Effect of fluorination on the performance of poly(thieno[2,3-f]benzofuran-co-benzothiadiazole) derivatives

Abstract

In order to shed light on the effects of different numbers of fluorine atoms in donor–acceptor (D–A) conjugated polymers on the photophysics and photovoltaic properties, three polymers named PTBFBT-0F, PTBFBT-1F, PTBFBT-2F were synthesized and thoroughly investigated. The nonfluorinated benzothiadiazole (BT) polymer (PTBFBT-0F) has a highest occupied molecular orbital (HOMO) energy level of −4.98 eV and a low bandgap of 1.64 eV. When one of the hydrogen atoms of the BT unit was substituted by a fluorine atom (PTBFBT-1F), a small blue-shift in UV-Vis absorption and a lower HOMO energy level of −5.11 eV were observed, thus a similar V_oc and J_sc were obtained. Nevertheless, the FF of PTBFBT-2F was further improved from 46% to 53% due to the relatively higher and balanced electron/hole charge transport mobility of 1.83 × 10⁻⁵ cm² V⁻¹ s⁻¹ and 1.52 × 10⁻⁵ cm² V⁻¹ s⁻¹. Using a Ca/Al top electrode, devices based on PTBFBT-0F, PTBFBT-1F, PTBFBT-2F as electron donor showed increasing power conversion efficiencies (PCE) of 3.0%, 3.6% and 4.2%, respectively. Furthermore, replacing Ca with a zirconium acetylacetonate film (ZrAcac) as the cathode buffer layer (CBL), a PCE of 5% with PTBFBT-2F as the donor was obtained.