Measurement Of Photoelectric Characteristics Of Organic Material MEH-PPV

Organic light-emitting materials and devices receive wide attention by the people because of its advantage that can't be compared in the modern display technological field. In improving the performance of the device, a very important factor is the carrier mobility of the thin film materials, which fundamentally determines the response time and photoelectric conversion efficiency of the electroluminescent device. Therefore, the effective measurement of the carrier mobility of the films becomes one of the important means for the research of the film luminous materials.At present, there are so many measurement methods of the carrier mobility, and compared against organic materials TOF method is recognized the best. In this paper, we firstly establish a TOF method experimental system, and explain the details of the principle of this method and the structures of the experimental system. Secondly, we study the carrier mobility of the SI semiconductor material and inorganic Se material separatly which show that we have successfully setted up a TOF experimental system. Thirdly, we carry on careful research to organic material MEH-PPV, in which different conditions are studied on the influence of the carrier mobility in the film. Then, we do some analysis of the results. At last, because of the time limitted, we do simple research on P3HT, caculate the carrier mobility of it. All of these are necessary and important for the further study of organic electroluminescent material.The design of the TOF experimental system:With the help of the photoconductive properties of organic material, TOF is a method which measures the transit time of carrier go through the film under the effect of electric field to obtain the carrier mobility. By this mechanism, we have designed a set of TOF experimental system, as shown in Fig.1, the signal on the oscillograph shows the information of the photocurrent change on time. Then, we can see the transit time of the carrier in the film from the signal and caculat the carrier mobility by The Hall effect is used to measure the carrier mobility of semiconductors, and when we use TOF method to measure it , there is some limitation. Therefore, we just give the qualitative description.of Si. Fig.2 and Fig.3 show the relationship between carrier mobility in Si with the electric field.. Comparing with the previous reports, we found that the results we obtained with the TOF method was basically exact, which firstly approved the veracity of experimental system we have setted up.The second test of the TOF experimental system(Se,for example): From Figure 4 (2) we can read out the time-of-flight and then calculate the carrier mobility of Se which consistent with the reported order of magnitude. From Figure 5 we can see that with the voltage reduced, the transit time of carrier in Se film gradually increased on the condition that the other experimental conditons are the same. And the voltage on the impact of more time. From Fig.6we can see that the sampling resistance have a greater impact on the transit time of carrier in the film. Basically, the transit time have a linear relationship with the sampling resistor. Therefor, we should select a appropriate sampling resistor to match the materials. Fig7(1) is the TOF curve of MEH-PPV,it is also dispersive, then, we use the method of Scherr-Montroll to get Fig.7(2), we can read out the transit time of the carrier in the film from the curve and caculate the carrier mobility ,which is the same with the reported order of magnitude.In the next experiment, we do more carfully research to MEH-PPV, we will look into the character of TOF curve on various experimental conditions separately.1:Different voltage: Fig.8:the Scherr-Montroll processes curve Fig.9:μ? EcurveFrom figure8 we can see that with the internal electric field intensity increased the flight time gradually reduce. We can caculate the carrier mobility from figure 8 and get the curve such as Figure 9. In Figure 9, the cruve accords with basically, then, we can caculate from the cruve and get PF factor In the organic semiconductor materials, the atoms in molecules combine together through covalent bond, and all the molecules are independent of each other, between each other only the weak van der Waals in existence.Therefor, there is small wave function overlap between the wave function of LUMO and HOMO. There is a big barrier to pass through for the electronic jump from one molecule to another, jumpping transmission is the only way, for which electronic field is a effective way to help the electronic to go through the barrier. And the carrier mobility will increase when the electronic field is stronger.2:Different Laser intensity: eWe can see from Fig.10 that with the light intensity increasing the current value significantly increases at the same voltage. It probably due to the increase of excitation light intensity results in more carrier be produced, and the charge which directional movement also incerase, leading to the current increase at the same time. Also, the biger the light powerful, the smaller the transit time at the same voltage. It is probably due to the big light intensity causes more deeper traps which catch more carrier make the transit time reduced. But the time reduced is a slight difference. In the high-voltage 108 V, the light intensity has little effect on the TOF curve, voltage play a major role at this time.3: Different Vacuum: Figure 11 tells us that the transit time of the sample placed in a vacuum room of 14 atmospheres is almost the same with the sample placed in a standard atmosphere,only slightly shorter. So the vacuum conditions is just to maintain the interference from the external environment, not to interact with the outside world to ensure that materials are not degenerate, and the carrier mobility has little effect on time.4:Different temperature: Looking into the TOF curve under different temperature conditions with the same voltage 193v such as Fig.12. We can see that the lower the temperature the more dispersive the TOF curve. Put the curve which is under condition of -1000C out and give it a sepatate observation, in Fig.13. From Fig.12, Fig.13 we can see that with the temperature decreased, the TOF curve are more dispersive. The structures of organic materials is belong to short-range order, the transition mechanism is jumping conduction. When we consider the dispersive or non-dispersive situation of TOF curve, the Gauss disorder model advanced by Borsenberger and Bassler is commonly used. There are two most important parameters in this model, one is energetic disorder parameter, and the other is location disorder parameter. For the charge excited by optics there is an energy release process before moving in the film, which will achieve to a quasi-equilibrium energy. When the time required for a carrier to settle to quasi-equilibrium less than their transit time, and there is no deep traps in the film there will be non-dispersive. Otherwise, there will be dispersive when there is a peak in the TOF signal. For small values of temperature the thermal energy is not adequate to enable the carriers to surmount the energy barriers, and the carriers cannot reach the quasi-equilibrium energy. Then, there is a peak in the TOF signal. The lower the temperature, the dispersiver the signal and when the Scherr-Montroll method can not be used.5:Different Sampling resistor:From Figure 14 we can observe that the sampling resistance has a great impact on the transit time of carrier in the film. When the sampling resistance becomes from 80Ωto 33.5KΩ, the transit time became longer. The character is the same with Se which have a linear relationship. Therefor, we should select a appropriate sampling resistor matched the materials. 6:Different solvent: Fig.15(1) Chlorobenzene as solvent, the TOF curve is non-dispersive, Fig.15(2) THF as solvent, the TOF curve is dispersive. The present results show that transport properties of carrier in the film depend not only on the chemical structures of the polymers but also on the solvents used for film perparation, solvent can affect the morphology of polymer to a certain extent. The plateaus shown in Fig.15(1) is not flat, which may be relate with the deep trapping of carriers in the film. The formation of these deep structural traps may depend upon the solvent used for device preparation.The carrier mobility measurement of other organic material: In our experiment, we use 355nm wavelength of ND-YAG lasers and get the hole TOF curve of P3HT, such as Fig.16, we can read out the transit time tT = 7.62×10?6S, the applied voltage 90.3V, the thickness of the film is 2μm, we can caculate the hole mobility isμh = 5.8×10 ?5 cm 2/vs.The testing result indicats that we have successfully established a set of TOF experimental system, which plays a supporting role in the study of luminous materials and can be effectively used on the optoelectronic properties research of organic materials. So, the experimental system has very important pratical value. At the same time, the detailed research of MEH-PPV will have great significance in the application of such material.