Increased Light Absorption in Dye-Sensitized Solar Cells with Light Harvesting Dye
Authors: Takekuma Yuya , Ochiai Tsuyoshi , Furukawa Rei , Nagata Morio 
Published: February 2020

In plants, photosynthetic protein, light harvesting complexes (LHC), holds reaction center called photosystem I or II (PSI or PSII). PSI and PSII receive light energy and convert it to electric flow. This reaction is done in high quantum efficiency. This light-harvesting system enables the plants to use the sun-light efficiently. The converted electron is spatially transferred in multiple chemical steps, and this is called Z scheme. The Z scheme is known to be highly efficient which have nearly no loss of electric energy.
Dye-sensitized solar cells (DSSCs) have attracted increasing attention because of its multiple advantages such as low-cost, lightweight, and flexibility. In DSSCs, photogenerated charge carriers in dye are separated and the electrons are injected to conduction band of semiconductor. The charge separating process of DSSCs is similar to that of photosynthesis.
Here, we report DSSCs further imitated photosynthesis. The DSSCs we reported have applied the light-harvesting system in photosynthesis. Perylene-based fluorophore was dispersed in electrolyte in order to imitate the roll of chlorophyll, the light harvesting dyes, in nature. This material was chosen due to the high stability against photo-oxidation. Phthalocyanine, the sensitizing dye, was adsorbed on the surface of photoanode to convert photoenergy into an electric flow. The two synthetic dyes are selected to imitate the light-harvesting complex and reaction center, respectively. The energy absorbed at the fluorophore undergoes Förster resonant energy transfer (FRET) to the sensitizing dye. This allows broader spectral absorption, which is beneficial as a characteristic of a solar cell. We have fabricated the above-mentioned imitated system using synthetic dyes, and evaluated its performances on FRET and following electron transfer.
Experiments were done as follows. Perylene-based fluorophore was dispersed in iodine/triiodide electrolytes. Three types of fluorophores with different emission wavelengths were used, and electrolytes with different fluorophore concentrations were prepared at the purpose of comparison. Two phthalocyanine-based sensitizing dyes, PcS2 and PcS20 (structures shown in Figure1 a and b), were chosen to investigate the impact of modifications around the main structure. Absorption characteristics of the two materials are nearly the same [1]. The TiO2electrode consisted of a 10 µm nanoporous layer with 3 µm scattering layer was prepared by screen printing. Solar cells were assembled using Surlyn film with platinized counter electrode. The electrolyte solution was introduced through two holes predrilled in the counter electrode. Evaluation of the samples was done using incident-photon-to-current conversion efficiency (IPCE).
Figure 2 shows IPCE result of the solar cell using one of the three fluorophore and PcS2. As a result, the DSSCs fabricated using fluorophore with emission well matching the absorption of the sensitizing dye had shown a notable photo-electric conversion around 400-600 nm. The overall photovoltaic performance was greater than the case of no use of fluorophore at all. The solar cells with other two types of fluorophores had not shown this characteristic. Since the emission band of these two fluorophores does not match the adsorption band of the sensitizing dye, it is suggested that FRET reaction is involved in the trend seen in Fig. 2.
In conclusions, DSSCs with plant-inspired structure was fabricated and their photovoltaic performance was investigated. Occurrence of FRET among the fluorophore introduced in the electrolyte and the sensitizing dye was suggested from the comparison between the evaluation using fluorophores with different emission band. Synthetizing dyes with different modification had shown the difference, in which suggests that the bulkiness of the dye affects the reaction in the photoanode surface. The overall photovoltaic performance was shown to improve by creating a structure that induces FRET reaction.
Ref.1 S. Mori, M. Nagata, Y. Nakahata, K. Yasuta, R. Goto, M. Kimura, and M. Taya J. Am. Chem. Soc.
2010, 132, 4054-4055

Figure 1