Novel Organic Electron Transport Layers Using NDI and PDI Derivatives Deposited via Thermal Evaporation for Perovskite Solar Cells

Organic transport layers are of significant interest in the field of hybrid perovskites due to their chemical adaptability and compatibility with industry favoured deposition techniques such as thermal evaporation. These methods are desired in industry given that they circumvent the need for undesirable process solvents and allow for the uniform coating of large area substrates. Hence, discovery and optimisation of organic Electron transport layers (ETL) provide an avenue for easier integration of perovskite photovoltaics into industry and eventual commercialisation. However, identification of effective ETLs is generally difficult owing to the necessity for both chemical and electrical compatibility with the perovskite layer.<br/>This study introduces a library of conjugated molecules, deposited via thermal evaporation, for use as ETLs in photovoltaic devices. The molecules, derived from naphthalene diimide (NDI) or perylene diimide (PDI), were modified with a range of terminal groups. These modifications tune the polarity and chemical nature of the molecules leading to a wide range of chemical behaviours such as hydrophobicity/philicity, hydrogen bonding and halogen bonding. Band alignment with the perovskite was also varied through extension of the conjugated system. The thin films were characterised using XPS, contact angle measurements, FTIR and ellipsometry. Our results show that some molecules exhibit stable evaporation profiles with no degradation, while others only partially evaporate or fail to evaporate entirely. Additionally, sensitivities to process solvents used in perovskite deposition were exhibited by some ETLs which manifested as changing chemical behaviours such as surface wettability and in some instances near total removal of the ETL. NIP devices were made to test the overall effectiveness of the organic ETLs and promising device results were obtained, with devices exhibiting unoptimised performances ranging from 7% to 14.8%.