Toward Silicon-Matched Singlet Fission: Energy-Level Modifications Through Steric Twisting of Organic Semiconductors

Singlet fission (SF) is a potential avenue for augmenting the performance of silicon photovoltaics, but the scarcity of SF materials energy-matched to silicon represents a barrier to the commercial realization of this technology. In this work, a molecular engineering approach is described to increase the energy of the S1 and T1 energy levels of diketopyrrolopyrrole derivatives such that the energy-level requirements for exothermic SF and energy-transfer to silicon are met. Time-resolved photoluminescence studies show that the silicon-matched materials are SF active in the solid state, forming a correlated triplet pair 1(TT) - a crucial intermediate in the SF process - as observed through Herzberg-Teller emission from 1(TT) at both 77 K and room temperature. Transient electron paramagnetic resonance studies show that the correlated triplet pair does not readily separate into the unbound triplets, which is a requirement for energy harvesting by silicon. The fact that the triplet pair do not separate into free triplets is attributed to the intermolecular crystal packing within the thin films. Nevertheless, these results demonstrate a promising route for energy-tuning silicon-matched SF materials. Two chromophores with desirable materials properties are modified so that their singlet and triplet energy levels are capable of both exothermic singlet fission (SF) and energy transfer to silicon semiconductors - features lacking in many well-studied SF materials. This energy-level tuning is accomplished through steric perturbation of their biradical character. Si-matched SF materials will greatly facilitate efficient solar harvesting and the subsequent energy transfer to silicon photovoltaic cells.image