Surface Modification And Mechanism Research On Monocrystalline Silicon Photocathode

Hydrogen production directly from water by sunlight is a promising approach to address the worldwide energy and environmental problems.p-Si has received more attention in tandem photoelectrochemical（PEC）systems as the photocathode for hydrogen evolution reaction（HER）because of its earth abundance,low-cost,suitable bandgaps and band alignment.However,p-Si suffers from two significant drawbacks,including 1)the instability in aqueous electrolyte and 2)limited photovoltage(V_oc)that requires a large external bias to drive the water splitting reaction.Therefore,we choose p-silicon as the photocathode due to its perfect light-absorbing performance.The protective layer and surface modification of the heterojunction are investigated to solve the problems of instability and insufficient photovoltage,and the mechanisms are also discussed.The first part of this paper highlights the importance of photo-inertness of protective layer on the superior stability of Ta₂O₅ protected Si photocathode under full spectrum.We compared the PEC performance and stability of TiO₂ and Ta₂O₅ coated Si photocathodes by vacuum atomic layer deposition（ALD）under AM 1.5G light(100mW cm^-2)and red light(35 mW cm^-2).The similar PEC performance show electron can readily travel through TiO₂ and Ta₂O₅ layers by small conduction band offset with Si.But pn⁺-Si/Ta₂O₅/Pt cathode is stable throughout a 200 h run at 0.3 V vs.RHE under simulated AM 1.5G full spectrum(100 mW cm^-2),which shows stronger stability than TiO₂ protected Si cathode.By comparing the photoelectrochemical and optical properties of TiO₂ and Ta₂O₅ based photocathodes in AM 1.5G light and red light illumination,the effect of spectral range on protective layers has been demonstrated.The second part of this paper mainly focuses on the application of p-Si/n-CdS heterojunction in water splitting and multifunction of TiO₂ layer.It describes the design and fabrication of a low-cost p-Si/n-CdS heterojunction photocathode using chemical bath deposition（CBD）method,which is further protected by a multifunctional TiO₂layer for efficient and stable PEC water splitting.With Pt as the surface co-catalyst,Si/CdS/TiO₂/Pt photocathode shows much higher photovoltage（425 mV）,photocurrent（21.9 mA cm^-22 at 0 V vs.RHE）.The solar-to-hydrogen conversion efficiency reaches2.07%,which is the highest among all Si heterogeneous p-n junction photocathodes in neutral electrolytes.By analyzing the electrode structure and electrode/electrolyte interface,the enhanced PEC performances are mainly attributed to the passivation effect and suitable band alignment of TiO₂.In addition,TiO₂ layer could prolong the time of stable PEC performance to 1 h by isolating electrolyte from Si and CdS.