The energy barrier in singlet fission can be overcome through coherent coupling and entropic gain

One strategy to improve solar-cell efficiency is to generate two excited electrons from just one photon through singlet fission, which is the conversion of a singlet (S-1) into two triplet (T-1) excitons. For efficient singlet fission it is believed that the cumulative energy of the triplet states should be no more than that of S-1. However, molecular analogues that satisfy this energetic requirement do not show appreciable singlet fission, whereas crystalline tetracene displays endothermic singlet fission with near-unity quantum yield. Here we probe singlet fission in tetracene by directly following the intermediate multiexciton (ME) state. The ME state is isoenergetic with 2 x T-1, but fission is not activated thermally. Rather, an S-1 double left right arrow ME superposition formed through a quantum-coherent process allows access to the higher-energy ME. We attribute entropic gain in crystalline tetracene as the driving force for the subsequent decay of S-1 double left right arrow ME into 2 x T-1, which leads to a high singlet-fission yield.