Study of the effects of shading in thin-film perovskite PV modules

Abstract

People worldwide have begun to feel the effects of climate change in recent years with unpredictable weather patterns, heatwaves, changes in precipitation, and sea level rise. To help mitigate climate change, photovoltaics (PV) is seen as a technology with the tremendous capability to satisfy the world’s electricity demands by tapping into renewable energy from the sun. There are various PV technologies available, each with its challenges; the newest upcoming material is perovskites. They have showcased an immense potential of further lowering the production costs of solar modules, lower material consumption, and promising high conversion efficiencies. The single cell efficiency has seen incredible improvement in the last decade to 25.7%. The next stage is the scaling up to module level and outdoor deployment. However, some significant challenges remain towards achieving this; one is its response to shading, which has been studied little thus far and will be investigated in this study. The increasing integration of PV into the close surroundings also means that integrated PV will be exposed to much more shading than a typical panel in a solar farm. It is imperative to understand the behavior of perovskite solar cells (PSCs) to shade, primarily due to the poor reverse bias behavior reported in recent years. Since the cells are typically connected in series, shaded cells dissipate energy by going into negative voltages. The response to shading of any material also depends on the interconnection method adopted for the solar cells. So, several interconnections were identified in the project, a few including bypass diodes to improve shade tolerance. Due to the poor reverse bias characteristics of the PSCs, the interconnections and shading were modeled using Matlab and Simulink to better understand the interconnection configuration with the least power mismatch losses. Total Cross Tied (TCT) with bypass diodes was identified as the optimal configuration. Furthermore, the TCT interconnection with bypass diodes was tested experimentally on a shading setup to validate the model. The experiments confirmed the modeled results. None of the modules degraded under shading for the interconnection as the cells did not experience large enough reverse bias voltages. Additionally, tests were conducted to gain deeper insights into the degradation mechanism of perovskites. Shading was tested on a single module, parallel and perpendicular to laser scribes, to confirm that the degradation was due to reverse bias voltage. When the module was exposed to an Electroluminescence (EL) test after shading, it revealed the degradation through the formation of dark spots. The degradation is speculated to form due to the formation of shunt paths caused by the migration of copper into the absorber layer. A similar test was conducted on semi-transparent modules that did not lead to degradation. Also, the modules were tested to find their reverse breakdown voltage. It was observed to breakdown at -6.5V, translating to a cell breakdown at -1.6V. Interestingly, the module’s performance was slightly regained under light soaking for one hour in forward bias due to the reversal of the ion movement, which occurred in the reverse bias condition. The research into the shading of PSCs is still in the early days; many further topics for the future are proposed that could be interesting for later research.