Application Research Of Quantum Dots In Solar Cells

Low photoelectric conversion efficiency of Photovoltaic (PV)technology, which is a renewable and clean energy technology, is still themain reason for restricting the development of Photovoltaic technology. Amajor problem limiting the conversion efficiency of PV cells is theirinsensitivity to a full solar spectrum. The spectral distribution of sunlightat Air Mass1.5global (AM1.5G) consists of photons with widewavelengths ranging from ultraviolet to infrared (280–2500nm,0.5–4.4eV), but current PV cells only utilize a relatively small fraction of thesolar photons. This is attributed to the fact that each PV material respondsto a narrow range of solar photons with energy matching thecharacteristic band-gap of the material. In principle, only photons withenergy higher than the band-gap are absorbed. And worse, the surfacerecombination and other factors make the external quantum efficiency(EQE) lower within the absorption range of300-500nm than over500nm.Quantum dots (QDs) have a large Stokes shift, after absorbing shortwavephotons can emit long-wave photons, and the emission peak is adjustable.When the quantum dots applied to solar cells, the external quantum efficiency is expected to be improved. In this paper, CdSe@ZnSe@ZnSquantum dots were incorporated evenly into the pre-polymerizationmethyl methacrylate, forming a kind of composite liquid namedPMMA/CdSe@ZnSe@ZnS. The composite liquid was spin-coated on amono-crystalline solar cell and on the420nm high-pass optical filterrespectively to form a down-shifting (LD) layer. The main conclusionsare described as follows:(1) when tested the EQE of the mono-crystallinesolar cell which was spin-coated by a PMMA/CdSe@ZnSe@ZnS LDlayer on its surface, no EQE improvement was observed in300-325nmband, the reason is that the fluorescence quantum efficiency (FQE) ofquantum dots is only9.25%, far below the ratio of EQE in shortwavelength to EQE in long wavelength; When test the EQE of anothermono-crystalline solar cell, which was covered by the LD layerspin-coated420nm high-pass optical filter(the optical filter makes thecell’s EQE almost zero in300~420nm and eliminate the influence of theLD layer’s antireflective effect),EQE improvement was observed in300-420nm, which means QDs can be applied to solar cells to enhancethe EQE;(3)According to the measured EQE values and AM1.5G photonflow distribution data, the QDs’s minimum FQE needed to improve theused solar cell’s EQE was calculated to be87.8%.