Journal of the Korean Physical Society, Vol. 53, No. 1, July 2008, pp. 376379
Investigation of Carrier Transport Mechanism in CdSe/PVK Hybrid Nanocomposites Fushan
Li, Han-Moe Cha, Seung-Mi Seo, Dong-Ick Son, Hyuk-Ju Kim and Tae Whan Kim Advanced Semiconductor Research Center, Division of Electronics and Computer Engineering, Hanyang University, Seoul 133-791
(Received 10 September 2007) The carrier transport mechanism in inorganic/organic hybrid nanocomposites consisting of core/shell type CdSe/ZnS nanoparticles dispersed in a poly(N-vinylcarbazole) (PVK) layer was investigated by using current density-electric eld (J-E) measurements. The J-E curves showed that the current density increased with increasing content of CdSe nanoparticles up to 0.2 wt% and that the current density decreased above a 0.5 wt% content of nanoparticles due to the aggregation of the PVK layer. A proposed theoretical model for the carrier transport mechanism is in reasonable agreement with the J-E results.
PACS numbers: 73.40.Qv, 73.63.Bd Keywords: Carrier transport mechanism, CdSe nanoparticle, PVK organic layer, Electrical property mechanism of the current density - electric eld (J-E) behavior of hybrid organic/inorganic LEDs have been provided. This paper presents data for the J-E characteristics of hybrid devices, which contain the necessary information to understand the charge carrier transport process in hybrid composites. Using indium tin oxide (ITO) for hole injection and Al for electron injection, we found that the J-E characteristics could be determined by using both the bulk-conduction properties of poly N-vinylcarbazole (PVK) and its interaction with CdSe nanoparticles.
I. INTRODUCTION
Semiconductor nanocrystallites, which are smaller than the bulk exciton diameter, have received enormous attention due to their strong light-emitting intensities [1]. The nanocrystallites are di erent from the traditional chromophores because the wavelength of the emission can be exactly controlled by selecting the diameter of the appropriate dots without the additional synthetic procedures often required to change the emission spectrum of organic chromophores [2{5]. The integration of organic layers and nanoscale inorganic materials into hybrid optoelectronic structures enables active devices that combine the diversity of organic materials with the excellent electrical and optical properties of inorganic nanocrystals. Nanocomposites consisting of functional polymers and inorganic nanoparticles have emerged as excellent candidates for possible applications in hybrid light-emitting diodes (LEDs), photorefractive devices and photovoltaic devices [6{11]. However, very few works concerning a fundamental understanding of the charge carrier transport in the hybrid materials have been performed. The emission eciency of LEDs fabricated utilizing hybrid composites is relatively low due to limits on the charge carrier transport process. Studies on the e ect of nanoparticles doping on the charge transport in the hybrid composites play a very important role for optimizing the electrical properties of the devices. However, very few studies concerning the carrier transport
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II. EXPERIMENTAL DETAILS
The devices in this study consist of a single CdSe/PVK mixed layer sandwiched between two electrodes on top
Fig. 1. Schematic structure of a CdSe nanoparticle and of the device for the J-V measurements. -376-
Investigation of Carrier Transport Mechanism in { Fushan Li
Fig. 2. Concentration dependence of the J-V characteristics in an ITO/CdSe:PVK/Al device. The CdSe nanoparticle concentrations of sample 1 to 4 are 0.05, 0.1, 0.2 and 0.5 wt%, respectively.
