Figure 9 (a) Chemical structures of the donor polymers (b)
Normalized ultraviolet–visible absorption of the donor polymers in
dilute chloroform solution and as neat films. (c) Energy level diagram
of thin films obtained from cyclic voltammetry measurements. Reproduced
with permission.[108] Copyright 2018, Springer
Nature Limited.
By fine-tuning the number of carbon atoms connected to the branched
alkyl group in TzBI unit, they synthesized a series of polymer, termed
P2F-EH, P2F-EHp and P2F-EO,
respectively.[108] As branching point was moved
away conjugated core, the HOMO energy level of the polymers is
consistently lowered (P2F-EH, -5.36 eV; P2F-EHp, -5.38 eV; P2F-EO, -5.41
eV). The energy levels of the materials are illustrated in Figure 9. The
P2F-EHp has the best photovoltaic performance (7.28 %), which is higher
than 6.13 % for P2F-EH: ITIC and 6.73 % for P2F-EO: ITIC. By blending
with IT-2F, the P2F-EHp:IT-2F device exhibited an optimized PCE of 12.11
%. By paring with BTPTT-4F and finely optimizing the morphology of the
blend film, P2F-EHp: BTPTT-4F displayed a PEC over 16
%.[109]
The polymer PTzBI-S and PTzBI-Ph with an alkylthiothienyl or alkylphenyl
chain on the BDT unit were also synthesized.[110]The PTzBI-S:ITIC and PTzBI-Ph:ITIC showed PCEs of 9.12 % and 10.19 %,
respectively. Zhang et al. [50] synthesized
polymer PBDTS-TzBI that have a PCE of 9.04 % with a highV OC of 0.94 V without using any processing
additive when blending with ITIC. Negash et
al. [111] synthesized PBDTTSi-TzBI and PBDTTS-TzBI
by replacing the alkyl side chain with the alkyl silyl or alkylthio side
chains in BDT. A PCE of 9.6 % was obtained for PBDTTS-TzBI:ITIC based
devices.
Li et al. [112] synthesized three TzBI-based
polymers, namely, PHT-EHp, PFT-EHp and PClT-EHp. Owing to the strong
electron-withdrawing ability of both imide group and halogen atoms, the
polymers substitutions exhibited slight blue-shift absorption and
deepened HOMO levels (-5.60 eV, -5.64 eV, and -5.71 eV, respectively).
By blending with ITIC, the PCl-EHp showed a high PCE of 15.8 % that is
higher than 7.7 % for PHT-EHp and 15.4 % for PFT-EHp. Additonally, the
PClT-EHp:Y6DT-based binary devices demonstrated an efficiency 16.4 %.
To explore the impact of the fluorine substitution points on thiophene
π-bridge on photovoltaic performance, Huang et
al. [113] synthesized PTzBI-p F and
PTzBI-d F. The PTzBI-d F has a more planar molecular
conformation, narrower bandgap, and higher hole mobility. The
PTzBI-d F: Y6-based device has an optimized PCE of 16.8 %, which
is much higher than 1.4 % for PTzBI-p F: Y6. Using L8-BO as the
second acceptor,[114] the PTzBI-d F:L8-BO:Y6
ternary devices exhibited a promising PCE of 18.26 % and excellent
long-term thermal stability under 85
°C.