Synthesis and characterization of new indeno[1,2-b]indole-co-benzothiadiazole-based π-conjugated ladder type polymers for bulk heterojunction polymer solar cells

We have developed a new ladder type, tetrafused, π-conjugated building block with indeno[1,2-b]indole (II) and extended the π-conjugation of the indeno[1,2-b]indole ring by incorporating a thiophene π-spacer in the 2nd and 8th positions to attain dithiophene-substituted indeno[1,2-b]indole (IIDT). The two newly synthesized donor monomers II and IIDT were polymerized with 2,1,3-benzothiadiazole (BT) acceptor to obtain poly[4-(5-ethyl-10,10-dioctyl-5,10-dihydroindeno[1,2-b]indol-2-yl)benzo[c][1,2,5]thiadiazole] (PIIBT) and poly[4-(5-(5-ethyl-10,10-dioctyl-8-(thiophen-2-yl)-5,10-dihydroindeno[1,2-b]indol-2-yl)thiophen-2-yl)benzo[c][1,2,5]thiadiazole] (PIIDTBT). The photophysical studies of PIIBT and PIIDTBT reveal the bandgaps 1.99 and 1.75 eV, respectively. Both the polymers showing a deep-lying highest occupied molecular orbital (HOMO) energy levels ∼−5.37 eV. The hole mobilities of PIIBT (4.03 × 10−6 cm2/V s) and PIIDTBT (1.94 × 10−4 cm2/V s) were studied by the space charge limited current (SCLC) method. Owing to the incorporation of the thiophene moiety as a π-spacer, PIIDTBT displayed extended π-electron conjugation and improved intermolecular π–π interaction in the polymer backbone. The bulk heterojunction polymer solar cells (BHJ PSCs) fabricated using PIIBT:PC71BM (1:3, 1.5 wt%) exhibited a power conversion efficiency (PCE) of 1.23%, which was enhanced to 3.72% for the BHJ PSCs based on PIIDTBT:PC71BM (1:3, 1.5 wt%) blend. The high hole mobility of PIIDTBT is in good agreement with its high short-circuit current density (Jsc) and fill factor (FF). This is the first report of an enhanced PCE using indeno[1,2-b]indole derivative as a donor material in BHJ PSCs.