Enhanced performance of thin-film amorphous silicon solar cells with a top film of 2.85nm silicon nanoparticles

The performance of thin-film amorphous silicon-based n⁺-i-p⁺ solar cells in the presence of an exterior top thin film of mono-dispersed ultra-small 2.85nm diameter silicon nanoparticles has been examined. The silicon nanoparticles have a 530-700nm red luminescence band with a band peak at 628nm. Reflectance and the external/internal quantum efficiency (EQE/IQE) of the devices were measured over the range of 300-800nm. With the Si NPs there is a reduction in reflectivity, which improves the coupling of light to the solar cell. The enhancement in IQE is found to be over the spectral slice 360-610nm lying between the confinement and the direct band gaps. An enhancement in the IQE in the range of 390-530nm with a maximum of 31% at 445nm and a smaller one in the range of 530-610nm with a maximum of 11% at 590nm was observed. The increase in the spectral response corresponds to 11.42% increase in short circuit current in agreement with our J-V measurements. The IQE enhancement is explained in terms of two competing channels: excitonic radiative recombination down-conversion in the range 530-610nm and charge separation and collection in the range 360-530nm.