Controlling Nonradiative Recombination in an Organic Semiconducting Polymer Donor-Acceptor System Using Organometallic Phosphors

Nonradiative recombination in organic semiconducting materials plays a significant role in energy loss pathways in organic solar cells and organic photodetectors. Here, we investigate a bilayer architecture using semiconducting conjugated polymers & horbar;poly(9,9-dioctylfluorene) (PFO) as the donor and poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) as the acceptor & horbar;that includes a red-emitting organometallic phosphor to modify nonradiative recombination. Photophysical investigations of the phosphor-doped donor-acceptor system give insight into the behavior of excitons and the subsequent recombination processes that they undergo. Enhancements in the photoluminescence quantum yield of the PFO layer in the presence of the phosphor indicate that nonradiative recombination within the PFO layer is suppressed because of triplet harvesting by the phosphor. However, in the bilayer with F8BT, the quantum yield is reduced because of triplet energy transfer from the phosphor to F8BT. Transient absorption spectroscopy indicates that the addition of the organometallic phosphor shortens the lifetime of excited singlet excitons in PFO, and increases the lifetime of excited singlet excitons in the bilayer. The photovoltaic and photodetector characteristics of devices incorporating the bilayer show an elevated open-circuit voltage and improved responsivity with the addition of the phosphor. These results demonstrate that phosphors can alter the degree to which nonradiative triplet states contribute to energy loss in organic semiconductor bilayers.