5,6-Difluorobenzothiadiazole and silafluorene based conjugated polymers for organic photovoltaic cells

Abstract

To achieve 5,6-difluorobenzothiadiazole and 2,7-linked silafluorene based soluble conjugated polymers, flexible side chains were attached at different positions of the conjugated polymers. Three soluble polymers PSiF-D(OT)DFBT, PSiF-TTDFBT, and PDOSiF-DTDFBT were prepared and used as donor materials for polymer solar cells. PSiF-D(OT)DFBT exhibits a band gap of 2.06 eV with a deep HOMO of −5.64 eV. PSiF-TTDFBT shows a band gap of 1.75 eV with the HOMO of −5.23 eV. PDOSiF-DTDFBT is of a band gap of 1.86 eV with the HOMO level of −5.37 eV. Among these three polymers, PDOSiF-DTDFBT shows the highest field effect transistor (FET) hole mobility up to 3.31 × 10⁻² cm² V⁻¹ s⁻¹, PDOSiF-DTDFBT:PC₇₁BM blend films show the highest SCLC mobility up to 5.10 × 10⁻⁴ cm² V⁻¹ s⁻¹, and polymer solar cells (PSCs) with the blend of PDOSiF-DTDFBT:PC₇₁BM (1 : 1, by weight) as the active layer gave a power conversion efficiency (PCE) of 4.03% with an open circuit voltage (V_oc) of 0.73 V, a short circuit current (J_sc) of 8.55 mA cm⁻², and a fill factor (FF) of 0.65. Our studies also reveal the structure–property relationship of 2,7-linked silafluorene and 5,6-difluorobenzothiadiazole based conjugated polymers.