Figure 1 The chemical structure of various imide-functionalized
building units.
3.Imide-functionalized
polymer donors
3.1.Naphthalenothiophene
Imide-Based Polymers
Scheme 1 Synthetic route of NTI and schematic reactions to
form two Csp2 -Csp2 bonds
by simple one-step reaction.
Six-membered imide has strong
electron-withdrawing ability, and monomers with two six-membered imides,
such as NDI and PDI, are widely used to construct n-type semiconducting
materials for OFETs or electron acceptors for
OSCs.[63-65] However, the electron absorption
ability of these diimides is too strong to be used as building units for
polymer donors. Compounds with one six-membered will have weaker
electron withdrawing ability and may be used as strong electron
deficient monomers to construct polymer donors. Wu et al.reported the synthesis of naphthalenothiophene imide (NTI), which bears
six-membered imide and a five-membered aromatic
ring.[22] The NTI compound was synthesized by
Suzuki coupling between
6,7-dibromo-2-(2-hexyldecyl)-1H-benzo[de]-isoquinoline-1,3(2-H)-dione
and 3,4-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)thiophene in a
very high yield of 82 %. Notably, this is a new and efficient approach
to construct five-membered aromatic ring, forming double
Csp2-Csp2 bonds in a simple one-step
reaction. The NDTI was also synthesized in a similar method from dibromo
and ditin compounds. Because there are various dibromo and ditin/diBpin
compounds, numerous fused-ring units for polymers could be created by
using this method to form double
Csp2-Csp2 bonds in a simple one-step
reaction. The chemical structures of NTI-based conjugated polymers are
illustrated in Figure 2 and the photovoltaic properties are summarized
in Table 1.
Compared with other widely used electron-deficient units, such as BDD,
TzBI, BTZ, the NTI has stronger electron-withdrawing ability and could
significantly lower the HOMO energy levels of the
polymers.[15] Thus, the NTI-based polymer PNTB1
has a low-lying HOMO energy level (−5.42 eV) without introducing any F
or Cl groups into the polymer backbone. The non-fullerene OSCs
fabricated from PNTB1:Y6 exhibited the highest PCE of 15.18 %. They
studied the impact of the thiophene π-bridge on photovoltaic performance
through designing two NTI-based polymer donors PNTB and
PNTB-2T.[22] PNTB-2T has two more thiophene units
in the polymer chain than PNTB, and exhibited better long-range
coplanarity of polymer conjugated chains, resulting in closer π-π
stacking, more ordered polymer chain packing, and higher hole transport
of PNTB-2T. The PNTB-2T:Y6-based OSCs exhibited PCE of 16.72 %, while
the PCE for PNTB:Y6 was only 3.81 %. The photovoltaic performance of
PNTB-2T-based solar cells could be enhanced to 17.35 % by using
PC71BM as second acceptor. Importantly, NTI-based
polymer donors exhibit excellent batch-to-batch reproducibility because
the twisted polymer conjugated backbone make the polymer packed through
weak short interactions rather than π-π stacking, which minimizes the
impact of molecular weight on phase separation and blend film
morphology.
Wu et al. also synthesized two NTI-based polymers PNTB6-Cl and
PNTB-Cl by exploring the effect of employing linear or branched alkyl
chains in NTI unit on photovoltaic
performance.[66] PNTB-Cl exhibited good solubility
in chloroform, however, due to stronger intermolecular interactions and
shorter π-π stacking distance,
the polymer PNTB6-Cl with linear alkyl chains is insoluble in chloroform
(Figure 3a, b). They fabricated layer-by-layer polymer solar cells
(LBL-PSCs) with PNTB6-Cl as donor and N3 as acceptor by sequential
spin-coating method (Figure 3c, d). Thus, PNTB6- Cl:N3 based LBL devices
exhibited a remarkable PCE of 17.59 %, which is much higher than 15.24
% for PNTB-Cl:N3 based devices.