Theoretical Investigation Of The Properties Of A Category Of Novel Metal-free Sensitizers And The Effect On The Performance Of Dye-sensitized Solar Cell

In this present work, we designed a series of novel coumarin-based dyes by adopting different number of pyrimidine or quinoline units as π-bridges. DFT (density functional theory) and TDDFT (time dependent density functional theory method) were used to investigate the related properties of all systems. To provide reliable molecule design guideline, we investigated the performance of light havesting efficiency, charge collection efficiency, charge injection efficiency and dye regeneration efficiency for newly designed dyes relative to their model dyes. Calculated results showed that pyrimidine unit was more favorable for improving the performance of DSSCs than quinoline unit. Meanwhile, increasing the number of electron-deficient units maked for improving the absorption properties of dyes in short-wavelength, while was unfavorable for improving the absorption properties of dyes in long-wavelength. Therefore, increasing the amount do electron-deficient units would decrease the optical properties of dyes. Based on the above investigations, we considered that the π-spacer plays a vital role on changing the efficiency of DSSCs, so we would introduce an auxiliary electron-withdrawing unit into common D-π-A structure of dyes, forming D-A-π-A featured dyes. By exploring how the auxiliary electron-withdrawing units affect the performance of DSSCs, we wished to provide theoretical guidance and basis to design effective sensitizers.1. An efficient strategy to explore the photophysical properties of metal-free coumarin sensitizers:how to improve the efficiency of dye-sensitized solar cellsDye-sensitized solar cells (DSSCs) based on metal-free coumarin have caused considerable concern in recent years. However, the power conversion efficiency (η) of coumarin-based DSSCs is still very low. One of the reasons is the narrow and weak optical absorption of coumarin sensitizers in short-wavelength region (B band). In the present work an efficient strategy was provided to achieve a broad absorption with high extinction coefficient in short-wavelength region by introducing different electron-deficient units (pyrimidyl and quinolyl) into parent coumarin sensitizers, which eventually improve the performance of DSSCs. Density functional theory calculations using several functionals were performed on both isolated coumarin dyes and dye-titania complexes. As expected, the introduction of single electron-deficient unit results in a positive influence on the performance of optical absorption (φLHE), charge separation (φCC), electron injection (φinject) and dye regeneration (φreg). Further, with a second electron-deficient unit adopted, a red-shifted and strengthened absorption in short-wavelength region and a blue-shifted absorption in long-wavelength region are obtained. The introduction of more pyrimidine units facilitates the electron transfer from donor to acceptor and favors the effective electron injection, whereas the second quinoline displays opposite effect. The influence of electron-deficient units on the electronic features and ultrafast electron injection of dye/TiO2systems was also studied. Despite the introduction of electron-deficient unit makes for electronic coupling between dyes and TiO2, but can not alter the indirect mechanism of electron injection for these investigated systems. The present strategy gives a guidance to improve the performance of DSSCs, and the newly designed sensitizers will be promising candidates for the future solar cell applications.2. Probing structure-property relationship of D-A-π-A featured metal-free sensitizers with different additional electron-withdrawing units for high-performance dye-sensitized solar cellsIn the past two decades, high performance and low cost of dye-sensitized solar cells(DSSCs) have drawn great interest from both academic and industrial domains. Metal-free organic dyes with D-π-A structure were the major topics of research. Recently, researchers introduced an additional electron-withdrawing unit into the π-spacer, and then formed effective dyes with newly D-A-π-A configuration. In this paper, we used density functional theory and time-dependent density functional theory to investigate the effect of different electron-withdrawing ability of this additional acceptor on the performance of optical absorption, charge separation, the efficiencies electron injection and dye regeneration. Our calculated results showed that the introduction of additional acceptor with weak electron-withdrawing ability leads to red-shifted absorption and is favorable for charge separation, whereas the introduction of additional acceptor with strong electron-withdrawing ability displays opposite effect. Therefore, we should select additional acceptor with suitable electron-withdrawing ability to realize a balance in both promising the performance of optical and charge separation. In addition, with the electron-withdrawing ability of additional acceptor increasing, the efficiency of electron injection would be decreased, while the efficiency of dye regeneration would be increased. The presented results would provide reliable guidance for probing structure-property relationship of D-A-π-A featured metal-free sensitizers to design effective dyes.3. Theoretical design of effective triphenylamine sensitizers with different substituents modified the donor part of dyesDye-sensitized solar cells (DSSCs) drew intense attention in recent decades. However, the maximum power conversion efficiency (η) was insufficient high until now. In this contribution, to improve the performance of DSSCs, we introduced different types of substituents (electron-donating and electron-withdrawing substituents) into the donor part of dye. By comparing the performance of optical absorption and charge separation of these investigated systems, we could provide an efficient strategy to obtain high η. Density functional theory (DFT) calculations were applied to study the absorption spectra and the electronic structures of these dyes. Based on the calculations, we proposeed that a red-shifted absorption spectrum and favorable charge separation could be achieved by adopting electron-donating substituents. However, the introduction of the electron-withdrawing substituents displayed opposite effect. Our calculations indicated that the introduction of electron-donating substituents into dyes is a feasible approach to boost the performance of DSSCs.