of a glass substrate. Core/shell-type CdSe nanoparticles with a diameter of 6 nm were purchased commercially and their schematic structure is shown in the inset of Figure 1. ITO-coated glass, which acted as a hole injection layer in our devices, was pretreated according to a regular chemical cleaning procedure using trichloroethylene, acetone and methanol in sequence. The CdSe/PVK mixed layers with a thickness of 900 nm were formed by using a spin-coating technique, followed by a thermal evaporation of the Al layer as a cathode. III. RESULTS AND DISCUSSION
Figure 2 shows the J-E characteristics of the Al/CdSe:PVK/ITO devices with di erent CdSe nanoparticles concentrations. The slope of the logJ logE plot for the devices with relatively lower CdSe nanoparticle concentrations (0.05 wt% for sample 1 and 0.1 wt% for sample 2) shows that the current density J depends quadratically on the electric eld E. This behavior is characteristic for a space-charge-limited current (SCLC), which should follow Child's law as [12, 13]:
J = 9=8"0 "r E 2 =L ;
(1)
with "0 "r being the permittivity of the hybrid composite and L the thickness of the device. Considering the small volume concentration of the nanoparticles and the relatively large barrier o set (2.4 eV) between the electrode work function level and the lowest unoccupied molecular orbital of PVK, should be the hole mobility of the hybrid layer. If "r = 3 is assumed, the J-V characteristics of devices 1 and 2 with CdSe nanoparticles concentration of 0.1 and 0.05 wt%, respectively, can be well described by Eq. (1) using = 0.48 10 6 cm2 /Vs
et al.
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and = 3.8 10 6 cm2 /Vs, respectively. The observed mobility is not very di erent from the hole mobility of the CdSe/PVK composite measured by using the timeof- ight technique [14]. The hole mobility of the hybrid composite increases with increasing volume concentration of the CdSe nanoparticles, which is consistent with the result reported by Choudhury et al. [15]. NonMarkoan transport can be introduced here to explain such an increase in mobility. An important issue of this theory is that carriers injected back from a nanoparticle into the matrix have their velocity enhanced compared to the average velocity of the rest of the population. The acceleration of carriers by such encounters with nanoparticles can explain the observed increase in mobility. The absence of a sharp increase in the current (trap- lled limit) indicates that the hole transport in the hybrid composite can be regarded as being trap free. The importance of the observation of SCLC in our devices is that it clearly demonstrates that the hole current is bulk limited and dominated by the PVK molecules because of the relatively low concentration of CdSe nanoparticles. It is very interesting that a strong increase appears for the sample 3 with a higher CdSe nanoparticle concentration (0.2 wt%) under the forward bias voltage. The occurrence of an abrupt increase in the current at a certain critical electric eld is characteristic of an insulator with traps [16], which indicates that the nanoparticles are inevitably involved in the carrier transport process in this device. Increasing the bias results in a rapid increase in the injected charge, thereby lling a limited number of traps. Because the reduction in empty traps leads to a rapid increase in the e ective carrier mobility, a rapid increase of the power-law current (I / Vm ) appears in an very short electric eld range from 1.0 104 to 1.2 104 V/cm. However, the m value was found to be as large as 50, which is drastically larger than the values for commonly used organic materials [17]. Thus, the enhancement of average mobility induced by the traps e ect may be only partial. Nanoparticles can be expected to in uence the charge transport in thin lms [15]. Therefore, the nanoparticles in the device actually act as extended trapping regions that compete with native traps. This behavior is modeled by assuming that the carrier transport property of matrix polymer (PVK) was modi ed by the extra electric eld produced by trapped charges in the CdSe nanoparticles. We have proposed before that the holes captured by quantum dots generate an internal electric eld along a direction that is the same as that of the applied voltage [18], as shown in Figure 4. When a positive voltage is applied, the injection of holes from the ITO into the highest occupied molecular orbital occurs through a Fowler-Nordheim tunneling process and the holes are transported along the direction of the applied voltage through a hopping mechanism among the PVK molecules [19, 20]. It can be intuitively proposed that a hole actually encounters a CdSe nanoparticle on its way when traversing the sample and is, consequently, captured in the valence band of the CdSe nanoparticles,
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Journal of the Korean Physical Society, Vol. 53, No. 1, July 2008
Fig. 3. Atomic force microscopy images of hybrid thin lms with various CdSe nanoparticle concentrations: (a) sample 2, (b) sample 3 and (c) sample 4. All the pictures have sizes of 5 5 m2 . IV. SUMMARY AND CONCLUSION
J-E measurements can be used to estimate the hole mobility of a hybrid nanocomposite consisting of CdSe nanoparticles and a PVK matrix. The carrier transport property is improved by increasing CdSe nanoparticles content and a strong current increase appears at a certain CdSe nanoparticle concentration due to the charge captured in the CdSe nanoparticles. However, excess CdSe nanoparticles leads to aggregation of PVK and to a degradation of the carrier transport properties. A simple model is proposed to explain this character and the theoretical analysis is consistent with the experimental results. Fig. 4. Schematic diagram for the process of charge capturing in the CdSe nanoparticles embedded in a PVK layer.
