Side chain engineering on a small molecular semiconductor: Balance between solubility and performance by choosing proper positions for alkyl side chains

Small molecular (SM) semiconductors with sufficient solubility and high performance are highly desired in solution-processed organic electronics. In order to address the poor solubility that a reported high performance molecular semiconductor (BDT(ThBTTh)(2)) with a benzodithiophene (BDT) core and two thiophene-benzothiadiazole-thiophene arms (ThBTTh) suffers from, two additional hexyl side chains have been designed into its scaffold and different attaching fashions yield two new SM semiconductors, SM1 and SM2. The integration positions in SM1 are terminal thiophene units, while that of SM2 are located on the inner thiophene units neighbor to the central BDT unit. With expectation, the so-prepared SM1 and SM2 possess much better solubility than BDT(ThBTTh)(2). However, the integration of additional alkyl side chains sacrifices its optoelectronic performance and the sacrifice extent is highly dependent on their attaching fashions. The molecule SM2 having new additional side chains in two inner thiophene units displayed much better performance (hole mobility: 1.80 x 10(-2) cm(2) V-1 s(-1), solar cell efficiency: (2.57 +/- 0.17)%) than SM1 that integrates them at the two terminal thiophene units (hole mobility: 2.18 x 10(-4) cm(2 )V(-1) s(-1), solar cell efficiency: (0.87 +/- 0.11)%). For understanding the origination of such performance differences, detail characterizations including differential scanning calorimetry, ultraviolet-visible absorption spectroscopy, cyclic voltammetry, density function theoretical calculation, X-ray diffraction and atomic force microscopy have been carried out. Finally, benefiting from their enhanced good solubility in non-chlorinated solvents, toluene has been proved capable as a green processing solvent for organic solar cell fabrication. The so-obtained SM2 devices displayed an optimized efficiency of (2.35 +/- 0.19)%.