The Experts below are selected from a list of 136473 Experts worldwide ranked by ideXlab platform
Ben F. Spencer - One of the best experts on this subject based on the ideXlab platform.
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dynamics in next generation solar cells time resolved surface photovoltage measurements of quantum dots chemically linked to zno 100
Faraday Discussions, 2014Co-Authors: Ben F. Spencer, Darren M. Graham, Elaine A. Seddon, Samantha J O Hardman, Matthew J. Cliffe, Karen L Syres, Andrew G ThomasAbstract:The charge dynamics at the surface of the transparent conducting oxide and photoanode material ZnO are investigated in the presence and absence of light-harvesting colloidal quantum dots (QDs). The time-resolved change in surface potential upon photoexcitation has been measured in the m-plane ZnO (100) using a laser pump-synchrotron X-Ray Probe methodology. By varying the oxygen annealing conditions, and hence the oxygen vacancy concentration of the sample, we find that dark carrier lifetimes at the ZnO surface vary from hundreds of μs to ms timescales, i.e. a persistent photoconductivity (PPC) is observed. The highly-controlled nature of our experiments under ultra-high vacuum (UHV), and the use of band-gap and sub-band-gap photoexcitation, allow us to demonstrate that defect states ca. 340 meV above the valence band edge are directly associated with the PPC, and that the PPC mediated by these defects dominates over the oxygen photodesorption mechanism. These observations are consistent with the hypothesis that ionized oxygen vacancy states are responsible for the PPC in ZnO. The effect of chemically linking two colloidal QD systems (type I PbS and type II CdS–ZnSe) to the surface has also been investigated. Upon deposition of the QDs onto the surface, the dark carrier lifetime and the surface photovoltage are reduced, suggesting a direct injection of charge carriers into the ZnO conduction band. The results are discussed in the context of the development of next-generation solar cells.
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time resolved surface photovoltage measurements at n type photovoltaic surfaces si 111 and zno 101 0
Physical Review B, 2013Co-Authors: Ben F. Spencer, Darren M. Graham, Stuart K. Stubbs, Elaine A. Seddon, Samantha J O Hardman, Fausto Sirotti, Matthew J. Cliffe, Andrew G Thomas, Karen L Syres, Mathieu G. SillyAbstract:The time-resolved change in the surface potential upon photoexcitation has been measured in two n-type photovoltaics, Si (111) 7×7 and ZnO (101[over ¯]0), using two different laser pump-synchrotron X-Ray Probe methodologies. Taken together, these experiments allow the dynamics of the surface photovoltage (SPV) to be monitored over timescales of subnanoseconds to milliseconds. The timescales for the photoinduced change in the SPV are dramatically different in the two samples, with measured SPV decay time constants of 6.6 μs for Si and up to 1.2 ms (dependent on surface oxygen concentration) for ZnO. The carrier dynamics at the Si (111) 7×7 surface are well modeled by a self-decelerating relaxation model involving the recombination of carriers by thermionic emission across the surface depletion layer on nanosecond timescales. In the case of ZnO (101[over ¯]0), a persistent photoconductivity (PPC) is observed, which is influenced by oxygen annealing conditions during sample preparation. Persistent photoconductivity is also observed when the surface is illuminated with subband-gap (405 nm) radiation, revealing that defect states approximately 340 meV above the valence band edge are directly associated with the PPC. We demonstrate that, under the conditions of our experiment, PPC mediated by these defects dominates over the oxygen photodesorption mechanism. These observations are consistent with the hypothesis that ionized oxygen vacancy states are responsible for PPC in ZnO. Time-resolved surface photovoltage measurements at n-type photovoltaic surfaces: Si(111) and ZnO(101[over ¯]0) - ResearchGate. Available from: http://www.researchgate.net/publication/258782678_Time-resolved_surface_photovoltage_measurements_at_n-type_photovoltaic_surfaces_Si(111)_and_ZnO(101over_0).
