Based On D-A And D-A-D Structure Of Organic Solar Cells Of Density Functional Theory Study

Organic solar cells have attracted much attention in recent years due to their low-cost fabrication, light weight, as well as excellent compatibility with flexible substrates. In the past decade, the power conversion effect (PCE) has exceeded9%, However, compared with Inorganic semiconductors solar cells, the low PCE is still an obstacle to their large-scale promotion. The quantum chemical calculations not only can explain the photoelectric conversion mechanism of polymer solar cells, but also can predict new polymers with excellent photovoltaic performance. This is a very important task to improve the performance of the organic solar cells. Therefore, we employed the density functional theory (DFT) to explain the photoelectric conversion principle and photoelectric properties of fluorene compounds and triphenylamine-benzothiadiazole derivatives.On the result, we design a series new-type compounds and predict their photovoltaic properties.1. Density functional study of Dithienosilole derivativesPDTSDPP (Dithienosilole-3,6-diaryl-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione) and PDTSPh (dithienosilole-phthalimide) are based on Dithienosilole (DTS) unit polymers, and it have been synthesized and used in solar cells. We used density functional theory to calculated the electronic structure and optical properties of them at the molecular level and designed two new molecules. we also calculated the frontier molecular orbitals level, band gap, absorption spectra and hole mobility and other parameters.The results shows that, PDTSDPP, PDTSPh have lower HOMO energy level and narrow band gap, and they also have some better ability to capture photons, smaller hole recombination and better photovoltaic properties, it is consistent with their experimental results. Additionally, compared with PDTSDPP, PDTSPh, both PDTSFDPP and PDTSFPh have more lower HOMO levels and more narrow band gap, and they are potential photovoltaic materials.2. Density functional study of D-A-D structure of organic solar cell materialsThe triphenylamine(TPA) is extensively applied to the small molecular donor materials due to it can increase the film forming ability and the absorption cross section for the incident light as well as the charge transport efficiency through the active layer.Furthermore, TPA possesses a high oxidation potential, and thus a high Voc could be expected. As is well known, benzothiadiazole derivatives which are amenable to facile ring modification have high electron affinities. The D-A-D structure of the molecules is to reduce the band gap of the materials for improving the absorption and enhance the charge mobility.A series of D-A-D donors have been designed based on TPA (D), DBT (A) with different electron-withdrawing unit toward solar cell materials. The structure and electronic properties, frontier molecular orbital energies, optical properties and charge transport properties have investigated by DFT methods. The calculated results reveal that HOMO, LUMO energy levels and Eg, the electron transfer property is obviously influenced by doped nitrogen atom in the electron-withdrawing unit of molecules. It not only can reduce the HOMO, LUMO energy levels and Eg, and improve the electron transfer ability of molecules, but also can lead the wavelength of maximum absorption red-shifted, and assures a relatively high open circuit potential(Voc). The analysis of the ionization potential and electron affinity demonstrate that the ability of electron dissociation and injecting are affected by the HOMO and LUMO levels. The studied results indicate all studied molecules (except TPABO) possess good electron transfer property(KeCT are from4.71to12.30×1013s-1) comparing with the TPABT, which have been synthesized and applied in solar cells.