Acceptor Gradient Polymer Donors for Non-Fullerene Organic Solar Cells

In organic solar cells, maximizing the open-circuit voltage (V_OC) via minimization of the ionization energy or electron affinity offsets of the blended donor and acceptor often comes at the expense of achieving a considerable amount of short-circuit current (J_SC). To explore a hypothesis for the design of materials that may circumvent this tradeoff, eight structurally similar polymers were synthesized consisting of a fluorinated/non-fluorinated benzothiadiazole (BTDF/BTD) strong acceptor moiety, a thiophene ester (TE) weak acceptor, and various donor units composed of bithiophene (T2), biEDOT, and benzodithiophene (BDT) to form six acceptor gradient and two nongradient polymers. The acceptor gradient motif was designed and theorized to induce more facile exciton dissociation in low driving force solar cells by creating a further separated intramolecular charge-transfer state between the strong BTD acceptor and various donor units through a bridging TE component. Solar cells were fabricated using the eight polymers blended with phenyl-C₇₁-butyric-acid methyl ester (PC₇₁BM) to reveal two top performing isomeric polymers, T2-BTDF-(TE2) and TE2-BTDF-(T2), which were further tested with several non-fullerene acceptors (NFAs): EH-IDTBR, ITIC, and ITIC-4F. In order to fabricate optimally performing solar cells, a 0.2 eV ionization energy offset was found to be essential or the short-circuit current of the NFA cells diminished dramatically. Ultimately, optimized NFA solar cells were fabricated using ITIC-4F paired with each of the top performing polymers to produce an average PCE of 7.3% for TE2-BTDF-(T2) (nongradient) and 3.6% for T2-BTDF-(TE2) (gradient). The acceptor gradient effect was not shown to reduce the amount of charge recombination in NFA solar cells mainly due to the inability to fabricate solar cells, with minimal ionization energy or electron affinity offsets along with morphological complications. This work stresses the importance of acquiring accurate ionization energies and electron affinities when characterizing solar cell energetics, as differences as small as 0.1 eV in the offsets can make a significant impact on overall charge collection.