Synthesis and Characterisation of Novel Semiconducting Small Molecules for Applications in Organic Solar Cells.

Abstract

Five novel semiconducting organic small molecules have been synthesised, fully characterised, and their photovoltaic performance tested in fabricated organic solar cells. Three (molecules 5, 7 and 9) were acceptor-donor-acceptor (A-D-A) small molecules bearing an electron-rich (3,4-ethylenedioxythiophene) EDOT central core with different terminal groups. The additional two (molecules 20 and 21) were truxene derivatives which have been differentiated by the alkyl chain length on the molecular backbone. The physical properties of molecules 5, 7, 9, 20 and 21were determined by TGA, DSC, UV/Vis spectroscopy and cyclic voltammetry. Their optically measured HOMO-LUMO energy gaps in the solid state were in the range 1.57 – 2.33 eV, and in solution 1.88 - 2.15 eV. The electrochemical HOMO-LUMO energy gaps determined by cyclic voltammetry were in the range 1.97 - 2.5 eV. A conventional bulk-heterojunction device architecture was employed in the fabrication and testing of the photovoltaic performance for the electron donating molecules 7 and 9. The processing solvent and fabrication conditions were optimised along with the device morphology by addition of 1,8-Diiodooctane (DIO) solvent to the donor/acceptor (D/A) blend. The devices showed better performance when processed with chloroform compared to chlorobenzene. The optimised processing conditions were 1:3 weight ratio of the D/A blend for 7:PC71BM devices that was annealed at 90°C. For 9:PC71BM devices 1:4 D/A blend thermally annealed at 60°C was optimal. 1% DIO more than doubled the power conversion efficiency (PCE) of the best performing device for compound 7 (0.63 vs 1.36%), but only slightly improved that for compound 9 (1.03 vs 1.05%). Fabricated and tested devices for molecules 20 and 21 showed a bifunctional property of the two in bulk-heterojunction organic solar cells. The molecules with longer side chains, 21, displayed a better device performances compared to 20, even though they were not impressive.