The
thiophene, thieno[3,2-b]thiophene, selenophene, and furan moieties
have been widely used as π-bridges to tune TPD-based polymers’
photophysical and photovoltaic properties.[83-87]Facchetti et al . reported a series of polymers TPD-1-3, and
explored the effects of alkyl side chain length and fluorine atom
substitutions on physical properties of TPD-based
polymers.[88] The TPD-1-3 exhibited good
solubilities, high hole transport, and matched frontier energy levels
with IT-F4. The efficiency of the devices decreases as the chain length
increase, and the efficiency of 11.7 %, 11.8 % and 12.1 % were
obtained for TPD-1, TPD-2 and TPD-3-based PSCs by using o-xylene
solvent. The presence of the F atom on the BDT deepens the HOMO energy
level, resulting in TPD-3F: IT-4F based OSC with a PCE of 13.8 % and a
high V OC 0.91eV. However, by matching with
non-fullerene acceptor Y6, the TPD-3 exhibited an PCE of 15.2 %, while
that for TPD-3F is 11.4 % probably due to the mismatch of HOMO energy
level.[89] Recently, Min et al . designed a
TPD-based polymer donor PTTB-F, which showed an decent PCE of 18.06 %
in binary OSCs by blending with L8-BO.[77] By
employing thieno[3,2-b]thiophene as the π-bridge, Liang et
al. synthesized TPD-based polymers PBDTT-6ttTPD and
PMOT16.[47] By blending with IDIC, the
PMOT16-based OSCs exhibited a PCE of 10.04 %. Later, they synthesized
polymers PMTT56, PMOT39 and PEHTT by introducing different side chains
in BDT unit.[92] By matching with IT-2F, PMTT56,
PMOT39 and PEHTT based OSCs achieved the highest PCEs of 12.6 %, 10.5
% and 10.8 %, respectively. It should be noted that the high PCEs were
obtained by using non-halogenated solvent toluene. By adding
PC71BM as the second acceptor, the PCE of PMTT56-based
ternary devices was further enhanced to 13.2 %. Later, Kim et
al. developed three polymers PBDT-H, PBDT-F and PBDT-Cl based on
thienothiophene π-bridgedN -octylthieno[3,4-c]pyrrole-4,6-dione (8ttTPD) and
benzo[1,2-b :4,5-b’ ] dithiophene (BDT)
units.[90] They found that the incorporation of
highly planar structured 8ttTPD unit could improve crystallinity and
hole mobilities of the BDT-based polymers. Besides, the introduction of
the halogen atoms on BDT unit could tune the crystallinity and energy
levels. Synergistic effects of incorporated 8ttTPD unit and halogen
atoms could significantly facilitate the charge transporting properties
and charge recombination process, which is stemmed from the enhanced
crystallinity and hole mobility of the polymers. Therefore, the
PBDT-Cl:Y6-based OSCs achieved the highest PCE of 15.63 %, which
out-performs the 11.84 % for PBDT‐H:Y6 and 14.86 % for PBDT‐F:Y6. To
investigate the effect of molecular weight on the photovoltaic
performances, Hwang et al. [91] synthesized
three batches of PBDTT-8ttTPD with different molecular weights. The
higher molecular weight could improve the ordering of polymer packing,
π–π stacking distance, absorption coefficient, and nanomorphology of
the blend films. The batch with highest molecular weight demonstrates
the highest PCE of 11.05 %, which is much higher than 8.27 % and 5.34
% for the batches with the medium and lowest molecular weight.
By introducing the third unit (D2 or A2) into the D-A copolymer, the
random terpolymer strategy have been proved to be an efficient method to
fine-tune the frontier energy levels, optical properties, and
film-forming properties of the polymer.[83,96,97]The TPD unit was also broadly used to in terpolymers. Chen and coworkers
designed and synthesized a D–A1–D–A2-type terpolymer (PM6-TPD-5 %)
via random copolymerization.[93] The addition of
TPD results in polymers with lower HOMO, wider light absorption, optimal
molecular packing, and more ideal morphology of blend film. PM6-TPD-5
%: Y6-based binary organic solar cells showed an encouraging PCE of
16.3 %, which out-performs the 14.8 % for PM6-TPD-10 %: Y6 and 15.6
% for PM6: Y6. Besides, a PCE of 17.1 % was obtained in
PM6: PM6-TPD-5 %: Y6 based ternary OSCs. Cao et al. synthesized
a set of random terpolymers PTAZ-TPD10-Cn , which are composed of
an electron-rich unit BDT, and two electron-deficient units of TAZ and
TPD.[94] The PCE of the N2200 and ITIC-based
devices decreases monotonically when shortening the side chain of
PTAZ-TPD10-Cn from decyl to hexyl. However, the PCEs of
PTAZ-TPD10-Cn :PC61BM-based OSCs increase
monotonically. Toppare and coworkers synthesized two random copolymers
P1 and P2 by using benzodithiophene (BDT) and thiophenes as donor
moiety, TPD as acceptor, and selenophene as π-bridging
unit.[95] Compared with polymer P2, P1 containing
alkyl thiophene have a larger molecular weight and better solubility,
resulting in a higher PCE of 7.94 % for P1: ITIC which is much higher
than 1.96 % for P2: ITIC.