Study On Synthesis And Strcture-performance Of The Pyrazino[2,3-g]quinoxaline-based D-π-A-π-A Metal-free Organic Dyes For Dye-sensitized Solar Cells

Dye-sensitized solar cells(DSSCs) have attracted considerable attention since the first report by Gr?tzel et al. in 1991. In the past two decades, plenty of research on novel dyes made great contributions to improve the power conversion efficiency(PCE) of DSSCs. It is reported that auxiliary acceptor is beneficial for modulating the energy levels, extending absorption wavelength, thus greatly improving photovoltaic performances. However, additional acceptor with strong electron withdrawing ability can be treated as an ‘‘electron trap’’ which suppresses the process of transporting electron from donor to acceptor and leads to mismatch in energy level. Discovering a balance between the two aspects is a matter of cardinal significance. But it lacks systematical investigation on the effect of different electron-withdrawing auxiliary units on photovoltaic performances.For this, two series and 4 metal-free organic dyes for DSSCs are designed, synthesized and confirmed by the standard spectroscopic methods. The relationship between dyes molecule structure and their photovoltaic performance is discussed. Our work can be divided into 3 parts:(1) On the basis of D–A–π–A framework, a series of novel triphenylamine-based D-π-A-π-A featured dyes with cyano acetate as the electron acceptor have been designed, synthesized and characterized by NMR(1H and 13C), HRMS. Pyrazino[2,3-g]quinoxaline(PZQA), quinoxaline unit(QA) were incorporated as auxiliary electron acceptors. Benzene(BZ) was also incorporated to compare with them.(2) We study the relationship between different electron-withdrawing auxiliary acceptor and photophysical, electrochemical as well as photovoltaic performance in DSSCs systematically via Q1-Q3. Q2 with quinoxaline as the auxiliary acceptor exhibited the best photovoltaic performance due to its highest short-circuit photocurrent density(17.22 mA cm-2), leading to a power conversion efficiency of 7.42% under simulated AM 1.5 G illumination.Q1 with the strongest electron withdrawing group(pyrazino[2,3-g]quinoxaline) as the auxiliary acceptor showed the weakest performance, pyrazino[2,3-g]quinoxaline acted an ‘‘electron trap’’ which suppresses the process of transporting electron from donor to acceptor as well as leads to mismatch in energy level.It’s harmful to short-circuit current and open circuit voltage.(3) We study the relationship between different donors and photophysical, electrochemical as well as photovoltaic performance in DSSCs systematically via Q2 and Q4. By comparing the Uv-vis spectrogram, we can find that increasing the electron donating ability can lead to red-shift in solution and result in more negative value of HOMO.In addition, when incorporating an auxiliary acceptor with moderate electron-deficient ability, triphenylamine is a better donor than alkyl substituted triphenylamine.