Enhancing performances of organic photovoltaics by incorporating small molecule stereoisomers as the third component†

Abstract

In the pursuit of high power conversion efficiency (PCE) for organic photovoltaics (OPVs), we synthesized four stereoisomeric 2,2′-((2Z,2′Z)-((4,4,9,9-tetrahexyl-4,9-dihydro-s-indaceno[1,2-b:5,6-b′]dithiophene-2,7-diyl)bis(methanylylidene))bis(3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (IDIC) type small-molecule acceptors that combine the indaceno[1,2-b:5,6-b′]dithiophene (IDT) core with two different electron deficient (3-oxo-2,3-dihydro-1H-indene-2,1-diylidene) dimalononitrile peripheral end groups, with hexyl or hexyloxyl side chains, namely cis-ID-OR, trans-ID-OR, cis-ID-OR-4Cl, and trans-ID-OR-4Cl, respectively, as the third component for being incorporated into the PM6:Y6 active layer for the fabrication of OPVs. The PM6 : Y6 : cis-stereoisomer—cis-ID-OR or cis-ID-OR-4Cl—(1 : 1 : 0.2 wt ratio) ternary blend that incorporates the cis-stereoisomer with either hexyl or hexyloxyl side chains oriented in the same direction imparted a larger crystalline correlation length (CCL), as compared to that in the cases of the trans-stereoisomer—trans-ID-OR or trans-ID-OR-4Cl—with hexyl or hexyloxyl side chains oriented in opposite directions, with the PM6:Y6:cis-ID-OR ternary blend providing the largest CCL of 25.8 Å, which facilitates carrier transport in the system. As a result, the PM6 : Y6 : cis-ID-OR (1 : 1 : 0.2 wt ratio) device displays the highest short-circuit current density (J_sc) value of 25.4 mA cm⁻², and an improved PCE value of 17.1%, over 15.8% for the device with the PM6:Y6 active layer (J_sc = 23.8 mA cm⁻², CCL = 19.8 Å), whereas the PCE value of PM6 : Y6 : trans-ID-OR (1 : 1 : 0.2 wt ratio) only reached 14.7%. The enhancement in the PM6:Y6:cis-ID-OR device results from the higher J_sc and fill factor values that were affected by CCL values. Therefore, this approach of using the side-chain orientation of the chiral-isomer acceptor to induce variations in CCL values can provide the enhancement of the PCE values of the devices when incorporating the cis-stereoisomer rather than the trans-stereoisomer.