In this study, a new methodology is proposed for the cosensitization of dye-sensitized solar cells by synthetic and natural dyes to increase their optical absorption. Synthetic dyes are efficient but expensive. In contrast, natural dyes are cheap but have a low adsorption rate in competition with synthetic dyes. Therefore, the conventional cosensitization method which is based on dye mixing prevents complete adsorption of natural dyes. In this work, a bilayer deposition of TiO2 and separated sensitization of each layer by one type of dye was performed. The N719 was used as a synthetic dye and betanin, crocin, acacetin, and indigo were used as natural dyes, which are extracted from plants inexpensively. The dyes were evaluated by UV-visible and FT-IR analyses. The results showed that a broad spectrum can be achieved due to different peaks in the photoabsorption spectra of these dyes. The bottom layer was sensitized by natural dyes while the top layer was sensitized by N719. Due to the effect of temperature on dyes, a low-temperature method based on acid-assistant sintering has been used for the top layer. The results of XRD and FESEM analyses indicated that a layer with proper crystalline phase, acceptable morphology, and good porosity can be achieved in the low-temperature process by tunning the acid concentration. The electrical properties of the fabricated solar cells were investigated by EIS analysis and J-V characteristics. The results showed that charge injection and transportation were improved via the ability of the proposed approach in separated dye adsorption. An efficiency of 3.48% was provided by the proposed method which demonstrates its better performance in comparison with the cocktail and sequential methods that showed efficiencies of 1.88% and 2.29%, respectively. The results indicated that separated sensitization of each layer can improve the dye loading leading to spectral expansion.

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