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)-4H-cyclopental[2,1-b;3,4-b']-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) blended with [6,6]-Phenyl-C71-butyric acid methyl ester (PC71BM). 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 (ELQE). The Urbach energy varies from 80 meV to 39 meV for bulk and 39 meV to 51 meV for D:A interface. An interesting feature is that changes in the Urbach energy of the D:A 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 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 cells processed from 0 vol.%. This increase in effective 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, JSC and EU-bulk measurements. Henceforth, we provide a functional relationship between the additive induced bulk heterojunction morphology and the optoelectronic properties of PCPDTBT-based solar cells.