## 3 Results and Discussion  
The \(\tau_{\text{eff}}\) with various solution conditions was measured to distinguish between BQ and HQ as the dominant passivation molecule. The results of the measurement of the \(\tau_{\text{eff}}\) of Si samples after passivation with BQ/MeOH and HQ/MeOH solutions (Figure 2a) show that BQ/MeOH produced \(\sim\)1.7 ms at a 2.0 \(\times\) 10\({}^{-15}\) cm\({}^{-3}\) carrier injection level after a 1-h treatment while with the sample with HQ/MeOH only 68 \(\upmu\)s was measured. From repeated measurements, we found that the \(\tau_{\text{eff}}\) of HQ/MeOH treated samples never exceeded that from the HF pre-cleaned-only sample, which indicates that HQ did not make any contribution to passivation under our experimental conditions. However, when we measured the \(\tau_{\text{eff}}\) while varying HQ ratios with a fixed 10 mM of BQ (Figure 2b), the highest \(\tau_{\text{eff}}\) of 3.2 ms was achieved at a 1:1 mixture ratio of BQ and HQ, while the \(\tau_{\text{eff}}\) of the other BQ:HQ ratios (1:0.5 and 1:1.5) was lower at 2.0 ms and 1.6 ms, respectively. These results lead us to assume that while HQ has little involvement in the passivation reaction when it exists alone, it can enhance the effectiveness of passivation under an optimal mixture ratio with BQ. In order to understand how HQ affects the Si surface passivation, the Si surface morphologies after passivation treatments in the solutions with different HQ mixture ratios were investigated using tapping mode AFM, which is a direct way to observe the uniformity of the passivation layer. Because the molecular passivation layer is ultra-thin, AFM scans were carried out with a super-sharp probe (probe diameter \(<\)2 nm) to generate high resolution AFM (HR-AFM) images. The HR-AFM images shown in Figure 3a-d were obtained from the samples passivated with (a) pure BQ, (b) BQ:HQ = 1:0.5, (c) BQ:HQ = 1:1, and (d) BQ:HQ = 1:1.5, which had 10 mM of fixed BQ in 500 mL MeOH. Figure 2: (a) Measured \(\tau_{\text{eff}}\) of BQ (solid-square) and HQ (solid-triangle) treated Si compared with HF pre-cleaned-only sample (open-circle); and (b) measured \(\tau_{\text{eff}}\) at a minority carrier density of \(2.0\times 10^{15}\) cm\({}^{-3}\) with 100% BQ, HQ, and various HQ mixture ratios into a fixed 10 mM BQ in 500 mL MeOH. It is evident that the higher HQ mixture ratios produced more prominent and larger molecular islands on the surface, which indicates higher molecular densities on the local surface (Figure 3). However, the uniformity of the islands deteriorated with the 1.5 HQ mixture ratio (Figure 3d). The cross-section profile directly below the AFM image in Figure 3 also supports the idea that excess HQ degrades the uniformity of the passivation layer, and this corresponds with the decrease in the \(\tau_{\text{eff}}\) shown in Figure 2b. To better understand the reasons for the variation of the passivation layer morphologies and \(\tau_{\text{eff}}\) with different HQ mixture ratios, the molecular interaction among BQ, HQ, and MeOH must be clarified. Therefore, FT-IR measurements were taken for the various solution conditions and the results, including pure MeOH for comparison, are shown in Figure 4a. In particular the characteristic absorptions for the alcohol O-H bond at 3390 cm\({}^{-1}\) (stretch) and 1400 cm\({}^{-1}\) (bending) are apparent and have been marked with vertical lines in Figure 4a. BQ in MeOH produces a highly hydroxyl (O-H) deficient solution because BQ is a strong oxidizing agent, which deprives a proton (H\({}^{+}\)) from MeOH as shown in Figure 4b. Considering that there is only 10 mM of BQ in MeOH, the reaction between BQ and MeOH must be extremely fast and dynamic because the O-H peaks at 3390 cm\({}^{-1}\) and \(\sim\)1400 cm\({}^{-1}\) have nearly disappeared. (Note: the C-H peak at \(\sim\)2900 cm\({}^{-1}\) is fairly ubiquitous, so the origin of the C-H peak intensity change is hard to specify) [20].