Efficient and Stable Quasiplanar Heterojunction Solar Cells with an Acetoxy-Substituted Wide-Bandgap Polymer

Intermolecular
interactions have fundamental importance in the
control of active layer morphology, exciton generation, charge transport,
and, thus, the overall photovoltaic performance. This is especially
true for quasiplanar heterojunction (Q-PHJ) polymer solar cells, because
the bilayer device structure requires larger exciton diffusion lengths.
However, little effort has been made to design polymer donors with
additional organic functional groups intended to control intermolecular
hydrogen-bonding interactions. Herein, we report two new copolymers
for Q-PHJ solar cells synthesized by the addition of hydroxy (PNTB-OH)
and acetoxy groups (PNTB-OAc) onto electron-deficient units. We have
systematically investigated the influence of the hydrogen bond on
electro-optical behaviors, crystallinity, photovoltaic properties,
energy losses, photostability, and storage stability in both types
of polymers. The single-crystal data reveals more regular stacking
and order orientation driven by hydrogen bonding, of the acetoxy-substituted
electron-deficient units. Q-PHJ organic solar cells (OSCs) were fabricated
for both polymers with a high-performance nonfullerene acceptor N3.
PNTB-OAc-based Q-PHJ OSCs realized the highest photovoltaic performance
of 16.53%, which is ∼2.4 times higher than 6.79% obtained from
the PNTB-OH-based Q-PHJ OSCs. This high performance is attributable
to low nonradiative energy losses, high and balanced electron/hole
mobility, and better crystallinity. In contrast, the PNTB-OAc film
has a longer crystal coherence length, which is calculated from grazing-incidence
wide-angle X-ray scattering (GIWAXS). Furthermore, the PNTB-OAc device
demonstrated superior photostability and storage stability, retained
more than 85% of the initial PCE after illumination for 1050 h, and
90% of the initial PCE under nitrogen for 1600 h. This work highlights
the importance of the acetoxy group to significantly control packing
and crystallinity by hydrogen bonding, thus realizing efficient OSCs
with durable device stability.