Negative Correlation between Intermolecular vs Intramolecular Disorder in Bulk-Heterojunction Organic Solar Cells.

By varying the concentration of a solvent additive, we demonstrate the modulation of intermolecular (donor/acceptor (D/A) interface) and intramolecular (bulk) disorder in blends of the low-band gap polymer poly[2,6-(4,4-bis(2-ethylhexyl)-4 H-cyclopental[2,1- b;3,4- b']-dithiophene)- alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) blended with [6,6]-phenyl-C ₇₁ -butyric acid methyl ester (PC ₇₁ BM). Using the solvent additive concentration of 1,8-diiodooctane (DIO) in the host-processing solvent, the disorder in the bulk and at the interface is studied in terms of Urbach energy, electroluminescence (EL) broadening, and EL quantum efficiency (EL _QE ). The Urbach energy varies from 80 to 39 meV for bulk and 39 to 51 meV for D/A interface. An interesting feature is that changes in the Urbach energy of the D/A interface are opposite to those of the Urbach energy of bulk; i.e., the disorder at the D/A interface increases as the disorder in the bulk decreases with increase in DIO concentration. Our study evidently suggested a negative correlation between intermolecular and intramolecular property in a bulk-heterojunction solar cell. Furthermore, scanning photocurrent microscopy measurements show that the effective hole transport length is double in magnitude for cells processed from 3 vol % DIO in comparison to that in cells processed from 0 vol %. This increase in effective hole transport length is explained by an increase in the delocalization of the electronic states involved in charge transport, as confirmed by dark J- V knee voltage,  J _SC and E _U-bulk measurements. Henceforth, we provide a functional relationship between the additive-induced bulk-heterojunction morphology and the optoelectronic properties of PCPDTBT-based solar cells.