Effect of an ultrathin TiO(2) layer coated on submicrometer-sized ZnO nanocrystallite aggregates by atomic layer deposition on the performance of dye-sensitized solar cells.

Adv. Mater. 2010, 22, 2329–2332 2010 WILEY-VCH Verlag G Since the advent of dye-sensitized solar cells (DSCs), which have achieved 11% of power conversion efficiency (PCE) in TiO2-based photoelectrodes, a lot of efforts have been devoted to make low-cost, light-weight, high-performance photovoltaic devices. Nanostructured metal oxides are one of key factors in determining the PCE of DSCs, because the nanostructured networks provide a huge surface area to accommodate a large quantity of dye molecules that relate to the light harvesting of a photoelectrode in DSCs. ZnO is a good alternative of TiO2 because it has a similar band gap but higher electron mobility than TiO2. [4–7] The mobility of ZnO is about 115–155 cmV 1 s , much higher than that of TiO2, 10 5 cmV 1 s . Recently, DSCs with photoelectrodes made of submicrometer-sized aggregates of ZnO nanocrystallites demonstrated a PCE of 5.4% due to much enhanced light scattering without compromising the surface area for dye molecule adsorption. A porous structured ZnO aggregates of nanocrystallites were thought to be helpful to retain their high surface area. Although this PCE is still lower than that of TiO2 DSCs, it doubled the PCE of ZnO nanocrystallite DSCs. Atomic layer deposition (ALD) has been used to introduce extremely thin and conformal coating due to its unique self-limiting nature and low growth temperature; lots of semiconductor materials like TiO2, ZnO, SnO, and Al2O3 can be grown by ALD. In this study, we utilized ALD to deposit ultrathin TiO2 layer on the porous structure of ZnO aggregates and demonstrated much enhanced PCE of ZnO DSC with photoelectrodes made of submicrometer-sized aggregates of ZnO nanocrystallites. As illustrated schematically in Figure 1a–c, TiO2 ultrathin layer deposited by ALD would form a complete and conformal coverage on the surface and even inside pores of ZnO that would otherwise be exposed to dye electrolyte during the dye loading. Consequently, all the dye molecules would adsorb onto the surface of TiO2 coating. Such an ultrathin and conformal ALD coating would not change the morphology the underline ZnO structures as shown in Figure 1e and 1f. The coating of TiO2 layer on the surface of ZnO by ALD is presumably so thin that would not affect any detectable change in the morphology by means of scanning electron microscopy (SEM). Brunauer Emmett Teller (BET) results demonstrate that micropores inside each aggregate still remain after ALD, indicating that the porous structure of ZnO is preserved. As shown in Table 1, the slight decrease in the size and volume of the micropore was observed due to the introduction of ALD-TiO2 layer. In addition, the connections between adjacent ZnO nanocrystallites would retain to ensure a favorable electron motion through ZnO (as suggested in Fig. 1d). Such structure would improve the surface stability with enhanced dye loading on the ZnO surface, while retains the advantage of high electron mobility in ZnO. It is reported that the growth rate of TiO2 at the substrate temperature of 200–250 8C ranges from 0.03 to 0.06 nm per one cycle so the thickness of TiO2 grown by ALD for 10 cycles in the present investigation is estimated to be 0.3–0.6 nm. X-ray diffraction (XRD) analysis did not reveal any diffraction peak associated with TiOx including TiO2, which corroborates with the fact that XRD is incapable of detecting an ultrathin layer of TiO2. It is not possible to tell if the ultrathin ALD-TiO2 coating is of crystallites or amorphous, though the literature strongly suggests that it is crystallites, especially anatase phase when it is annealed above 400 8C. X-ray photoelectron spectroscopy (XPS) was carried out to verify the presence of ALD-TiO2 coating on the surface of ZnO (see Fig. S1 in Supporting Information). Only a weak peak of Ti 2p with a binding energy of 459 eV was observed. Ti 2p spectrum could be resolved in three spin-orbit components with binding energies of 455.9, 456.7, and 458.5 eV and each of them corresponds to TiO, Ti2O3, and TiO2 fractions, respectively. [18,19]