Acceptor-donor-acceptor small molecules based on derivatives of 3,4-ethylenedioxythiophene for solution processed organic solar cells

Three simple semiconducting acceptor-donor-acceptor (A-D-A) small molecules based on an electron-rich (3,4-ethylenedioxythiophene) EDOT central core have been synthesised (DIN-2TE, DRH-2TE, DECA-2TE) and characterised. Organic photovoltaic (OPV) devices incorporating these materials have been prepared and evaluated. The physical properties of the molecules were characterised by TGA, DSC, UV/vis spectroscopy and cyclic voltammetry. The optical HOMO-LUMO energy gaps of the molecules in the solid state were in the range 1.57-1.82 eV, and in solution 1.88-2.04 eV. Electrochemical HOMO-LUMO energy gaps determined by cyclic voltammetry were found to be in the range 1.97-2.31 eV. The addition of 1% 1,8-diiodooctane (DIO) to photoactive blends of the A-D-A molecules and PC₇₁BM more than doubled the power conversion efficiency (PCE) in the case of DRH-2TE:PC₇₁BM devices to 1.36%.