p/n Switching of Ambipolar
Bithiazole–Benzothiadiazole-Based
Polymers in Photovoltaic Cells
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Abstract
Two new alternating copolymers <b>P1</b> and <b>P2</b>, of bithiazole (BT) and benzothiadiazoles (BTZ), differing
in their
side chain positioning at the thiophene units which sandwich the BT
unit, were designed and synthesized. Both polymers exhibited broad
absorption ranging from 300 to 700 nm with a narrow optical bandgap
in the film state. Control over structural ordering of polymer chains
was achieved in <b>P1</b> by treating with a small amount of
additive (1,8-octanedithiol, ODT) as evident by a large red shift
of absorption peak and also from the XRD measurements. In contrast,
no such effects were observed in the case of <b>P2</b> in the
presence of additive. Flash-photolysis time-resolved microwave conductivity
(FP-TRMC) experiments revealed that the transient photoconductivity
of <b>P1</b> is far superior to that of <b>P2</b>, which
is further increased when processed with ODT. The charge carrier mobility,
as determined by the space-charge-limited current (SCLC) technique,
indicates that <b>P1</b> exhibits both electron and hole mobilities
with a clear dominance of the latter. The charge carrier mobilities
become higher and more balanced for ODT-modified <b>P1</b> films
compared to that of <b>P1</b> alone. TRMC analysis revealed
that the photoconductivity of <b>P1</b> reduced when blended
with PCBM in the absence of additive, whereas significant enhancement
was obtained in presence of additive. The blend with P3HT exhibited
an increase in photoconductivity in both the presence and absence
of additive. In complete accordance with the TRMC results, in the
absence of additive, <b>P1</b> acted as an n-type material (P3HT
as donor), whereas in presence of additive, it exhibited ambipolar
nature acting as both n-type and p-type (P3HT as donor and PCBM as
acceptor, respectively) material. Switching of the major charge carrier
species was demonstrated simply by the presence of additive for <b>P1</b> in the present paper