Near 0 eV HOMO offset enable high-performance nonfullerene organic solar cells with large open circuit voltage and fill factor

Abstract

For nonfullerene acceptor (NFA) based organic solar cell (OSC), a small HOMO offset favors the high open-circuit voltage (V_oc), while a large one facilitates the charge separation. To simultaneously achieve efficient charge separation and low voltage loss in OSCs, we synthesized four 10,11-dihydrodiindeno[1,2-b:2′,1′-d]thiophene (DIDT)-based NFAs, namely S1-N1F, S1-N2F, S2-N1F, and S2-N2F, by modifying the end groups and side chains and obtaining tunable HOMO offsets of 0, 0.03, 0.04, and 0.06 eV, respectively, when blended with PBDB-T. S1-N1F:PBDB-T-based OSCs with a HOMO offset of 0 eV show the lowest short circuit current density (J_sc) and fill factor (FF) values, resulting in an unsatisfactory power conversion efficiency (PCE) of 7.94%, due to insufficient driving force for effective charge separation. Regardless of near zero HOMO offset (0.03 eV), S1-N2F:PBDB-T-based OSCs give the highest PCE of 13.59% with a high V_oc of 0.94 V along with a comparable FF up to 74.30%. S2-N1F- and S2-N2F-based OSCs give moderate PCEs of 10.35 and 11.59% with J_sc values of 17.07 and 17.82 mA cm⁻², and FF values of 62.14 and 70.33%, respectively. Femtosecond transient absorption (fs-TA) measurements indicate that S2-N1F and S2-N2F with large HOMO offsets have faster interfacial hole transfer and charge recombination processes. This study demonstrates that high-performance OSCs can be achieved by blending the donor and acceptor with a near zero HOMO offset, which can provide valuable experience for designing highperformance NFAs.