The Electronic Structures And Properties Of Several Systems Related To Organic And Dye Sensitized Solar Cells

Due to solar cells can convert solar power into electricity,therefore,has attracted more and more attention.For organic solar cells,the power conversion efficiency(PCE)mainly determined by photoactive layer which is composed of electron donor material and electron acceptor material.And fullerenes and their derivatives who have better electron affinity and electron transport capacity are still the most commonly used electron acceptor materials.The properties of dye are directly related to the PCE of dye sensitized solar cells.Although ruthenium(Ru)-based dye sensitizer,porphyrin dye sensitizer have high energy conversion efficiency,but ruthenium resource limited and difficult to prepare as well as porphyrin dye purification difficult and other problems against their development.In contrast,the organic dye sensitizer has the advantages of high molar extinction coefficient,easy structure modification and purification,and become a new hope for improving the performance of dye-sensitized solar cells.Dye sensitizer is mainly through the adsorption on the semiconductor surface,and the adsorption morphology and coverage of the dye on the semiconductor surface to affect the PCE of dye-sensitized solar cells.Therefore,it is important to study the structure and properties of the fullerene derivative and the sensitizer as well as the adsorption behavior and adsorption properties of the dye sensitizer on the semiconductor surface.As electronic acceptor materials in heterojunction,fullerene derivatives(FDs)can significantly affect the power conversion efficiency of organic solar cells(OSCs).Here,in order to investigate the number and size effects of fullerene-cage in FDs for OSCs,the geometries,electronic structures and related properties,as well as photovoltaic parameters of FDs called as MP,PCBM,BP,TP,and PC71 BM were analyzed based upon density functional theory(DFT)and time-dependent density functional theory(TD-DFT)calculations.The results indicate that the fullerene-size is more important than the number of fullerene-unit to affect the local geometrical parameters,energy level alignments,dipole moments,hyper-polarizabilities,and optical absorptions in visible region.Furthermore,the lowest unoccupied molecular orbital(LUMO)energies are almost same for these FDs,and the orbital energies near frontier molecular orbitals for BP and TP exhibit two-and three-fold quasi-degenerate,respectively.The transition configurations and molecular orbitals reveal the absorption bands in visible region of MP,PCBM,PC71 BM and TP are local excitations,and that of BP are charge transfer excitations.The similar open-circuit voltages and fill factors of OSCs based upon P3HT/MP,P3HT/PCBM,P3HT/BP,P3HT/TP,and P3HT/PC71 BM blend films result from the similar LUMO energy levels of these FDs.As the important component of dye sensitized solar cells(DSSCs),the performance of dye sensitizer is directly relate to the DSSCs’ stability,power conversion efficiency and so on.Therefore,in addition to experiment research,it is also important to use theoretical calculations to analyze the structure and properties of dye sensitizers in depth.In this paper,we use DFT and TD-DFT to calculate and study the geometry structures,electron structures and excited properties of the metal-free dye sensitizers C201,C203,C204,C205 and the designed dyes C20x-1,C20x-2,C20x-3,C20x-GF,C20x-GF-BTD which have common electron donor,with different π-conjugated linker and D-π-A structure.Based on the analysis of the results,we know that the π-conjugated linker plays a very important role in the structure and properties of dyes.Dyes C201 and C203 have nearly equal energy gaps,but with the number increase of thiophene group which in the "fusion" way to combine,reduce the light absorption properties of the dye,improve the electron injection efficiency of dye.The π-conjugated linker number of dye C205 is one more EDOT group than C204,result in C205 has a better light absorption spectrum,but C204 possesses better electron injection efficiency and dye regeneration performance.The results of dyes C20x-2 and C20x-3 show that the introduction of hexyl chains into π-conjugated linker has little effect on dye performance.Dye C20x-1 has a good optical absorption spectrum,charge transfer as well as electron injection properties with dihexylcyclopentadithiophene as π-conjugated linker.The results of dyes C20x-2 and C20x-3 indicate that the introduction of π-conjugated linker have little effect on the dyes properties.While the dye C20x-GF and C20x-GF-BTD have lower energy gap,wider light absorption range,but lack of effective charge transfer excitation,resulting in lower electron injection and dye regeneration free energy change.A group containing carboxyl group usually used as electron acceptor(in general also act as fixed group)in organic dye sensitizers,so study the electron acceptor group adsorption on photoanode can predict the adsorption behavior and adsorption properties of organic dyes,and also the foundation to optimize dye-sensitized solar cells performances.α-cyanoacrylic acid(CAA)is the typical anchor moiety coupled electronic acceptor in dye sensitizer.To understand the interaction between dye sensitizer based upon CAA and rutile TiO2 photo-anode in dye sensitized solar cells,the atomic configurations,energies and electronic properties of CAA adsorbed on rutile TiO2(110)surface were studied by using DFT calculations.The results indicate CAA prefers to dissociatively adsorb on rutile TiO2(110)surface as bidentate configuration in which the O atoms of CAA bind with two adjacent surface five-coordinate Ti atoms along [001] direction,and the H atom in hydroxyl of CAA adhere to the nearest neighbor surface O atom at bridge site,generating a hydroxyl species.The calculated adsorption energy is about 1.480 eV.The analysis of geometrical parameters,density of states and electron density suggests the bonds between rutile TiO2(110)surface and CAA are formed.The density of states and orbital character at the gamma-point bands support that the adsorption of CAA on the surface provides feasible mode for photo-induced electron injection.