Thiadiazoloquinoxaline-Based Narrow Energy Gap Molecules for Small Molecule Solar Cell Applications

Polymer bulk heterojunction (BHJ) solar cells consisting of the photoactive layer containing the interpenetrating network of electron donating polymer and electron accepting PC71BM has been proved to be quite effective in converting the solar energy into electrical energy. After many efforts, the power conversion efficiency (PCE) of the polymer solar cells (PSCs) was improved up to 9.2%. Unfortunately, the PCEs of the PSCs were found to be highly affected by the synthetic characteristics of the donor polymers. At present, the poor reproducibility of the synthetic polymer characteristics such as purity, molecular weight, regioregularity and polydispersity of each batch of the polymerization limits the commercial application of PSCs. On the other hand, small molecule organic solar cells (SMOSCs) fabricated from the photoactive layer containing organic small molecule as an electron donor and PC71BM as an electron acceptor also showed promising performances in terms of PCE. The PCE of the solution-processed SMOSCs was reached up to 7%. and that for the vacuum-processed tandem SMOSCs was improved up to 10.7%. The overall PCE of SMOSCs is quite close to the maximum PCE obtained from the PSCs. This inspires us to develop new low band gap small molecules for SMOSCs application. It is well known that the PCE of solar cell devices is strongly dependent on the light harvesting ability of the photoactive layer. To improve the light harvesting ability of the active layer, it is essential to utilize the donor molecules which can absorb the sun light from 300 nm to 1000 nm, along with that the absorption band of donor molecules should be located at the maximum solar flux region (500 nm to 800 nm) of the solar spectra. In our attempt to prepare narrow energy gap small molecules, we were interested in utilizing thiadiazoloquinoxaline units, because certain polymers containing thiadiazoloquinoxaline units have been known to show their absorption band from 300 nm to 1200 nm. In this study, we prepared two new thiadiazoloquinoxaline-based low band gap small molecules, TDQ-SM1 and TDQ-SM2, shown in Scheme 1 and studied their optical, electrical and photovoltaic properties. In this study, to recognize the influence of the substituent bonded to the quinoxaline ring of the thiadiazoloquinoxaline unit, isobutyl and thiophene substituted thiadiazoloquinoxaline compounds 1 and 2 were coupled with commercially available trans-2-(4-pentylphenyl)vinylboronic acid pinacol