Hybrid Steroid-[60]Fullerene as n-Type Material for Organic Photovoltaics

Fullerene derivatives have been used as electron acceptor and transport materials in organic photovoltaics as well as in perovskite solar cells. Among them, [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) has been one of the most widely used, in combination with poly-3-hexylthiophene (P3HT) as an electron donor semiconducting polymer, for the fabrication of organic bulk heterojunction solar cells (BHJ-OSC). In this work, a steroid-fullerene hybrid was synthesized through the Bingel-Hirsch reaction to compare its properties with PC61BM as the reference structure. Different spectroscopic techniques were employed to characterize the new fullerene derivative, namely 1D and 2D nuclear magnetic resonance (NMR), Fourier Transform Infrared Spectroscopy (FTIR), and high-resolution mass spectrometry (HRMS). In addition, thermogravimetric analysis (TGA), and cyclic voltammetry were also performed. The characterization was completed by transmission electron microscopy (TEM), which explore the hybrid's morphology, where the presence of the steroid moiety increases the solubility of the fullerene material. DFT/PBE-D3(BJ)/6-311G(d,p) calculations for both the steroid-fullerene hybrid and PC61BM allowed to obtain geometry optimizations and electronic structure data. A comparative study to analyze the nature of the electronic properties of the synthesized steroid-[60]fullerene hybrid and PC61BM with an oligomer of ten thiophene units, used in the active layer of organic solar cells, was carried out. Electron density maps using the [CIS|CNDOL/21] approach illustrate the photoexcitation of the donor followed by an electron-charge transfer to the acceptor fullerene. A new steroid-[60]fullerene hybrid obtained through a cyclopropanation reaction has been synthesized and its optoelectronic properties compared with PC61BM as reference acceptor material in organic solar cells. The structure of monoadducts were fully determined and, amazingly, theoretical calculations, together with the experimental results obtained, demonstrate that the new compound can be considered an appealing candidate for photovoltaic applications.image