Photoconductivity and charge trapping in porous nanocrystalline titanium dioxide

Nelson, Jenny; Eppler, Anuradha M.; Ballard, Ian

doi:10.1016/s1010-6030(02)00035-7

Cited by 93 publications

(85 citation statements)

References 21 publications

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“…Similar observations of high dark and long-lived dark currents have been made in nanoporous titanium dioxide electrodes, 24 in which the conductivity is dependent on the oxygen partial pressure and Ti 3+ /oxygen vacancy surface defects are associated with enhanced recombination in a dye-sensitized liquid electrolyte cell. 29 Nevertheless, further experiments are planned to confirm the presence of trapped charge.…”

Section: Dark Currentsupporting

confidence: 74%

“…The two equivalent circuit models used to analyse the PV response do not consider any time dependent processes and are therefore inadequate to describe these very long-lived transient phenomena. There could be a number of different reasons for an increase in dark current in the device, for example trapped charge in the nanoporous titanium dioxide layers 16,23,24 or the MEHPPV, the presence of mobile ions, photo-oxidation of the polymer or an increase in temperature.…”

Section: Dark Currentmentioning

confidence: 99%

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Nanocomposite titanium dioxide/polymer photovoltaic cells: effects of TiO 2 microstructure, time, and illumination power

et al. 2004

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Nanocomposite titanium dioxide/polymer photovoltaic cells have been fabricated using poly[2-(2-ethylhexyloxy)-5-methoxy-1,4-phenylenevinylene] (MEHPPV). Two different types of titanium dioxide were used, one synthesized using a sol-gel method, the other was a commercial paste. The crystal structure, porosity and absorption spectra of the titanium dioxide layers were measured, and the titanium dioxide synthesized using the sol-gel method had a much lower level of anatase. The photovoltaic properties of the ITO/TiO 2 /MEHPPV/Au cells, which were similar for both types of TiO 2 , were measured as a function of illumination power and compared with equivalent circuit models. A simple equivalent circuit model incorporating a diode, two resistances and a light induced current was inconsistent with the illumination -dependent data and was improved by adding an illumination dependent shunt resistance. A very long lived, photo-induced increase in dark current was observed, which could not be explained by a polymer degradation mechanism or an increase in temperature under illumination, but was more likely to be due to trapped charge.

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Section: Dark Currentsupporting

confidence: 74%

Section: Dark Currentmentioning

confidence: 99%

Nanocomposite titanium dioxide/polymer photovoltaic cells: effects of TiO 2 microstructure, time, and illumination power

et al. 2004

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“…It is acknowledged that the presence of water is important for photocatalysis, 10-11 but its exact role in the process is poorly understood. 12-15 Numerous models 13,[16][17][18][19] have been proposed for the influence of adsorbed water on charge carrier trapping but consensus has still to be reached.Surface photovoltage (SPV) spectroscopy has been used to study both polycrystalline and single crystal (110) rutile TiO 2 . [20][21][22] For both types of material, it was reported that water modified the surface work function of rutile TiO 2 surfaces.…”

