Development of thiazolo [5,4-d]thiazolebased novel organic chromophores and their photovoltaic applications

Organic solar cells have recently received a great deal of interest owing the advantages of inexpensive materials, easy to process and high power conversion efficiency. OPV materials in bulk heterojunction (BHJ) solar cells basically consist of a phase-separated mixture of a p-type conjugated polymer or small organic molecule as a donor and the fullerene derivative; phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM) as an acceptor material. Organic chromophores display numerous advantages such as low bandgap, ease and reproducibility of synthesis, ease of purification process and high solubility in common organic solvents. Various π-conjugated small molecules of D-A-D type architecture have been designed to improve the light harvesting ability, charge-separation property, energy levels and photovoltaic properties. Thiazolo [5, 4-d] thiazole is a rigid and fused heterocyclic ring moiety which ensures a coplanar structure of a π-conjugated system and a strong π-π stacking ability. Thiazolo [5,4-d] thiazole as electron-acceptor unit has aroused much interest in efficient small organic molecules of D–A–D or A–D–A type and accomplishes good photovoltaic properties. The both side terminal n-hexyl side chains in these molecules lead to good solubility in common organic solvents, enhances the energy levels and induces self-assembly behavior. From the morphological analysis, the presence of different liquid-crystal phases affects the film-morphology of the devices during the solution-processed fabrication and leads to self-organization behavior. Due to the incorporation of triphenyl amine donor, small molecule presents high absorption, good film morphology and the solubility which eventually lead to better performances of small molecule organic solar cells (SMOSCs). The overall improved performance of the devices might be attributed to their improved absorption, electrochemical-properties and the presence of strong e1ectron-withdrawing furan moieties. Moreover, the presence of furan spacer in small molecule in blend thin film substantially improves the interface area for an exciton dissociation, which results in the high photocurrent density and PCE of the devices. The enhanced JSC and PCEs are attributed to the presence of terminal alkyl units and the homogenous-miscibility of the blend thin film which assists the ultrafast intermolecular charge transfer from donor to acceptor. This chapter includes some of the promising electron-donor organic small molecules and their applications in organic solar cells. The synthesis methods, morphology, physical and chemical properties of the materials have shown that the further architecting of these organic materials would surely their applications in solar cells and other opto-electronic devices due to enhanced optical, electrochemical and photovoltaic properties.