## 3 Results and Discussions  
An affordable and facile method for synthesizing different Au NPs is reported. The particles can be dissolved with both non-polar and polar solvents due to the powder form of the nanoparticles. TEM was carried out to investigate the size and morphology of the prepared Au NPs, and the results are reported in Figure 2a-c. Au NPs are mostly spherical and highly monodisperse; however, the smaller particle size shows the narrower size distribution. In detail, Au NPs 1 with the oleylamine concentration at \(3\times 10^{-2}\) M has the largest average diameter of 24.8 \(\pm\) 2.1 nm with a broad size distribution. Meanwhile, the highest concentration of oleylamine at \(6\times 10^{-2}\) M produces Au NPs 3 with the smallest core particle size of 12.3 \(\pm\) 0.6 nm. Additionally, the normalized absorption spectra of Au NPs (Figure 3a) with the plasmon absorption peaks of 524 nm, 523 nm, and 521 nm, in accordance with the Au NPs 1, Au NPs 2, and Au NPs 3. With increasing oleylamine concentration, the slight blue shift of Au NPs plasmon peak (from 524 nm to 521 nm) strongly confirms the particle size decrease. Thus, the particle diameters can be inferred from the oleylamine concentration. In the formation of oleylamine capped Au NPs complex, oleylamine plays a crucial role as both reducing and stabilizing agent [26]; as a result, the particle size and distribution can be tunned by the concentration of oleylamine. Figure 2: (**a–c**) TEM images of Au NPs. The insert shows the particle size distribution histogram of Au NPs. (**d**) TEM images of CdSe/ZnS QDs-528 nm. In term of QDs, the green CdSe/ZnS core/shell QDs was employed. TEM image in Figure 2d indicates the narrow size distribution of QDs with an average diameter of \(\sim\)3.6 nm. The absorption spectra and the PL emission spectra of QDs with the wavelength at 528 nm are presented in Figure 3b. The full width at half maximum (FWHM) is smaller than 35 nm. The quantum yield (QY) is about 85%, indicating high efficiency. The absorbance peak of QDs located at the wavelength of 508 nm, corresponding to the energy bandgap of 2.180 eV [27]. In the solar radiation spectrum, photons around 520 nm have the highest irradiance energy intensity that is useful for Silicon-based solar cells [28; 29]. Compared to other CdSe QDs, the green one has a greater band gap, leading to a lower recombination rate in solar cells. Thus, CdSe/ZnS (green) is an potential candidate for enhancing PV performance [30; 31]. To investigate the dependence of the LSPR effect on Au NPs and QDs, the absorption spectra of bare QDs and different Au NPs layers are measured, as shown in Figure 3c. With the addition of Au NPs, the overall absorbance intensity has increased. The SPR peaks and the accompanied FHWM are also obtained. The spectra peaks are located at 527 nm, 526 nm, and 523 nm. Correspondingly, the FWHM values are 68 nm, 80 nm, and 43 nm for Au NPs 1, Au NPs 2, and Au NPs 3. The lowest FHWM of Au NPs 3 indicates highly monodisperse particles. Besides, the variation in the absorbance intensity can be attributed to the surface coverage of the particles on thin film. It was reported that the spin coating Au NPs (\(<\)20 nm) gave the superior uniform shape and concentration on both glass and silicon substrates. To understand the effect of Au/QD layer on PV performance, thin layers of Au NPs 2 (with a diameter of 18.3 nm) and CdSe QDs are adopted on the front side of the Schottky-based solar cell. Figure 4a displays the current density-voltage (J-V) characteristics of the devices under light illuminating conditions. Overall, the performance of solar cells has enhanced clearly after coating Au/QDs layer. After coating Au NPs layer, the considerable increase in J\({}_{\text{SC}}\) from 14.86 mA/cm\({}^{2}\) to 17.07 mA/cm\({}^{2}\) and the slight growth in V\({}_{\text{OC}}\) from 0.36 V to 0.37 V are observed. These improvements can be attributed to the enhanced IPCE (by 11%) in the wavelength segment ranging from 600 nm to 850 nm, presented in Figure 4b.