Improvement of efficiency for crystalline silicon (c-Si) with nanopillar arrays (NPAs) solar cell was demonstrated by
deployment of CdS quantum dots (QDs). The NPAs was fabricated by colloidal lithography of self-assembled
polystyrene (PS) nanospheres with a 600 nm in size and reactive-ion etching techniques, and then a colloidal CdS QDs
with a concentration of 5 mg/mL was spun on the surface of c-Si with NPAs solar cell. Under a simulated one-sun
condition, the device with CdS QDs shows a 33% improvement of power conversion efficiency, compared with the one
without QDs. Additionally, we also found that the device with CdS QDs shows a 32% reduction in electrical resistance,
compared with the one without QDs solar cell, under an ultraviolet (UV) light of 355nm illumination. This reduced
electrical resistance can directly contribute to our fill-factor (FF) enhancement. For further investigation, the excitation
spectrum of photoluminescence (PL), absorbance spectrum, current-voltage (I-V) characteristics, reflectance and
external quantum efficiency (EQE) of the device were measured and analyzed. Based on the spectral response and
optical measurement, we believe that CdS QDs not only have the capability for photon down-conversion in ultraviolet
region, but also provide extra antireflection capability.
A free-standing nanopillar with a diameter of 300 nm, and a height of 2 μm is successfully demonstrated by focused ion
beam milling. The measured micro-photoluminescence (μ-PL) from the embedded InGaN/GaN multiple quantum wells
shows a blue shift of 68 meV in energy with a broadened full-width at half maximum, ~200meV. Calculations based on
the valence force field method suggest that the spatial variation of the strain tensors in the nanopillar results in the
observed energy shift and spectrum broadening. Moreover, the power-dependent µ-PL measurement confirms that the
strain-relaxed emission region of the nanopillar exhibits a higher radiative recombination rate than that of the as-grown
structure, indicating great potential for realizing high-efficiency nano devices in the UV/blue wavelength range.
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