In
this paper we report the rationale and implementation of a new building
block for organic electronics based on 4,8-di(2-thienyl)-benzo[1,2-d:4,5-d′]bis([1,2,3]thiadiazole)
(iso-BBT-T2) and realization of two alkyl-functionalized iso-BBT-tetrathiophene (T4) alternating copolymers
(P1 with alkyl = 2DT and P2 with alkyl = 2DH). Compared to the previously investigated
small molecules/polymers based on the conventional 4,8-di(2-thienyl)-benzo[l,2-c:4,5-c′]bis[ l,2,5]thiadiazole
(BBT-T2), the use of the iso-BBT heterocycle
widens the polymer band gap to a region (∼1.4 eV) compatible
for use in single junction solar cells. The influence of iso-BBT vs BBT structural variation on the molecular structure,
electronic characteristics, and optical properties was accessed by
DFT computations, single-crystal determination, optical absorption,
and electrochemical measurements. In-plane charge transport for P1 and P2 was investigated in an organic thin-film
transistor (OTFT) structure demonstrating hole mobilities approaching
1 cm2 V–1 s–1 and further
enhanced by off-center spin-coating method to 1.32 cm2 V–1 s–1. Using PC61BM as
acceptor, a remarkable PCE of 10.28% was achieved for P1 along with a current density > 20 mA/cm2. The substantial
PCEs of these devices, despite the relatively narrow donor energy
gap, is due to retention of high open circuit voltages ( V
oc > 0.8 V) as the result of the small energy
loss (E
loss < 0.6 eV). Atomic force
microscopy, transmission electron microscopy, and X-ray diffraction
characterization further support the solar cell trends and rationalize
structure–property correlations. These results demonstrate
that iso-BBT-T2-based polymers are promising candidates
for both organic electronics and photonic applications.