mentioning

confidence: 99%

Apparent Semiconductor Type Reversal in Anatase TiO₂ Nanocrystalline Films

et al. 2007

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The electronic properties of anatase and rutile TiO 2 polycrystalline particle films have been studied using surface photovoltage (SPV) and infrared (IR) spectroscopies. The films were prepared from aqueous suspensions using a range of particle sizes (8-400 nm) and were examined under different ambient conditions. The results show that all of the examined films exhibited the expected n-type semiconductor characteristics in dry nitrogen ambient. Films created from the 30 and 400 nm anatase particles exhibited the largest surface photovoltage and also a broad mid-IR absorption attributed to shallowly trapped electrons. The latter was absent from films made from rutile particles. When examined under as-prepared "wet" conditions, the SPV was reduced in magnitude and the IR signal was absent. Further, the films formed from 8 and 30 nm particles exhibited an apparent p-type behavior in their SPV spectra. Interestingly, small anatase particles are known to exhibit enhanced photocatalytic activity when compared to larger anatase particles. These correlations indicate that water, adsorbed on TiO 2 particles of nanodimensions, induce surface states and enable redistribution of photogenerated charge carriers, which is conducive to photocatalysis.Photocatalysis at semiconductor surfaces has been extensively investigated over recent decades. 1-3 Titanium dioxide has usually been the photocatalyst of choice and Degussa P25, which contains about 80% anatase and 20% rutile, has emerged as the benchmark material. 3 Photocatalysis studies have spanned well-characterized, single-crystal (usually rutile) surfaces under ultrahigh vacuum (UHV) conditions 4-6 to polycrystalline powders under catalysis operational conditions. 7-9 The latter have generally involved the catalyst in contact with an aqueous solution or water vapor. It is acknowledged that the presence of water is important for photocatalysis, 10-11 but its exact role in the process is poorly understood. 12-15 Numerous models 13,[16][17][18][19] have been proposed for the influence of adsorbed water on charge carrier trapping but consensus has still to be reached.Surface photovoltage (SPV) spectroscopy has been used to study both polycrystalline and single crystal (110) rutile TiO 2 . [20][21][22] For both types of material, it was reported that water modified the surface work function of rutile TiO 2 surfaces. There appear to have been no corresponding studies of anatase TiO 2 surfaces. It has been observed that adsorbed water influences a broad mid-infrared spectral absorption between 4000 and 700 cm -1 , which arises when TiO 2 polycrystalline films are irradiated with UV light 13,17,23,24 and that has been assigned to excitation of shallowly trapped electrons. 24 Panyatov and Yates 17 observed that this signal decreases when water is chemisorbed to the surface of a thermally reduced P25 TiO 2 film. They propose that the water depletes conduction band electrons during the formation of Ti-OH groups in the early stages of chemisorption. In this paper, we report the results...

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“…The latter correlates well with the notable drop of V oc in Figure 2 for the 420 nm long-pass filter measurement. It is worth noting that the increase in dark current is reversible, but its return to the original level takes place over a matter of several days when the PV device is maintained in vacuum 13 . The fact that the dark current returns to its original level and that there is no deterioration in photovoltaic performance when the device is illuminated again indicates that the observed changes in V oc and dark current and are not due to chemical degradation of the polymer.…”

Section: Resultsmentioning

confidence: 99%

“…The photo-generation of free charge carriers in the TiO 2 film follows its absorption profile and the increase in dark current even when a 420 nm filter is present can be related to the fact that the film has an absorption tail extending well into the visible light range. The UV-Vis spectrum of the TiO 2 layer has been presented previously 12 and the mechanism of photoconductivity and charge trapping in TiO 2 has been recently discussed 13 .…”

Section: Resultsmentioning

confidence: 99%

Non-steady-state operation of polymer/TiO 2 photovoltaic devices

et al. 2004

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We present data on the initial period of operation of Gilch-route MEH-PPV/TiO 2 composite solar cells (CSCs) which show that during this period the CSCs operate in a non-steady state regime. The behavior is complex and may include a gradual rise of the open circuit voltage (V oc ) and of the short-circuit current density (J sc ) with time, a passage through a maximum of either or both parameters, and even a sign reversal. The mechanisms most probably contributing to the transient processes are: i) diffusion driven redistribution of charges resulting in the build up of a quasi steady state charge density profile across the device; ii) photo-doping resulting in a relatively slow increase of the average charge carrier concentration and consequently of the conductivity of the device. The latter is responsible for a strong decrease in V oc , and is evidenced by the significant increase in dark current after device illumination.

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Photoconductivity and charge trapping in porous nanocrystalline titanium dioxide

Cited by 93 publications

References 21 publications

Nanocomposite titanium dioxide/polymer photovoltaic cells: effects of TiO 2 microstructure, time, and illumination power

Nanocomposite titanium dioxide/polymer photovoltaic cells: effects of TiO 2 microstructure, time, and illumination power

Apparent Semiconductor Type Reversal in Anatase TiO₂ Nanocrystalline Films

Non-steady-state operation of polymer/TiO 2 photovoltaic devices

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Photoconductivity and charge trapping in porous nanocrystalline titanium dioxide

Cited by 93 publications

References 21 publications

Nanocomposite titanium dioxide/polymer photovoltaic cells: effects of TiO 2 microstructure, time, and illumination power

Nanocomposite titanium dioxide/polymer photovoltaic cells: effects of TiO 2 microstructure, time, and illumination power

Apparent Semiconductor Type Reversal in Anatase TiO2 Nanocrystalline Films

Non-steady-state operation of polymer/TiO 2 photovoltaic devices

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Apparent Semiconductor Type Reversal in Anatase TiO₂ Nanocrystalline Films