Interfacial Energetics in Organic and Hybrid Solar Cells—Low-Light, Oxides, Light-Soaking and More!

Interfacial energetics play a key role in defining the operation and performance of organic and perovskite photovoltaics. Of particular importance is the interface between the photoactive layer (PAL) and the hole transport or electron transport layers (HTL and ETLs). This interface affects charge extraction, and influences charge recombination mechanisms in devices. Understanding and optimising these interfaces is therefore of prime importance to improving device efficiencies and achieving market realisation. Here we present several examples of how interfacial energetics affect device performance and determine the slow processes observed in these devices under illumination (i.e. light-soaking effects).<br/>Organic photovoltaics (OPVs), now exceed 18% power conversion efficiency at 1 sun illumination. However, efficiencies still lag behind Si and perovskite solar cells which limits the potential market for OPVs. One promising route to market for OPVs is for targeted low-light and indoor applications where illuminance is orders of magnitude smaller than 1 sun. Under low-light OPVs perform significantly better than silicon solar cells. However, devices considered in the literature so far employ materials or fabrication methods which are not suitable for large scale-up.<br/>We demonstrate the commercial viability of OPVs for low-light applications by utilizing a fully scalable device architecture with upscalable OPV materials. We show superior device performance to silicon and discuss important optimisation parameters. Critically, we identify and characterise a critical light-soaking (LS) effect that is ruinous for low-light performance. Using surface photovoltage (SPV) and energetics measurements we find that this LS effect results from poor electron extraction at the electron transport layer (ETL)/photoactive layer interface due to high density of mid-gap states in the ZnO ETL and poor energetic alignment. We effectively remove this LS effect by employing SnO₂ nanoparticles as ETL, which we then use to demonstrate scale-up potential by fabricating a 21.6 cm² module (1).<br/>We further investigate the properties of the ETL and look at controlling the LS effect by controlling the properties of ZnO. Here we characterise this oxide and compare it to other oxides using wavelength and atmosphere dependent SPV measurements to develop our understanding of these oxide interlayers and their role in device performance.<br/>Additionally, we apply energetics measurements to perovskite solar cells, to understand how the interface between interlayers and perovskite affects device performance (2,3). Perovskite solar cells are also an exciting PV technology due to rapid improvements in power conversion efficiencies, now exceeding 25%. To further improve performance, it is necessary to improve V_OC which is sensitive to the choice of interlayer and the recombination processes at the interface.<br/>Here, we look at a range of interlayers, and control their properties e.g. the doping level, or energetics, to modulate the interface with the perovskite. Doing so we are able to pinpoint an interlayer dependent light-soaking effect in large-area blade-coated perovskite cells, and suggest how to avoid this. We also investigate the energetics of an integrated organic/perovskite solar cell, demonstrating the loss mechanisms involved at the different interfaces, and how these affect device performance.<br/>In summary, this presentation will highlight the importance of understanding the energetics and interfaces in a range of optoelectronic devices, and also the key role of SPV measurements to achieve this.<br/><br/>(1) J. Luke, L. Corrêa, J. Rodrigues, J. Martins, M. Daboczi, D. Bagnis, J. S. Kim, Adv. Energy Mater. 2021, 2003405.<br/>(2) Y. C. Chin, M. Daboczi, C. Henderson, J. Luke, J. S. Kim, ACS Energy Lett., 2022 7 (2), 560-568<br/>(3) M. Daboczi, I. Hamilton, S. Xu, J. Luke, S. Limbu, J. Lee, M. A. McLachlan, K. Lee, J. R. Durrant, I. D. Baikie, J. S. Kim, ACS Appl. Mater. Interfaces 2019, 11, 46808.