A new 2D-naphtho[1,2-b:5,6-b’]dithiophene based donor small molecules for bulk-heterojunction organic solar cells

Abstract

We present design and synthesis of three new symmetrical and linear A-D-A type π-conjugated donor small molecules (2D-NDT(TPD)2, 2D-NDT(Ester)2 and 2D-NDT(Amide)2) containing two dimensional (2D) naphthodithiophene (NDT) unit as the central donor core, end-capped with electron deficient unit such as thieno[3,4c]pyrrole-4,6-dione (TPD), 2-ethylhexyl 2-cyanoacetate (Ester) and 2-cyano-N-(2-ethylhexyl)acetamide (Amide) group respectively. We characterized these small molecules and further investigated the optical, electrochemical, morphological and photovoltaic properties. When solution–processed bulk heterojunction organic solar cells are fabricated using these small molecules, the morphology of 2D-NDT(Ester)2 or 2D-NDT(Amide)2 and [6,6]-PhenylC71-butyric acid methyl ester (PC71BM) blend film was optimized using 1,8 Diiodooctane (DIO) additive. DIO additive promotes the formation of nanoscopically well-connected molecular domains in the active blend film. A device based on (1% DIO, 1:1) 2D-NDT(Ester)2:PC71BM exhibited highest the efficiency of 1.22% with a short-circuit density (Jsc) of 3.75 mA/cm2, an open circuit voltage (Voc) of 0.91 V and fill factor (FF) of 35.50. Similarly for (1% DIO, 1:3) 2D-NDT(Amide)2:PC71BM device efficiency of 0.55%, with Jsc of 2.36 mA/cm2, Voc of 0.64 V and FF of 36.95 was observed. Whereas for (1:2) 2D-NDT(TPD)2:PC71BM device, due to the improper blending and phase separation between donor and acceptor efficiency restricted to 0.33% with Jsc of 1.66 mA/cm2, Voc of 0.73 V and FF of 27.2.