## 3 Results and Discussion
### Solar Cell Characterization  
Besides reduced parasitic absorption and moderate resistance losses, transition metal oxides must also generate a potential difference that facilitates the separation of photogenerated carriers. Figure 5 depicts the current density-voltage (_J-V_) characteristic of the oxide/n-Si solar cells under standard AM1.5g illumination and in the dark (inset). All oxide solar cells show the typical rectifying behavior of a p-n junction except for ReO\({}_{3}\), whose surface quality was severely deteriorated and produced a Schottky diode between ITO and silicon. The cell dark response at high voltage shows larger series resistance (_Rs_) than the a-Si:H reference, while at low voltage the so-called recombination diode indicates inadequate passivation of the oxide/n-Si interface. Under illumination, the highest open-circuit voltage (_VOC_) is achieved by the V\({}_{2}\)O\({}_{5}\) cell with 593 mV, followed by MoO\({}_{3}\) (581 mV) and WO\({}_{3}\) (570 mV). These _VOC_ values are notable given the chemical dissimilarity between materials, the thinness of the oxide layer and the simple solar cell design, and even though higher _VOC_ s have been reported by use of (i) a-Si:H passivating interlayers [7,10], they introduce an additional processing step (PEVCD). Moreover, a correlation could be inferred between the reported work functions values (Figure 1(b)) and the measured _VOC_ s (with larger work functions exerting a stronger surface inversion on n-Si), but further research is necessary. As to the short-circuit current densities (_JSC_), all cells achieved a relatively constant value of \(\sim\)29 mA/cm\({}^{2}\), with the reference being slightly lower due to its higher parasitic absorption (lower _Egap_). The observed Fill Factors (_FF_) were similar for all cells (\(>\)70%) and could be improved by decreasing _Rs_, which is dominated by ITO's sheet resistance and probably by the high contact resistivity amongst n-Si/oxide/ITO. By considering the obtained solar cell parameters (Table 2), the Power Conversion Efficiencies (_PCE_) were 12.7% (V\({}_{2}\)O\({}_{5}\)), 12.6% (MoO\({}_{3}\)) and 11.8% (WO\({}_{3}\)), compared to a reference 13.1%. Figure 5: Current density-voltage (_J-V_) response of the fabricated oxide/n-Si solar cells under standard AM1.5g illumination, including an a-Si:H reference device. Inset shows _J-V_ under dark conditions. The External Quantum Efficiency (EQE) response of the solar cells is shown in Figure 6. The curves show considerable reflectance losses in the ultraviolet and near-infrared wavelengths, which could be lowered by fabricating the device on a texturized wafer. Likewise, the reflectance minimum (\(\sim\)750 nm) could be shifted to shorter wavelengths by reducing the ITO thickness, matching the solar spectrum power peak (\(\sim\)525 nm) and enhancing photon absorption. A notable feature of the oxide cells is the improved response in the 300-550 nm range when compared to a-Si:H, explained by the lower parasitic absorption of the oxide films and predicted by the optical simulation of the device, accounting for a net gain in \(J_{SC}\) of \(\sim\)0.5 mA/cm\({}^{2}\). Finally, it is well known in a-Si:H/c-Si heterojunction technology that ion bombardment occurring during ITO sputtering degrades the passivation of a c-Si surface [24], increasing carrier recombination (lower \(J_{SC}\)) and decreasing the effective minority carrier lifetime (lower \(Voc\)). A short post-fabrication annealing at 160 \({}^{\circ}\)C is sufficient to recover the damage, as evidenced by a \(Voc\) and \(J_{SC}\) enhancement of the reference a-Si:H cell to 645 mV and 32.7 mA/cm\({}^{2}\). In search of a similar boost in performance, the oxide/n-Si cells were annealed under the same conditions, obtaining varied results as summarized in Figure 7. The V2 Os device decreased its \(PCE\) to 10.8% as a consequence of a drop in \(FF\), even though