Establishment Of The Transient Photovoltage Equipment And The Application In The Study Of Photogenerated Carriers In Functional Materials

The organic, inorganic functional materials have been widely used in theelectronic, optical and optoelectronic semiconductor devices as well as thephotocatalysis. The performance of the devices as well as the photochemicalreactivity of photocatalyst is directly related to the optoelectronic properties ofthe semiconductor materials. Therefore, the understanding of thephoto-generated charge carriers properties is very important for theoptimization of the devices performance and the photochemical reactivity. Asa result, the study of the photo-generated charge carriers properties is one ofthe most active researches at present. The nondestructive characterization andqualification of semiconductor materials as well as the devices is a crucial stepin the fabrication of modern day electronic, optical, and optoelectronicsemiconductor devices. Improvements in the quality and yield ofsemiconductor devices, with a resultant increase in performance and reducedcost, will rely on characterization methods that are informative, nondestructive,convenient, easy to use, and inexpensive. A photovoltage arises wheneverphoto-generated charge carriers are separated in space. Therefore, we canobtain the fundamental properties of light absorption, transport of excesscarriers in a semiconductor material and the electronic structures of the surfaceand interface from the study of the surface photovoltage (SPV). Themeasurements of the photovoltage do not need the Ohmic contact between themeasured sample and the electrode and may be performed under differentatmosphere and temperature. In addition, the high sensitivity of SPV for thedetection of the surface charge makes it be a powerful method to characterizethe semiconductor materials and devices.The present paper is about the establishment of the transient photovoltagewhich may detect the evolution of the SPV with the time, and the study of thephotogenerated charge carriers in functional materials using the transientphotovoltage technique. The results in this paper provide the theory evidencefor optimization of the performance of the semiconductor devices,dye-sensitized solar cells, and for improvement of the photochemicalreactivity of the photocatalyst and the light emission quantum yield of theluminescence materials.The main results are illuminated as follows:1. Transient photovoltage (PV) technique has been applied to investigate theseparation and the transport mechanism of the photo-induced charge carriers innano-TiO2 film electrode as well as the dye-sensitized nano-TiO2 filmelectrode. It was directly observed the drift of the photo-generated chargecarriers at TiO2/ITO interface, which demonstrates the existence of thepotential barrier at the interface. The transport dynamic process of thephoto-generated charge carriers in the dye-sensitized nano-TiO2 film electrodeis determined by the electronic property of the TiO2 film. This results providea new experiment for the understanding of the work mechanism of thedye-sensitized solar cells.2. The study of the optoelectronic properties of the TiO2 single crystals (111,110, 001). It was observed that the TiO2 single crystals with different surfaceorientation had difference SPV transient response. This indicates that thetransient SPV directly reflects the optoelectronic properties of thesemiconductor surface. Combining the physical properties of the TiO2 singlecrystals with different surface orientation, the factors that may influence theSPV response have been discussed. This implies that there are some tightrelations between the the formation of the SPV and the photochemicalreactivity.This result provides a new point for understanding the mechanism of thephotocatalysis.3. The study of the optoelectronic properties of nano ZnS:Cu2+ with differentlight emission. It is found that the samples with different light emission havedifferent SPV transients. The different light emission results from the differentdepth of the defect level from the conduction band. So, this indicates that theSPV responses are significantly affected by the electronic structures of thesemiconductors. This result provides a new point for understanding the lightemitting mechanism of the luminescence materials.4. The study of the transport mechanism of the photo-generated charge carriersin the organic molecular semiconductors films. The SPV results from the driftof the photo-generated charge carriers at the semiconductors/ITO interface. Ithas been discussed on the formation of the interface Schottky barrier, and thetransport process of the excess carrier at the surface and bulk of thesemiconductor films.This result provides the scientific experiment fact for the understanding ofwork mechanism of the semiconductor devices made from the organicsemiconductor films, such as the organic light emitting diodes (OLED). Also,this result may provide the theory evidence for the design of the highperformance optoelectronic devices.5. The study of the optoelectronic properties of the GaN and GaN/AlGaNheterostructure using the transient photovoltage technique. It has beenobserved that the SPV may result from the photo-generated charge carriers atthe surface and interface. By analyzing the SPV transients, the electronicstructures of the devices were obtained. It is the first time to observe thetransport of photo-generated charge carriers in the two dimensional electrongas (2DEG) in GaN/AlGaN heterostructure. The bimolecular recombinationprocess was observed at higher excitation level for the photo-generated excesscarriers in 2DEG of GaN/AlGaN heterostructure.This result provides the scientific experiment fact for the understanding ofwork mechanism of the semiconductor devices based on the GaN, such as,high electron mobility transistor (HEMT), ultraviolet detection devices, etc..