Defects in Chemically Synthesized and Thermally Processed ZnO Nanorods: Implications for Active Layer Properties in Dye-Sensitized Solar Cells

Abstract

We have carried out the effect of post annealing temperatures on the performance of solution-grown ZnO rods as photoanodes in dye-sensitized solar cells. Keeping our basic objective of exploring the effect of native defects on the performance of DSSC, we have synthesized ZnO rods having length in the range of 2-5 μm by a modified sonication-induced precipitation technique. We performed extensive characterization on the samples annealed at various temperatures and confirmed that annealing at 300 °C results in ZnO rods with minimum native defects that have been identified as doubly ionized oxygen vacancies. The electron paramagnetic resonance measurements on the samples, on the other hand, confirmed the presence of shallow donors in the low temperature annealed samples. We also carried out electrochemical impedance measurements to understand the transport properties at different interfaces in the solar cell assembly. We could conclude that solution-processed ZnO rods annealed at 300 °C are better suited for fabricating DSSC with improved efficiency (1.57%), current density (5.11 mA/cm(2)), and fill factor (45.29%). On the basis of our results, we were able to establish a close connection between the defects in the metal oxide electron transporting nano system and the DSSC performance.

Solution grown ZnO rods: Synthesis, characterization and defect mediated photocatalytic activity

“…S4). In our previous work, we have attributed the band at 560 nm to double ionized oxygen vacancy (V o •• ) and the band at 618 nm to chemisorbed/interstitial oxygen [28,[34][35][36]. The different reaction conditions also induce variation in defect concentration as revealed from the intensity of the deconvoluted peaks D1 and D2 in the PL spectra of each sample (Fig.…”

“…All the samples exhibited diamagnetism at RT, giving an indirect evidence of the absence of either paramagnetic singly ionized oxygen vacancies or zinc interstitials in our samples. The observed EPR signal at 1.954 ± 0.001 could be assigned to shallow donor centres related to the incorporation of hydrogen in the solution processed ZnO [28,[40][41][42][43]. It is anticipated that the incorporation of hydrogen during growth process was higher in S-4 sample (as revealed from experimental results) which acts as shallow donor and passivate the double ionized oxygen vacancy site [43] giving rise to less vacancy defect states as compare to S-3 sample.…”

“…For this purpose, we have used sonication assisted chemical precipitation process using a weak base like NH 4 OH to prepare zinc hydroxide. As a part of our on-going program on DSSC, we prepared ZnO flower, cubes and rods by both precipitation and sonochemical techniques and studied their performance as photoanodes in DSSC [25][26][27][28]. Further, we have also investigated the photocatalytic application of the synthesized materials for environmental applications.…”

Photochemical performance of ZnO nanostructures in dye sensitized solar cells

“…on the device performances [23,24]. Here, we have extended the work further to understand the influence of different shapes of ZnO, the structural defects present in them and their performance as photoanodes in DSSC.…”

“…ZnO rods have been prepared by a sonochemical method using ammonium hydroxide and zinc acetate as the precursors maintaining the pH around 9 as discussed in our earlier work [23]. We have prepared quasi-spherical ZnO nanoparticles (NPs) again by a sonochemical process using 250 mL 0.05(M) aqueous solution of zinc nitrate hexahydrate (Zn(NO 3 ) 2 .6H 2 O, 99.9%, Merck Ltd. India) as zinc source.…”