Mediating Solar Cell Performance
by Controlling the
Internal Dipole Change in Organic Photovoltaic Polymers
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Abstract
We report synthesis and characterizations of two novel
series of
polymers, namely the PBTZ and PBIT series. The PBTZ1 polymer was synthesized
as a copolymer of 4,8-bis(2-butyloctyl)benzo[1,2-<i>b</i>:4,5-<i>b</i>′]dithiophene (BDT) along with 2,5-bis(2-ethylhexyl)-3,6-bisthiazol-2-yl-2,5-dihydropyrrolo[3,4-<i>c</i>]pyrrole-1,4-dione (TzDPP), while PBTZ2 was a copolymer
of TzDPP and 2-(1-butylheptyl)thieno[3,4-<i>d</i>]thiazole
(TTz). The PBIT series based on dithienopyrrolobenzothiadiazole (DPBT),
and BDT was also synthesized. The PBIT series of polymers showed enhanced
ground and excited state dipole moments (μ<sub>g</sub> and μ<sub>e</sub>) when compared to the previously reported PBB3 polymer, while
PBTZ1 showed the largest dipole change (1.52 D) from ground to excited
state (Δμ<sub>ge</sub>) in respective single polymer units.
It was found that the power conversion efficiencies of the polymer
series were strongly correlated to Δμ<sub>ge</sub>. The
results reported demonstrate the utility of the calculated parameter
Δμ<sub>ge</sub> of single units of the polymers to predict
the performance of donor–acceptor copolymers in photovoltaic
devices. We rationalize this result based on the large degree of polarization
in the excited state, which effectively lowers the Coulomb binding
energy of the exciton in the excited state and leads to faster charge
separation kinetics, thus facilitating the full separation of electron
and hole