Abstract
The persistent nature of the increased conductivity upon removal of incident illumination, described by the term persistent photoconductivity (PPC), in ZnO films is sensitive to their defect states. PPC can be viewed as a process of charge storage with relevant defects. To evaluate charge storage quantitatively, in this work, some thought-provoking characteristic quantities were derived from a photocurrent–time curve acquired by testing the photoelectric properties of ZnO under on and off UV illumination. Quo was defined as the obtained charge number per unit voltage during the light-on phase, while Qus was defined as the storage charge number during the light-off phase. η was acquired by dividing Qus by Quo to measure the storage efficiency after the removal of UV light. On the basis of previous work, it was assumed that the PPC of ZnO originated from the unique property of V0O. Meanwhile, this report reveals that the intrinsic defects VO2+, VO+, V0Zn will enhance Quo and Qus but decrease η in the pure ZnO nanorod array film. The extrinsic defect Cu0Zn introduced by coating the ZnO nanorod array film in an ethanol solution of copper acetate suppresses Quo and Qus but promotes the increase of η. Since the whole methodology originated from a series of physical definitions, it can be easily extended to other materials with similar PPC effects.
Highlights
The unique phenomenon of the long relaxation of photoconductance a er removing illumination observed in many semiconductor crystals (GaN,[1,2] SiC,[3] ZnO,[4,5] SnO2,6 Bi2S3,7 WO3,8), known by the term persistent photoconductivity (PPC), has stimulated the profound interest of researchers
There was no obvious difference in morphology between the Cu(CH3COO)2-coated ZnO nanorods and the pure ZnO nanorods, indicating that the addition of copper acetate had no distinguishable in uence on the morphology of the ZnO nanorods
We provided a whole new perspective on the property of PPC which concerns the storage of photogenerated charges a er illumination, possibly affecting optical energy storage in materials showing PPC
Summary
The unique phenomenon of the long relaxation of photoconductance a er removing illumination observed in many semiconductor crystals (GaN,[1,2] SiC,[3] ZnO,[4,5] SnO2,6 Bi2S3,7 WO3,8), known by the term persistent photoconductivity (PPC), has stimulated the profound interest of researchers. A method for the quantitative characterization of long-term carrier (holes for p-type semiconductors and electrons for n-type semiconductors) storage through persistent photoconductance is proposed for the rst time based on the physical de nitions of current. To obtain the characteristic values for the quantitative characterization of long-term carrier storage, we focused on mathematical treatment of the photocurrent–time curve based on general physics. According to eqn (1), the number of charges for the photoresponse phase (Qo) can be obtained by integrating the I–t curve in the time interval of the stage of illumination-on, ð t1. T0 while the number of charges for the decay phase (Qs) can be given by integrating the I–t curve in the time interval of the stage of illumination-off as follows, ðt[2]. T1 here, the number of charges for the photoresponse phase (Qo) is the total number of photogenerated carriers, while the number
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