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time resolved surface photovoltage measurements at n type photovoltaic surfaces si 111 and zno 101 0
Physical Review B, 2013Co-Authors: Ben F. Spencer, Darren M. Graham, Elaine A. Seddon, Samantha J O Hardman, Matthew J. Cliffe, Andrew G Thomas, Karen L Syres, Stuart Stubbs, Fausto SirottiAbstract:The time-resolved change in the surface potential upon photoexcitation has been measured in two $n$-type photovoltaics, Si (111) 7\ifmmode\times\else\texttimes\fi{}7 and ZnO ($10\overline{1}0$), using two different laser pump-synchrotron X-Ray Probe methodologies. Taken together, these experiments allow the dynamics of the surface photovoltage (SPV) to be monitored over timescales of subnanoseconds to milliseconds. The timescales for the photoinduced change in the SPV are dramatically different in the two samples, with measured SPV decay time constants of 6.6 \ensuremath{\mu}s for Si and up to 1.2 ms (dependent on surface oxygen concentration) for ZnO. The carrier dynamics at the Si (111) 7\ifmmode\times\else\texttimes\fi{}7 surface are well modeled by a self-decelerating relaxation model involving the recombination of carriers by thermionic emission across the surface depletion layer on nanosecond timescales. In the case of ZnO ($10\overline{1}0$), a persistent photoconductivity (PPC) is observed, which is influenced by oxygen annealing conditions during sample preparation. Persistent photoconductivity is also observed when the surface is illuminated with subband-gap (405 nm) radiation, revealing that defect states approximately 340 meV above the valence band edge are directly associated with the PPC. We demonstrate that, under the conditions of our experiment, PPC mediated by these defects dominates over the oxygen photodesorption mechanism. These observations are consistent with the hypothesis that ionized oxygen vacancy states are responsible for PPC in ZnO.
Elaine A. Seddon - One of the best experts on this subject based on the ideXlab platform.
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dynamics in next generation solar cells time resolved surface photovoltage measurements of quantum dots chemically linked to zno 100
Faraday Discussions, 2014Co-Authors: Ben F. Spencer, Darren M. Graham, Elaine A. Seddon, Samantha J O Hardman, Matthew J. Cliffe, Karen L Syres, Andrew G ThomasAbstract:The charge dynamics at the surface of the transparent conducting oxide and photoanode material ZnO are investigated in the presence and absence of light-harvesting colloidal quantum dots (QDs). The time-resolved change in surface potential upon photoexcitation has been measured in the m-plane ZnO (100) using a laser pump-synchrotron X-Ray Probe methodology. By varying the oxygen annealing conditions, and hence the oxygen vacancy concentration of the sample, we find that dark carrier lifetimes at the ZnO surface vary from hundreds of μs to ms timescales, i.e. a persistent photoconductivity (PPC) is observed. The highly-controlled nature of our experiments under ultra-high vacuum (UHV), and the use of band-gap and sub-band-gap photoexcitation, allow us to demonstrate that defect states ca. 340 meV above the valence band edge are directly associated with the PPC, and that the PPC mediated by these defects dominates over the oxygen photodesorption mechanism. These observations are consistent with the hypothesis that ionized oxygen vacancy states are responsible for the PPC in ZnO. The effect of chemically linking two colloidal QD systems (type I PbS and type II CdS–ZnSe) to the surface has also been investigated. Upon deposition of the QDs onto the surface, the dark carrier lifetime and the surface photovoltage are reduced, suggesting a direct injection of charge carriers into the ZnO conduction band. The results are discussed in the context of the development of next-generation solar cells.
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time resolved surface photovoltage measurements at n type photovoltaic surfaces si 111 and zno 101 0
Physical Review B, 2013Co-Authors: Ben F. Spencer, Darren M. Graham, Stuart K. Stubbs, Elaine A. Seddon, Samantha J O Hardman, Fausto Sirotti, Matthew J. Cliffe, Andrew G Thomas, Karen L Syres, Mathieu G. SillyAbstract:The time-resolved change in the surface potential upon photoexcitation has been measured in two n-type photovoltaics, Si (111) 7×7 and ZnO (101[over ¯]0), using two different laser pump-synchrotron X-Ray Probe methodologies. Taken together, these experiments allow the dynamics of the surface photovoltage (SPV) to be monitored over timescales of subnanoseconds to milliseconds. The timescales for the photoinduced change in the SPV are dramatically different in the two samples, with measured SPV decay time constants of 6.6 μs for Si and up to 1.2 ms (dependent on surface oxygen concentration) for ZnO. The carrier dynamics at the Si (111) 7×7 surface are well modeled by a self-decelerating relaxation model involving the recombination of carriers by thermionic emission across the surface depletion layer on nanosecond timescales. In the case of ZnO (101[over ¯]0), a persistent photoconductivity (PPC) is observed, which is influenced by oxygen annealing conditions during sample preparation. Persistent photoconductivity is also observed when the surface is illuminated with subband-gap (405 nm) radiation, revealing that defect states approximately 340 meV above the valence band edge are directly associated with the PPC. We demonstrate that, under the conditions of our experiment, PPC mediated by these defects dominates over the oxygen photodesorption mechanism. These observations are consistent with the hypothesis that ionized oxygen vacancy states are responsible for PPC in ZnO. Time-resolved surface photovoltage measurements at n-type photovoltaic surfaces: Si(111) and ZnO(101[over ¯]0) - ResearchGate. Available from: http://www.researchgate.net/publication/258782678_Time-resolved_surface_photovoltage_measurements_at_n-type_photovoltaic_surfaces_Si(111)_and_ZnO(101over_0).
