Observing electron transport and percolation in selected bulk heterojunctions bearing fullerene derivatives, non-fullerene small molecules, and polymeric acceptors

We systematically study how electrons achieve percolation pathways in a bulk-heterojunction (BHJ) blend as the weight fraction of the electron acceptor is altered. Three different classes of BHJs are explored: BHJs that contain (1) fullerene-based acceptors, (2) non-fullerene small molecules, and (3) polymer acceptors. The nano-topologies of acceptors in (1) to (3) can be approximately associated with spheres, oblates (plates) and prolates (long rods), respectively. We discover that the weight fraction of the acceptor for electron percolation to complete is successively reduced in the order of fullerenes (0.32), non-fullerene (NF) small molecules (0.25), and polymers (0.11). A new parameter, namely, the effective volume fraction, chi, is introduced to quantify the fraction of the acceptor moieties that can contribute to electron transport in the BHJs. The electron percolation regions can be fitted with a model that considers the shapes of the acceptors and their chi in the BHJs. An all-polymer BHJ containing PTB7-Th:N2200 exhibits the largest chi (0.6-0.8), indicating N2200 acceptors are the most effective in transportation of electrons. All-polymer BHJs can better preserve their electron transport capability arising from composition variations, and they are immuned from the introduction of inert materials or the exposure of moisture, in stark contrast to fullerene-based BHJs. Our results correlate the electron transport behaviors to the nano-topologies of acceptors for BHJ cells, and offer guidance to tune the compositions and understand the nano-morphology of BHJ cells using different classes of acceptors.