Perovskite solar cells represent one of the most promising emerging photovoltaic technologies due to their high power conversion efficiency. However, despite of the huge progress made not only in terms of the efficiency achieved, but also fundamental understanding of relevant physics of the devices and issues which affect their efficiency and stability, there are still unresolved problems and obstacles on the path towards commercialization of this promising technology. In this roadmap, we aim to provide a concise and up to date summary of outstanding issues and challenges, and progress made towards addressing these issues. While the format of this article is not meant to be a comprehensive review of the topic, it provides a collection of the viewpoints of the experts in the field which covers a broad range of topics related to perovskite solar cell commercialization, including those relevant for manufacturing (scaling up, different types of devices), operation and stability (various factors), and environmental issues (in particular the use of lead). We hope that the article will provide a useful resource for researchers in the field and that it will facilitate discussions and moving forward towards addressing the outstanding challenges in this fast developing field.
Submicron periodic nanostructures have great potential for light trapping in ultra-thin silicon solar cells. In addition to period, aspect ratio, and structure geometry, the symmetry of the periodic nanostructures also has an impact on their light trapping properties. It has been generally agreed that breaking of symmetry in such structures can enhance light trapping. However, the quantitative relation of asymmetry and light trapping is still an open question. In this work, we suggest a method to quantify the impact of structure symmetry using periodic inverted nanopyramids (PiNPs) as a case study. Different degree of asymmetry is introduced into the structure by changing the cross section of the baseline PiNPs into hexagon, octagon, or circle and by skewing the PiNPs to different degree. We then present a systematic discussion of the impact of broken symmetry on absorption in the context of ultra-thin silicon solar cells. The results demonstrate that the light trapping effects of periodic nanostructures increase with the degree of asymmetry. For the investigated configurations, breaking of symmetry could improve the absorbed photocurrent density by up to 3 mA/cm2. We also propose explanations for the enhanced absorption due to breaking of symmetry from the perspective of diffraction and near-field enhancement.
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