Impact of Molecular Design and Conjugation in Diammonium A′‐Cations on the Photovoltaic Performance of Quasi‐2D Perovskite Solar Cells

Bulky organic spacers are widely employed to synthetize quasi‐two‐dimensional (quasi‐2D) metal halide perovskites (MHPs), which display enhanced stability with respect to three‐dimensional (3D) MHPs. The chemical nature and size of the bulky spacers play a key role in determining the performance of the solar cells by controlling the orientation of the octahedral layers and the charge transport. This work investigates how π‐conjugation versus nonconjugation in bulky spacers impacts the structure and properties of quasi‐2D MHPs and their photovoltaic performance in lead halide perovskite solar cells. Thus, diammonium spacers with related chemical structures and based on diphenylacetylene units ( 1 , 2 ) or biphenyl flanked with alkyne moieties ( 3 ) are synthesized and investigated in comparison with the nonconjugated 4,4′‐ethylenedianiline ( ET ) spacer (SP). Quasi‐2D Dion–Jacobson MHPs are fabricated with a nominal n = 5, i.e., blocks of five octahedral layers are separated by the organic spacers and a chemical composition of SP(FA 0.9 Cs 0.1 ) 4 Pb 5 I 16 (SP = ET , 1 , 2 , or 3 ). The preferential orientation of the octahedral layers was studied by X‐ray diffraction measurements, revealing better alignment in the quasi‐2D MHPs containing the conjugated spacers. Defect concentration was estimated from space‐charge limited‐current (SCLC) measurements resulting in lower values for the quasi‐2D MHPs with the conjugated cations with respect to that prepared with the nonconjugated ET spacer. The lowest defect density was found for the film with the conjugated spacer 1 , which is in line with the slower photoluminescence (PL) decay for this sample. The solar cells prepared with the quasi‐2D MHPs incorporating the nonconjugated ET spacer display power conversion efficiencies (PCEs) of around 11%, while approx. 13% are reached in the devices prepared with the conjugated spacers 1 , 2 , and 3 . Our study demonstrates that the incorporation of conjugation in bulky spacers decreases the defect density and ion mobility leading to higher PCEs.