INFLUENCE OF OPTICAL VIBRATIONS ON ENERGY ACTIVATION OF THE CHARGE CARRIER TRAPS AND THE THERMOLUMINESCENCE OF SILICON ORGANIC POLYMER

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INFLUENCE OF OPTICAL VIBRATIONS ON ENERGY ACTIVATION OF THE CHARGE CARRIER TRAPS AND THE THERMOLUMINESCENCE OF SILICON ORGANIC POLYMER

Post by Admin on Wed Mar 14, 2018 12:27 pm

V. Sugakov1,
N. Ostapenko2,
Yu.Ostapenko2,
O. Kerita2,
V. Strelchuk3,
O. Kolomys3,
A. Watanabe4
Б. Мінаєв, д.х.н., професор5,
В. Мінаєва, к.х.н., доцент5
1Institute for Nuclear Research, National Academy of Science of Ukraine, Kyiv, Ukraine
2Institute of Physics of NASU, Kiev, Ukraine
3Lashkaryov Institute of Semiconductor Physics of NASU, Kiev, Ukraine
4Institute of Chemical Reactions, Tohoku University, Sendai, Japan
5B. Khmelnitsky National University, Cherkasy, Ukraine


INFLUENCE OF OPTICAL VIBRATIONS ON ENERGY ACTIVATION OF THE CHARGE CARRIER TRAPS AND THE THERMOLUMINESCENCE OF SILICON ORGANIC POLYMER

Poly(di-n-hexylsilane) (PDHS) belongs to silicon organic polymer which consists of -conjugated Si-backbone and hexyl side groups (Fig.1).

Fig. 1. The optimized structure of the simplest short poly(di-n-hexylsilane) oligomer.
A simple structure of the short oligomer, optimized by PM3 method is shown in Fig. 1. Here one H atom at one Si center has to be substituted by hexyl group; the other one simulates the long
Si backbone of the polymer chaine. Due to -bonds delocalization in the Si-backbone [1, 2] PDHS has several unique photophysical characteristics, namely: strong absorption in the UV range, high quantum fluorescence yield and high hole mobility. These characteristics determine the possibility of its usage as transport [3] and light-emitting layers [4] in electroluminescent devices like organic light-emitting diodes (OLED). The PDHS macromolecule, which consists from the segments of different length, has mainly an ordered trans-conformation at room and lower temperatures. The rotation of the polymer chain segment around the Si-Si bonds leads to a formation of the conformational defects (Fig. 2). These defects create energy barriers for the holes, so that at low temperatures the hole is delocalized within a segment [1]. Thus, the segments of different length are the traps for holes. The model of quasi-continuous distribution of traps in polymers is widely accepted nowadays [4]. A number of photo-physical properties of polysilanes are related to delocalization of electronic excitations on the segments of a polymeric chain [2], which consist of silicon atoms and organic molecules as side groups. As a result, the strong absorption in the UV range, strong dependence of the electron transition energy on the conformation of a polymeric chain, the phenomenon of thermochromism and high mobility of charge carriers in these polymers are observed and described. The high mobility of charge carriers determines the use of polysilanes as transport [3] and luminescence [4] layers in OLED and other organic electroluminescent devices.
The study of the fractional low temperature thermostimulated luminescence provides important information about the presence and the nature of traps and defects for charge carriers. One of the processes determining the properties of TSL is the carriers escape from the traps. In order to leave a trap and transit to a state with the energy above the mobility threshold a carrier has to acquire additional energy.
The connection between the activation energies of charge carriers traps and the energies of optical vibrations of the polymer backbone observed in the Raman or IR spectra can be very informative.

Fig. 2. The optimized structure of the poly(di-4-hexylsilane) oligomer with few traps (this is not the all-trans type).
This has been recently received in [6] using the TSL temperature dependence of the PDHS films and Raman spectrum at 300 K. The corresponding model [7, 8] has been proposed for the explanation of these results. It was assumed that the processes of the hole release from traps are activated via resonant energy transfer to carriers from Si-Si vibrations of the polymer chain that are excited as the temperature increases [7]. The model also predicts the appearance of the additional structure on the TSL curve. Therefore, we focus herein on vibration analysis by quantum chemical calculation of finite models of certain PDHS oligomers. We also want to support the detailed correlation between the activation energies of hole traps and the frequencies of Si-Si Raman modes of the polymer chain in the low temperature region of TSL spectra shown before [7,9] as well as the found dependence of the structure in TSL curve. To provide this correlation the TSL spectrum of PDHS in the 5–40 K and Raman spectrum at 300 K on the same polymer were investigated [9]. It was found that TSL curve has fine structure and energy spectrum of traps consists of six horizontal shelves which coincide with the frequencies of the Si-Si vibrations in the polymer chain, being active in the Raman spectrum. The experimental results [7-9] provide a proper confirmation of the proposed model.
Method of calculations. A number of PDHS oligomer models have been calculated by self-consistent field (SCF) method with the semiempirical PM3 approximation [10] which provides quite reasonable heat of formation, geometry structure of molecules upon optimization of all 3N-6 parameters of the Hessian matrix, force field and vibrational frequencies. A simple PDHS model for oligomer H3Si-[Si(CH3)2]6-SiH3 is presented in Fig. 3. A big number of similar models with n=4-16 have been optimized and their vibrational spectra are analized. Termination of the polymer is necessary (n is final, limited and finished) and methyl groups are a good and simple model for the bulky hexyl groups.

