Perylene-Diimide Based Non-Fullerene Solar Cells with 4.34% Efficiency through Engineering Surface Donor/Acceptor Compositions

Although solvent additive has been observed to positively effect the nanomorphology and, in turn, the electric performance of organic small molecule acceptor based cells, how they act on the donor/acceptor (D/A) compositions throughout the active layer and, consequently, on the electron and hole injection/extraction property is still largely unknown yet. Herein, we use a D/A combination of PBDTTT-C-T (polymer donor) and a perylene diimide (PDI) dimer (small molecule acceptor) and find that the additive of 1,8-diiodooctane (DIO) effectively controls the surface D/A compositions. As pointed out by Xray photoelectron spectroscopy experiments, the air surface of the active layer is donor-rich and the buried surface is acceptor-rich, while there is a homogeneous distribution of D/A composition across the active layer between the air and the buried surfaces. The donor abundance, i.e., D/A weight ratio, in the acceptor-rich buried surface plays a crucial role in tuning the injection property of selective carrier from the buried contact-as the donor abundance is increased from 0.3 ([DIO] <= 5%) to 0.7-0.84 ([DIO] > 5%), the favorable carrier injection from the bottom contact is switched from electrons to holes, as revealed by the hole-only and electron-only volt-ampere characteristics. Through tuning the surface D/A compositions and along with modulating the phase separation and phase size, a best efficiency of 4.34% is obtained from an inverted cell when using a low [DIO] of 2%, while a best efficiency of 3.28% is achieved from a conventional cell structure when using a high [DIO] of 7%. The results presented herein clearly demonstrate that a small change in the surface D/A compositions of the active layer may give a significant difference in the device performance of PDI based nonfullerene cells. The tuning of the D/A compositions in the buried surface is primarily explained by the different boiling points of DIO and main processing solvent, different solubility of the donor and acceptor in DIO, and different wettability of the donor DIO and acceptor DIO solutions on the PEDOT:PSS or ZnO substrates.