Effects Of Electron Correlation On Photoelectirc Characteirstics In Organic Polymers

Conjugated polymers have attracted considerable attention in the world because theirunique properties, such as they can conduct and glow, and have advantages of light, simpletechnology, flexibility, low cost, etc., make them suitable candidates for a wide range ofapplications in optoelectronics. There is different from inorganic materials, the organicpolymers have constructions of conjugated-electrons and are quasi-one-dimensional system.Which are not steady and generate Peierls phase transition, resulting in dimerize of the latticeconstructions and generate energy gap for the ground states. The carriers in polymers are nottraditional electrons and holes, but the nonlinear elementary excitations induced byself-trapping effects, such as polaron,(polaron-)exciton, bipolaron etc..With a deeper understanding of the physical properties about excited states in organicconjugated polymers, studies of the physical mechanisms of organic light-emitting diode andorganic solar cell etc. have become important and amusing. Due to recombination of oppositecharged polarons is a dominating process in organic light-emitting diode, so there are a lot ofworks reported in recent years, such as electric field, impurity, temperature, or interchaincoupling effects on dynamic processes of the recombination of opposite charged polarons.While the photoelectric conversion efficiency and photoelectric conversion mechanism arestill disputable, and need more studies and a deeper understanding. At1999, Yong Cao et al.have found that the electroluminescence efficiency can reach50%of the photoluminescenceefficiency, and even higher, which is much higher than theoretical calculation25%. There arelots of experimental and theoretical works to explain the results. They have found that yield ofexcitons is not only depended on the excitation energy and quantum size, but also dependedon the recombination and exchange processes between excitons and charge carriers. It isimportant to note, Zhigang Shuai’s research group has reported that the form of excitationstates is closely related to spin in organic polymers. On the other side, exciton dissociation isan important physical process in organic solar cell. It has been found that impurity,temperature, or interchain coupling etc. also can influence the electric field strength requiredto dissociate the excitons. Energy of the triplet excitons is lower about a few tenths ofelectron volt than that of the singlet excitons, so they have different dissociation velocities.One can adjust photocurrent by regulating the ratio of yields of the singlet/triplet excitons. In recent years, the dissociations of the singlet/triplet excitons have attracted considerableattention, some results have found that the electric field required to dissociate the tripletexcitons is much higher than the singlet excitons. Actually, the organic materials not onlyhave strong electron-phonon interactions, but also important electron-electron interactions.However, that in previous works, only the electron-lattice interactions have been consideredand the effects of electron correlation have been ignored.In our works, we have chosen Su-Schrieffer-Heeger+Pariser-Parr-Pople (SSH+PPP)model and the multi-configurational time dependent Hartree-Fock (MCTDHF) method, whichpartly takes into account electron correlations and allows us to investigate the properties ofsinglet and triplet excited states, respectively, and to discover the difference between them.Main works as follows: Using this method, we have respectively investigated dynamics ofsinglet/triplet exciton transfer and singlet/triplet opposite charged polarons recombination incoupled polymer chains. Which can provide more theoretical basis for newly understandingthe photoelectric conversion processes. In order to increase the light-emitting efficiency oforganic light-emitting diode, probability of conversing from triplet to singlet exciton shouldbe increased. Because of the conversing probability is closely depended on energy differencebetween singlet and triplet exciton, we have investigated effects of some factors on the energydifference between singlet and triplet exciton, such as impurities, electric field, molecularstructure, chain length etc.. The results can provide new idea and direction for improving theoptoelectronic efficiency. In addition, temperature effects on the dynamics of photoexcitationsin conjugated polymers are investigated based on our works during my master’s stage. In thework, a tight-binding SSH model modified to include temperature effects and a semi-classicalnonadiabatic evolution method are used. Detailed contents are as follows:1\Dynamics of exciton transfer in coupled polymer chains: There are interchain interactionsbetween polymer chains, and interchain dynamics of carriers are important to understand thephotoelectric conversion processes. In fact, along with the forming of excitons, the transferexcitons also are formed. Which are important intermediates in the photoelectric conversionprocesses. Considering the excitons with spin singlet and triplet are not degenerate when theelectron interactions are taken into account, the interchain coupling strength required togenerate the charge transfer excitons with spin singlet and triplet are different between each other. The results as follows:(1) In order to explain the processes involved, the effects ofon-site and