Abstract
Singlet fission has emerged as a promising way to overcome the Shockley-Queisser limit in solar energy conversion devices, and a few studies have claimed proof-of-principle results using dye-sensitized photoelectrodes. However, a detailed understanding of what factors govern the fate of the excited state on mesoporous surfaces is still lacking. Here, we have studied how the excitation progresses into singlet fission, electron injection, or formation of molecular charge separated states in diphenylisobenzofuran derivatives with flexible carbon linkers attached to nanocrystalline mesoporous ZrO2, TiO2, and SnO2 thin films. We show that singlet fission occurs for the molecule attached to ZrO2 films when the assembly is immersed in nonpolar solvents, and that singlet fission is hampered by the formation of a molecular charge separated state in more polar solvents. On TiO2 surfaces, direct electron injection from the singlet excited state outcompetes the singlet fission. Instead, triplet formation occurs via charge recombination from the conduction band of TiO2 in nonpolar solvents. When the molecule is attached to SnO2 films, singlet fission partly outcompetes electron injection from the singlet excited state and the two processes occur in parallel. Subsequent to singlet fission on SnO2, triplet injection into the conduction band of SnO2 is observed. The results presented here provide a detailed picture of the singlet fission dynamics in molecules attached to mesoporous semiconductor surfaces, demonstrating that both the semiconductor substrate as well as the environment around the molecules have a large impact, which can be useful in the design of future devices.
Highlights
Singlet fission (SF), the spin-allowed process where a molecule in its singlet excited state (S1) interacts with a neighboring molecule in the ground state to produce two triplet excited states (T1),[1] was first discovered in the 1960s2−5 and has attracted a lot of attention over the past 15 years due to the potential for it to increase the efficiency of solar cells.[6,7] If an SF material could be properly integrated into a solar cell, one incident high-energy photon could result in two charge carriers if both created triplets are utilized
A high singlet injection yield on TiO2 together with the relatively high conduction band (CB) energy level of TiO2 does not allow for singlet fission followed by triplet injection
Our results suggest that utilizing semiconductors with a lower CB energy level could make SF
Summary
Singlet fission (SF), the spin-allowed process where a molecule in its singlet excited state (S1) interacts with a neighboring molecule in the ground state to produce two triplet excited states (T1),[1] was first discovered in the 1960s2−5 and has attracted a lot of attention over the past 15 years due to the potential for it to increase the efficiency of solar cells.[6,7] If an SF material could be properly integrated into a solar cell, one incident high-energy photon could result in two charge carriers if both created triplets are utilized. The same appears to be true for excimer states: They can either be detrimental to[20,21] or enable SF.[22−25] The exact mechanisms of SF have been thoroughly studied but are not yet fully understood.[16,19,22,26−29] Despite the lack of a Received: July 20, 2020 Revised: August 23, 2020 Published: August 25, 2020
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