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Significance Statement
Most researchers have focused on enhancing the power conversion efficiency of crystalline solar cells via innovative methods. They have explored technical methods such as metallization, surface texturing, plasma doping, using anti-reflection coating, designing new structures, etc. Irrespective of these efforts, silicon solar cells still post power conversion efficiency of approximately 25%, therefore leaving a narrow window for improvement.
Generally, silicon solar cells have the ability to absorb photos with energy higher than their band gap, but can only convert the photons to photovoltaic power if the photon energies are closer to the silicon band gap. In simple terms, if the photons energy is higher than the silicon bandgap, the photons will be absorbed but the excess energy, which is the difference between the two, will be lost into the lattice owing to scattering effects. For this reason, conversion efficiencies of silicon solar cells in the UV region is weaker than in the visible region.
The use of energy-down-shift applying quantum dots in silicon solar cells has been proposed recently to absorb the wasted energies in the UV region and emit them in the visible light region. This is in the quest for improved power conversion efficiency. Researchers led by professor Jea-Gun Park at Hanyang University in Republic of Korea implemented the produced Manganese Cd0.5Zn0.5S/ZnS quantum dots on textured mc-p-Si solar cell. The solar cell designed with an antireflective surface, using the benefit of external quantum efficiency as well as the energy tuning effect of the nano quantum dots. This was in a bid to improve power conversion efficiency and short-circuit-current-density. Their work is published in Nano Energy.
The authors doped and spin coated various concentrations of the prepared Manganese: Cd0.5Zn0.5S/ZnS quantum dot solution, onto the surface of the textured Silicon Nitride film of a synthesized mc-p-Si solar cell. They then annealed the quantum dot-coated solar cells to eliminate the remaining solvents from the quantum dot-coated layers. They then obtained the final solar cell devices.
The researchers confirmed that the fabricated Mn2+:Cd0.5Zn0.5S/ZnS quantum dots absorbed UV wavelength light and emitted longer wavelengths visible light via the mechanism of an energy-down-shift and energy tuning effect. The quantum dots were applied to improve absorption and photovoltaic performance of the silicon solar cells. They then investigated the effect of energy-down-shit/energy tuning effect quantum dot layer on the performance. They did this using UV-visible spectroscopy and J-V characterization under solar simulation.
The research team observed that the best-improved performance was reported at 0.3% concentration. This was due to high UV light absorption by the zinc sulfide shell and reduced re-absorption of the visible light with its extensive Stokes shift having an energy tuning effect. Less concentration led to weaker absorption of UV light, while higher concentration increased the direct interaction between adjacent quantum dots. Therefore, higher re-absorption in the visible region.
The best enhancements of power conversion efficiency and short-circuit-current-density were recorded for the solar cells coated with quantum dots layers when the concentration was 3.22 and 4.02%, respectively. The outcomes of their study will have potential in advancing commercial solar cell applications.
Reference
Mohammed Jalalaha, Yun-Hyuk Ko, Farid A. Harraz, M.S. Al-Assiri, Jea-Gun Park. Enhanced efficiency and current density of solar cells via energy-down-shift having energy-tuning-effect of highly UV-light-harvesting Mn2+-doped quantum dots. Nano Energy, volume 33 (2017), pages 257–265.
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