Exploring Carrier-Driven Exciton Formation in Upconverting Perovskite/Rubrene Bilayers Using Drift-Diffusion Simulations

Photon upconverters based on thin film lead halide perovskites combined with small molecule organic semiconductors are an emerging topic in perovskite-based optoelectronics. In these systems, photoexcited carriers in a perovskite thin film induce triplet exciton formation in an adjacent layer of molecules, such as rubrene, which then undergo triplet-triplet annihilation upconversion (TTA-UC). By replacing what would ordinarily be an excitonic triplet sensitizer with a perovskite film, problems associated with low triplet exciton diffusion length in excitonic materials can be mitigated. In addition, many of the favorable characteristics of perovskite thin films, such as large carrier diffusion lengths, strong light absorption, and bandgap tuneability are available to be exploited.<br/><br/>The key internal mechanism in such systems, wherein carriers in the perovskite film generate ~1.2eV triplet excitons in the organic layer, is still a matter of discussion. A definitive identification of this mechanism is important for aiding rational device design, as well as establishing an upper bound on the possible upconversion efficiency obtainable with these systems.<br/>We conducted extensive time-resolved and continuous wave photoluminescence measurements of bilayer upconverters, consisting of various perovskite films layered with rubrene annihilator molecules. We developed a drift-diffusion numerical simulation and quantitatively modelled the experimental data, allowing us to visualize the time-dependent carrier and exciton fluxes induced within the sample under various experimental conditions.<br/>Two distinct simulation modules were developed, corresponding to two contending exciton generation mechanisms, surface exciton transfer and sequential charge transfer. In the former, electron-hole recombination at the perovskite/rubrene interface generates an exciton current within the rubrene layer. In the latter, excitons are formed in the rubrene layer by hole transfer from the perovskite film, followed by electron capture to yield a bound exciton.<br/>By comparing the two exciton formation models with our experimental data, we generate new insights into the likely exciton formation process operating in these bilayer upconversion samples. We will present our latest results in this space, and discuss ongoing challenges and opportunities for drift-diffusion modelling of these perovskite-based optoelectronic systems.