D-π-A-Type Pyrazolo[1,5- a ]pyrimidine-Based Hole-Transporting Materials for Perovskite Solar Cells: Effect of the Functionalization Position.

Donor-acceptor (D-A) small molecules are regarded as promising hole-transporting materials for perovskite solar cells (PSCs) due to their tunable optoelectronic properties. This paper reports the design, synthesis and characterization of three novel isomeric D-π-A small molecules PY1 , PY2 and PY3 . The chemical structures of the molecules consist of a pyrazolo[1,5- a ]pyrimidine acceptor core functionalized with one 3,6-bis(4,4'-dimethoxydiphenylamino)carbazole (3,6-CzDMPA) donor moiety via a phenyl π-spacer at the 3, 5 and 7 positions, respectively. The isolated compounds possess suitable energy levels, sufficient thermal stability (T d > 400 °C), molecular glass behavior with T g values in the range of 127-136 °C slightly higher than that of the reference material Spiro-OMeTAD (126 °C) and acceptable hydrophobicity. Undoped PY1 demonstrates the highest hole mobility (3 × 10 -6 cm 2 V -1 s -1 ) compared to PY2 and PY3 (1.3 × 10 -6 cm 2 V -1 s -1 ). The whole isomers were incorporated as doped HTMs in planar n-i-p PSCs based on double cation perovskite FA 0.85 Cs 0.15 Pb(I 0.85 Br 0.15 ) 3 . The non-optimized device fabricated using PY1 exhibited a power conversion efficiency (PCE) of 12.41%, similar to that obtained using the reference, Spiro-OMeTAD, which demonstrated a maximum PCE of 12.58% under the same conditions. The PY2 and PY3 materials demonstrated slightly lower performance in device configuration, with relatively moderate PCEs of 10.21% and 10.82%, respectively, and slight hysteresis behavior (-0.01 and 0.02). The preliminary stability testing of PSCs is also described. The PY1- based device exhibited better stability than the device using Spiro-OMeTAD, which could be related to its slightly superior hydrophobic character preventing water diffusion into the perovskite layer.