Photo-stable substituted dihydroindolo[2,3-b]carbazole-based organic dyes: tuning the photovoltaic properties by optimizing the π structure for panchromatic DSSCs

Three dihydroindolo[2,3-b]carbazole (DDC) donor based organic dyes (DDC6-DDC8) with long conjugated systems were successfully synthesized and characterized by optical, electrochemical, computational analysis, and photovoltaic methods. It was found that the elongated π system as well as the high molar extinction coefficients (4.2-5.3 M−1 cm−1) sufficiently reinforced the light harvesting ability with the IPCE response of one dye DDC8 extended to 850 nm, which resulted in large Jsc of these dyes based devices (high as 16.41 mA cm−2). The attached alkyl chains suppress the dark current to some extent, resulting the DDC6 and DDC7 devices in high Voc, while DDC8 displayed low Voc because BTD in DDC8 could attract the iodine ion, which could accelerate the charge recombination. Among the three dyes, DDC7 displayed the best performance with an η value 6.53% under 4 μm thick scattering layer (condition 1+1), which was boosted to 7.49% under 8 μm thick scattering layer (condition 1+2). The photo-stability measurements indicated that all the three dyes are fairly photo-stable.

Three metal-free dyes based on dihydroindolo[2,3-b]carbazole donor DDC6-DDC8 have been synthesized and applied in DSSCs. It was found that these dyes have high molar extinction coefficients, very broad IPCE spectra, high performance, and good photo-stability.