Improving the Conductivity of Amide-Based Small Molecules through Enhanced Molecular Packing and Their Application as Hole Transport Mediators in Perovskite Solar Cells

Abstract

Organic-inorganic hybrid halide perovskite solar cells (PSCs) have attracted substantial attention from the photovoltaic research community, with the power conversion efficiency (PCE) already exceeding 26%. Current state-of-the-art devices rely on Spiro-OMeTAD as the hole-transporting material (HTM); however, Spiro-OMeTAD is costly due to its complicated synthesis and expensive product purification, while its low conductivity ultimately limits the achievable device efficiency. In this work, we build upon our recently introduced family of low-cost amide-based small molecules and introduce a molecule (termed TPABT) that results in high conductivity values (∼10^-5 S cm^-1 upon addition of standard ionic additives), outperforming our previous amide-based material (EDOT-Amide-TPA, ∼10^-6 S cm^-1) while only costing an estimated $5/g. We ascribe the increased optoelectronic properties to favorable molecular packing, as shown by single-crystal X-ray diffraction, which results in close spacing between the triphenylamine blocks. This, in turn, results in a short hole-hopping distance between molecules and therefore good mobility and conductivity. In addition, TPABT exhibits a higher bandgap and is as a result more transparent in the visible range of the solar spectrum, leading to lower parasitic absorption losses than Spiro-OMeTAD, and has increased moisture stability. We applied the molecule in perovskite solar cells and obtained good efficiency values in the ∼15% range. Our approach shows that engineering better molecular packing may be the key to developing high-efficiency, low-cost HTMs for perovskite solar cells.

Figure 1

Reaction scheme and molecular structure of TPABT.

Figure 2

(a) View of the structure of TPABT-CHCl₃ in the [001] direction highlighting the solvent-accessible channels. Hydrogen atoms and solvent molecules were omitted for clarity. (b) Typical hydrogen-bonded dimer of TPABT molecules as observed in each TPABT structure.

Figure 3

Frontier orbital distributions of TPABT (top left: HOMO; bottom left LUMO) and EDOT-Amide-TPA (top right: HOMO; bottom right: LUMO) in dichloromethane, from DFT (PBE0/def2-SV(P)).

Figure 4

(a) Normalized UV–visible absorption spectra of TPABT (50 nm thickness), EDOT-Amide-TPA (45 nm), and Spiro-OMeTAD (200 nm) as a thin film that was applied by spin coating under the same conditions as the solar cells (below). (b) Normalized cyclic voltammogram of TPABT and EDOT-Amide-TPA measured in anhydrous dichloromethane under N₂ with 0.1 M TBAPF₆.

Figure 5

(a) Thermogravimetric analysis of pristine TPABT, EDOT-Amide-TPA, and Spiro-OMeTAD at a heating rate of 5 °C min^–1 under N₂ atmosphere. (b) Differential scanning calorimetry (first cycle of heating) for TPABT, EDOT-Amide-TPA, and Spiro-OMeTAD at the heating rate of 5 °C min^–1.

Figure 6

(a) Conductivity of the film vs concentration of LiTFSI. 20% error is added to account for the deviation in film thickness across the samples (± 10 nm). (b) J–V measurements of hole-only devices.

Figure 7

(a) J–V curves collected under AM 1.5 simulated sunlight of the champion device comprising TPABT and Spiro-OMeTAD in combination with FAMACs perovskite on SnO₂. (b) Energy level diagram of TPABT with perovskite and ETL.