New Alternating Copolymers of 3,6‐Carbazoles and Dithienylbenzothiadiazoles: Synthesis, Characterization, and Application in Photovoltaics

Abstract

Three new low‐bandgap copolymers containing 3,6‐linked carbazole units are synthesized and characterized using a combination of spectroscopic and electrochemical techniques and DFT calculations. The electron‐withdrawing effect of the BTD units leads to a significant decrease of the optical bandgap from 2.59 eV for PBTC to 1.81 eV for PCDTBT, whereas pendant octyl chains on the thiophene ring lead to an increase of the gap to 2.05 eV for PCDOTBT due to steric hindrance. PBTC:PCBM photovoltaic devices exhibit a PCE of 0.35% with an EQE reaching 35% in the blue region of the solar spectrum whereas PCDTBT:PCBM blends are efficient over a wider spectral range due to the lower bandgap of PCDTBT.magnified image

Synthesis and structure–property relationship of carbazole‐alt‐benzothiadiazole copolymers

“…However, this trend was not observed when comparing PCBT3 and 4 for which purity of monomers along with more twisted reactants could be incriminated. While some of the materials ( PCBT1 and 4 ) showed short chain lengths, such molar masses could be sufficient to show interesting opto‐electronic features, since it was already reported that even oligomers with a suitable ICT could lead to small band‐gaps . Most of the measured dispersities were below 2, which is the theoretically expected for a step‐growth polymerization mechanism.…”

“…For instance, carbazole‐ alt ‐benzothiadiazole copolymers have attracted particular attention since Witker and Reynolds demonstrated that this combination allows a significant decrease of the band gap . In the literature, the band gaps of carbazole‐ alt ‐benzothiadiazole copolymers using thiophene units as spacers are weakly dependent of the molar masses and the linkage position (2,7‐ or 3,6‐) on the carbazole . This can be explained by an equivalent effective intramolecular charge transfer.…”

Synthesis and Photovoltaic Properties of 1,8‐Carbazole‐Based Donor–Acceptor Type Conjugated Polymers

“…This value was much lower than that reported for the copolymer of 2,7-carbazole and dithienylbenzothiadiazole (PCE: 3.6%) [ 5a ] . Combined with the PCE of 0.35% reported for the copolymer of 3,6-carbazole and dithienylbenzothiadiazole, [ 9 ] it would be reasonable to state that rod-like polymer structures are preferred over zigzag-type polymers for use as p-type semiconductors in solar cells. However, the effects of the alkyne spacer between the donor and acceptor moieties need to be elucidated in the future.…”

“…[ 9 ] In contrast to the 3,6-carbazole structure, the 1,8-carbazole connectivity possesses an effective conjugation length comparable to that of the 2,7-carbazole derivatives. [ 10 ] Furthermore, the 1,8-positions of the carbazole are the second highest electron density position and can be readily functionalized if the 3,6-positions of the carbazole with the highest electron density are appropriately protected.…”

New small molecules with thiazolothiazole and benzothiadiazole acceptors for solution-processed organic solar cells

“…[24][25][26] High performance OSCs were achieved by blending BT-based copolymers and fullerene derivatives, such as benzo [1,2- 27 diketopyrrolopyrrole-BT, 28 and carbazole-BT. 29 Thus, Tz and BT units could be employed in the design of a Institute of Material Science and Engineering, Ocean University of China, semiconductive small molecules for solution-processed OSC applications.…”