Bifacial Indoor OPVs: Investigating The Influence of Vertical Stratification on Performance Symmetry of Semi-Transparent OPVs

The demand for sensors in building digitalization is growing rapidly due to the increased need for better monitoring. However, Internet-of-Things (IoT) sensors typically rely on batteries, leading to high maintenance costs and environmental concerns related to battery recycling. One of the innovative solutions is harvesting ambient light to power these sensors. Organic Photovoltaics (OPVs) offer a promising approach due to their semi-transparency and high power-per-weight ratio, making them suitable for integration with IoT devices like electronic shelf labels and asset tracking labels.The synthesis of organic semiconductor materials for OPV gives flexibility which allows to design material systems which surpass state-of-the-art inorganic PVs for indoor applications.<br/>Recent advancements in solution-processable, flexible, and semi-transparent indoor OPVs made by roll-to-roll and lamination technologies have opened the door to large-scale industrialization of IOPVs¹, reducing manufacturing costs by eliminating vacuum processing and scarce materials. Nevertheless, semi-transparent IOPVs may exhibit performance asymmetry when illuminated from different sides (cathode/anode illumination), which is problematic for IoT sensors requiring bifacial use.<br/>In our study, we investigate the link between performance asymmetry and vertical phase segregation in semi-transparent IOPVs featuring various photoactive layers, including fullerene and non-fullerene-based materials. Through a combination of optoelectronic characterizations and simulations, we identify that a filtering layer within the photoactive layer does not contribute to photocurrent extraction and is the source of observed cathode/anode performance asymmetry².<br/>Our investigation delves into the vertical arrangement of the cathode and anode stack, utilizing techniques such as Neutron and X-Ray Reflectivity, Time-of-Flight Secondary Ion Mass Spectrometry, and Ellipsometry. We find that certain photoactive layers, while highly efficient for indoor use, tend to phase separate when deposited on the cathode electrode, creating a physical barrier and causing trap-assisted recombination when illuminated from the anode side. Furthermore, we found that some photoactive layers remain vertically homogeneous, regardless of the electrode used, presenting the potential for electrically symmetric semi-transparent IOPVs.<br/>Through thorough screening, we identify a photoactive layer system that not only delivers high and air-stable indoor performance but also exhibits perfect symmetry when illuminated from either the anode or cathode side. These findings offer a comprehensive understanding of how photoactive layers affect electrical symmetry in semi-transparent IOPVs, especially for bifacial applications. Our research contributes valuable insights for designing photoactive layers to achieve air-stable, and electrically symmetric semi-transparent IOPVs for various applications in the field of IoT sensors.<br/><br/>References:<br/>1. Bergqvist, J. <i>et al.</i> Asymmetric photocurrent extraction in semitransparent laminated flexible organic solar cells. <i>npj Flex. Electron.</i> <b>2</b>, (2018).<br/>2. Rodríguez-Martínez, X. <i>et al.</i> Air Processing of Thick and Semitransparent Laminated Polymer:Non-Fullerene Acceptor Blends Introduces Asymmetric Current–Voltage Characteristics. <i>Adv. Funct. Mater.</i> 2301192 (2023).