Photoelectrochemical Conditioning of MOVPE p-InP Films for Light-Induced Hydrogen Evolution: Chemical, Electronic and Optical Properties

Abstract

Homoepitaxial p-InP(100) thin films prepared by MOVPE (metallorganic vapor phase epitaxy) were transformed into an InP/oxidephosphate/Rh heterostructure by photoelectrochemical conditioning. Surface sensitive synchrotron radiation photoelectron spectroscopy indicates the formation of a mixed oxide constituted by In(PO 3 ) 3 , InPO 4 and In 2 O 3 as nominal components during photo-electrochemical activation. The operation of these films as hydrogen evolving photocathode proved a light-to-chemical energy conversion efficiency of 14.5%. Surface activation arises from a shift of the semiconductor electron affinity by 0.44 eV by formation of In-Cl interfacial dipoles with a density of about 10 12 cm −2 . Predominant local In 2 O 3 -like structures in the oxide introduce resonance states near the semiconductor conduction band edge imparting electron conductivity to the phosphate matrix. Surface reflectance investigations indicate an enhanced light-coupling in the layered architecture. Solar hydrogen generation from water represents a viable route for establishing a carbon-neutral energy infrastructure based on renewable energy resources.1-4 To achieve this long-term objective, numerous approaches are currently being pursued comprising adapting systems derived from photosynthesis, 5-7 identification of appropriate catalysts, 8 development of transition metal oxide photoelectrodes 9,10,11 as well as devising efficient semiconductor tandem structures.12-14 Because biomimetic systems inspired by natural photosynthesis are characterized by rather low theoretical efficiencies, 6 the use of photoresponsive semiconductor materials for the splitting of water appears currently most promising. It can be shown that dual-bandgap systems reach theoretically efficiencies well above 40% at AM1.5. 15 The development horizon suggests therefore the use of technologically advanced semiconductor materials which would allow comparably fast technical realization. Further, the orthogonalization of charge carrier and photon pathways as well as an increased built-in potential by interfacial doping was recently exploited to achieve efficiencies near 10% using Si as substrate. 16 p-type InP is one of the most efficient photocathode materials for hydrogen evolution available. Heller and Vadimsky 17 have shown three decades ago that hydrogen evolution can occur with an efficiency of 12% if rhodium is deposited as catalytically active and optically transparent thin-film on top of an In 2 O 3 /InP structure. In this work, a new approach based on thin film photoelectrodes is presented. The photocathode is realized by homoepitaxial growth of thin films onto InP wafers which allows the use of liftoff procedures already known for photovoltaic systems. 18,19 The removal of the thin film photocathodes from the growth substrate is thereby possible after fabrication of the devices. This approach reduces production costs, which is a decisive factor for many III-V devices. In this report, we evaluate the applicability of homoepitaxial devices with emphasis in...

Efficient direct solar-to-hydrogen conversion by in situ interface transformation of a tandem structure

“…While AlInP exhibits χ =3.8 eV (ref. 18), it changes to χ ≈4.4 eV for indium oxide31. Such differences in electron affinity between III–V semiconductors and their oxides with their potential impact on the photovoltage have already been addressed in the literature37.…”

Efficient and Stable TiO2:Pt–Cu(In,Ga)Se2 Composite Photoelectrodes for Visible Light Driven Hydrogen Evolution

“…The stability at pH14, however, is limited (about 2 h). In comparison to the most effi cient devices for photoassisted evolution of hydrogen published so far, planar Si-Pt and InP-Rh heterojunctions [ 25,26 ] with effi ciencies of 9.6% and 14.5%, respectively, the TiO 2 :Pt-Cu(In,Ga)Se 2 system shows higher saturation photocurrent densities but operates less active. However, our CIGSe substrates are characterized by uniform doping while both the Si and InP absorbers were fabricated as homojunctions with a highly doped surface-near region, improving thereby considerably the achievable photovoltage.…”

Current-voltage characteristics of coupled photodiode-electrocatalyst devices

“…3(a). 12 The photodiode is a heterojunction of p-InP and n-InO x P y , where the photoactive layer is p-type InP and the voltage is maximized by the use of a thin, highly doped layer of n-InO x P y . Figure 3(b) compares the modeled and experimental current density versus voltage behavior.…”