long-range e-e interactions on the locality of the singlet and triplet excitons are frst investigated on an isolated chain. It is found that the locality of the singlet excitondecreases, while the locality of the triplet exciton increases with an increase in the on-site e-einteractions. On the other hand, an increase in the long-range e-e interaction results in a morelocalized singlet exciton and triplet exciton;(2) In coupled polymer chains, we thenquantitatively show the yields of singlet and triplet exciton transfer products under the sameinterchain coupling. It is found that the yield of singlet interchain excitons is much higherthan that of triplet interchain excitons, that is to say, singlet exciton transfer is signifcantlyeasier than that for triplet excitons. This results from the fact that the singlet exciton is moredelocalized than the triplet exciton. In addition, hopping of electrons with opposite spinsbetween the coupled chains can facilitate the transfer of singlet excitons.2\Dynamics of opposite charged polarons recombination in coupled polymer chains:Recombination of opposite charged polarons is a dominating process in organic light-emittingdiode, we have investigated the dynamic properties of singlet and triplet excited states incoupled polymer chains respectively, and to discover the difference between them, byconsidering both electron correlations and electron-phonon interactions. The results as follows:(1) Firstly, under a fixed interchain coupling, the effects of on-site and long-rangeelectron-electron (e-e) interactions on the dynamics of the charged polarons recombination inthe singlet and triplet channels are investigated respectively. By calculating yields of thesinglet and triplet intrachain excitons, it is found that the yields of the singlet and tripletintrachain excitons decrease with increasing of the on-site e-e interactions. On the other hand,an increase in the long-range e-e interaction results in an increase of the yield of the singletintrachain exciton, while decrease of the yield of the triplet intrachain exciton;(2) We thentake polyacetylene as an example, the yields of the singlet and triplet intrachain excitons asa function of the interchain coupling are studied, which find that the formation of singletexcitons is easier than triplet excitons within the interchain coupling range of0-0.1eV;(3) Inour work, influence of polymer chain length on the yield of the singlet and triplet intrachainexciton also has been discussed. It is found that an increase of the chain length induces anincrease of the yield of the intrachain excitons both for singlet and triplet states. 3\Effects of some factors on the energy gap between singlet and triplet exciton in polymers:In recent years, heat assisted transforming from triplet exciton to singlet exciton is aneffective approach. Considering the probability of conversing from triplet to singlet excitondepends on the energy difference between singlet and triplet exciton, some factors effects onthe energy difference between singlet and triplet exciton in polymers are investigated.(1)Firstly, effects of impurities on the energy difference between singlet and triplet exciton inpolymers are investigated. It is found that with increasing of the impurity potential andimpurity number, the energy difference between singlet and triplet exciton decreases.However, the decrease of the energy difference effected by the impurity or chain-length isvery small, and it is difficult to increase the probability of conversing from triplet to singletexciton through the methods discussed above;(2) Electric field effects on the energydifference between singlet and triplet exciton are then investigated. One can find that anincrease of the electric field strength induces an obvious decrease of the energy difference;(3)Finally, a Donor-Accepter structure is designed, and this structure can availably decrease theenergy difference between singlet and triplet exciton.4\Temperature effects on the dynamics of photoexcitations in conjugated polymers: Theproperties of conjugated polymers have been widely investigated, due to their attractiveperformance in optoelectronic devices. To deeply understand the photoelectric conversionmechanisms, theoretical studies of photoexcitation as one of the fundamental physicalprocesses are necessary. The results as follows:(1) For the photoexcited state from thehighest occupied energy level (HOMO) to the lowest unoccupied energy level (LUMO), it isfound that with increasing temperature, the stability of the polaron-excitons becomes weak.Furthermore, by showing time evolution of the localized energy levels at differenttemperatures, a blue-shift in the photoluminescence spectrum is observed. Yields of thepolaron-excitons are calculated over the temperature range5300K. It is found that the yieldof polaron-excitons decreases with increasing temperature, and involves two differentphysical processes at lower and higher temperatures;(2) For the photoexcited state fromHOMO1to LUMO+1, we focus on the charged polaron pair lifetimes and the yield ofpolaron-excitons. The results show that the temperature effects shorten the lifetime of thecharged polaron pair and cause the charged polaron pairs to easily merge into polaron-excitons, while the stability of the polaron-excitons formed from merging of thecharge polaron pairs is influenced by temperature. As a result, the yield of thepolaron-excitons first increases and then decreases in the processes.