Time- and Frequency-Resolved Photoelectrochemical Measurements on Dye Sensitized Solar Cells Employing (102) and (100) Dominated ZnO Nanocrystals

Abstract

We have succeeded in microwave-assisted hydrothermal synthesis of ZnO nanocrystals in distinctively different structures by use of structure directing agents (SDAs). Use of triethanolamine resulted in hexagonal bi-pyramidal (HBP) shape with 84% with predominant exposoure of (102) facet, whereas benzene tetracarboxylic acid yielded nanosheet (NS) with 76% (100) facet. They were compared as photoelectrodes of dye sensitized solar cells (DSSCs) employing D149 and eosinY as sensitizers. Dye coverage at the highest density was found for HPB for both dyes, whereas NS the lowest. Denser packing resulted in higher current, not because of the difference of absolute light harvesting efficiency, but improved efficiency of electron injection. On the other hand, highest voltage was always observed for NS. The values of MZ appeared always between those of HBP and NS, in fact, resulting in the highest efficiency among the three due to its best-balanced performance. These observations left us some questions. Why does the (102) facet densely covered with dyes become leaky to result in low voltage, while loosely covered (100) was able to block the back reaction to achieve high Voc? In this paper, we have carried out intensity modulated photocurrent/voltage spectroscopies (IMPS/IMVS) of the DSSCs employing these different ZnO nanocrystals to elucidate their transport and back reaction kinetics. Pastes made of different ZnO nanoparticles were doctor bladed on FTO glass substrate using Scotch® tape to control their thickness to 10 mm. After drying, the geometric area of the films were regulated to be 0.25 cm2 by scratching off the redundant part. Then the films were dipped into 60°C warm water to promote necking for 10 min. and dried in an oven at 100°C for 30 min. The films were immersed into 0.5 mM DN216 (same chromophore as D149 but with improved adsorption stability due to extra anchor, developed by Chemicrea) in tert-butanol/acetonitrile (1/1 in volume) solution for sensitization. Sandwich cells were made by attaching platinized FTO glass counter electrodes in a face-to-face geometry with a thermoplastic film. The cells were sealed with a cover glass after injecting electrolyte of 0.5 M propyl-methyl-imidazolium iodide and 0.05 M iodine in acetonitrile. Monochromatic LED (625 nm) were used as light sources for IMPS and IMVS. The background light intensity was varied between 1 and 250 W m-2, and the amplitude of the light modulation was set to 10% of the back-ground light intensity, where frequency range from 0.3 Hz to 100 kHz was set for IMPS and IMVS. The electron transit time and lifetime were calculated from the frequency of the minimum in the imaginary component from IMPS (short circuit condition) and IMVS (open circuit condition), respectively. The fitting values were plotted against light intensity in log scale, as shown in Fig. 1 where open marks correspond to transit time and solid ones to lifetime. From IMPS results, longer transit times were revealed for HBP and NS compared to MZ, which means it takes longer for the photoinjected electrons to reach the back contact. This factor is influenced by the film thickness and dye loading. As the film thickness was constant at 10 µm, the difference may rather come from the total amount of dye loading. As described earlier the dye loading was largest on MZ because of the larger surface area. Also, the transit time depends on the trap concentrations in the films. Due to the larger particle size in HBP and NS films, the particles in the film may not be as well connected as in the films of small MZ particles, which will create more traps to limit electron transport. In the IMVS results, NS shows longer electron lifetime than MZ and HBP. A slower back reaction of photogenerated electrons to tri-iodide is thereby indicated under open circuit conditions. Hence, a stronger passivation against this recombination reaction in the NS film than in MZ and HBP is observed, which gives one reason why NS cells always exhibit higher Voc than the others. In order to quantify this passivation behavior, electrochemical impedance spectroscopy (EIS) was also carried out to clarify the factors such as recombination resistance, chemical capacitance, etc and will be discussed in this study. Figure 1