Effects of molecular structure of electron acceptors on the performances in polymer/fullerene and polymer/polymer blended solar cells

Abstract

The ability to tune the device function via controlling the lowest unoccupied molecular orbital (LUMO)/highest occupied orbital (HOMO) levels of fullerene derivatives used as electron acceptors is crucial in controlling the optical/electrochemical properties and the open circuit voltage ($V_{OC}$) of polymer solar cells (PSCs). Here, we report a various fullerene multiadducts in which different numbers of solubilizing groups are attached to the fullerene molecules. The addition of units to fullerene raised its LUMO and HOMO levels, resulting in higher $V_{OC}$ in P3HT:fullerene binary blended PSC device. Despite demonstrating the highest $V_{OC}$ value, the P3HT:fullerene tris-adducts (TRIS-) exhibited lower efficiency than P3HT:fullerene bis-adducts (BIS-). This result was explained by the lower electron mobility of TRIS- due to presence of too many regio-isomers which hampers fullerene crystallization. Likewise, although the TRIS- have the potential of achieving high $V_{OC}$ in PSCs, no successful examples of the use of TRIS- as electron acceptors have been reported. To achieve it, we developed a ternary blend approach for the use of TRIS- to fully exploit the merit of their high LUMO level. To explore the effect of TRIS- in ternary-blend PSC devices, the photovoltaic behavior of the P3HT:fullerene mono-adduct (MONO-):TRIS- based devices was investigated in terms of the weight fraction of TRIS- ($W_{TRIS-}$). When $W_{TRIS-}$ is small (<0.3), TRIS- can generate a synergistic bridging effect between P3HT and MONO-, leading to simultaneous enhancement in both $V_{OC}$ and short-circuit current ($J_{SC}$). For example, the ternary blend PSC device of P3HT:(MONO-:TRIS-) with $W_{TRIS-}$ of 0.1 and 0.3 exhibited power-conversion efficiencies (PCEs) of 3.91% and 3.96%, respectively, which were significantly higher than the P3HT:MONO- device. Interestingly, for $W_{TRIS-}$ > 0.7, both $V_{OC}$ and PCE of the ternary-blend PSCs exhibited non-linear compositional de...