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time resolved surface photovoltage measurements at n type photovoltaic surfaces si 111 and zno 101 0
Physical Review B, 2013Co-Authors: Ben F. Spencer, Darren M. Graham, Elaine A. Seddon, Samantha J O Hardman, Matthew J. Cliffe, Andrew G Thomas, Karen L Syres, Stuart Stubbs, Fausto SirottiAbstract:The time-resolved change in the surface potential upon photoexcitation has been measured in two $n$-type photovoltaics, Si (111) 7\ifmmode\times\else\texttimes\fi{}7 and ZnO ($10\overline{1}0$), using two different laser pump-synchrotron X-Ray Probe methodologies. Taken together, these experiments allow the dynamics of the surface photovoltage (SPV) to be monitored over timescales of subnanoseconds to milliseconds. The timescales for the photoinduced change in the SPV are dramatically different in the two samples, with measured SPV decay time constants of 6.6 \ensuremath{\mu}s for Si and up to 1.2 ms (dependent on surface oxygen concentration) for ZnO. The carrier dynamics at the Si (111) 7\ifmmode\times\else\texttimes\fi{}7 surface are well modeled by a self-decelerating relaxation model involving the recombination of carriers by thermionic emission across the surface depletion layer on nanosecond timescales. In the case of ZnO ($10\overline{1}0$), a persistent photoconductivity (PPC) is observed, which is influenced by oxygen annealing conditions during sample preparation. Persistent photoconductivity is also observed when the surface is illuminated with subband-gap (405 nm) radiation, revealing that defect states approximately 340 meV above the valence band edge are directly associated with the PPC. We demonstrate that, under the conditions of our experiment, PPC mediated by these defects dominates over the oxygen photodesorption mechanism. These observations are consistent with the hypothesis that ionized oxygen vacancy states are responsible for PPC in ZnO.
Andrew G Thomas - One of the best experts on this subject based on the ideXlab platform.
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dynamics in next generation solar cells time resolved surface photovoltage measurements of quantum dots chemically linked to zno 100
Faraday Discussions, 2014Co-Authors: Ben F. Spencer, Darren M. Graham, Elaine A. Seddon, Samantha J O Hardman, Matthew J. Cliffe, Karen L Syres, Andrew G ThomasAbstract:The charge dynamics at the surface of the transparent conducting oxide and photoanode material ZnO are investigated in the presence and absence of light-harvesting colloidal quantum dots (QDs). The time-resolved change in surface potential upon photoexcitation has been measured in the m-plane ZnO (100) using a laser pump-synchrotron X-Ray Probe methodology. By varying the oxygen annealing conditions, and hence the oxygen vacancy concentration of the sample, we find that dark carrier lifetimes at the ZnO surface vary from hundreds of μs to ms timescales, i.e. a persistent photoconductivity (PPC) is observed. The highly-controlled nature of our experiments under ultra-high vacuum (UHV), and the use of band-gap and sub-band-gap photoexcitation, allow us to demonstrate that defect states ca. 340 meV above the valence band edge are directly associated with the PPC, and that the PPC mediated by these defects dominates over the oxygen photodesorption mechanism. These observations are consistent with the hypothesis that ionized oxygen vacancy states are responsible for the PPC in ZnO. The effect of chemically linking two colloidal QD systems (type I PbS and type II CdS–ZnSe) to the surface has also been investigated. Upon deposition of the QDs onto the surface, the dark carrier lifetime and the surface photovoltage are reduced, suggesting a direct injection of charge carriers into the ZnO conduction band. The results are discussed in the context of the development of next-generation solar cells.
