Relationship between HOMO energy level and open circuit voltage of polymer solar cells

Abstract

A series of pi-conjugated polymers (PDHF-BT and PDHF-TBT) with 4-(3,4-ethylenedioxythienyl)-2,1,3-benzothiadiazole (BT), 4,7-bis(3,4-ethylenedioxythienyl)-2,1,3-benzothiadiazole (TBT), and 9,9'-dihexylfluorene were synthesized by the Suzuki coupling reaction. The HOMO energy level of PDHF-BT was -5.47 eV, which was lower than that of PDHF-TBT (-5.22 eV), while the LUMO energy level of PDHF-BT (-3.45 eV) was very similar to that of PDHF-TBT (-3.42 eV). These energy levels of PDHF-BT and PDHF-TBT were also supported by a DFT calculation. The power conversion efficiency (PCE) of the polymer solar cell (PSC) with a structure of ITO/PEDOT:PSS/PDHF-BT:PCBM (1:1)/Al was determined as 0.34% and it was larger than that of the device based on PDHF-TBT (0.22%). Correspondingly, the V-oc of the PSC based on PDHF-BT (0.71 V) was much larger than that of the device based on PDHF-TBT (0.40 V). The results support that the V-oc of polymer based PSCs is strongly related to the HOMO energy level of the active polymers. (C) 2012 Elsevier B.V. All rights reserved.