Excitonic spin engineering in optoelectronic devices

Abstract

Despite decades of research, solar cell efficiencies struggle to get higher than 25%. This is due to two fundamental losses in the device: thermalization of high energy photons and transmission of low energy photons. In this work, we demonstrate efforts to improve both these losses, which, when fully realized, could increase power efficiencies to 35% or higher. First, we utilize singlet exciton fission as a downconverting layer. Singlet exciton fission is a process in which a single high energy exciton fissions into two excitons of half the energy. Here, we first demonstrate the potential of singlet fission in an all-organic solar cell. We measure an EQE as high as 109%, breaking the conventional limit of 100%. We utilize the magnetic field effect of fission to characterize and quantize the fission yield in these devices, demonstrating that an increase in absorption should lead to even higher EQE values. Next, we utilize an optical light trapping scheme to increase the absorption, driving the EQE as high as 126% with no external optics. Finally, we demonstrate the ability to orthogonalize singlet fission from the normal OPV functions such as absorption and charge transport with a small interfacial layer of a fission material. With the efficiency of singlet fission established, we then demonstrate how it can be utilized by building an optical downconverter with tetracene as the fission material and PbS colloidal nanocrystals as the acceptor. We demonstrate that low energy excitons generated in the fission material transfer to the nanocrystal with 90% efficiency before fluorescing. This fluorescence will be able to transfer energy to inorganic solar cells such as silicon. To combat the transmission loss, we turn to the reverse process of singlet fission: triplet-triplet annihilation. We utilize colloidal nanocrystals as the sensitizer and rubrene as the annihilator. The use of colloidal nanocrystals as the sensitizer allows us to minimize energetic loss and extend deeper into the infrared as compared to state of the art devices, while allowing for facile construction of a solid state geometry. We characterize this process and demonstrate the potential it holds for future solar cells. Finally, we characterize the charge transfer state in organic solar cells. We demonstrate that intersystem crossing plays a key role, defining device performance and recombination. We further show that these states are mobile and can diffuse via an 'inchworm' hopping motion.