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time resolved surface photovoltage measurements at n type photovoltaic surfaces si 111 and zno 101 0
Physical Review B, 2013Co-Authors: Ben F. Spencer, Darren M. Graham, Stuart K. Stubbs, Elaine A. Seddon, Samantha J O Hardman, Fausto Sirotti, Matthew J. Cliffe, Andrew G Thomas, Karen L Syres, Mathieu G. SillyAbstract:The time-resolved change in the surface potential upon photoexcitation has been measured in two n-type photovoltaics, Si (111) 7×7 and ZnO (101[over ¯]0), using two different laser pump-synchrotron X-Ray Probe methodologies. Taken together, these experiments allow the dynamics of the surface photovoltage (SPV) to be monitored over timescales of subnanoseconds to milliseconds. The timescales for the photoinduced change in the SPV are dramatically different in the two samples, with measured SPV decay time constants of 6.6 μs for Si and up to 1.2 ms (dependent on surface oxygen concentration) for ZnO. The carrier dynamics at the Si (111) 7×7 surface are well modeled by a self-decelerating relaxation model involving the recombination of carriers by thermionic emission across the surface depletion layer on nanosecond timescales. In the case of ZnO (101[over ¯]0), a persistent photoconductivity (PPC) is observed, which is influenced by oxygen annealing conditions during sample preparation. Persistent photoconductivity is also observed when the surface is illuminated with subband-gap (405 nm) radiation, revealing that defect states approximately 340 meV above the valence band edge are directly associated with the PPC. We demonstrate that, under the conditions of our experiment, PPC mediated by these defects dominates over the oxygen photodesorption mechanism. These observations are consistent with the hypothesis that ionized oxygen vacancy states are responsible for PPC in ZnO. Time-resolved surface photovoltage measurements at n-type photovoltaic surfaces: Si(111) and ZnO(101[over ¯]0) - ResearchGate. Available from: http://www.researchgate.net/publication/258782678_Time-resolved_surface_photovoltage_measurements_at_n-type_photovoltaic_surfaces_Si(111)_and_ZnO(101over_0).
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time resolved surface photovoltage measurements at n type photovoltaic surfaces si 111 and zno 101 0
Physical Review B, 2013Co-Authors: Ben F. Spencer, Darren M. Graham, Elaine A. Seddon, Samantha J O Hardman, Matthew J. Cliffe, Andrew G Thomas, Karen L Syres, Stuart Stubbs, Fausto SirottiAbstract:The time-resolved change in the surface potential upon photoexcitation has been measured in two $n$-type photovoltaics, Si (111) 7\ifmmode\times\else\texttimes\fi{}7 and ZnO ($10\overline{1}0$), using two different laser pump-synchrotron X-Ray Probe methodologies. Taken together, these experiments allow the dynamics of the surface photovoltage (SPV) to be monitored over timescales of subnanoseconds to milliseconds. The timescales for the photoinduced change in the SPV are dramatically different in the two samples, with measured SPV decay time constants of 6.6 \ensuremath{\mu}s for Si and up to 1.2 ms (dependent on surface oxygen concentration) for ZnO. The carrier dynamics at the Si (111) 7\ifmmode\times\else\texttimes\fi{}7 surface are well modeled by a self-decelerating relaxation model involving the recombination of carriers by thermionic emission across the surface depletion layer on nanosecond timescales. In the case of ZnO ($10\overline{1}0$), a persistent photoconductivity (PPC) is observed, which is influenced by oxygen annealing conditions during sample preparation. Persistent photoconductivity is also observed when the surface is illuminated with subband-gap (405 nm) radiation, revealing that defect states approximately 340 meV above the valence band edge are directly associated with the PPC. We demonstrate that, under the conditions of our experiment, PPC mediated by these defects dominates over the oxygen photodesorption mechanism. These observations are consistent with the hypothesis that ionized oxygen vacancy states are responsible for PPC in ZnO.
Fausto Sirotti - One of the best experts on this subject based on the ideXlab platform.
