Table 2 Photocurrent density-voltage characteristics of TiO 2 nanofiber cells Cell ZnO thickness (nm) J sc(mA/cm2) V oc(V) FF η (%) τ d(ms) τ n(ms) L n(μm) II 0 14.5 0.825 0.53 6.34 1.88 107.7 138.3 IV 4 15.0 0.828 0.54 6.71 1.43 119.5 166.9 V 10 16.5 0.833 0.54 7.42 1.21 154.3 206.4 VI 15 17.3 0.842 0.55 8.01 1.08 179.7 235.7 VII 20 14.8 0.825 0.53 6.47 4.62 354.5 159.9 TiO2 nanofiber cells with ZnO layer of different thicknesses, the transit time (τ d) and electron lifetime (τ n), and diffusion length (L n). The schematic view of electron transfer with ZnO layer is shown in Figure  8. The interfacial processes involved

in charge transportation in the cell are depicted in Figure  8b. As exciton dissociation occurs, SB202190 concentration electrons injected into the TiO2 conduction band will transport to the FTO by diffusion [33]. Because the conduction band edge of ZnO is a little more negative than that of TiO2, an energy barrier is introduced at the interface of FTO/TiO2, in which ultrathin ZnO layer can effectively suppress the back electron transfer from FTO to electrolytes or may block injected electron transfer from TiO2

to FTO. The back reaction was studied using IMVS measurements. The electron lifetime τ n obtained from IMVS (as shown in Table  2) is 107.7 ms for the cell without ZnO layer but is significantly increased from 119.5 to 354.5 ms with ZnO layer thickness increasing from 4 to 20 nm. The striking increase in the lifetime shows direct evidence that ultrathin ZnO layers prepared by ALD method dipyridamole successfully suppress the charge recombination between electrons emanating from the FTO substrate and I3 − ions in

the electrolyte. The transit times of electrons calculated learn more from IMPS measurements reflect charge transport and back reaction. Although an energy barrier is induced by introduction of ZnO layer between the TiO2 and FTO, the electron transit time estimated from IMPS measurement is decreased from 1.88 to 1.08 ms for cells with ZnO layer thickness increasing from 0 to 15 nm. However, when the thickness of ZnO layer further increases, the change trend is reverse, and electron transit time for the cell with 20-nm-thick ZnO layer is markedly increased to 4.62 ms. It is put forward that relative to the cell without ZnO blocking layer, the electron transport in the cells with ZnO layers is determined by the two competition roles of the suppression effect of recombination with I3 − and potential barrier blocking effect. The increased electron lifetime has verified that ultrathin ZnO layer effectively slows the back recombination of electrons at the interface of FTO/electrolyte, so the decreased electron transit time reveals that the suppression effect is stronger than the potential barrier effect when the ZnO layer thickness is smaller than 20 nm. The obtained values of L n/d of cells IV to VII are shown in Table  2, which are all larger than that of the reference cell without ZnO layer, with the largest value of 8.