Squaraine (SQ) dyes have gained significant attention in the scientific community due to their versatility, tunable structures, photothermal stability, and exceptional photophysical properties, including their ability to exhibit fluorescence under specific conditions.[1] SQ dyes feature a donor-acceptor-donor (D-A-D) architecture, where the donor part is an electron-rich group, typically an aromatic or conjugated structure, that donates electrons upon excitation. The central core, typically a butanedione group, is electron-deficient and serves as the acceptor unit (A), enabling efficient electron transfer from the donor to the acceptor upon excitation.[2,3]  To stabilize the D-A-D system, strong electron-donating groups, such as heterocycles or electron-rich aromatic units like aniline derivatives, are often incorporated into squaraine dyes. Symmetric SQ dyes have two equivalent donor units, while unsymmetrical squaraine (USQ) dyes feature two different donor units.[4]

In this study, several unsymmetrical squaraine (USQ) dyes were synthesized using a simple two-step method involving a condensation reaction between electron-rich aromatic amines and squaric acid, which minimizes byproducts and simplifies purification. The synthesized products were characterized using ¹H-NMR, ¹³C-NMR and FT-IR to confirm their structures. The absorption and emission properties of the dyes were evaluated through UV-Vis and fluorescence spectroscopy.

Photovoltaic (PV) devices were fabricated by depositing the cell components using thermal vapor deposition (< 5.0x10^-6 Torr) on pre-patterned indium tin oxide (ITO) glasses (c.a. 100 nm).[5] The structure of the assembled cell consist of 8 nm layer of MoO₃ as hole-extracting layer (HEL), a 40 nm co-deposition of the SQ dye with fullerene (C60) as the active layer, a 8 nm layer of BPhen as the electron transport layer (ETL) and 8-hydroxyquinolinato lithium (Liq) as electron extracting layer (EEL) and a final 100 nm aluminum electrode. These devices exhibited homogeneous morphologies that reduced charge recombination. The resulting PV cell achieved an average power conversion efficiency (PCE) of 3.38%, attributed to the optimal alignment of HOMO/LUMO levels and the spectral synergy between SQ dyes and C60 fullerenes. This study provides essential guidelines for optimizing these materials to further enhance the performance of SQ-based photovoltaic cells.