Later, the effect of alkyl chains on optical and photovoltaic properties
was further
studied.[101]The GIWAXS test results indicate that as the alkyl side chain length
decreases from C56 to C48, the crystal surface orientation and molecular
order increases significantly. As
a result, PBDT-BiTPD-C48 with the shortest alkyl side chains achieved
the best photovoltaic performance with the PCE of14.1 %, while that for
other two polymers (C52 and C56) are 11.1 % and 7.8 %, respectively.
Cai et al. [102] synthesized three polymers
2HD/C8, C8/2BO/C6, and 2BO/C6/C6 by introducing side alkyl chains into
different positions. Introducing alkyl side chains to the thiophene (T)
motifs in C8/2BO/C6 or to the BDT motifs in 2BO/C6/C6 decrease the
crystallinity and face-on crystallite orientation, resulting in serious
charge recombination loss and decreased FF andJ SC in BHJ devices. Hwang et
al. [31] synthesized two biTPD-based polymers
PBDT-biTPD(BO) and PBDT-biTPD(HD)with different lengths of the alkyl
side chain. Due to the strong electron-withdrawing strength of biTPD,
both polymers show deeper HOMO and LUMO energy levels than those for
TPD-based polymers. By blending with IT-4F, PBDT-biTPD(BO) and
PBDT-biTPD(HD)-based OSCs showed the PCE of 9.49 % and 9.32 %,
respectively.
3.3. TzBI-Based
Polymers.
Scheme 3 Synthetic Route to
TzBI
The imide-functionalized benzotriazole (TzBI) unit was firstly reported
by Huang et al [51]. The Diels-Alder
reaction of 1, 3-di-2-thienothiophene [3,4-c][1,2,5]
thiadiazole-2-S (IV) (1) with dimethyl acetylenedicarboxylate yield
4,7-Di(thien-2-yl)-2,1,3-benzothiadiazole-5,6-Dicarboxylate (2).
Followed by alkaline hydrolysis, anhydride and imide formation,
N-alkyl-4,7-di(thien-2-yl)-2,1,3- benzothiadiazole-5,6-dicarboxylic
imide (5) was prepared accordingly. The reduction reaction of compound 5
by iron powder, the cyclization reaction with excess
NaNO2 and condensation reaction with amine give the
compound
4,8-di(thien-2-yl)-6-octyl-2-octyl-5H- pyrrolo[3,4-f ]benzotriazole-5,7(6H )-dione
(TzBI) successfully (Scheme 3). Presented in Figure 8 are the chemical
structures of some TzBI-based polymers and their corresponding
photophysical properties and photovoltaic performance are summarized in
Table 4. They synthesized TzBI-based WBG polymer PTZBIBDT by coupling
with BDT unit.[51] The
PTZBIBDT:PC71BM OSCs exhibited a PCE of 8.63 %. By
blending with a high molecular weight polymer acceptor N2200, the
PTzBI:N2200-based all-PSC devices exhibited a high PCE of 9.16 %. It
should be noted that this efficiency was obtained from the OSCs that was
prepared from the environmentally friendly solvent
MeTHF.[103] They also synthesized a wide-bandgap
polymer PTzBI-DT by coupling with DTBDT unit.[104]Devices based on PTzBI:ITIC and PTzBI-DT:ITIC achieved PCEs of 10.24 %
and 9.43 %, respectively.
Incorporating appropriate side chains into the TzBI groups can affect
the optoelectronic properties of polymers. Huang et
al. [105] synthesized a polymer PTzBI-O, by
replacing the alkyl side chain in TzBI with a polar substituent. The
addition of oxygen atom enhanced the electron-withdrawing ability of
theTzBI, resulting in a red-shifted absorption and strong preaggregation
of PTzBI-O. A PCE of 7.9 % was achieved for PTzBI-O:N2200 based all-PSC
devices. They further synthesized a copolymers PTzBI-Si that has a
siloxane-terminated side-chain in TzBI.[106]Because the siloxane-functionalized side chains endow PTzBI-Si with
excellent solubility in the green-solvent MeTHF, the photovoltaic
performance of PTzBI-Si:N2200-based OSCs processed from green-solvent
MeTHF was improved to 10.24 %. The increased the molecular weight of
PTzBI-Si could optimize the morphology of the active
layer,[107] leading to a PCE of 11.5 % in
PTzBI-Si:N2200-based devices.