Investigation and characterization of crystalline silicon solar cells for indoor and low light applications

Energy harvesting systems are electronic systems which harvest the needed energy from the ambiance and which are thus self-sufficient. Such systems find application in rather remote outdoor areas (e.g. in forests) or indoors. In both application scenarios, the available illumination densities are two to three orders of magnitude below the given illumination densities in standard photovoltaic applications. Therefore, the solar cell behavior under low illumination densities (0.1 mW/cm² to 10 mW/cm²) and under artificial light sources is of importance for the application of solar cells in energy harvesting systems. Within this work, the current state of the art for solar cells in indoor and low light applications is presented with the focus on solar cell characterization and optimization. This work offers new findings and approaches in both fields. For the characterization of solar cells under low illumination densities and under artificial light sources, three different characterization methods or approaches are presented in this work. Each method addresses a different issue of indoor and low light solar cell characterization. For the characterization of solar cells under low illumination densities, a new method is presented which is based on common measurement techniques. This new method simplifies the characterization of solar cells under low illumination densities compared to the existing method which is rather arduous. The new method is based on the finding that the series resistance effects are vanishing towards low illumination densities. Furthermore, the new method can also be used for the evaluation of non-metallized test structures providing a reduction in processing. A second method focuses on the characterization of solar cells under indoor conditions. Here, the behavior of the solar cell under different (artiffcial) spectra and illumination densities is of interest. The aim of the new approach is avoiding an arduous characterization where the whole IV curve of a solar cell is measured under various spectra for different illumination densities. Therefore, the spectral scaling factor is introduced. This factor allows the estimation of the illumination-dependent performance of a solar cell under various spectra. The third method aims on a unified description of indoor conditions. The here presented approach is based on the separation of given illumination densities and given spectral distributions. The given spectral distributions are described as a superposition of known spectra. This allows the comparison of different given indoor conditions which is not possible so far. For the investigation of the influences of differing spectra and illumination densities on the behavior of a solar cell, simulation and experimental results are evaluated. It is shown that the Shockley-Queisser-Limit is not an appropriate model to describe solar cells under low illumination densities because the important limiting effects are not included. For the optimization of solar cells for indoor and low light applications, those limiting effects need to be minimized. It has been shown that the edge recombination has a major impact on the performance of crystalline silicon solar cells under the low illumination densities. The investigation and minimization of this loss is therefore another focus of this work. The formation of emitter windows (local emitter area which are not bordering the sample edge) is known to reduce the edge recombination and to improve the performance under low illumination densities. In this work, the impact of different passivation layer (silicon dioxide, aluminum oxide, and silicon nitride) on the edge recombination is firstly investigated in combination with emitter windows. Therefore, test structures are fabricated, characterized and evaluated. Applying the new characterization method, it is shown that the edge recombination quality of emitter windows dependents on the passivation layer outside of the emitter window. The minimum edge recombination current of 0.6 nA/cm was achieved with aluminum oxide as passivation layer outside of the emitter window. This is a reduction in edge recombination by a factor of eight compared to an unpassivated edge. In contrast, the silicon nitride passivated sample shows edge recombination losses which cause a worse performance than an unpassivated edge. This behavior and its causes are firstly discussed in this work. Those losses are attributed to the inversion layer due to the fixed positive charges which cause a conductive connection of the emitter window and the edge. The formation of an inversion layer has already been reported earlier for silicon nitride in another context. A similiar effect is observed for silicon dioxide as passivation outside of the emitter window when reducing the distance between emitter window and edge. Furthermore, it is firstly shown that the losses can be modeled in analogy to the resistance-limited recombination. Furthermore, small screen-printed solar cells are processed and evaluated with respect to their performance at low illumination densities. It is shown that the need of contact structure optimization vanishes at low illumination densities. In agreement with the results gained on symmetric test structures, the edge passivation is of importance for the performance at low illumination densities. Low edge recombination currents of 0.7 nA/cm and 1.0 nA/cm are achieved by emitter windows passivated by aluminum oxide or silicon dioxide in the outside region, respectively. Considering these values, the edge passivation by the formation of emitter windows outperforms other published passivation techniques. With these results, all-screen-printed solar cells are firstly optimized for low-illumination densities yielding 13.9% effciency at an illumination of 0.15 mW/cm².