Tetra(phenoxy)perylenediimide Polycrystalline Films: Symmetry-Breaking Charge Separation in the Solid State for Open-Circuit Voltage Enhancement in Organic Photovoltaics

The light-absorbing layer in an organic photovoltaic (OPV) device is made of separate electron donor and acceptor molecules. Finding suitable materials which optimize exciton generation but are also electronically similar enough to minimize defects is an engineering challenge. Poor band gap matching can lead to exciton recombination losses which lower the open-circuit voltage and efficiency. With symmetry-breaking charge separation (SB-CS), only one material is needed to generate excitons and separate charges. SB-CS occurs when one molecule is photoexcited while coupled to a neighbor and the complex undergoes charge transfer (CT) resulting in one anion and one cation. We show SB-CS in films of tetraphenoxy-PDI (tpPDI) with different tails at imide positions: octyl-tpPDI and H-tpPDI. Single crystal and grazing incidence X-ray diffraction reveal pi-pi stacking in both derivatives and intermolecular hydrogen-bonding only in H-tpPDI, resulting in H-tpPDI films being most ordered. A mixture of singlet and CT character is observed by transient absorption spectroscopy, resulting in confirmation of an excimer-like state. Stronger CT character is present in H-tpPDI suggesting it is the strongest candidate for OPVs.