Cathode Interface Layer-Free Sequential-Deposited Nonfullerene Acceptor Organic Solar Cells Via a...

Cathode interface layers (CILs), which are often built between the active layer and the metal cathode to facilitate electron collection and extraction, are highly necessary to develop stable and efficient organic solar cells (OSCs). However, most CILs are usually very sensitive to film thickness and work only in ultrathin films. In this study, 4,8-bis((N,N-dimethylamino)propyloxy)benzo[1,2-b,4,5-b']dithiophene (BDT-N) is utilized as an electron-attracting additive for sequential-deposited (SD) OSCs made from the nonfullerene acceptor (NFA) Y6 paired with the polymer donor PM6. Upon addition of 0.3 wt % of BDT-N to the Y6 layers, the power conversion efficiencies (PCEs) of the CIL-free SD-NFA OSCs with the device configuration ITO/PEDOT:PSS/PM6/Y6 without/with BDT-N/Ag improved from 6.93 to 15.13%, alongside a remarkable improvement in the stability of the devices. Meanwhile, the possible reasons behind the lowering of the work function of Ag metal cathodes, facilitating the electron attraction of the devices, etc. through the addition of the BDT-N additive are supported and discussed by a set of comparative experiments, such as ultraviolet photoelectron spectroscopy (UPS), capacitance-voltage (C-V) measurements, charge-carrier recombination measurements, etc. This work outlines a facile and promising approach to avoid the utilization of ultrathin CILs in the SD-NFA OSCs and to simplify the device fabrication.