As aresult, the electron mobility of NFAs can be tuned to matchthe hole mobility of the donor. Early stage studies showed thatplanar monomeric PDI would induce strong intermolecular p–pstacking, leading to large crystalline domains and severe phaseseparation [108]. Three-dimensional PDI acceptors were lateron developed to manipulate the molecular stacking and crystallinity of the films, thus the phase separation. Yu synthesizedTPB acceptor based on a BDT-Th core with four PDI groups[121].