Enhancement in the inherent photostability of small molecule-based BHJ device by molecular architecturing

Three small molecules of D-A-D architecture were synthesized with benzothiadiazole as basic acceptor unit. It has been showed previously that the fluorination on the benzothiadiazole acceptor moiety and incorporation of fused thienothiophene spacer have increased the photovoltaic properties [1]. Hence the same strategies have applied here on small molecules to study both the photovoltaic and to tailor the inherent photostability of the material. The first small molecule, TBT (4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole) was designed with benzothiadiazole- acceptor, and thiophene moiety as the donor unit. The effect of fluorine substitution (TDFBT- 5,6-difluoro-4,7-di(thiophen-2-yl) benzo [c][1,2,5]thiadiazole) and incorporation of fused donor moiety (TTBTT- 4,7-bis(thieno[3,2-b] thiophene-2-yl) benzo [c][1,2,5]thiadiazole) on the optical electrochemical, photovoltaic and photostability of the molecules were evaluated. The photostability study on the structurally modified molecules and blend with PCBM shows promising results. Structural modifications of the p-type molecules exhibit an improved photostability of neat films by significantly. The photostability of the active layer comprised of small molecule- PCBM blend has increased by >13.8 times on incorporation of fused thienothiophene (TTBTT) and by a factor of >5.2 on fluorination of acceptor (TDFBT). The results suggest that the structural modification p-type materials not only improve the optical, electrochemical and photovoltaic properties, but also the photostability of BHJ active layer. A careful selection of p-type materials with increased electron affinity with a sensible balancing with ambient stability and band edge matching with that of the n-type material would lead to stable photovoltaic devices.