Electronic structures and charge transport properties of the organic semiconductor bis[1,2,5]thiadiazolo-p-quinobis(1,3-dithiole), BTQBT, and its derivatives

We analyze the correlation between crystal and film structures and charge transport of an important organic semiconductor, bis[1,2,5]thiadiazolo-p- quinobis(1,3-dithiole) (BTQBT), and its derivatives 4,8-bis(1,3-dithiol2- ylidene)-4H,8H-[1,2, 5]selenadiazolo[3,4-f]-2,1,3-benzothiadiazole, 4,8-bis(1,3-diselenol-2-ylidene)-4H,8H-benzo[1,2-c:4,5-c′]bis[1,2,5] thiadiazole, and tetramethyl-BTQBT. We present first-principles density functional theory (DFT) calculations that agree well with earlier angle-resolved photoelectron spectroscopy (ARPES) experiments on BTQBT films, strongly supporting that the BTQBT films adopt the same layered structure as in the single crystals. Qualitative charge transport properties based on presented DFT results agree with experiments regarding the sign of the charge carriers and the unusually small anisotropy of conductivity. These agreements indicate that accurate electronic structure calculations, when coupled with ARPES, help establish the correlation between intermolecular packing and charge transport, which is one of the central but elusive aspects of organic molecular materials. Predictions are made for derivatives of BTQBT, and calculations agree with available experimental information on the conductivities. Comparisons are made with pentacene, one of the most widely studied organic molecular materials.