Contents
1. Introduction Page No.2
2. Imide-functionalized building units Page No.2
3. Imide-functionalized polymer donors Page No.2
3.1. NTI-Based Polymers Page No.3
3.2. TPD-based polymers Page No.5
3.3. TzBI-based polymers Page No.8
3.4. BTI-Based Polymers Page No.11
3.5. PhI-based polymers Page No.12
3.6. Other Imide-Based Polymers Page No.13
4. Conclusions and Outlook Page No.17
1. Introduction
In the past few years, the field of organic solar cells (OSCs) has
experienced a significant increase in power conversion efficiency (PCE),
surpassing 19 %, primarily due to the development of high-performance
nonfullerene electron acceptors (NFAs).[1-3] Zhanet al. , Zou et al. and Sun et al. made significant
contributions by developing benchmark NFAs
ITIC,[4] Y6,[5] and
L8-BO.[6] Compared with fullerene derivatives,
these NFAs have different molecular geometry, strong absorption in the
near-infrared (NIR) range, moderate HOMO energy levels, and high
crystallinity.[6-13] Because the photophysical
properties and morphology compatibility between NFAs and polymer donors
significantly affect photovoltaic performance of OSCs, these new NFAs
demand matchable physical properties of polymer
donors.[12,14-19] Currently, the most widely used
polymer donors are donor-acceptor (D-A) copolymers which were
synthesized by coupling between electron-rich (D) and electron-deficient
(A) units. Although several D-A polymer donors, such as PM6, D18, have
shown a power conversion efficiency (PCE) over 19
%,[1,2,20,21] their high cost and poor
batch-to-batch reproducibility highlight the importance to develop
alternative types of polymer donors.[22-24]
Among various of D-A copolymers, the imide-functionalized polymers have
drawn considerable research attention and their use in organic
field-effect transistor (OFET) and fullerene-based OSCs have been
extensively explored.[25-28] The imide building
units have the following distinctive characteristics: 1) the strong
electron-withdrawing ability of imide group could downshift the HOMO
energy levels which benefit high V OCvalues;[29-31] 2) good π-conjugation of the planar
core and interactions between O and other atoms are beneficial for
intramolecular interaction and charge transport;[22,32,33] 3) the N-alkyl side chain can adjust
solubility and aggregation tendency of the polymer. Additionally, the
N-alkyl chains, distant from the aromatic core, could minimize polymer
chain π-π stacking and exert little influence on charge carrier
mobility.[34-37] 4) Last but not least, synthetic
routes of most imide building units are straightforward from facile
accessible materials.[38,39] These advantages make
imide-functionalized polymer donors really fascinating in nonfullerene
OSCs.
Because the imide-functionalized polymers used in OFET and
fullerene-based OSCs have been well summarized in previous comprehensive
reviews,[25,26,40] here, we focus especially on
the recent advances of imide-functionalized polymer donors for
non-fullerene OSCs. This review article first summarizes the latest
structural evolution and common synthesis routes of classical
imide-containing electron-deficient building blocks, then investigate
the effects of polymer structure on their physical and optoelectronic
properties, and introduce the application and photovoltaic performances
of imide functionalized polymers in nonfullerene OSCs. In this review,
the polymers were classified according to the types of imide-containing
electron-deficient units, and the emphasis on the relationships between
the polymer structures, physical and photovoltaic properties will
provide the guidelines on how to rationally design imide-functionalized
polymer donors. It is hoped that this article provides ideas for future
innovation on imide-functionalized polymers, especially towards
high-performance, low-cost, and stable OSCs.
2. Imide-functionalized building units
The chemical structure, number of imide groups, and component of
electron-deficient imide-functionalized building units significantly
affect the photophysical and film-forming properties of their polymers,
such as frontier energy levels, bandgap, absorption coefficient,
crystallinity and charge transport.[41-43]Historically speaking, the creation of new imide-functionalized unit
played a crucial role in the evolution of imide-functionalized polymers.
Till now, many imide building units have been created or developed,
including naphthalene diimide (NDI),[44] perylene
diimide (PDI),[26,45]thieno[3,4-c]pyrrole-4,6-dione
(TPD),[39,46,47] phthalimide
(PHI),[48,49] pyrrolo [3,4-f ]
benzotriazole-5,7(6H )-dione (TzBI),[50,51]dithienophthalimide (DPI),[35,52]naphthalenothiophene imide (NTI),[22] bithiophene
imide (BTI) units,[53,54] naphthodithiophene imide
(NDTI),[55] thieno[3,4-f]isoindole-5,7-dione
unit (TID),[25,56]5,9-di(thiophen-2-yl)-6H -pyrrolo[3,4-g ]quinoxaline-6,8(7H )-dione
(PQD),[57,58]N-alkyl-4,7-di(thien-2-yl)-2,1,3-benzothiadiazole-5,6-dicarboxylic imide
(DI),[59,60] pyromellitic diimide
(PMDI),[61] tetraazabenzodifluoranthene diimide
(BFI).[62] The chemical structure of various
imide-functionalized building units are illustrated in Figure 1. Due to
strong electron-withdrawing ability, diimides building units are
commonly employed in the construction of n-type semiconducting polymers
for OFETs, as well as electron acceptors for OSCs, which have been well
documented in other review articles.[26,34] In
this article, we aim to present the latest advancements in
imide-functionalized polymer donors, categorizing them based on the
specific types of imide units. The chemical structures of acceptor
materials used below are shown in Figure 15.