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Determining the Conduction Band-edge Potential of Solarcell-relevant NbO Fabricated by Atomic Layer Deposition 2
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William L. Hoffeditz, Michael J Pellin, Omar K. Farha, and Joseph T. Hupp Langmuir, Just Accepted Manuscript • Publication Date (Web): 12 May 2017 Downloaded from http://pubs.acs.org on May 15, 2017
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Langmuir
Determining the Conduction Band-edge Potential of Solar-cell-relevant Nb2O5 Fabricated by Atomic Layer Deposition William L. Hoffeditz†, Michael J. Pellin†,‡, Omar K. Farha†,§, Joseph T. Hupp*,†,‡ †
Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States. ‡Material Science Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States. §Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia. *Joseph T. Hupp:
[email protected] KEYWORDS: photovoltage, atomic layer deposition, ALD, niobium pentoxide, titanium dioxide, conduction band, Mott-Schottky. ABSTRACT: Often key to boosting photovoltages in photoelectrochemical and related solarenergy-conversion devices is the preferential slowing of rates of charge recombination – especially recombination at semiconductor/solution, semiconductor/polymer, or semiconductor/perovskite interfaces. In devices featuring TiO2 as the semiconducting component, a common approach to slowing recombination is to install an ultrathin metal-oxide barrier layer or trap-passivating layer atop the semiconductor, with the needed layer often being formed via atomic layer deposition (ALD). A particularly promising barrier layer material is Nb2O5. Its conduction-band-edge potential ECB is low enough that charge injection from an adsorbed molecular, polymeric, or solid-state light absorber and into the semiconductor can still occur, but high enough that charge recombination is inhibited. While a few measurements of ECB have been reported for conventionally synthesized, bulk Nb2O5, none have been described for ALD-fabricated versions. Here, we specifically determine the conduction-band-edge energy of ALD-fabricated Nb2O5 relative to that of TiO2. We find that, while the value for ALD-Nb2O5 is indeed higher than that for TiO2, the difference is less than anticipated based on measurements of conventionally synthesized Nb2O5 and is dependent on the thermal history of the material. The implications of the findings for optimization of competing interfacial rate processes, and therefore photovoltages, are briefly discussed. Introduction The open circuit photovoltage (Voc) is a key parameter in determining the power conversion efficiencies of photovoltaic devices, including photoelectrochemical devices such as dyesensitized solar cells (“Gratzel cells”); consequently, methods of increasing Voc have been the focus of numerous studies.1-7 Voc is defined by the potential difference between the front and back contacts of a photovoltaic device when the net current flow is 0 (i.e., current is flowing as fast in the desired “forward” direction as in the deleterious “backward” direction). In dye-
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sensitized cells, this potential difference is the difference between the quasi-Fermi level (Eqf) in the semiconductor (usually TiO2) and the redox shuttle formal potential at the dark electrode;8, 9 the Voc in p-i-n junction devices is limited by the potential difference between the valence band of the p-type semiconductor and the conduction band of the n-type semiconductor.10-13 Thus, regardless of device design, the theoretical maximum Voc is material dependent, and one of the most straightforward approaches to increasing Voc in all device architectures is to substitute for the n-type semiconductor another n-type semiconductor featuring a higher conduction-band-edge energy (ECB).14-16 (Recall that higher energies correspond to more negative potentials on an electrochemical scale. Recall also that for well-behaved and moderately doped n-type photoelectrodes, the quasi-Fermi-level at open-circuit, under illumination, and in the presence of a well-chosen redox shuttle, will tend to vary with ECB.17, 18) The substitution of a given photoelectrode material with one with higher ECB can also engender undesirable effects. For example, for n-type DSCs, substitution of the photoelectrode material for an alternative material with higher ECB will decrease the driving force for electron injection from a photo-excited dye molecule into the electrode’s conduction band.19-21 The diminished driving force, in turn, may slow the rate of charge injection, potentially decreasing the injection yield, and ultimately decreasing the cell’s photocurrent.22 Given the typically exponential relationship between driving force and injection time (a consequence of Marcus’ and similar theories), large increases in band-edge energy, e.g. several hundred meV or more, can have disastrous consequences with regard to energy conversion efficiency.23 In contrast, smaller changes may offer the possibility of increasing Voc without appreciably degrading the cell’s short-circuit current density, Jsc, or fill-factor (FF). The archetypal photoelectrode material for n-type DSCs is TiO2. Molecular sensitizers (dyes, D) are typically designed or selected with some attention to positioning the formal potential for the photo-excited dye (the D*/D+ couple) proximal to, but slightly more negative than ECB for TiO2 under cell operating conditions.24 The energy matching, however, is seldom exact and, therefore, many dyes that yield high APCE values overshoot the needed excited-state potential and thus waste an otherwise recoverable portion of the dye excited-state energy. 25 (APCE = absorbed-photo-to-current-conversion efficiency, sometimes termed the internal quantum efficiency.) For these dyes, replacing the photoelectrode material with one offering a somewhat more negative ECB could translate to greater overall efficiency for light-to-electrical energy conversion, or in dye-sensitized photosynthetic cells (DSPCs), greater efficiency for light-tochemical energy conversion. A sizable number of oxides have been installed as ultrathin (
