Emission from Charge-Transfer States in Bulk Heterojunction Organic Photovoltaic Cells Based on Ethylenedioxythiophene-Fluorene Polymers

Abstract

We investigated the electroluminescence (EL) from charge-transfer (CT) in the interface state of PEDOTF/PC₇₀BM, known to be empirically related to V_oc, to elucidate a difference in V_oc of 0.24 V in bulk heterojunction (BHJ) organic photovoltaic cells (OPVs) based on two polymers (H-PEDOTF and S-PEDOTF with a high and low molecular weight, respectively) having almost the same HOMO energy levels. The difference in the energies of CT EL peaks is relatively small (0.06 eV) compared to the difference in the V_ocs. Consequently, we mainly attribute the excessive loss of V_oc in S-PEDOTF-based OPVs to a trap assisted recombination mechanism.

Keywords:

• bulk heterojunction
• organic
• photovoltaic
• polymer
• electroluminescence