Investigation Of Additives For Hole Transporting Materials In Solid-State Dye Sensitized Solar Cells

Conversion of the infinite solar energy to electric power provides an effective approach for solving energy shortage and environmental problems.Dye sensitized solar cells(DSCs)are devices that can convert solar energy to electric power.However,there still remain some challenges for traditional liquid state DSCs such as electrolyte leakage and electrode corrosion.Solid state dye sensitized solar cell by replacing liquid electrolyte with 2,2',7,7'-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9-spirobifluorene(Spiro-OMeTAD)offers a solution to the abovementioned problems.Due to its spiro-linked center,molecular stacking in film is significantly hindered.As a result,intrinsic Spiro-OMeTAD suffers poor conductivity,hole mobility as well as low power conversion efficiency of ssDSCs.Doping is an effective strategy to enhance the charge transporting properties of Spiro-OMeTAD.However,the application of most existing additives working as dopants for ssDSCs is limited by poor solubility,sensitivity to ambient environment and difficulties in synthesis and purification.The development of new type additives for Spiro-OMeTAD in ssDSCs is definitely essential.2,2,6,6-tetramethyl-l-oxopiperdine bromide(TEMPO-Br)was selecteded as an additive for Spiro-OMeTAD in ssDSCs.UV-vis and carrier transporting properties measurements indicated that Spiro-OMeTAD could be oxidized by TEMPO-Br,and the resulting cationic species could improve the conductivity and hole mobility of Spiro-OMeTAD.Compared with the ssDSC without TEMPO-Br,a boosted power conversion efficiency(PCE)from 3.99%to 6.83%in terms of the short circuit current(Jsc)and fill factor(FF)was achieved by introducing TEMPO-Br into hole transporting material of ssDSC based on Y123 sensitizer.2,3-Dichloro-5,6-dicyano-1,4-benzoquinone(DDQ)was introduced into hole transporting material of ssDSCs.The experimental results showed that there is an increase in the conductivity and hole mobility due to a charge transfer complex formed by Spiro-OMeTAD donating an electron to DDQ.A PCE of 6.37%was achieved by ssDSCs sensitized with Y123 sensitizer.DDQ in Spiro-OMeTAD could promote charge carrier transportation,reduce electron recombination,increase the regeneration of sensitizers from cations and improve the stability of the corresponding solid state devices.Another factor playing an important role in device performance is the pore filling fraction of Spiro-OMeTAD in mesoporous TiO2 photoanode.The pore filling fraction can be improved by increasing the concentration of Spiro-OMeTAD in the solution for device fabrication,which,however,may cause a huge waste of Spiro-OMeTAD during spin-coating.Decreasing the concentration of Spiro-OMeTAD in the solution for hole transporting material deposition by 17%,by the addition of 20%chloroform in the solvent could lead to similar pore filling fraction compared with the concentrated solution.Because of the partly positive charge of carbon atom in chloroform,an increase in conductivity could be observed in Spiro-OMeTAD due to the Spiro-OMeTAD radical cation caused by charge transportation of Spiro-OMeTAD with chloroform,and the corresponding ssDSCs showed a 20%rise in PCE.This may provide a new approach for reducing the cost of ssDSCs in the future.