Silicon is the dominating solar cell material, therefore add-ons on the silicon solar cell that can improve on the power conversion efficiency are urgently needed. In certain organic materials singlet fission generates two triplet (spin 1) excitons from one singlet (spin 0) exciton. If the triplet excitons are harvested in a silicon solar cell the efficiency could be dramatically increased, as we show computationally. There are different transfer pathways between the organic singlet fission material and silicon. We have simulated the achievable efficiency for each transfer path with realistic assumptions such as a singlet fission quantum efficiency of 1.7 (1.7 e-h pairs per high energy photon), a transmission loss of 5%, and different entropy gains of the Singlet Fission process.

Even with these realistic assumptions, the efficiency of a silicon/singlet fission solar cell can be as high as 34% when combined with the current record silicon solar cell of 27%. We found that dissociating the triplet excitons at the interface leads to a large potential efficiency gain because a triplet energy lower than the silicon bandgap still leads to charge generation, and allows for high current generation. We also find that current singlet fission materials do not absorb light strongly enough, motivating sensitization schemes.  A direct triplet exciton transfer shows lower overall efficiencies because the energy level requirements are more strict, however the solar cell architecture is more elegant since there are no additional contacts needed. Finally, we compare the singlet fission/silicon solar cells to the efficiency potential of perovskite/silicon tandem solar cells. We find that tandem cells are particularly beneficial for a silicon base cell with low efficiency, while a highly efficient silicon solar cells benefits less from the perovskite top cell. In contrast, the efficiency gain from the singlet fission layer is almost constant for all silicon base cells, and for highly efficient silicon cells would clearly outperform a high-efficiency perovskite top cell.

We also tried to realize the direct exciton transfer solar cells we simulated by fabricating a silicon solar cells with a top layer of tetracene. The silicon base cell is back-contacted, so we can HF-etch from one side to have direct access to the silicon <111> sides of pyramidally textured silicon. The photocurrent under a magnetic field can differentiate between photocurrent contributions of singlet and triplet excitons. A newly improved magnetic field dependent photocurrent setup allows us to measure current changes on the order of 0.01% and is a vital tool for a precise attribution of the origin of the photocurrent. We find that after deposition of the tetracene layer we see an injection of singlets or photons into silicon, but after aging the solar cell we see evidence for triplet transfer. The characteristic Merrifield curve (photocurrent as function of applied magnetic field ) inverts, which suggest the injection of triplet excitons from tetracene into silicon. We observe a triplet injection curve for tetracene-silicon solar cells that have been aged for five days in air or six weeks encapsulated in nitrogen atmosphere. We discuss different possible mechanisms for this behavior, a thin layer of silicon dioxide growing between tetracene and silicon and a changing orientation of the tetracene molecules. A better understanding of the energy transfer processes at the interface will be important for increasing the injection efficiency.