as shown in Figure 4. The hybrid composite switched to a \high conductance state" due to the increased holetransport property of PVK under the internal electric eld. A sharp increase in the current can still be observed in the J-E dependence when the CdSe nanoparticle concentration is 0.5 wt%. However, above that value, a more gradual increase in the current appeared rapidly, as shown in Figure 2. On the other hand, we found an apparent decrease in the current density for this device when compared with device 3. Figure 3 shows atomic force microscopy images of the hybrid thin lm for various CdSe nanoparticles concentrations. Along with the increase in the CdSe nanoparticle concentration, an obvious aggregation of PVK molecules can be observed, which degrades the carrier transport property in the hybrid composite. Thus, we may conclude that a higher CdSe nanoparticle concentration does not always lead to a better charge carrier transport property. In this case, the carrier mobility of the hybrid composite reaches a maximum value when the CdSe nanoparticle concentration is about 0.2 wt%.
ACKNOWLEDGMENTS
This work was supported by the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea government (MOST) (No. R0A-2007-000-200440).
REFERENCES
[1] D. H. Oh, S. Lee, W.-J. Cho, J.-H. Kim and T. W. Kim, J. Korean Phys. Soc. 50, 1755 (2007). [2] R. Rossetti, R. Hull, J. M. Gibson and L. E. Brus, J. Chem. Phys. 82, 552 (1985). [3] L. E. Brus, J. Phys. Chem. 90, 2555 (1986). [4] M. G. Bawendi, P. J. Carroll, W. L. Wilson and L. E. Brus, J. Chem. Phys. 96, 946 (1992). [5] B.-C. Wang, H.-R. Liao, W.-H. Chen, Y.-M. Chou, J.T. Yeh and J.-C. Chang, J. Molecular Structure 716, 19 (2005). [6] B. O. Dabbousi, M. G. Bawendi, O. Onistuka and M. F. Rubner, Appl. Phys. Lee. 66, 1316 (1995). [7] S. Coe, W.-K. Woo, M. Bawendi and V. Bulovic, Nature 420, 800 (2002). [8] W. U. Huynh, J. J. Dittmer and A. P. Alivisatos, Science 295, 2425 (2002).
Investigation of Carrier Transport Mechanism in { Fushan Li [9] J. G. Winiarz, L. Zhang, J. Park and P. N. Prasad, J. Phys. Chem. B 106, 967 (2002). [10] H. Kim, W.-W. So and S.-J. Moon, J. Korean Phys. Soc. 48, 441 (2006). [11] C. Lee, S. Kim, H. Kim and W. Lee, J. Korean Phys. Soc. 50, 568 (2007). [12] M. A. Lampert, Phys. Rev. 103, 1648 (1956). [13] I. D. Parker, J. Appl. Phys. 75, 1656 (1994). [14] K. R. Choudhury, J. G. Winiarz, M. Samoc and P. N. Prasad, Appl. Phys. Lett. 82, 406 (2003). [15] K. R. Choudhury, M. Samoc, A. Patra and P. N. Prasad,
et al.
-379-
J. Phys. Chem. B 108, 1556 (2004). [16] M. A. Lampert and P. Mark, Current Injection in Solids (Academic, New York, 1970). [17] P. W. Blom, M. J. M. de Jong and J. J. M. Vleggaar, Appl. Phys. Lett. 68, 3308 (1996). [18] F. Li, D.-I. Son, H.-M. Cha, S.-M. Seo, B.-J. Kim, H.-J. Kim, J.-H. Jung and T. W. Kim, Appl. Phys. Lett. 90, 222109 (2007). [19] D. A. Seanor, Electrical Properties of Polymers (Academic Press, New York, 1982). [20] X. Y. Zhu, J. Phys. Chem. B 108, 8778 (2004).
