External Radiative Efficiency of Organic Semiconductor Materials Doped with Organometallic Phosphors

Organic photovoltaics (OPVs) can have certain desirable properties, such as, lightweight, adjustable bandgap, semitransparency, low toxicity, and solution processability, that differentiate them from inorganic and perovskite solar cells. However, the current record efficiency of OPVs (~19%) is still less than for many other solar cell technologies, which limits their application. Shockley-Queisser theory helps to elucidate the efficiency limit of a solar cell, and it assumes that the highest efficiency can be achieved when the external radiative efficiency (ERE) of the solar cell is equal to unity. ERE quantifies the radiative recombination of excitons under total dark current recombination. For the power conversion efficiency (PCE) of solar cells, the best performing solar cells display high EREs due to low nonradiative recombination. One major nonradiative loss pathway in OPVs that suppresses ERE, and hence PCE, is non-emissive triplet states in organic semiconductors.<br/>In this work, we introduce organometallic phosphors into organic semiconducting donor:acceptor OPV films with the goal of converting non-emissive triplet states into emissive phosphorescence and, thereby, increasing ERE. An iridium-based organometallic phosphor with a low triplet energy level, tris(1-phenylisoquinoline) iridium, Ir(piq)₃, is incorporated into bilayer OPV films composed of polyfluorene polymers - poly(9,9-di-n-octylfluorenyl-2,7-diyl), PFO, as the electron donor, and poly(9,9-dioctylfluorene-alt-benzothiadiazole), F8BT, as the electron acceptor. The photophysical properties of PFO, F8BT and PFO/F8BT donor:acceptor bilayer films with Ir(piq)₃ phosphor doping are studied. Additionally, both OPV and photodetector devices are fabricated and tested with and without Ir(piq)₃.<br/>Photoluminescence (PL) spectroscopy confirms that photoexcitation of PFO/Ir(piq)₃ films results in efficient triplet exciton transfer from PFO to the triplet state of the Ir(piq)₃ phosphor. In addition, when the phosphor doping concentration is increased, there is an increase in PL quantum yield (PLQY), from 38% to 56%, attributed to reduced nonradiative recombination from the PFO triplet state. In contrast, the PLQY of PFO/F8BT and [PFO/Ir(piq)₃]/ F8BT bilayer films decreases with increasing doping concentration, which indicates that the photogenerated triplet state of Ir(piq)₃ is quenched by the presence of the F8BT acceptor. In addition, time-resolved PL measurements show that the average lifetime of PFO decreases from 964 ps to 219 ps in the presence of phosphor, which further indicates efficient energy transfer. We also perform PL lifetime measurements of PFO/F8BT and [PFO/Ir(piq)₃]/F8BT bilayer films and the lifetime changes from 930 ps to 770 ps in the presence of phosphor, which show that efficient energy transfer occurs from the donor to the acceptor. Transient absorption spectrum of PFO and PFO(Ir) films shows that there is singlet exciton absorption peak at a wavelength of 826 nm, around 1.5 eV.<br/>OPV devices are fabricated with the following configuration, indium-tin oxide (ITO)/PEDOT: PSS/PFO/F8BT/Ca/Al, with and without phosphor doping in the PFO layer. The OPV devices are tested under one sun illumination. Due to the bilayer structure of PFO/F8BT, the PCE is extremely low, which is ~0.02%; however, the open-circuit voltage is improved by 0.1 V in the presence of the phosphor, suggesting reduced nonradiative recombination. The devices are also tested as organic photodetectors (OPD). PFO(Ir)/F8BT has a responsivity of 0.64 mA/W, the detectivity of 3.25E10 Jones. As for PFO/F8BT devices, their responsivity is 0.48 mA/W, and the detectivity is 3.28E10 Jones. The OPD test indicates that their responsivity is comparable with other photodetector devices.