A comparative study of the influence of N,N′-dialkyl vs. N,N′-diaryl-based electron donor ancillary ligands on photocurrent and photovoltage in dye-sensitized solar cells (DSSCs)

Abstract

In this study, we report the synthesis of a novel heteroleptic Ru(II)-sensitizer, (Ru(2,2′-bipyridine-4,4′-dicarboxylic acid)-4,4′-bis(4-piperidin-1-yl)phenyl ethenyl)-(2,2′-bipyridine) (NCS)₂, denoted as SD-1; moreover, its photophysical, electrochemical, and photovoltaic performances were compared with those of N719 and K77-7 (N,N′-diaryl Ru-sensitizer, namely Ru(2,2′-bipyridine-4,4′-dicarboxylic-acid)-4,4′-bis(2-(4-N,N′-diphenylaminophenyl)ethenyl)-2,2′-bipyridine (NCS)₂). The photovoltaic performance of SD-1 outperformed those of N-719 and K77-7, particularly in the red region, and the overall efficiency of SD-1 was 8.5% as compared to 8.0% of K77-7 and 7.7% of N719 under the same experimental device conditions. The superior light harvesting efficiency of SD-1 can be attributed to the strong electron donor sp³-nitrogen, which is attached to two sp³-carbons (dialkyl), whereas in the case of K77-7, all carbon atoms attached to the sp³-nitrogen are sp², which decrease the electron density on the latter and minimize the electron-donating power of the ancillary ligand in K77-7. To gain a quantitative understanding of the electron density on nitrogen in SD-1 and K77-7, first-principle calculations using molecular and thermodynamic descriptors, such as frontier molecular orbitals, ground-state oxidation potential (GSOP), excited-state oxidation potential (ESOP), optical gap (E_0–0), and charge distributions, were conducted in solution. In addition, for understanding the anchored structures of dyes on Ti₂₄O₄₈, density functional theory (DFT) and time-dependent density functional theory (TD-DFT) were utilized. Results of computational studies are in excellent agreement with the experimental results, which can be used as a screening tool for the design of more efficient molecular motifs for DSSCs.