## 3 Results
Ultra-violet diffuse reflectance spectroscopy (UV-DRS) has been carried out to evaluate the optical properties of vertically arranged ZnO NRs on Si wafers with reaction times of 3 h, 4 h, 5 h, and 6 h. In Figure 6, the bare Si wafer displays an average reflectance of \(\sim\)38% in the wavelength range of 400-1000 nm, whereas after texturing the Si wafers, the average reflectance has reduced to 14%. The vertically arranged ZnO NRs on the Si wafer detected an average reflectance of \(\sim\)12.36% for 3 h, 12.69% for 4 h, 9.58% for 5 h, and 14.95% for 6 h, covering a large wavelength range from 400-1000 nm. As seen in Figure 6, the significant reduction in the reflectance to \(\sim\)9.58% was recorded in the 5 h reaction time on the Si wafer, which might be due to the generation of multiple reflections of the incident light by the vertically arranged nanorods. The optimized average reflectance of 9.58% at 5 h of treatment time (experimental) is well matched with the average reflectance of 9.79% obtained by the simulation, which is clearly validated in the simulation data as shown in Figure 6.  
The other optical properties, such as the refractive index (n) and extinction coefficient (k) of ZnO NRs on Si wafers, are crucial to develop efficient Si solar cells, as explored in Figure 7, and are extracted from the reflectance results [37; 38; 39]. As compared with other reaction times, the highest refractive index of 1.92 and lowest extinction coefficient are observed by the vertically arranged ZnO NRs on Si wafers with an optimized reaction time of 5 h, as shown in Figure 7a,b. Noticeably, the low k value in the whole visible region represents the weak absorption and large photonic energy of light via the black colored surface of the Si wafer. Based on the above structural and optical properties of ZnO NRs AR layers, a simulation has been carried out using PC1D simulation tools to investigate the PV properties by simulating the current (I)-voltage (V) and incident photon-to-electron conversion efficiency (IPCE) data. Table 2 summarizes the extracted PV parameters, which were taken to perform the simulation for ZnO NRs AR-based Si solar cells. In this work, the PCE and fill factor (FF) of the solar cells were calculated in accordance with the report's articles [40]. Figure 8a shows the simulated I-V characteristics and P-V curve of the Si solar cell based on ZnO NRs AR. As seen in Table 2, the maximum I\({}_{\text{sc}}\) = 0.49 A and V\({}_{\text{oc}}\) = 0.59 V with a conversion efficiency of 14.88%, observed at a length of 1.1 \(\upmu\)m for ZnO NRs and with a minimum reflectance of 9.58%. Solar cells with ZnO NRs 1.1 \(\upmu\)m in length show the highest power of \(\sim\)0.238 W.  
Figure 6: UV-Vis spectra in reflectance mode of the bare Si wafer, textured Si wafer, and vertically arranged ZnO NRs on textured Si wafers. A similar trend is noticed in the PV parameters, as obtained in reflectance and other optical results. Interestingly, all the simulated data of solar cells are extracted by putting in the information of the ZnO NRs AR layer, which means vertically arranged NRs are crucial in this system. Furthermore, the plots of internal quantum efficiency (IQE) and external quantum efficiency (EQE) versus wavelength for the ZnO NRs AR-based Si solar cell is shown in Figure 8b. The EQE of a Si solar cell based on ZnO NRs demonstrated over 80% absorption, covering a 400-1200 nm wavelength range, and the integrated photocurrent was consistent with the Isc extracted from the I-V curve. The absorption of blue photons might usually occur at the topmost part of ZnO NRs, where the charge carriers are supposed to move the entire length of the ZnO NRs to reach the p-n junction. Therefore, the enhancement of the PCE of a Si solar cell occurs by controlling the AR behavior of ZnO NRs.