A Partially-Planarised Hole-Transporting Quart-p-Phenylene for Perovskite Solar Cells

Herein, we describe the synthesis of a hole transporting material based on a partially planarised quart-p-phenylene core incorporating tetraketal and diphenylamine substituents that show optimal energy levels and solubility for perovskite solar cell applications. Triple-cation perovskite devices incorporating such quart-p-phenylene derivative show power-conversion efficiencies, short circuit currents, open circuit voltages, and fill factors that are comparable to those of spiro-OMeTAD. Abstract Herein, we describe the synthesis of a hole transporting material based on a partially planarised quart-p-phenylene and substituents that show optimal energy levels and solubility for perovskite solar cell applications. Triple-cation perovskite devices incorporating such quart-p-phenylene derivative show power-conversion efficiencies, short circuit currents, open circuit voltages, and fill factors that are comparable to those of spiro-OMeTAD.

In recent years, perovskite solar cells (PSCs) have rapidly emerged as a promising technology in photovoltaics. [1][2][3][4][5][6][7][8][9][10] Current state-of-the-art PSCs have surpassed the power-conversion efficiencies (PCEs) of organic solar cells [11][12][13][14] and dyesensitized solar cells, 2,[15][16][17] and recently surpassed those reached with the current market leader polycrystalline silicon. [18][19][20][21] In PSCs, light is harvested by organicinorganic metal halide perovskites, which show strong absorption in the UV-visible-near infrared range, large free charges diffusion lengths, small exciton binding energy, and low-cost fabrication. The free charges formed in the perovskite after the absorption of light are separated in electrons and holes, which are respectively transported to the electrodes by an electron transporting material (ETM) and a hole transporting material (HTM) to close the circuit. 8,22,23 Currently, the most commonly used small molecule as HTM is 2,2′,7,7′tetrakis(N,N-bis(p-methoxyphenyl)amino)-9,9′-spirobifluorene 20 (spiro-OMeTAD) ( Figure 1). However, the long synthetic route of spiro-OMeTAD limits its suitability for large-scale industrial applications. Furthermore, the exploration of other HTM provides key and valuable information for the design of new materials with improved processability and performance for PSCs. Spiro-OMeTAD is constituted by two fluorene units in a spiro configuration with two triphenylamines at the 2 and 7 positions. Its electronic structure is analogous to that of a planarised p-biphenylene. Oligo-and poly-p-phenylenes 24,25 are a class of conjugated materials that have received much interest as hole transporting materials, [26][27][28][29][30][31] since the HOMO level can be controlled with the number of rings in the p-phenylene chain. 24,25 Nevertheless, the phenylene rings adopt a nonplanar conformation with a dihedral angle that (i) partially interrupts the longitudinal conjugation resulting in a lower effective conjugation, and also (ii) interferes with the co-facial packing optimal for charge transport. Therefore, planarised p-phenylenes with a higher effective conjugation are attractive candidates for the design of efficient HTM.
Herein, we report the synthesis and characterisation of a novel HTM based on a partially planarised quart-p-phenylene structure (1), in which the two central rings are embedded on a planar tetrahydropyrene core with four lateral tetraketals ( Figure 1) that enhance the stability of the tetrahydropyrene core against oxidation and also that can potentially passivate the perovskite surfaceby coordination [32][33][34][35][36]  Quart-p-phenylene 1 is easily obtained in a gram-scale via Suzuki crosscoupling reaction between tetrahydropyrene 2 and triarylamine 3 in a good yield (63%). Precursors 2 37-40 and 3 41 were prepared from commercially available compounds in three and one steps, respectively. Quart-p-phenylene 1 showed a high stability under ambient conditions without any sign of decomposition after several months. Also, thermogravimetric analysis of 1 reveals a thermal stability up to circa 280 °C ( Figure S4). Moreover, quart-p-phenylene 1 was soluble in common organic solvents (toluene, chlorobenzene, CH2Cl2, and CHCl3).
The structure of quart-p-phenylene 1 was determined by 1 H-NMR, 13  On the other hand, the torsion angle of the more external phenyl rings of the quart-p-phenylene core with the tetrahydropyrene reside is substantially higher (24°) as an effect of the free rotation. The distance of the central C-C bond between the phenyl rings of the tetrahydropyrene core was 1.468 Å, in agreement with the bond distance observed in tetrahydropyrene (1.470 Å) 42 for the same bond, which is slightly larger than that in pyrene (1.422 Å).
The optical properties of 1 were investigated in solution. Figure 2b shows the UV-Vis absorption and the normalized photoluminescence spectra of 1 in  Table S1). In addition, the natural transition orbitals were also computed to aid the  To test the behavior of quart-p-phenylene 1 as a HTM, we prepared regular mesoporous PSCs with the triple-cation perovskite 7

Perovskite precursor solution and film preparation
The perovskite films were deposited from a precursor solution containing FAI (1 M The substrates were then annealed at 100°C for 45 min in the glove box.

Hole transporting layer and top electrode
After perovskite annealing, the substrates were cooled down for few minutes, and