University of Cambridge > > Physical RIG > Excitonic Light Management for Solar Cells beyond Shockley-Queisser

Excitonic Light Management for Solar Cells beyond Shockley-Queisser

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Single-junction solar cells are limited to an energy conversion efficiency of 33.7% under unconcentrated sunlight. Crystalline silicon, with a band gap of 1.12 eV, is limited to 29%, and UNSW researchers and others have pushed this beyond 25%. To go beyond 30% requires careful management of photon energies. The principal inefficiencies of a solar cell can be divided into two phenomena: The inability to absorb photons below the band gap, and the thermalisation of carriers generated with photon energies exceeding the bandgap. Both of these shortcomings can be addressed using excitonic phenomena in organic materials.

Upconversion: Sub-bandgap losses can be remedied by the application of photochemical upconversion, whereby transmitted light is converted to light of higher energy, which can then be harvested by the cell and contribute to current generation. We achieved unprecedented upconversion efficiencies and incorporated upconverters into amorphous silicon and organic solar cells. These proof-of-principle experiments motivated the development of more efficient upconverters which harvest light deep in the infrared – beyond silicon. In our most recent work using quantum dot sensitizers we achieved upconversion from below the silicon bandgap for the first time, therefore establishing a low energy record for photochemical upconversion. This was achieved in the presence of oxygen, usually the nemesis of molecular triplet states.

Singlet fission is a process whereby two triplet excitons can be produced from one photon, potentially increasing the efficiency of photovoltaic devices by mitigating thermalisation losses. Endothermic singlet fission is desired for a maximum energy-conversion efficiency, and such systems have been considered to form an excimer-like state with multiexcitonic character prior to the appearance of triplets. However, the role of the excimer as an intermediate has, until now, been unclear. Here we show that, rather than acting as an intermediate, the excimer serves to trap excited states to the detriment of singlet-fission yield. We show that singlet fission and its conjugate process, triplet–triplet annihilation, occur at a longer intermolecular distance than an excimer intermediate would impute. These results establish that an endothermic singlet-fission material must be designed to avoid excimer formation, thus allowing singlet fission to reach its full potential in enhancing photovoltaic energy conversion

References [1] Elham M. Gholizadeh, Shyamal K. K. Prasad, Zhi-Li Teh, Thilini Ishwara, Sarah Norman, Anthony J. Petty II, John E. Anthony, Shujuan Huang and T. W. Schmidt, ChemRxiv, DOI : 10.26434/chemrxiv.7834838 [2] Cameron B. Dover, Joseph K. Gallaher, Laszlo Frazer, Patrick C. Tapping, Anthony J. Petty II, Maxwell J. Crossley, John E. Anthony, Tak W. Kee and Timothy W. Schmidt, Nature Chemistry 2018, 10, 305-310.

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