Singlet fission in organic semiconductors is a promising and potentially inexpensive approach to surpass the Shockley-Queisser limit of single-junction solar cells. By converting one high-energy excitation into two lower-energy excitons, singlet fission can efficiently down-convert high-energy light of the solar spectrum. The coupling between molecules has proven to be critical for efficient singlet fission, but exactly how the intermolecular distances and stacking will influence the rate and mechanism of singlet fission is not clear.
Here we use hydrostatic pressure as a simple and clean tool to tune the intermolecular distance and stacking in thin films of tetracene, a highly efficient singlet fission material and rubrene, which has a relatively low singlet fission efficiency. In the case of tetracene, a slower singlet decay rate is observed at higher pressure and may be attributed to a higher energy barrier between the first excited singlet state and the multi-exciton state. In rubrene, the rate of singlet fission is increased at higher pressure, which is accompanied by a significant drop in photoluminescence intensity, indicating a higher singlet fission efficiency.
Our study shows that increasing intermolecular coupling may have different effects for different singlet fission materials and provides insights for designing new efficient singlet fission materials.