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time resolved surface photovoltage measurements at n type photovoltaic surfaces si 111 and zno 101 0
Physical Review B, 2013Co-Authors: Ben F. Spencer, Darren M. Graham, Stuart K. Stubbs, Elaine A. Seddon, Samantha J O Hardman, Fausto Sirotti, Matthew J. Cliffe, Andrew G Thomas, Karen L Syres, Mathieu G. SillyAbstract:The time-resolved change in the surface potential upon photoexcitation has been measured in two n-type photovoltaics, Si (111) 7×7 and ZnO (101[over ¯]0), using two different laser pump-synchrotron X-Ray Probe methodologies. Taken together, these experiments allow the dynamics of the surface photovoltage (SPV) to be monitored over timescales of subnanoseconds to milliseconds. The timescales for the photoinduced change in the SPV are dramatically different in the two samples, with measured SPV decay time constants of 6.6 μs for Si and up to 1.2 ms (dependent on surface oxygen concentration) for ZnO. The carrier dynamics at the Si (111) 7×7 surface are well modeled by a self-decelerating relaxation model involving the recombination of carriers by thermionic emission across the surface depletion layer on nanosecond timescales. In the case of ZnO (101[over ¯]0), a persistent photoconductivity (PPC) is observed, which is influenced by oxygen annealing conditions during sample preparation. Persistent photoconductivity is also observed when the surface is illuminated with subband-gap (405 nm) radiation, revealing that defect states approximately 340 meV above the valence band edge are directly associated with the PPC. We demonstrate that, under the conditions of our experiment, PPC mediated by these defects dominates over the oxygen photodesorption mechanism. These observations are consistent with the hypothesis that ionized oxygen vacancy states are responsible for PPC in ZnO. Time-resolved surface photovoltage measurements at n-type photovoltaic surfaces: Si(111) and ZnO(101[over ¯]0) - ResearchGate. Available from: http://www.researchgate.net/publication/258782678_Time-resolved_surface_photovoltage_measurements_at_n-type_photovoltaic_surfaces_Si(111)_and_ZnO(101over_0).
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time resolved surface photovoltage measurements at n type photovoltaic surfaces si 111 and zno 101 0
Physical Review B, 2013Co-Authors: Ben F. Spencer, Darren M. Graham, Elaine A. Seddon, Samantha J O Hardman, Matthew J. Cliffe, Andrew G Thomas, Karen L Syres, Stuart Stubbs, Fausto SirottiAbstract:The time-resolved change in the surface potential upon photoexcitation has been measured in two $n$-type photovoltaics, Si (111) 7\ifmmode\times\else\texttimes\fi{}7 and ZnO ($10\overline{1}0$), using two different laser pump-synchrotron X-Ray Probe methodologies. Taken together, these experiments allow the dynamics of the surface photovoltage (SPV) to be monitored over timescales of subnanoseconds to milliseconds. The timescales for the photoinduced change in the SPV are dramatically different in the two samples, with measured SPV decay time constants of 6.6 \ensuremath{\mu}s for Si and up to 1.2 ms (dependent on surface oxygen concentration) for ZnO. The carrier dynamics at the Si (111) 7\ifmmode\times\else\texttimes\fi{}7 surface are well modeled by a self-decelerating relaxation model involving the recombination of carriers by thermionic emission across the surface depletion layer on nanosecond timescales. In the case of ZnO ($10\overline{1}0$), a persistent photoconductivity (PPC) is observed, which is influenced by oxygen annealing conditions during sample preparation. Persistent photoconductivity is also observed when the surface is illuminated with subband-gap (405 nm) radiation, revealing that defect states approximately 340 meV above the valence band edge are directly associated with the PPC. We demonstrate that, under the conditions of our experiment, PPC mediated by these defects dominates over the oxygen photodesorption mechanism. These observations are consistent with the hypothesis that ionized oxygen vacancy states are responsible for PPC in ZnO.
Samantha J O Hardman - One of the best experts on this subject based on the ideXlab platform.
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dynamics in next generation solar cells time resolved surface photovoltage measurements of quantum dots chemically linked to zno 100
Faraday Discussions, 2014Co-Authors: Ben F. Spencer, Darren M. Graham, Elaine A. Seddon, Samantha J O Hardman, Matthew J. Cliffe, Karen L Syres, Andrew G ThomasAbstract:The charge dynamics at the surface of the transparent conducting oxide and photoanode material ZnO are investigated in the presence and absence of light-harvesting colloidal quantum dots (QDs). The time-resolved change in surface potential upon photoexcitation has been measured in the m-plane ZnO (100) using a laser pump-synchrotron X-Ray Probe methodology. By varying the oxygen annealing conditions, and hence the oxygen vacancy concentration of the sample, we find that dark carrier lifetimes at the ZnO surface vary from hundreds of μs to ms timescales, i.e. a persistent photoconductivity (PPC) is observed. The highly-controlled nature of our experiments under ultra-high vacuum (UHV), and the use of band-gap and sub-band-gap photoexcitation, allow us to demonstrate that defect states ca. 340 meV above the valence band edge are directly associated with the PPC, and that the PPC mediated by these defects dominates over the oxygen photodesorption mechanism. These observations are consistent with the hypothesis that ionized oxygen vacancy states are responsible for the PPC in ZnO. The effect of chemically linking two colloidal QD systems (type I PbS and type II CdS–ZnSe) to the surface has also been investigated. Upon deposition of the QDs onto the surface, the dark carrier lifetime and the surface photovoltage are reduced, suggesting a direct injection of charge carriers into the ZnO conduction band. The results are discussed in the context of the development of next-generation solar cells.
