Theoretical Simulation Of The Excited States Of Fused-Ring Electron Acceptors For High Performance Organic Solar Cells

Bulk-heterojunction organic solar cells with active layers solution-processed from blends of fused-ring electron acceptors（FREAs）and donor polymers have been the foundation of significant inventions when compared with the fullerene ones in terms of efficiency,flexibility,stability,lightweight,easy synthesis and solution-processing.The magnificent designing and persevering progress of acceptor-donor-acceptor FREAs have pushed the power conversion efficiencies up to 18 to 20%.Understanding the molecular design and excited states of the most efficient organic photovoltaic materials is the key to designing novel molecules and improving the power conversion efficiency of organic solar cell.Calculations employing density functional theory（DFT）and linear-response time-dependent density functional theory（TD-DFT）yield transition energies at comparatively low computing costs and have hence become established approaches for exploring electronically excited states of big organic molecules.This thesis herein evaluates the performance of theoretical methods in the framework of DFT and TD-DFT for the excited states of FREAs,aiming to deeply understand their excited-states nature,get an insight into the intra-molecular charge transfer process,and propose the most suitable methods for evaluating their excited states to provide rationalized recommendations.（1）The accuracy of TD-DFT was determined for FREAs by comparing their theoretically predicted vertical absorption wavelength values(λ_ver-theo)with the experimental maximum absorption(λ_max).Theλ_ver-theo values of 50 molecules obtained from major types of FREAs were investigated using TD-DFT by considering the solvent effects.The values ofλ_ver-theo predicted with a pure density functional（PBE）,global hybrids（GHs）such as B3LYP and PBE0,and range-separated hybrids such as CAM-B3LYP and LC-?PBE follow the exact exchange percentage included at an intermediate inter-electronic distance.The GH functionals outperform all other schemes.The mean absolute error（MAE）provided is 22 nm by PBE0 and38 nm by B3LYP for the whole set of molecules.The maximum deviation of 92 nm provided by B3LYP and 69 nm provided by PBE0 confirms that PBE0 is the more appropriate for predicting the absorption wavelengths when designing new FREAs.By applying linear regression analysis to obtain the calibration curve,it was found that the range-separated hybrids provide an equal or even more consistent description of FREAs excited states.For the whole set of molecules,linearly corrected data yield an average error of 25 and 27 nm for CAM-B3LYP and LC-?PBE,respectively.Consequently,when a statistical analysis technique is applicable for a certain series of FREAs,a theoretical method permits a chemically comprehensive and empirically good explanation of UV-vis spectra for newly designed FREAs.（2）By employing TD-DFT,the effect of the Hartree-Fock exact exchange（HFX）was evaluated on the performance of 16 GHs for computing theλ_ver-theo and E_ver-theo of 34 A-D-A FREAs and comparing them with the experimental ones.The computedλ_ver-theo values strictly follow inverse proportionality to HFX percentage.The performance of functionals with the same HFX is almost identical,such as B3LYP,B3PW91,and m PW3PBE with 20%HFX.The performance enhances when moved towards a higher HFX ratio of 21%in X3LYP,B971,B972,and 22%in B98 through giving smaller deviations.For the whole set of FREAs,APF and APFD with 23%HFX give the smallest deviations,with a mean signed error limited to-4nm（0.02 e V）and an MAE of 21 nm（0.06 e V）.The performance drops with M06 and M05containing higher HFX ratios.The M06-2X with 54%HFX provided the largest MAE value of0.35 e V.The 23 to 25%HFX in most of the GH functionals give MAE values of 21 and 22 nm.These results show that these are the optimal values for the best prediction of FREAs excitation energies.It was also found that the GHs seem to be more efficient for larger-sized FREAs with a smaller band gap.（3）The most efficient new versions of the GHs were developed by optimizing the HFX ratio in the standard functionals.All the newly developed GHs were used to compute theλ_ver-theo and E_ver-theo of the first excited states of 34 FREAs using TD-DFT.Most of the new GH functionals were found to be more efficient than the original standard functionals such as,B3PW91-24HFX and X3LYP-23HFX.The smallest MAE of 21 nm was obtained with most of the new functionals,such as B3LYP-24HFX and M06-24HFX.Theλ_ver-theo mean signed error-shift from positive to negative also occurs above 23%when the HFX changes in most GH functionals.The performance drops when we move towards the GHs with higher HFX ratios.The new GHs with 23 to 25%HFX have a tendency to control the overestimation and the underestimation of transition energies.（4）The DFT,TD-DFT,and an intra-molecular charge transfer index were used to evaluate the nature of the excited states of FREAs.Typically,several efficient electronic transitions contribute to the absorption spectra of FREAs.An investigation of every efficient electronic transition of each FREA was performed based on the electronic density variation in the donor and acceptor moieties of the molecules upon absorbing solar photons.Not all these transitions are equivalent for light-to-electricity conversion.The first transition contributes the most to the absorption spectra.This transition is intense and extremely efficient for light-to-electricity conversion,giving a higher value of intra-molecular charge transfer.The phenyl rings in the donor unit behave as the electron-donating units for certain effective transitions of FREAs.The foremost finding of the present research work is that the furthermost strong electronic transitions sometimes may be the less effective ones for the conversion of sunlight into electricity.