Improving Thickness Independence Through Counter-Anion Size Engineering in Polyfluorene-Based Cathode Interlayers

In organic photovoltaics and tandem devices, poly [(9,9-bis(3'-(<i>N</i>,<i>N</i>-dimethylamino) propyl)-2,7-fluorene)-alt-2,7-(9,9-ioctylfluorene)] (PFN) is commonly used as a cathode interlayer material (CIM) due to its alcohol solubility. However, the representative PFN-based CIM, PFN-Br, exhibits limitations in scalability due to significant variations in efficiency with changes in cathode interlayer thickness. To improve the thickness tolerance of the cathode interlayer, we propose a simple substitution of the anion from Br^- to TFSI^-. PFN-TFSI, containing the bulkier counter anion (7.9 Å), disrupts interchain interactions, resulting in an amorphous property. Devices employing PFN-TFSI maintain 90% of their initial efficiency when the cathode interlayer thickness is increased from 5 to 15 nm, with consistent efficiency across the device area. Moreover, PFN-TFSI facilitates uniform film formation, even on large surfaces up to 25 cm2. This study demonstrates the potential of PFN-TFSI for scalable device applications.