ANNOUNCEMENTS
Many of the commercially available conventional single-junction crystalline silicon solar cells show a noticeably low spectral response or low quantum efficiency at short-wavelengths (300-500 nm). To improve the low spectral response or low quantum efficiency at short-wavelengths, different methods such as the design of very narrow junctions, employing low doping levels with selective doping and using very thin window/buffer layers, etc. have been explored. However, some of these modifications are difficult and expensive to integrate with the existing conventional solar cell process line. The downshifting technique, as an alternative additional step and with a possibility of easy integration in the existing process line, has been suggested as a method for improving the short wavelength response of the solar cells. The downshifting layer contains luminescent materials dispersed in a transparent medium. The short-wavelength photons are absorbed by the luminescent species and re-emitted at a red-shifted wavelength. The re-emitted photons having low absorption coefficients are absorbed away from the front surface and, therefore, reduce the recombination losses near the front surface and improve the spectral response.
In this work, both zinc sulfide [ZnS] and erbium-doped zinc sulfide [ZnS: Er] nanomaterials have been used as downshifting material using the coprecipitation method. Co-precipitation is one of the well-known techniques to synthesize trivalent lanthanide or transition material doped nanoparticles. Ethylenediamine (EN) is used as the stabilizing agent for the synthesis of zinc sulfide [ZnS] nanoparticles at room temperature. The quantity of ethylenediamine (EN) in the synthesis process is optimized based on structure, morphology as well as fluorescence quantum yield. The absorption spectra of zinc sulfide [ZnS] nanoparticles have performed a blue shift from the bulk zinc sulfide [ZnS] due to the quantum confinement effects while they show a wide emission spectrum. As the ethylenediamine quantity is increased, the size of the particle and the lattice structure has been changed from nanometer to micro-meter and cubic to hexagonal, respectively. The maximum value of quantum yield is obtained with 15 ml ethylenediamine in the synthesis process. The bandgap of ZnS nanoparticles is calculated as 4.0 eV from Tauc’s plot.
A series of erbium-doped ZnS nanoparticles are also prepared by the coprecipitation method. The developed nanoparticle has exhibited emission properties in the visible range after excitation with a wavelength of 250 nm. Based on the material characterization, the suitable doping concentration for erbium-doped ZnS nanoparticles is determined to be 10% to enhance the spectral response in the short wavelength. The doped and undoped ZnS nanomaterials are coated on 3 cm2 conventional solar cells, originally laser-cut from commercially produced 6-inch mono and multi-crystalline solar cells, to characterize its impact on solar cell performance. PMMA is used as the binding agent. Two elaborate schemes of optimizations are performed by varying the relative quantity of nanoparticles to PMMA and the doping concentrations of Erbium in ZnS.
The maximum EQE enhancement in the short wavelength is observed for a mono c-Si solar cell with ZnS: 10%Er NP/PMMA (8 mg/ml) downshifting layer and a multi c-Si solar cell with ZnS:10%Er NP/PMMA (3 mg/ml) downshifting layer. The relative improvement in short circuit current density of mono c-Si solar cell with ZnS: 10%Er NP /PMMA downshifting layer is 7.18 %, whereas 4.76% relative enhancement is noted in the short circuit current density of multi c-Si solar cell with ZnS: 10%Er NP /PMMA downshifting layer.
It can be concluded that ZnS: 10% Er NP is a suitable material for the downshifting technique in silicon solar cells. It is used to improve the short wavelength response as well as the current density for both mono and multi c- Si solar cell. However, it is important to test these improvements on conventional 6-inch solar cells, which might not address in this work due to the limitations of experimental facilities. Moreover, the stability of the solar cell with the downshifting layer under the sun becomes an important study for the future. The absorption efficiency of erbium-doped ZnS nanoparticles by surface plasmonic can also be studied as part of future work.
