Enhanced performance of polymer bulk heterojunction solar cells employing multifunctional iridium complexes†
We report on the enhanced performance of polymer bulk heterojunction solar cells composed of an iridium complex with pendant sodium cations (pqIrpicNa) as an energy donor, poly(3-hexylthiophene) (P3HT) as an energy acceptor, polyethylene oxide (PEO) as an ion channel, and PCBM as an electron acceptor. With the iridium complex and PEO as additives, we observe a 20% increase in the current density, from 8.57 mA cm−2 to 10.24 mA cm−2, and a photoconversion efficiency of up to 3.4%. The observed enhancement in current density comes primarily from an efficient triplet–singlet energy transfer from the iridium complex to P3HT. Transient photoluminescence studies reveal triplet–singlet energy transfer efficiency from pqIrpicNa to P3HT of over 99%. Because of this high energy transfer efficiency, an enhancement is observed in the incident photon-to-conversion efficiency spectrum between 350 and 550 nm, which overlaps with the absorption range of the iridium complex. We also observe enhanced nanophase segregation of the active layer with pqIrpicaNa and PEO by atomic force microscopy. We propose that the observed enhancement in the current density stems not only from the enhancement in the morphology with the iridium complex, but also from the enhanced mobility of the sodium cations toward the metal electrodes through the ion channel of PEO under sunlight, which results in an increased charge collection at the electrodes.