Interface Regulation Of Silicon-Based Photoelectrode For Photoelectrochemical Water Splitting

Photoelectrochemical(PEC)water splitting is one of the crucial technologies to convert intermittent solar energy into high energy density and storable green hydrogen,and an important way to address the issues about energy shortage and environmental pollution.Typically,by using semiconductor as light absorbing materials,PEC systems enable a line of processes,such as the generation,separation and transport of photogenerated charges,and surface catalytic reactions.Crystalline silicon has attracted extensive attention of researchers owing to these significant advantages such as high abundance in crustal,outstanding properties of light harvest,long carrier diffusion length and mature manufacture process.However,the noticeable disadvantages,poor stability and low catalyst activity,remain in the silicon material,which requires that silicon electrodes are usually loaded with a protective and catalyst layer.Therefore,the light absorption and catalytic reaction take place on the different material in general during the operation of silicon–based photoelectrodes for practical solar-fuel production.The separation and transport of charge were sensitively affected by the multi-interface and interlayer.Therefore,the understanding of the role of interlayer,the exploration of interfacial properties and its effects before and after interlayer modification,and the regulation of charge transfer by the interlayer are the keys to improve the electrode performance and reduce the loss of energy.The main results of the dissertation are summarized as follows:(1)The n-type silicon(n-Si)based photoelectrode with metal-oxide-semiconductor(MOS)structure was fabricated by using alumina(Al O_x)and metallic nickel(Ni),which deposited by atomic layer deposition(ALD)and magnetron sputtering techniques,respectively.The Ni thin film is a multifunctional layer,which collects the photogenerated charge,provides well protection for the underlying n-Si substrate,and serves as a precursor of OER catalyst for water oxidation.It was found that Al O_x has superior properties for interfacial passivation of n-Si photoelectrode than that of Ti O_x and Si O_x.The existed interfacial negative charge of Al Ox was identified and calculated through a comparative study,i.e.,here with n-Si and p-Si as substrates.Thus,Al O_x has both excellent field effect passivation and chemical passivation.In addition,in view of the dilemma that the thick interlayer enables favorable interface passivation but greatly hinder the charge transfer,the experiment of decoupling regulation is carried out through controlling the deposition power of Ni.It is speculated that,for the electrode that Ni obtained at high deposition power,the insertion of some nickel particles into thick oxide layer provides a channel for charge transport.(2)By taking the n-Si/oxide(MO_x)/Ni as the prototype,heterointerface with different characteristics and its effects on charge transportation and corresponding photoelectric/photoelectrochemical behaviors were clearly clarified.It is found that the direct contact of silicon and nickel arouse interfacial states with high density,which pinned the Fermi level of n-Si and limited the improvement of photovoltage.The introduction of a thin layer Al O_x can effectively diminish the pinning at n-Si/Ni interface,and largely enhance the barrier height and negatively shift the onset potential of photoanode.Donor-like defects with deep energy level appeared with Al O_x interlayer were ionized under positive potential/reverse bias,leading to the photoinduced charge recombination instead of surface reaction.Fortunately,these deep defects can be further eliminated by cooperating Al O_x with a thin Au layer.Then,all obstacles for electric field to drive the charge extraction from Si for surface reaction can be almost removed by using the Al O_x/Au dual-interlayer.Eventually,the n-Si/Si O_x/Al O_x/Au/Ni/Ni Fe O_xphotoanode behaved an applied bias photon-to-current efficiency(ABPE)of 3.71%for PEC water oxidation,and a record fill factor of 0.75 for corresponding photoelectric device.(3)A silicon-based MOS junction photoanode was designed and fabricated by using Mo O_x as interlayer for efficient PEC water oxidation.We investigated the effects of amorphous sub-stoichiometric Mo O_x on interfacial passivation between n-Si and metal Ni and on charge transport behaviors.A featured photocurrent plateau in current-potential curve was observed for n-Si/Si O_x/Mo O_x/Ni/Ni Fe O_x photoanode,which is originated from charge transport process instead of redox of Mo species.The interfacial passivation effects of Mo O_x were investigated by testing the carrier behaviors of solid state electrode and PEC performance.Here,an electronic states mediated charge transfer process is proposed by combining transient photoelectrochemical response,impedance spectroscopy and intensity modulated photocurrent spectroscopy.Furthermore,for the PEC systems,the regulation of interfacial states and interlayer can also affect the distribution of applied potential on semiconductor side for band bending.These contribute to the understanding of interlayer affected charge transport process and provides an insight on the design and development of remarkable MOS junction photoelectrode.Eventually,after coating with the excellent oxygen evolution catalyst,the n-Si/Si O_x/Mo O_x/Ni/Ni Fe O_x photoanode achieves an onset potential of 0.95 V vs.RHE,and ABPE of 3.2%.