Development of novel naphtho[1,2-b:5,6-b′]dithiophene and thieno[3,4-c]pyrrole-4,6-dione based small molecules for bulk-heterojunction organic solar cells

Abstract

Two new small molecules, composed of naphthodithiophene (NDT) donor core and thienopyrroledione (TPD) group acceptor group end-capped with and without an alkyl-bithiophene, defined as NDT(TPD)2 and NDT(TPDTT)2 were designed and synthesized by stille coupling reactions. The thermal and electrochemical analyses carried out for both the small molecules revealed good thermal stability along with high decomposition temperature (>350 °C). NDT(TPD)2 showed a deep HOMO level (−5.38 eV), compared to slightly upshifted HOMO (−5.26 eV) of NDT(TPDTT)2. While X-ray diffractometry suggests crystalline and amorphous nature of NDT(TPD)2 and NDT(TPDTT)2 respectively, the space charge limited current analysis revealed high hole mobility in the former and appreciable charge balance in the later. The conventional organic solar cell (OSC) devices fabricated using NDT(TPD)2 and NDT(TPDTT)2 as donor show power conversion efficiency (PCE) of 0.26% and 0.8% respectively. While NDT(TPDTT)2 device after blending with additive, owing to the improved D-A heterojunction yielded maximum PCE of 1.31% resulting from enhanced Jsc 3.32 mA/cm2, Voc 0.75 V and FF of 52.44.