Study On The Charge Separation Mechanisms Induced By Molecular Configuration And Thermal Effect In Organic Solar Cells

The transitional dependence on fossil fuels makes us face severe energy crisis and environmental problems.To deal with these problems,it is imperative to develop and utilize clean renewable energy.As an inexhaustible clean energy,solar energy is not only widely distributed in the region,but also has the advantages of no mining and transportation.These advantages give it great application value.In addition to the direct use of solar radiation heat,we can also use solar cells to convert light energy into electrical energy.According to the materials used,solar cells can be divided into inorganic solar cells and organic solar cells.Silicon solar cells are the typical representatives of inorganic solar cells.Their high power conversion efficiency(PCE)and good stability enable them to be commercialized,but their large-scale promotion is limited by the high manufacturing cost and complex preparation process.Organic solar cells have attracted continuous attention due to their advantages such as light weight,low cost,flexibility,translucency and large area preparation using wet film forming method.However,relatively low PCE and poor stability have been restricting their commercial application.Over the past few decades,thanks to the progresses in material synthesis,structural optimization,morphology control ternary strategy and so on,photovoltaic efficiency of single junction organic solar cells has already broke through the 17%,since efficiency of the large-area photovoltaic devices is still on the low side(~ 10%),further enhance efficiency of organic photovoltaic solar cells is still a key project in the field of organic photovoltaics.The low photovoltaic efficiency of organic solar cells is rooted in the strong electron-lattice interaction and low dielectric constant of organic materials,which determines that organic materials cannot directly generate free charge after absorbing photons,but the bound "electron-hole" pairs,called excitons.In organic systems,photocurrent is obtained by separating the positive and negative charges in excitons.Since excitons in the organic system usually have a larger binding energy(～0.5eV),it is obviously that they can not be separated by the thermal energy at room temperature(～0.026eV).Therefore,photovoltaic layer of the organic photovoltaic device often employ the donor/acceptor(D/A)material blended bulk heterojunction structure.As such the photogenerated excitons can be separated into charge transfer(CT)state by the driving force induced by the different electron affinity of these two materials after they reached the D/A interface.As such the CT state can be further separated into free charges.After that the free charges are transported to the electrodes until they are eventually collected by the electrodes.Charge separation is a key process in the above mentioned organic photovoltaics,but there is still a lack of comprehensive understanding on the physical mechanism behind charge separation,which limits our targeted design and regulation of the photovoltaic devices.Based on this,in this thesis,we adopted the extended SSH model and combined with specific experimental phenomena to study the charge separation mechanism in organic solar cells.The specific research contents and results are as follows:1.Intramolecular exciton delocalization induced by the intramolecular disorder of polymerThe phenomenon of ultrafast charge separation observed experimentally(<1ps)is a challenging problem in organic photovoltaics.At present,there are mainly two understandings on this phenomenon.One is based on delocalization of the excited states,which is typically represented by the work of the Kaake’s group.Kakke et al.found first in the organic heterojunctions and then in the bilayer membranes,that most of the free charges were generated on a sub ps timescale.According to the exciton theory,the time scale required for exciton transport to the interface should be in the order of lps,which obviously cannot explain charge separation phenomenon in the ultrafast time scale.Therefore,they attributed the reason of such ultrafast charge separation phenomenon to the delocalization of the initial excited state.It should be noted that the excited state delocalization proposed here is mainly an intermolecular delocalization behavior.Considering that the intramolecular exciton delocalization may also contribute to ultrafast charge separation,we further investigated the influence of intramolecular disorder on intramolecular exciton delocalization.In the model,the intrinsic disorder effects in polymer molecules,such as the bending and torsion of carbon skeleton,were attributed to the transition integral disorder,and the vibration of hydrogen atoms and random rotation of side chains were attributed to the on-site energy disorder.By calculation,we found that the intramolecular disorder can effectively induce the delocalization of excitons and reduce the binding energy of excitons.In particular,when the intramolecular disorder reaches a certain intensity,the exciton can be dissociated into free charge under the driving of the internal electric field,indicating that when the intramolecular disorder is strong,the intramolecular disorder induced intramolecular exciton delocalization may also contribute to ultrafast charge separation.In addition,we also calculated the impact on the exciton creation energy caused by these two kinds of disorders,and found that in both cases the exciton creation energy are reduced as the disorder strength increases.Considering that the closer to the D/A interface,the more disordered of the polymer chains,we can deduce that differences of the intramolecular disorder can induce a force to drive the exciton transport to the interface,which would promote the exciton