## 2 Experimental Details  
The oxide/c-Si solar cells were fabricated on double-side polished mono-crystalline (float zone, 100 orientation) n-Si wafers (\(\sim\)2.5 \(\Omega\)cm, \(N_{\mathrm{D}}\)\(\sim\) 1.8 \(\times\) 10\({}^{15}\) cm\({}^{-3}\)) of 280 \(\upmu\)m thickness. After the standard RCA cleaning process and 1% HF bath (1 min) to remove native SiO\({}_{2}\), samples were introduced in a Plasma-Enhanced Chemical Vapor Deposition (PECVD) system (Elettrorava, Turin, Italy) for deposition of three hydrogenated amorphous-silicon carbide layers (a-SiC\({}_{x}\):H, 0.2 \(<\)\(x\)\(<\) 1) on the rear side of the wafer. This passivating/n\({}^{+}\) doped/reflective stack was then laser-fired (infrared nanosecond laser) to create a matrix of point contacts (0.5% contacted area) with a measured contact resistivity \(<\)0.5 m\(\Omega\)cm\({}^{2}\) and an effective surface recombination velocity \(\sim\)50 cm/s (before metallization) [12]. After a second 1% HF bath, the different transition metal oxides (Sigma Aldrich, Dorset, England;99.99%, powdered) were deposited by vacuum thermal evaporation from a tantalum boat at \(<\)8 \(\times\) 10\({}^{-6}\) mbar at a rate \(<\)0.2 A/s. Even though ReO\({}_{3}\) was evaporated without difficulty, the film proved unstable when exposed to ambient air and some of the devices could not be measured. The oxide thickness was fixed at 15 nm and controlled by quartz microbalance. The samples were then briefly exposed to air for transferring into a RF-magnetron sputtering system for deposition of an 80 nm Indium-Tin Oxide (ITO) conductive/anti-reflective layer (ITO target: [PERSON], Hastings, England), followed by lithographic patterning of 1 cm\({}^{2}\) cell area. Contacting of the rear side was done by electron-beam assisted evaporation of Ti (15 nm)/Al (1000 nm) followed by a front grid (4.3% shadow) of thermally evaporated Ag (metals: [PERSON], Hastings, England). A reference device was also fabricated by replacing the transition metal oxide layer with an intrinsic (i) a-Si:H (4 nm)/(p) a-Si:H (15 nm) stack contacted by ITO and using the same point contact strategy on the rear side. Figure 1(a) depicts the structure of the oxide/n-Si heterojunction solar cells.  
Characterization of the solar cells was performed before and after a 10 min 160 \({}^{\circ}\)C annealing in N\({}_{2}\) atmosphere. The current-voltage characteristics were measured (Keithley Instruments 2601B, Cleveland, OH, USA) both in dark and AM1.5g standard illumination (Oriel Instruments, Irvine, CA, USA), extracting the relevant cell performance parameters. The External Quantum Efficiency (EQE) response was measured by a commercial instrument (PV Measurements QEX10, Boulder, CO, USA) and the total reflectance by spectrophotometry (Shimadzu 3600, Kyoto, Japan) with an integrating sphere. Optical simulation of the solar cell structure was done by the tool _Wafer Ray Tracer_[15] (Version 1.6.4, PV Lighthouse Pty. Ltd., Australia) at 20 nm intervals and 5000 incident rays. Transfer Length Method (TLM) measurements of the oxide films (15 nm thick) were also performed with 1 \(\times\) 0.1 cm Au electrodes at various distance intervals, measuring the current-voltage response (Keithley Instruments 2636, Cleveland, OH, USA; 1 fA resolution) in the dark and under vacuum. The elemental composition of MoO\({}_{3}\) films was determined from Gaussian-Lorentzian peak fittings of X-ray Photoelectron Spectroscopy (XPS) spectra measured at \(<\)3 \(\times\) 10\({}^{-9}\) mbar using a non-monochromatic Al-K\(\alpha\) source (SPECS, Berlin, Germany; Phoibos 150 detector).  
Figure 1: **(a)** Schematic of the transition metal oxide/n-Si heterojunction solar cell; **(b)** reported work functions for ReO\({}_{3}\)[13], V\({}_{2}\)O\({}_{5}\), MoO\({}_{3}\) and WO\({}_{3}\)[14]. The valence and conduction band energy levels of c-Si are also shown.