New alkylthienyl substituted benzo[1,2-b:4,5-b′]dithiophene-based polymers for high performance solar cells

Abstract

Three new alkylthienyl substituted benzodithiophene (BDT)-based polymers, poly{4,8-bis(2′-ethylhexylthiophene)benzo[1,2-b;3,4-b′]dithiophene-alt-5,5-(4′,7′-di-2-thienyl-5′,6′-dioctyloxy-benzo[c][1,2,5]oxadiazole)}(PBDTTDTBO), poly{4,8-bis(2′-ethylhexylthiophene)benzo[1,2-b;3,4-b′]dithiophene-alt-5,5-(4′,7′-di-2-thienyl-5′,6′-dioctyloxy-2′,1′,3′-benzothiadiazole)}(PBDTTDTBT) and poly{4,8-bis(2′-ethyl hexylthiophene)benzo[1,2-b;3,4-b′]dithiophene-alt-5,5-(4′,7′-di-2-thienyl-2-octyl-2′,1′,3′-benzotriazole)} (PBDTTDTBTz), were synthesized by Stille coupling polymerization reactions. All of the polymers were found to be soluble in common organic solvents such as chloroform, tetrahydrofuran and chlorobenzene with excellent film forming properties. Their structures were verified by elemental analysis and NMR spectroscopy, the molecular weights were determined by gel permeation chromatography (GPC) and the thermal properties were investigated by thermogravimetric analysis (TGA). The polymers exhibited tunable absorptions and energy levels on incorporation of different electron accepting units. All the copolymers showed high field hole mobility up to 10⁻² order, and their blends with PCBM exhibited mobility as high as 10⁻¹ order by the space-charge-limited current (SCLC) method. Preliminary photovoltaic cells based on the device structure of ITO/PEDOT:PSS/PBDTTDTBO:PC₇₁BM (1 : 1.5, w/w)/Ca/Al showed a power conversion efficiency of 5.9% with a high open-circuit voltage (V_oc) of 0.84 V and a short circuit current density (J_sc) of 11.45 mA cm⁻². To the best of our knowledge, this is the highest efficiency for dithienyl benzooxadiazole (DTBO)-based polymer solar cells.