Photoelectrochemical Investigation of Ultrathin Film Iron Oxide Solar Cells Prepared by Atomic Layer Deposition

Klahr, Benjamin M.; Martinson, Alex B. F.; Hamann, Thomas W.

doi:10.1021/la103541n

Cited by 189 publications

(235 citation statements)

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“…More work is needed to clarify these points, however, which is the subject of 40 The identity of these dopants has not been confirmed, due to the very small amount of impurity needed to produce these modest doping levels in the thin films studied. The typical assignment of oxygen vacancies for metal oxide electrodes, which can be related to the annealing and cooling of iron oxide independent of preparation method, may be applicable.…”

Section: Resultsmentioning

confidence: 99%

“…40 The modification consisted of using both ozone and water as the oxidation source instead of just ozone, which results in increased growth rate and uniformity of the hematite films compared to those made using only ozone as the oxygen source. 41 A single precursor-oxidation cycle consisted of a 20 second ferrocene pulse followed by an oxidation sub-cycle which included 10 cycles of a 0.015 s H 2 O pulse followed by a 6 s ozone pulse, where each sub-cycle was separated by a 5 s purge.…”

Section: Methodsmentioning

confidence: 99%

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Water Oxidation at Hematite Photoelectrodes: The Role of Surface States

et al. 2012

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Published asKlahrHowever, the physical-chemical mechanisms responsible for the photoelectrochemical performance of this material ( J(V ) response) are still poorly understood. In the present study we prepared thin film hematite electrodes by Atomic Layer Deposition to study the photoelectrochemical properties of this material under water splitting conditions. We employed Impedance Spectroscopy to determine the main steps involved in photocurrent production at different conditions of voltage, light intensity and electrolyte pH. A general physical model is proposed, which includes the existence of a surface state at the semiconductor/liquid interface where holes accumulate. The strong correlation between the charging of this state with the charge transfer resistance and the photocurrent onset provides new evidence of the accumulation of holes in surface states at the semiconductor/electrolyte interface, which are responsible for water oxidation. The charging of this surface state under illumination is also related to the shift of the measured flat band potential. These findings demonstrate the utility of Impedance Spectroscopy in investigations of hematite electrodes to provide key parameters of photoelectrodes with a relatively simple measurement. 3 I ntroductionAs part of the quest to develop better and cleaner energy conversion and storage systems, the direct conversion of sunlight into chemical fuels has become a subject of renewed interest.One attractive example is the use of semiconductors to harness solar photons to split water, thereby producing hydrogen as a chemical fuel. In order to achieve this, a given material must satisfy a number of stringent requirements including visible light absorption, efficient charge carrier separation and transport, facile interfacial charge-transfer kinetics, appropriate positions of the conduction and valence band energy levels with respect to required reaction potentials and good stability in contact with aqueous solutions. 1 While such systems were heavily investigated several decades ago, no material so far has fulfilled all the required conditions. 2,3 Recent advances in nanotechnology and catalysis, however, greatly increase the prospects of developing a combination of materials capable of efficient conversion of sunlight to chemical fuels. 4 Hematite ( -Fe 2 O 3 ) is a very promising material for photoelectrochemical (PEC) water splitting due to its combination of sufficiently broad visible light absorption, up to 590 nm, and excellent stability under caustic operating conditions. 5,6 However, hematite electrodes are adversely affected by a number of factors including a long penetration depth of visible light due to its indirect band gap transition and a very short minority carrier lifetime and mobility; this combination hinders efficient collection of the minority carriers via the required interfacial charge-transfer reactions. Considerable effort has been devoted to improving the actual efficiency by employing nanostructuring strategies, which disconnects the ligh...

