Optical Probes of Triplet Exciton Sensitization of Silicon

To match the growing global energy demand while meeting space and cost limitations, efficiencies of solar cells need to improve. However, the efficiencies of crystalline silicon solar cells, the current industry standard, are approaching the maximum theoretical limit. One method of going beyond this limit is to sensitize the silicon (Si) by using a material that can perform singlet exciton fission (SF), a carrier multiplication process that can create two triplet excitons (electron-hole pairs) from a single photon. Successful transfer of these two triplet excitons to silicon can result in increased photocurrent and improved efficiencies.<br/> <br/>Recent work has shown coupling between Si and the archetype SF material tetracene (Tc) in the presence of passivating interfacial layers of Hafnium oxynitride [1]. Excitation spectra show a boost in the photoluminescence from Si when Tc is photoexcited that may be caused by energy transfer or changes in the silicon passivation [1]. To experimentally distinguish between these phenomena and understand the complex dynamics of excited states and charges at silicon/SF interfaces, we have developed a spectroscopy technique that is robust to the weak and intensity-dependent photoluminescence from silicon. Using combinations of biasing optical pumps and selective modulation of SF rates using a magnetic field, we study structural variations at the interface to probe the mechanism of coupling at Hafnium oxynitride interfaces and other rationally-designed heterostructures. We demonstrate positive contributions from tetracene to silicon photoluminescence that suggest a key role for charge transfer states in realizing solar cell efficiency enhancements from singlet exciton fission.<br/> <br/>[1] Einzinger, M., Wu, T., Kompalla, J.F. <i>et al.</i> Sensitization of silicon by singlet exciton fission in tetracene. <i>Nature</i> <b>571,</b> 90–94 (2019).