Effect of an Ultrathin TiO₂ Layer Coated on Submicrometer‐Sized ZnO Nanocrystallite Aggregates by Atomic Layer Deposition on the Performance of Dye‐Sensitized Solar Cells

Abstract

Since the advent of dye-sensitized solar cells (DSCs), which have achieved $11% of power conversion efficiency (PCE) in TiO 2 -based photoelectrodes, a lot of efforts have been devoted to make low-cost, light-weight, high-performance photovoltaic devices. [1][2][3] 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 TiO 2 because it has a similar band gap but higher electron mobility than TiO 2 . [4][5][6][7] The mobility of ZnO is about 115-155 cm 2 V À1 s À1, much higher than that of TiO 2 , $10 À5 cm 2 V À1 s À1. 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. [8][9][10] 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 TiO 2 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 TiO 2 , ZnO, SnO, and Al 2 O 3 can be grown by ALD. [11][12][13] In this study, we utilized ALD to deposit ultrathin TiO 2 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, TiO 2 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 TiO 2 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 TiO 2 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-TiO 2 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 TiO 2 at the substrate tempera...

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