dissociation.2.Charge separation dynamics induced by the intermolecular configurationAnother understanding of the ultrafast charge separation phenomenon is closely related to morphology of the photovoltaic layer,represented by the work of Banerji’s research group.In an efficient organic photovoltaic system,the photovoltaic layer generally consists of three phases,namely pure polymer phase,pure small molecule phase,and cross-mixed polymer/small molecule phase at the molecular level.Banerji’s group experimentally observed that timescale of the charge separation is closely related to photovoltaic layer morphology.Exciton generated in the pure polymer or small molecule phase usually has a charge separation timescale of ps,while exciton generated in the mixed polymer/small molecule phase can be separated in an ultrafast timescale(<100fs),and the ultrafast charge separation is likely to be through ultrafast charge transport along polymer chains,but for the details of this ultrafast charge separation is still not clear.To address this problem,we consider that in the mixed polymer/small molecule phase,polymer chains are intercalated by small molecules,and the interchain distance between polymer chains is larger than that in the pure polymer phase.Thus,in the transition region from "mixed polymer/small molecule phase" to the "pure polymer phase",the interchain distance between polymer chains tends to decrease gradually.However,the non-uniform packing of polymers in the transition region may affect the dynamical evolution behavior of the CT state and the excitons.Based on this,we then simulated the influence of the non-uniform polymer packing on the dynamical evolution behavior of the cold CT state and the excitons by using the non-adiabatic dynamical method.2.1 Charge separation dynamics form cold CT state induced by the non-uniform intermolecular packing of polymersWe first simulated the effect of the non-uniform packing configuration on the dynamic evolution of the cold CT state.For simplicity,we used a linear configuration of between polymers to describe the non-uniform packing of polymers,and found that the non-uniform packing of polymers can induce the force to drive the ultrafast charge separation of the cold CT state.The whole charge separation process of the cold CT state includes two stages:(1)delocalization of the positive charges between polymer chains;(2)Ultrafast transport of the positive charges along polymer chains.On this basis,we further considered the influence of small molecule aggregation on the charge separation dynamics of the cold CT state,and found that small molecule aggregation can reduce the binding energy of the cold CT state and thus to improve its charge separation efficiency.2.2 Complete exciton-to-charge dynamics induced by the non-uniform intermolecular packing of polymersConsidering that the actual charge separation may come from excitons initially excited in the pure phase.We then further investigate the effect of the non-uniform polymer packing on the exciton evolution dynamic.By adjusting band gap of the system,it is found that the dynamic evolution behavior of excitons is determined by band offset of the system.As the band offset is comparative to the exciton binding energy,the exciton can be separated to the interfacial CT state through charge transfer As band offset of the system is larger than the critical band offset of charge separation,the CT state formed by exciton dissociation can be further separated into free charges,thus to realize the complete exciton-to-charge transformation.In addition,we found that the complete exciton-to-charge dynamics includes charge transfer driven by the band offset,charge delocalization between polymers,and the ultrafast charge transport process along polymers driven by the non-uniform packing of polymers,in particular,transition of the charge state accompany to these processes,which would result in the energy loss.In addition,by simulating the complete transformation of the exciton-to-charge dynamics under different band offsets,we found that energy loss related to this process is approximately linear with the band offset of the system.The larger the band order is,the larger the energy loss will be.Similarly,we also studied the regulation of exciton dynamics by small molecule aggregation,and found that small molecule aggregation can reduce the binding energy of excitons and the critical band offset of charge separation,and thus reduce the energy loss caused in the complete process of the exciton-to-charge conversion.3.Delocalization of intramolecular excitons and interfacial CT states induced by the thermal effectActual photovoltaic devices always work at a certain temperature.In addition,the heat dissipated by the internal conversion through the high-energy excited state and the hot CT state will also increase temperature of the system.Therefore,the thermal effect may affect the organic photovoltaic process by affecting the excited state,interface CT state and charged state in the organic system.After considering the temperature,the state of the system is determined by the free energy of the system,which includes the contribution of electronic entropy and the vibrational entropy.Limited by the classic processing to the lattice in this model,we preliminary considers the contribution of the electronic entropy to free energy of the system through Fermi-Dirac distribution of the electron.And we then study influence of the electronic entropy on the intramolecular exciton and the interfacial CT state.We found that the electronic entropy can induce delocalization of the interfacial CT state and the intramolecular exciton generated in system with weak electron-lattice interaction,while it has nearly no impact on the intramolecular exciton generated in system with strong electron-lattice interaction due to the deeper local energy level of exciton.