Pranabesh Dutta

Novel naphtho[1,2-b:5,6-b′]dithiophene core linear donor–π–acceptor conjugated small molecules with thiophene-bridged bithiazole acceptor: design, synthesis, and their application in bulk heterojunction organic solar cells

This study involves the development of new solution processable organic small molecules for photovoltaic applications. We have rationally designed and synthesized two novel, symmetrical and linear D–A–D–A–D-type π-conjugated organic small molecules bearing a rigidly fused naphtho[1,2-b:5,6-b′]dithiophene core flanked by bithiazole (M3) or triphenylamine-capped thiophene(3-decanyl)-bridged bithiazole (M4) conjugated moieties through thiophene(3-decanyl) spacer. The resultant small molecules have been characterized by thermal analysis, UV-Vis spectroscopy, photoluminescence spectroscopy, X-ray diffraction, and cyclic voltammetry. Their applications in field effect transistors and solution processed bulk-heterojunction (BHJ) organic solar cells (OSCs) have also been explored. Due to the presence of an adequate number of 3-decanylthiophene moieties as short π-bridging units into the conjugated molecular backbone, both the small molecules have good solubility in common organic solvents and form highly ordered self-assembled π–π stacks in their solid states with long decyl chains organized by interdigitation. Additionally, they exhibit good thermal stability with decomposition temperatures exceeding 380 °C. Photophysical and electrochemical studies reveal that these molecular donors have comparable optical band gaps (∼1.99 to 2.02 eV) and nearly similar HOMO–LUMO energy levels, both of which are aligned with the PC61BM/PC71BM electron acceptors. The preliminary BHJ photovoltaic cells configured with the device structures of ITO/PEDOT:PSS/small molecule:PC71BM/Lif/Al were evaluated. The small molecule M3 was found to deliver the best power conversion efficiency of 1.09% when processing the active layer from chloroform solvent. In contrast, under identical device conditions M4 gave improved performance with a maximum efficiency of 1.62%. The morphological studies using atomic force microscopy showed that the PCE enhancement for M4 is mainly due to improvement in the nanoscale film morphology of the M4–PC71BM blend.

Synthesis characterization and bulk-heterojunction photovoltaic applications of new naphtho[1,2-b:5,6-b′]dithiophene–quinoxaline containing narrow band gap D–A conjugated polymers

Alternating donor-acceptor (D–A) π-conjugated copolymers, poly[2,7-bis(3-hexadecylthiophene-2-yl)naphtho[1,2-b:5,6-b′]dithiophene-5,5′-diyl-alt-5,8-bis(4-hexadecylthiophen-2-yl)-2,3-bis(4-(octyloxy)phenyl)quinoxaline-5,5′-diyl] (PTNDTT-QX-I) and poly[2,7-bis(3-hexadecylthiophene-2-yl)naphtho[1,2-b:5,6-b′]dithiophene-5,5′-diyl-alt-5,8-bis(thiophen-2-yl)-2,3-bis(3-(octyloxy)phenyl)quinoxaline-5,5′-diyl] (PTNDTT-QX2-II), were designed and synthesized based on the same thiophene-bridged naphtho[1,2-b:5,6-b′]dithiophene donor moiety, differing only at the quinoxaline acceptor counterpart by either additional electron-donating alkyl chain substitution in the thienyl ring attached to the quinoxaline base (in PTNDTT-QX-I) or a change in the location of the outward alkoxy side chain substituent of the phenyl rings (to the meta-position) adjoining the quinoxaline base (in PTNDTT-QX-II). The effect of alkyl chain positioning on the thermal, optical, and electrochemical properties, as well as field effect transistors and solar cell performances of the copolymers, were investigated and the results were compared with a previously published copolymer, PTNDTT-QX, which features a similar quinoxaline unit but is alkoxy substituted at the position para to its peripheral phenyl rings. Both polymers exhibited excellent thermal stability, with thermal decomposition temperatures over 400 °C. They absorbed light in the 300–700 nm range and exhibited optical band gaps of about 1.70 and 1.73 eV for PTNDTT-QX-I and PTNDTT-QX-II, respectively. Precise control of the alkyl/alkoxy chain positioning has made it possible to tune the HOMO energy levels between −5.14 and −5.29 eV and the LUMO energy levels between −3.44 and −3.55 eV. Bulk heterojunction photovoltaic devices of the structure ITO/PEDOT:PSS/polymer:PC71BM/LiF/Al were fabricated by using the polymers as the donors and [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) as the acceptor. Power conversion efficiencies (PCEs) of 1.28% and 1.61% respectively were achieved for the photovoltaic devices based on PTNDTT-QX-I/PC71BM and PTNDTT-QX-II/PC71BM under AM 1.5 G simulated 1-sun solar illumination.

Naphtho[1,2-b:5,6-b′]dithiophene-based conjugated polymer as a new electron donor for bulk heterojunction organic solar cells

We report for the first time the synthesis and photovoltaic application of a naphtho[1,2-b:5,6-b′]dithiophene (NDT) containing donor–acceptor conjugated copolymer. The combination of a thiophene-bridge NDT building block with a quinoxaline acceptor unit results in a new copolymer that has an optical band gap of 1.77 eV, a HOMO level of −5.24 eV, and a hole mobility of 5.0 × 10−5 cm2 V−1 s−1. Preliminary characterization of the bulk-heterojunction solar cell device exhibits a PCE of around 1.5%.

Synthesis and Photovoltaic Properties of Moderate Band Gap Diketopyrrolopyrrole Based Small Molecules for Solution Processed Organic Solar Cells

Two new small molecules, based on diketopyrrolopyrrole core flanked by cyanothiophene units (CN-DPP-CN, CN-TH-DPP-TH-CN) were synthesized using CuCN and palladium catalyzed Suzuki coupling and explored in organic solar cells (OSCs). The HOMO/LUMO energy levels of CN-DPP-CN, CN-TH-DPP-TH-CN having moderate band gap of 1.83 eV and 1.44 eV were estimated to be −5.63/−3.84 eV, −5.20/−3.75 eV respectively. The device efficiency was found to be 0.013, 0.21% for CN-DPP-CN, CN-TH-DPP-TH-CN respectively as donors for BHJ solar cells. When CN-DPP-CN (0.05%) was added in P3HT:PC60BM device, its PCE was enhanced to 2.45% from 2.08% signifying its ability to be used as potential n-type additive.