cells are shown. In CdTe cells, much research has been devoted to the activation treatment of the absorber, and to the technology of the back contact. Analysis of ample measurements has evidenced the crucial role of the profile of the (effective) doping density through the device. It will be illustrated how this relative simple (but hardly mastered) materials property has a far reaching influence to the cell characteristics such as roll-over and cross-over of I-V curves, also in dependence on illumination and voltage, conventional and apparent quantum efficiency, and finally fill factor and efficiency
[en] We demonstrate a self-consistent numerical scheme for simulating an electronic device which contains active defects. As a specific case, we consider copper defects in cadmium telluride solar cells. The presence of copper has been shown experimentally to play a crucial role in predicting device performance. The primary source of this copper is migration away from the back contact during annealing, which likely occurs predominantly along grain boundaries. We introduce a mathematical scheme for simulating this effect in 2D and explain the numerical implementation of the system. Finally, we will give numerical results comparing our results to known 1D simulations to demonstrate the accuracy of the solver and then show results unique to the 2D case
[en] We present a model describing the undesired roll-over which is a well-known phenomenon in the current-voltage characteristics of CdTe solar cells. Therein, the roll-over is ascribed to a Schottky barrier at the back contact which is effective as a reverse diode. The formation of this barrier is investigated depending on the CdTe absorber thickness as well as on the employed back contact metal. Computer simulations of the energy band diagram reveal that the back contact barrier can be reduced and even eliminated for sufficiently thin absorbers. The reason is the spatial overlap between the space-charge regions of the p-n heterojunction with the one of the back contact. This behaviour correlates with experimental current-voltage data of solar cells with a simple gold back contact. In the latter, the roll-over is considerable for absorbers with 3 to 5 μm thickness, diminishes when the absorber thickness is reduced and finally vanishes when the absorber thickness is approximately 1 μm. The investigations show that thickness reduction can be employed in order to suppress the roll-over phenomenon in CdTe solar cells.