Fig. 3. Simple PDHS model for oligomer H3Si-[Si(CH3)2]6-SiH3
For such system as an example we obtained 179 vibrational modes which are presented by few bunchs. The high-frequency bunch includes 30 quasi-degenerate C-H stretching modes in the range 2986-2950 cm-1 with rather weak IR absorption intensity. In the region 1900 cm-1 there is a small group of the terminal SiH3 vibrations. At 1320-1280 cm-1 we see a group of deformation C-H vibrations. The important finger-print region from 940 till 340 cm-1 includes intense IR bands of Si-C stretchings and CH3 twists, rocking; its low-frequency part contains a number of Si-Si assimmetric stretchs with very low IR intensity in the range of 338-400 cm-1, which are active in Raman spectra and observed in Ref. [9].
Determination of the activation energy of charge carrier traps
The fractional TSL measurements were carried out with automatic equipment over a temperature range 5–40 K with the heating rate of 0.25 K/s. The PDHS films were prepared by direct casting from toluene solution on sapphire substrates. The carriers in the PDHS were photogenerated by the sample excitation by unfiltered light of Hg lamp for 2 min at 5 K. As seen from Fig. 1 of Ref. [9], the additional structure is clearly observed on the TSL curve. The structure consists of spikes and dips and cannot be considered as a noise because it is repeated in the rerecording.
Fig. 3 in Ref. [9] indicates dependence of the activation energy of the traps on the fraction. The numbers on the horizontal shelves are shown the activation energies in eV. The frequencies of Raman spectrum in cm-1 are given in brackets. It is seen that the activation energy traps form six horizontal shelves. Comparison of the data obtained in the study of the TSL and Raman spectra (see Fig. 2 in Ref. [9]) showed that the activation energy of the traps correlates well with the frequency of Si-Si vibrations of the polymer chain.
This result allows to conclude, that although the energy spectrum of the traps is the quasi-continuous, the release of the holes in the polymer in proposed model is more probable from those traps, the depth of which corresponds to energy of Si-Si vibration of the polymer chain. Also we detected additional structural features on the TSL curve (Fig. 1 [9]), which was predicted in presented model in [7]. The suggested in [7] model cannot explain the coincidence of the activation energy with the energy of the vibration. Therefore we use the model suggested in [7], which explained this coincidence and predicted the appearance of the structure.

The foundation of the model [7] and discussion
The system of the charge carriers captured by traps after irradiation is non-equilibrium. The equilibration in the vibration subsystem happens much faster than the equilibration in the electronic subsystem (in the charge distribution). So, we may suggest that vibration subsystem is in equilibrium state, but the electronic subsystem is nonequilibrium. At such condition there is a process in which energy of the vibration quanta may be transferred to trapped carrier and pulls out it to the state above the mobility threshold. It occurs if localization center has trapped energy coinciding or less than the energy of vibration quantum. Subsequently, the carrier recombines with a charge of opposite sign and manifestoes itself in TSL.
The time dynamics of charge carrier density may be presented in the form of the Arrhenius formula which connects the density of localized charges and the energy of the trapped carrier with respect to the mobility threshold [8, 9]. This includes the term which describes the process of localized charge carrier release with the absorption of the vibration quantum and the term which describes contribution from the processes involving all other vibrations, including multi -phonon processes, into the processes of charge carrier delocalization. The obtained the temperature dependence of the radiation intensity [7] shows that in the PDHS disordered system the activation energies coincide with the energies of vibration Quanta. The second conclusion says that the vibronic fine structure arises in the TSL curve in the form of spikes and dips; therefore, in spite the fact that the spectrum of traps in polymer is quasy-continuous, only several of traps provide the studied effect on TSL, which activation energy is in resonance with the corresponding vibration quanta.
References
1. Pope M., Swenberg C. E., Electronic Processes in Organic Crystals and Polymers (Oxford University Press, N.Y., 1999), p. 877.
2. R.D. Miller and J. Michl, Chem. Rev. 89, 1359 (1989).
3. Suzuki H., Meyer H., Hoshino S., Haarer D., Appl J. Phys. 78, 2684 (1995).
4. Sharma A., Katiyar M., Deepak, Seki S. and Tagawa S., Appl. Phys. Lett. 8, 143511 (2006).
5. Ba¨ssler H. in Semiconducting Polymers: Chemistry, Physics and Engineering, edited by G. Hadziioannou and P. F. van Hutten. WileyVCH, Weinheim, 2000.
6. Gumenyuk A., Ostapenko N., Ostapenko Yu., Kerita O., Suto S. Chem. Phys. 394, 36 (2012).
7. Sugakov V. I., Ostapenko N. I., Chem. Phys. 456, 22 (2015).
8. Ostapenko N., Ostapenko Yu., Kerita O., Ukr. J. Phys. 2014. Vol. 59, No. 3.
9. Sugakov V., Ostapenko N., Ostapenko Yu., Kerita O., Strelchuk V., Kolomys O, Watanabe A. Contemporary material science (Poltava, 2017), p. 12.
10. Powel K. PM3 method.

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