Advances in physics The influence of electric fields on polaron excitons in macromolecules FU Lu-xin SUN Xin (State Key Laboratory of Infrared Physics, Shanghai Institute of Technology Physics, Chinese Academy of Sciences, where the thin line represents the ground state, and the dashed line represents an extra electron-hole, coarse The line represents two additional electron-holes, indicating that the additional electron-holes cause localized distortions in the lattice structure of the polymer, and that the extra electron-holes that cause this distortion of the crystal field fall into this field again. In the lattice distortion potential field of the domain, it becomes an electron-hole self-trapping bound state, which is very similar to solid state physics, in which electrons attract positive ions to make them move inward, and at the same time repel the negative ions to move outward, resulting in ion shifts. When polarization occurs, the potential energy of the electrons is reduced, and a potential trap is bound to the electrons, that is, the electrons cause lattice distortion and generate a local distortion potential field, and the electrons are bound by the local distortion potential field caused by themselves and form electrons. Self-trapped bound state polaron.
The difference is that not only electrons, but also holes, are formed here, which results in the trapped and bound states of electrons and holes. Since the electron-hole bound state is essentially an exciton, such an exciton is also called an trapped bound exciton or a polaron exciton. It should be pointed out here that the general inorganic material is Coulomb induced excitons, but for low-dimensional systems such as polymers and c, lattice distortion can also cause excitons. Even if there is no Coulomb effect, excitons can be formed due to the trapping effect caused by extra electrons and holes. Such excitons are called polaron exciton or trapped bound exciton. Of course, the Coulomb effect is also considered. The child will be more tightly bound. There is one electron-hole in the exciton that is a single exciton, and there are two electrons and two holes are double exciton.
The extra electrons and holes not only cause lattice distortion but also cause changes in the electronic state: the highest occupied molecular orbital (ie, the original valence band top level) and the lowest unoccupied molecular orbital (ie, the original guided bottom level) (without additional Electrons and holes have two electrons at the top of the original valance band, and there are no electrons at the bottom of the original conduction band. It separates from the original quasi-continuous energy level and enters the vicinity of the forbidden band, forming two energies near the middle of the band gap. Closely localized electronic state ehw and free. For polaron single excitons, there is one electron in the upper local state Shigh and the lower local state (see (b)); for the polaron double exciton, there are two electrons in the upper localized state eMgh, the lower domain There are no electrons on state '. (See additional electrons and holes to make the electronic state change. The new valence band is full, and the new conduction band is empty.
3 Polarization characteristics of polaron exciton in polymer under the action of electric field After the electric field is applied to the electric field, it is found that the weak or moderately strong electric field partially shifts the lattice charge in the polaron exciton.
Shows the lattice charge density distribution of the polaron and excitons of the polymer in the macromolecule under the electric field. The charge density distribution corresponding to the ground state under the same field strength has been subtracted from the figure. The electric field in the graph is plotted along the abscissa. The unit of the ordinate is the electronic charge value, the thin solid line represents U==0; the thick solid line indicates U=4eV, 7=0.6; the thin broken line is “=4,7=0; the thick dotted line is=2,7= 0.6.Indicates that the positive charge is transferred along the direction of the electric field, and the negative charge is transferred in the direction of the electric field, ie, the polaron is excited by the normal polarization, and the positive charge is shown to be transferred in the direction of the electric field, and the negative charge is transferred in the direction of the electric field, that is, the middle of the polymer. The bipolar exciton shows reversed polarization, and the reversed polarization is a new physical phenomenon.
The physical cause is to analyze the wave function of the occupied electronic state. Note that for single and double exciton polarons, the wave functions of their electronic energy levels are very similar, and their wave functions change in the same way as the electric field. Further note that in (b) and in the polymer shown, the same is true for the polaron single and double exciton; the valence band is filled with electrons and the conduction band is empty; and the difference between them is in the vicinity of the middle of the forbidden band. There are different electron filling conditions in the localized electronic states. For this purpose, the changes of wave functions corresponding to the two localized energy levels near the central region of the band gap under external electric fields are emphatically studied.
4 The polarization characteristics of the two localized electronic states near the forbidden band center are the wave functions corresponding to the two localized energy levels near the forbidden band center calculated by the method described above.
Where (1) represents 4=, 5=, (2) represents 4= 0, E=105V/cm, and the electric field is along the chain direction 15V/cm. The direction of the electric field along the chain has been pointed out before. A localized energy level can accommodate two electrons. , From (a) Visible to the electric field in the downward localized state eiw after the application of the electric field, this is the forward polarization; from (b) to the forbidden band, the two localized energy levels corresponding to the localized energy level are determined The electrons in the domain Wgh move in the direction of the electric field. This is the reverse polarization. The polarization characteristics of the two localized electronic states are explained as follows: The quantum theory of polarization shows that the polarizability of the wave function level corresponding to the first domain energy level is x because eiw and hgh are determined near the central region of the band gap. In the domain level, they are close to each other in energy and far away from other energy levels. Therefore, from the energy distribution, the high energy level has the largest contribution to Xw; while the hw energy level has the largest contribution to Xigh, and the other energy levels can contribute slightly. In addition, the wave function corresponding to the hw level is a singular parity (see (a)), and the wave function corresponding to the eh level is even parity (see (b)). Since the two wave functions are perfectly matched, the dipole transition matrix element is large; the wave functions of the valence band and the conduction band do not match each other, and the dipole transition matrix element is small. Therefore, from the perspective of dipole transition matrix elements, it is also the hw and hgh energy levels that contribute the most, then Xw and Xigh can be approximated by a value of 0. That is, the electrons on the lower energy localized energy level hw will move in the direction of the inverse electric field and are positive. Polarization; and Xgi> 0, that is, the electrons on the higher energy localized energy level hgh will move in the direction of the electric field and become the reverse polarization.
