Design Of Expanded Carbazole-benzothiadiazole Based Small Moleculars And Their ?-linkage Effect

The organic small molecule solar cells (OSM-SCs) become one of the current research hotspots on account of their well-defined chemical structure, easy purification and better batch-to-batch reproducibility. So far, the highest efficiency of OSM-SCs has achieved to 10.08%, suggesting that OSM-SCs has a bright prospect.Over the past few years, much attention has been paid to the OSMs with A-D-A-structure due to their diversity of structures, easy control of energy levels and the favorable PV properties. Currently, most of researchs'work for designing A-D-A-typed OSM are focus on design the donor and acceptor units within molecules, in contrast, less attention was paid to extension structure and ?-linkage effect of various bridges. Therefore, to investigated above contents, another donor units triphenylamine and different ?-bridges (phenyl, styryl, cyanostyryl and phenylethynyl) were introduced into the A-D-A-typed molecule FBT which containing 3,6-carbazole as centre donor units. As a result, expanded bridge-molecules FSBT, FEBT, FECNBT and FTBT were synthesized in this paper. After characterization, we found that on one hand the conformation of these molecules have been bent as it like "bow shape" On the other hand, benefited from introducting the triphenylamine units and different ?-bridge extending the molecular conjugated length, the absorption spectra of bridge molecules are greatly broadened and red-shifted. In addition, due to triphenylamine units and various ?-bridges, the electrochemical bandwidth of the molecules are greatly reduced, especially the electrophilic bridges such as cyanostyryl and phenylethynyl played more prominent role in reducing energy band gap. According to the PV data:(1) The PV performance has a huge improvement due to the introduction of the different bridges and triphenylamine donor groups; (2) ?-bridge of cyanostyryl and phenylethynyl with strong electrophilic effect successfully deepened the HOMO level of FECNBT and FTBT, thus improved the VOC value of the device, especially the device based on FECNBT reached a high VOC as 1.08V; (3) Owing to the non-rigid structure of phenyl and cyanostyryl reducing the steric and conformational constraint between the adjacent units, the tension of the molecules are more accommodating when they employed as ?-bridge, hence the device based on FSBT and FECNBT possessing larger Jsc and PCE. Finally, the molecule FSBT and FECNBT with relatively higher efficiency were chose for solvent vapor annealing (SVA) optimization. The SVA results showed that the device based on FSBT/PC61BM is suitable for using lower relative vapor density THF as annealing solvent. Through exploring the optimal annealing time, the FF value of the device was promoted from 0.33 to 0.60, and the efficiency is also increased from the original 2.66% to 6.59%, which means 2.5 times'growth. In addition, the device based on FECNBT/PC61BM was also suitable for using lower relative vapor density THF or DMK as annealing solvent, of which DMK with larger polarity having more obvious effect. As a result, when the DMK were using as annealing solvent, the FF value of device was increased from 0.34 to 0.52, and the efficiency was also promoted to 7.59%, which approximately increased 2.8 times.Research results indicate that the effective extension structures and an in-depth study of ?-Linkage effects possess vital significance for understanding structure-property relationships, thus developing high performance photovoltaic materials.