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Section: Resultsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

Water Oxidation at Hematite Photoelectrodes: The Role of Surface States

et al. 2012

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“…40 Films were determined to be hematite by Raman Spectroscopy and XRD measurements in a previous publication. 20 …”

Section: Methodsmentioning

confidence: 99%

“…[16][17][18][19] Thin films serve as excellent model systems for such nanostructured systems since the charge collection dimension can be readily controlled by the film thickness. 20 Further, the uniformity and simple geometry of thin films facilitate the elucidation of the relevant physical properties of hematite photoelectrodes under water splitting conditions. For example, we have recently utilized impedance spectroscopy (IS) to investigate water splitting with hematite by monitoring the evolution of the relevant resistances and capacitances with the onset of photocurrent.…”

Section: Introductionmentioning

confidence: 99%

Electrochemical and photoelectrochemical investigation of water oxidation with hematite electrodes

Klahr

Giménez

Fabregat‐Santiago

et al. 2012

Energy Environ. Sci.

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Atomic layer deposition (ALD) was utilized to deposit uniform thin films of hematite (α-Fe 2 O 3 ) on transparent conductive substrates for photocatalytic water oxidation studies.Comparison of the oxidation of water to the oxidation of a fast redox shuttle allowed for new insight in determining the rate limiting processes of water oxidation at hematite electrodes. It was found that an additional overpotential is needed to initiate water oxidation compared to the fast redox shuttle. A combination of electrochemical impedance spectroscopy, photoelectrochemical and electrochemical measurements were employed to determine the cause of the additional overpotential. It was found that photogenerated holes initially oxidize the electrode surface under water oxidation conditions, which is attributed to the first step in water oxidation. A critical number of these surface intermediates need to be generated in order for the subsequent hole-transfer steps to proceed. At higher applied potentials, the behavior of the electrode is virtually identical while oxidizing either water or the fast redox shuttle; the slight discrepancy is attributed to a shift in potential associated with Fermi level pinning by the surface states in the absence of a redox shuttle. A water oxidation mechanism is proposed to interpret these results.

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“…These thin films have potential for e.g. gas sensitive materials, 7,9 thermoelectric materials, 8 and photoanode materials in solar cells. 2,6 An obvious drawback of inorganic thin films in general is that they are brittle and cannot necessarily be employed in flexible applications.…”

Section: Introductionmentioning

confidence: 99%

Iron-based inorganic–organic hybrid and superlattice thin films by ALD/MLD

Tanskanen

Karppinen

2015

Dalton Trans.

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Here we present novel layer-by-layer deposition processes for the fabrication of inorganic-organic hybrid thin films of the (-Fe-O-C6H4-O-)n type and also superlattices where thicker iron oxide layers alternate with monomolecular-thin organic layers.The proc esses are based on a combi nation of at omic layer deposition ( ALD) a nd molec ular layer deposition (ML D) tec hni ques where cyclopentadienyl iron dicarbonyl dimer (Cp2Fe2(CO)4) is used as the iron source and hydroquinone (HQ) as the organic precursor. For the (-Fe-O-C6H4-O-)n hybrid films a growth rate value as high as 3.7 Å/cycle was achieved at 180 °C.Superlattices where thin crystalline iron oxide layers of the magnetite structure alternate with single organic layers consisting of benzene rings were moreover successfully fabricated from the same precursors at 160 o C using water as the source of oxygen in the ALD cycles for the magnetite layers. We foresee that our new ALD/MLD processes offer a valuable novel tool to modify the properties of magnetite thin films and even more widely possess the potential to boost the ALD/MLD research frontier on functional transition metal oxide based thin films.

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Photoelectrochemical Investigation of Ultrathin Film Iron Oxide Solar Cells Prepared by Atomic Layer Deposition

Cited by 189 publications

References 50 publications

Water Oxidation at Hematite Photoelectrodes: The Role of Surface States

Water Oxidation at Hematite Photoelectrodes: The Role of Surface States

Electrochemical and photoelectrochemical investigation of water oxidation with hematite electrodes

Iron-based inorganic–organic hybrid and superlattice thin films by ALD/MLD

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