5 The origin of polariton exciton in polymer and the dissociation of polaron exciton under strong electric field to mono-exciton state, due to the upper and lower local states each have one electron and upper localized state. The reverse polarization is canceled by the forward polarization of the lower-order domain state. Therefore, the positive polarization of the single exciton state originates from the filled valence band, indicating that the filled continuous valence band shows a weaker positive polarization. The ordinate of the ordinate is one order of magnitude smaller. r The double exciton state d has no electrons in the ttpi domain state, and the upper localized state has two electrons, so the polarization characteristic of the double exciton state should be the upper localized state. The difference between the strong reverse polarization and the weaker forward polarization of the filled continuous valence band results in a pronounced reverse polarization of the dipole state. Also, because the amount of charge transfer at the upper localized energy level eh is significantly larger with the interaction strength U between the different spin electrons and the interaction V between the neighboring grid points, U and V only slightly affect the filling. The total amount of charge transfer in the continuous valence band, which causes the interaction between the electrons to slightly change the positive polarization of the polaron exciton (see (a)); but it is obvious that the double exciton state of the strong polaron Polarization is shown in (b).
A strong electric field will dissociate the polaron excitons. It was found that there is a critical electric field Ec, and there will be a mutation in both the atomic configuration and the charge density distribution at the critical electric field. For five> helium atomic configuration and charge density distribution.
(a) It can be seen that the polaron exciton dissociates into two polarons in the fifth and the second polar ion; (b) shows that the strong electric field dissociates the polaron exciton into positive and negative polarons. From the perspective of nonlinear excited states, it can be understood that the energy for generating a polaron exciton is smaller than the energy for generating an anodizing and a negative polarizer. The energy required for the polaron to become a positive polarizer and the negative polarizer is External electric field supply, external electric field is not enough to provide dissociation energy for hours, so only polarization, as shown; only a sufficiently large electric field (ie, critical electric field) can provide 4E>Ec atomic configuration and (b) charge supply sufficient to make The polaron exciton dissociates into positive and negative polarizers.
After the dissociation, the positive and negative polarizers move in opposite directions under the action of an external electric field, and there is no chance of recombination, resulting in quenching of luminescence. These are the experimental results of the direct dissociation of macromolecule excitation elements by positive electric fields into positive and negative polarizers, and the results of experiments in which the electroluminescence of polymers is quenched by strong electric fields (11) and (10) (their denominators) It may be positive or negative that it has been explicitly stated that the static state of the quantum state can be positive or negative, indicating that the quantum state can have positive or negative polarization. In 1977, it was reported that the formulas (1) and (10) obtained from the np state of the highly excited sodium atom have shown that some microscopic states always have opposite polarizations. The wave function corresponding to the eMgh of the localized energy level is the reverse polarization. The reverse polarization of the polaron double exciton in the polymer is due to the reverse polarization of the microscopic ehlgh.
The reverse polarization reported so far has appeared in the excited state. The experimental fact so far is that the quasi-polarity of the ground state is always called the simultaneous quantum theory of polarization. The formula has clearly indicated that the local polarization of the electronic state can be negative. From the energy point of view, generally the electron state energy with reverse polarization is higher, and the electron state energy with positive polarization is lower, so when all the occupied states are summed, for example, the occupied state is “quasi-continuous†(here "Quasi-continuous" means that there is no empty state in the highest occupied state) and no reverse polarization is exhibited. From the perspective of energy, only the highest occupied state still has an empty state, ie, an excited state, under certain conditions (this condition is that the reverse polarization of the occupied state above the empty state is greater than the following The necessary condition for reverse polarization is exhibited, but it is not yet a sufficient condition. It has been found that the basic photoexcitation in a macromolecule is an intrachain exciton, and this forms an extraneous electron-hole in the polymer chain forming a polaron exciton. The computer simulation results are consistent. In macromolecules, reexcitation of intrachain excitons can form double excitations of 29, which is consistent with computer simulations of two additional electron-holes forming double exciton in the polymer chain. Computer simulations show that the double exciton in the polymer is reversely polarized, so it can be said that the double excitons in the polymer are physically achievable examples with negative static polarizability. This example shows that in the paramagnetic and antimagnetic magnetic There is indeed some similarity between the nature of the electrical properties of forward and reverse polarizations, and the existence of such similarities between electrical and magnetic properties will have potential application prospects.
Systems with inverse quiescent polarizabilities show novel electronic characteristics. For example, if the polariton single exciton in a macromolecule is normal polarization and the polaron double exciton is in the reverse polarization, absorption of one photon can change the exciton state in the polymer from a single exciton. Double exciton, so that the polarization is reversed. A new physical phenomenon, photoinduced polarization reversal, can thus be obtained. Dynamics study. The computer simulation method has been used to show that there can be reversed polarization. Burroughes 2 Fu Ruoli, Ye Hongjuan, Li Lei, Fu Rongtang, Yu Jian, Sun Xin, Zhang Zhilin. Acta Physica Sinica, 1998, Li Lei, Rao Xuesong, Sun Xin, Fu Rouli, Yan Junhao. Acta Physica Sinica, 1998, 47(9): Zhao Erhai, Fu Rongtang, Sun Xin, Fu Rouli, Yan Junhao. Chinese Journal of Physics, 1998, Fu Rouli, Yan Junhao, Li Lei, Sun Xin. Journal of Luminescence, 1998, Sun Xin, Wu Changqin, Liu Jingnan, Fu. Advances in Physics, 1990, 10 Fu Rouli, Ye Hongjuan, Fu Rongtang, et al. Journal of Infrared and Millimeter Waves, 1993
