Theoretical Estimation On Rate Of Charge Separation And Recombination In Organic Solar Cell System

For many organic sollar cell systems with high photoelectric conversion efficiency, theDonor materials in heterojunction interface always contain an intramolecularDonor-Acceptor structure, which means there are two coexisting charge transfer (CT)processes: intramolecular charge transfer and intermolecular charge transfer. Is thisintramolecular charge transfer process related to the high photoelectric conversionefficiency？We design two molecular model system models: one is D1/A2, the other isD1-A1/A2, corresponding to two comparative systems: only intermolecular CT, and bothintermolecular and intramolecular CT. By comparison between two systems, the electrondensity distribution, HOMO and LUMO energy, recognization energy, electron couplingand free energy are calculated and analyzed. According to Marcus–Levich–Jortner equation,we estimate the rates for charge separation and charge recombination, further to judgewhether the intramolecular charge transfer is in favor of highly efficient intermolecularcharge separation.This paper includes three parts: the first part introduce the progress of solar cell, thedevice structure, the basic principle and the reasons of the low efficiency. The second partexpatiated the basic theoretical methods, for example, Franck-Condon principle,Born-Oppenheimo principle, Adiabatic and nonadiabatic reaction, and some charge tranferclassical models (Marcus classical, semi-classical model and quantum mechanics model).And then we introduced a core equation: Marcus–Levich–Jortner equation, including thephysical meaning of all related parameters.The third part is about case simulation section. Firstly, we set NZP and TPA-NZP asthe electron donors, PCBM as the electron acceptor. They can form two kinds of the solarcell systems: D1/A2and D1-A1/A2. Because of the introduction of TPA， a strongelectron-donating group, the intramolecular charge transfer can form in TPA-NZP/PCBM complex. We hope, through the calculation for intermolecular charge transfer rate, that canget the information about the relationship between the intermolecular charge transfer andthe intra-molecular charge transfer. And then we designed two different kinds of interfacialgeometry models: the vertical structure and the horizontal structure. By comparisonbetween these two interfacial models， we can understand the physical mechanismresponsible for the cooperative between intramolecular CT and intermolecular CT. Basedon the calculation, we found that in the vertical structure system, the intramolecular CT canseparate the exciton to some extent before the intermolecular charge transfer take places,which can reduce the exciton binding energy greatly, and make the intermolecular chargeseparation easier. So for the coexisting intramolecular and intermolecular CT system, thecharge separation efficiency should be higher than the pure intermolecular CT system. Butfor the horizontal structure system, there doesn’t exist the electron hopping bridge-likeenergy level as that in vertical structure, and the intramolecular CT direction isperpendicular to the intermolecular CT. Even though the binding energy ofTPA-NZP/PCBM is also reduced, but compared with NZP/PCBM system, theintermolecular charge separation rate has no obvious change. On the contrary, the chargerecombination rate is found to be increased. It indicates that the interfacial geometry hassignificant effect on the intermolecular charge separation rate. As for D1-A1/A2verticalsystem, the consistent intramolecular and intermolecular CT direction can induce thecooperative intramolecular and intermolecular CT, which is sure to enhance thephotoelectrical conversion efficiency greatly.