(703e) Multiple Triplet Exciton Generation Mechanism in Acene Crystals
Multi-exciton generation (MEG) has been reported for several materials and may dramatically increase solar cell efficiency. In tetracene and pentacene MEG is speculated to proceed by singlet fission (SF), which converts one singlet exciton into two triplet excitons. Although SF likely governs MEG processes in a variety of materials, no fundamental mechanism for SF has yet been described. Because photoexcited states have single-exciton character, MEG to produce a pair of triplet excitons must involve an intermediate state that: (1) exhibits multi-exciton (ME) character, (2) is accessible from S1 and satisfies the fission energy requirement, and (3) efficiently dissociates into multiple e-h pairs. Using sophisticated ab initio calculations we show that MEG in pentacene and tetracene proceeds through a dark state (D) of ME character that lies near the optically allowed excited state and satisfies the SF energy requirement (ED>2ET0), and which subsequently splits into two triplets (2×T0). The energetic position of this dark state controls whether the singlet fission process is thermally activated.