In this work, we investigate a tetracene/Si singlet-triplet down-conversion solar cell geometry in which we control the directional emission of quantum dots (QDs). In this system, photons in the visible range (450-550 nm) excite high-energy singlet-excitons in tetracene that rapidly convert into two triplet-excitons at about half the energy. The triplet-exciton energy is transferred to the QDs that subsequently emit at 1000-1100 nm. A significant loss channel is the QD emission that is directed upwards, so anisotropic downward emission into the underlying Si cell is essential.
Here, we demonstrate directional light emission of CdSe/ZnS core-shell quantum dots (QDs) coupled to Si Mie resonators fabricated on a Si solar cell. By varying the shape and size of the Mie resonator, interference of the electric and magnetic multipolar modes supported by the resonator is controlled. Placing the QDs in the near-field of the resonator enables efficient coupling of the QD transition dipole with these multipolar modes. Using numerical FDTD calculations we show that the QD emission is efficiently directed into the solar cell.
We fabricate nanostructures on a Si substrate using electron-beam lithography and reactive-ion etching. Using soft-stamp imprinting we selectively print CdSe/ZnS QDs (peak emission 800 nm) on top of the nanostructures. We then map the QD emission anisotropy for different Mie resonator sizes, using photoluminescence spectroscopy. Photoluminescence lifetimes show a systematic increase from 7 ns to 17 ns, for Si nanocylinder diameter from 200 to 425 nm (cylinder height 125 nm), consistent with the varying nanostructure resonances as found in FDTD simulations.
The anisotropic downward emission demonstrated in this work of QDs coupled to Si nanostructures can enhance the efficiency of a tetracene/Si down-conversion system. Moreover, this work can impact a broad range of other applications in which directional emission is relevant, in solid-state lighting, integrated optics, and photovoltaics.