Chlorination of dithienobenzodithiophene (DTBDT) based polymers to simultaneously improve the VOC, JSC and FF of non-fullerene organic solar cells

Abstract

Two donor–π–acceptor (D–π–A) type polymers (PE64 and PE65) with pentacyclic dithieno[2,3-d; 2′,3′-d′]benzo[1,2-b; 4,5-b′]dithiophene (DTBDT) as the electron-donating (D) unit and quinoxaline (Qx) as the electron-accepting (A) unit were designed to extend the application of DTBDT in non-fullerene organic solar cells (OSCs). The impact of introducing chlorine atoms at the two-dimensional thiophene side chain of the DTBDT unit on the photovoltaic performance was investigated by blending with two distinct non-fullerene acceptors (NFAs) of ITIC and Y6. We found that chlorinated polymer PE65 exhibited superior charge separation and transport due to the balanced crystallinity and miscibility when blending with acceptors. Thus, PE65 exhibited higher power conversion efficiencies (PCE) of 13.01% (Y6 as the acceptor) and 9.13% (ITIC as the acceptor) than non-chlorinated polymer PE64 (4.69% for PE64:ITIC and 3.54% for PE64:Y6). It is noteworthy that all three parameters (open-circuit voltage, short-circuit current and fill factor, abbreviated as V_OC, J_SC and FF) are largely improved simultaneously, which can be attributed to the adjusted energy levels, molecular crystallinity, blend film morphology and charge carrier mobility. The magnitude of differences of photovoltaic performance between PE64 and PE65 demonstrates the significance of introducing chlorine atoms at the thiophene side chain of the DTBDT unit on tuning the photoelectric and morphological properties and constructing various promising p-type photovoltaic polymers.