Detailed balance analysis of advanced geometries for singlet fission solar cells

Abstract

Singlet fission is a process by which a single photon can be converted into a pair of triplet excitons, making it highly attractive for light harvesting technologies. Maximizing the efficiency of excitonic solar cells is a challenge requiring careful energy alignment among other things. We performed detailed balance calculations on excitonic solar cells that leverage endothermic singlet fission with an endothermicity of up to ten times thermal energy at room temperature. As expected, we find that the design surpasses the single junction (Shockley Queisser) limit, with a maximum at an endothermicity of 0.125 eV. However, the design is susceptible to the effects of exciton binding energy. Calculations suggest that including a third material to form a double heterojunction can help to overcome this challenge. For exciton binding energies of 0.5 eV, the singlet fission double heterojunction design can achieve an efficiency of 40.8%, a nearly 10% improvement over a single heterojunction. Practical implementations of this design are likely to encounter a number of challenges unique to this design, namely, unwanted tunneling currents and exciton-charge annihilation. Their effects on the output characteristics of the cell are described. It appears likely that these issues can be avoided, and that highly efficient, inexpensive solar cells that leverage the full promise of the singlet fission mechanism can be created.