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time resolved surface photovoltage measurements at n type photovoltaic surfaces si 111 and zno 101 0
Physical Review B, 2013Co-Authors: Ben F. Spencer, Darren M. Graham, Stuart K. Stubbs, Elaine A. Seddon, Samantha J O Hardman, Fausto Sirotti, Matthew J. Cliffe, Andrew G Thomas, Karen L Syres, Mathieu G. SillyAbstract:The time-resolved change in the surface potential upon photoexcitation has been measured in two n-type photovoltaics, Si (111) 7×7 and ZnO (101[over ¯]0), using two different laser pump-synchrotron X-Ray Probe methodologies. Taken together, these experiments allow the dynamics of the surface photovoltage (SPV) to be monitored over timescales of subnanoseconds to milliseconds. The timescales for the photoinduced change in the SPV are dramatically different in the two samples, with measured SPV decay time constants of 6.6 μs for Si and up to 1.2 ms (dependent on surface oxygen concentration) for ZnO. The carrier dynamics at the Si (111) 7×7 surface are well modeled by a self-decelerating relaxation model involving the recombination of carriers by thermionic emission across the surface depletion layer on nanosecond timescales. In the case of ZnO (101[over ¯]0), a persistent photoconductivity (PPC) is observed, which is influenced by oxygen annealing conditions during sample preparation. Persistent photoconductivity is also observed when the surface is illuminated with subband-gap (405 nm) radiation, revealing that defect states approximately 340 meV above the valence band edge are directly associated with the PPC. We demonstrate that, under the conditions of our experiment, PPC mediated by these defects dominates over the oxygen photodesorption mechanism. These observations are consistent with the hypothesis that ionized oxygen vacancy states are responsible for PPC in ZnO. Time-resolved surface photovoltage measurements at n-type photovoltaic surfaces: Si(111) and ZnO(101[over ¯]0) - ResearchGate. Available from: http://www.researchgate.net/publication/258782678_Time-resolved_surface_photovoltage_measurements_at_n-type_photovoltaic_surfaces_Si(111)_and_ZnO(101over_0).
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time resolved surface photovoltage measurements at n type photovoltaic surfaces si 111 and zno 101 0
Physical Review B, 2013Co-Authors: Ben F. Spencer, Darren M. Graham, Elaine A. Seddon, Samantha J O Hardman, Matthew J. Cliffe, Andrew G Thomas, Karen L Syres, Stuart Stubbs, Fausto SirottiAbstract:The time-resolved change in the surface potential upon photoexcitation has been measured in two $n$-type photovoltaics, Si (111) 7\ifmmode\times\else\texttimes\fi{}7 and ZnO ($10\overline{1}0$), using two different laser pump-synchrotron X-Ray Probe methodologies. Taken together, these experiments allow the dynamics of the surface photovoltage (SPV) to be monitored over timescales of subnanoseconds to milliseconds. The timescales for the photoinduced change in the SPV are dramatically different in the two samples, with measured SPV decay time constants of 6.6 \ensuremath{\mu}s for Si and up to 1.2 ms (dependent on surface oxygen concentration) for ZnO. The carrier dynamics at the Si (111) 7\ifmmode\times\else\texttimes\fi{}7 surface are well modeled by a self-decelerating relaxation model involving the recombination of carriers by thermionic emission across the surface depletion layer on nanosecond timescales. In the case of ZnO ($10\overline{1}0$), a persistent photoconductivity (PPC) is observed, which is influenced by oxygen annealing conditions during sample preparation. Persistent photoconductivity is also observed when the surface is illuminated with subband-gap (405 nm) radiation, revealing that defect states approximately 340 meV above the valence band edge are directly associated with the PPC. We demonstrate that, under the conditions of our experiment, PPC mediated by these defects dominates over the oxygen photodesorption mechanism. These observations are consistent with the hypothesis that ionized oxygen vacancy states are responsible for PPC in ZnO.