Intra-molecular Charge Transfer and Electron Delocalization in Non-fullerene Organic Solar Cells

Two
types of electron acceptors were synthesized by coupling two kinds
of electron-rich cores with four equivalent perylene diimides (PDIs)
at the α-position. With fully aromatic cores, TPB and TPSe have
π-orbitals spread continuously over the whole aromatic conjugated
backbone, unlike TPC and TPSi, which contain isolated PDI units due
to the use of a tetrahedron carbon or silicon linker. Density functional
theory calculations of the projected density of states showed that
the highest occupied molecular orbital (HOMO) and lowest unoccupied
molecular orbital (LUMO) for TPB are localized in separate regions
of space. Further, the LUMO of TPB shows a greater contribution from
the orbitals belonging to the connective core of the molecules than
that of TPC. Overall, the properties of the HOMO and LUMO point at
increased intra-molecular delocalization of negative charge carriers
for TPB and TPSe than for TPC and TPSi and hence at a more facile
intra-molecular charge transfer for the former. The film absorption
and emission spectra showed evidences for the inter-molecular electron
delocalization in TPB and TPSe, which is consistent with the network
structure revealed by X-ray diffraction studies on single crystals
of TPB. These features benefit the formation of charge transfer states
and/or facilitate charge transport. Thus, higher electron mobility
and higher charge dissociation probabilities under Jsc condition were observed in blend films of TPB:PTB7-Th
and TPSe:PTB7-Th than those in TPC:PTB7-Th and TPSi:PTB7-Th blend
films. As a result, the Jsc and fill factor
values of 15.02 mA/cm2, 0.58 and 14.36 mA/cm2, 0.55 for TPB- and TPSe-based solar cell are observed, whereas those
for TPC and TPSi are 11.55 mA/cm2, 0.47 and 10.35 mA/cm2, 0.42, respectively.