AbstractIn modern bulk heterojunction (BHJ) organic solar
cells, blends of low-bandgap polymer and [70]PCBM acceptor
are used in the active layer. In this combination, the
polymer absorbs photons from the red and near-IR parts
of the solar spectrum, while the blue and near-UV photons
are harvested by [70]PCBM. As a result, both electron
transfer from polymer to [70]PCBM and hole transfer
from [70]PCBM to polymer are of utmost importance
in free charge generation and have to be optimized simultaneously.
Here we study electron and hole transfer
processes in BHJ blends of two low-bandgap polymers,
BTT-DPP and PCPDTBT, by ultrafast photoinduced spectroscopy
(PIA). By tracking the PIA dynamics, we observed
substantially different charge separation pathways
in BHJs of the two polymers with [70]PCBM. From the photoinduced
anisotropy dynamics, we demonstrated that in
the PCPDTBT:[70]PCBM system both electron and hole
transfer processes are highly efficient, while in the BTTBPP:[
70]PCBM electron transfer is blocked due to the unfortunate
energy level alignment leaving hole transfer the
only pathway to free charge generation. Calculations at the
density functional theory level are used to gain more insight
into our findings. The presented results highlight the importance of the energy level alignment on the charge
separation process.
Hole‐Transfer Dependence on Blend Morphology and Energy Level Alignment in Polymer: ITIC Photovoltaic Materials
