Nonideal Charge Recombination and Conduction Band Edge Shifts in Dye-Sensitized Solar Cells Based on Adsorbent Doped Poly(ethylene oxide) Electrolytes

Abstract

Three adsorbents, that is, 4-N,N-dimethylaminopyridine (DMAP), tetrabutylammonium isonicotinate (TBAIN), and methyl isonicotinate (MIN), were employed for blocking charge recombination in the dye-sensitized solar cells (DSCs) with poly(ethylene oxide)/poly(ethylene glycol) blend electrolyte. Photovoltage–light intensity measurements showed the adsorbents not only decrease the recombination rate but also prompt the nonideality of recombination. The energy level of the TiO2 band edge (Ec) is elevated by the adsorbents, which also result in the increase in the characteristic temperature (T0), reflecting a deeper distribution of the electrons at the surface states. An electron transfer model on the basis of the Marcus theory was applied to calculate the rate of nonideal recombination as well as the ideality factor. The results indicate that the increases in Ec and T0 both foster the nonideal recombination. The decreased recombination is further attributed to the upward shift of the TiO2 band edge, which reduces the densities of the electrons in the conduction band and at the surface states. Another reason for that is the increase in T0, which results in the drop of the averaged possibility of electron transfer via the surface states. The enhancement of open-circuit voltage follows the sequence of DMAP > TBAIN > MIN, which is correlated to the negative charge of pyridine-N of the adsorbents in terms of the quantum calculations. By measuring IPCE and impedance spectra, the adsorbents are shown to improve the electron collection but impede the electron injection from the excited dye. As a result, the short-circuit current of the adsorbent-doped DSCs is enhanced with the sequence of TBAIN > DMAP > MIN.