Development of Iso-Triazatruxene-Based Hole-Collecting Monolayer Materials for Efficient Perovskite Solar Cells
Authors: Adachi Yuta , Truong Minh Anh , Wakamiya Atsushi 
Published: December 2024

Perovskite solar cells (PSCs) have attracted worldwide attention due to their high power conversion efficiency (PCE) and low-cost solution processing. Besides the film and interfacial engineering of the perovskite layer, the development of hole-collecting materials (HCMs) is a critical key to boosting the performance of PSCs.
In inverted PSCs, the use of chemically adsorbed monolayers that can efficiently collect photo-generated holes from the perovskite layer and transport them to a transparent conducting oxide (TCO) electrode as the hole-collecting layer (HCL) has become one of the mainstream directions.1

Recently, we developed a tripodal molecule composed of a triazatruxene core connected with three phosphonic acid anchoring groups (PATAT).2 We demonstrated that after being chemically adsorbed on the TCO surface, PATAT molecules tend to form a monolayer with a face-on orientation, resulting in improved hole-collection compared to their monopodal counterpart, and the corresponding inverted PSCs using the PATAT monolayer as HCL exhibited PCEs up to 23%. However, PATAT still has its own drawbacks that need to be improved, such as its difficulty in further functionalization and the hydrophobicity of its monolayer.
In this work, we designed a series of isotriazatruxene derivatives bearing three phosphonic acid anchoring groups, iso-PATAT and its tribromo-substituted analogue, iso-PATAT-Br (Figure 1a), as hole-collecting monolayer materials. Isotriazatruxene is an isomer of triazatruxene, which has two indole moieties facing each other, leading to a steric hindrance when bulky substituents such as alkyl phosphonic acid groups were introduced into –NH positions. Taking advantage of this point, after being chemically adsorbed, instead of binding to the TCO surface, some phosphonic acid groups would point upward, leading to a hydrophilic monolayer and improved surface wettability.
The target molecules were synthesized using a facile three-step synthetic route (Figure 1), including Cu-catalyzed cyclization, nucleophilic substitution, and deprotection reactions from commercially available corresponding indole derivatives as starting materials.
Monolayers of the iso-PATAT series were fabricated by spin coating their DMF solution onto ITO substrates. The contact angle of water droplets on treated ITO substrates was measured to evaluate the surface wettability of the resulting monolayers. The contact angle of the water droplet was estimated to be 40 and 60° for iso-PATAT and iso-PATAT-Br-treated ITO, respectively, confirming better surface wettability compared to our previously reported PATAT-based ITO with a water contact angle of 65°. These results also indicate the existence of some hydrophilic phosphonic acid groups on the surface of the monolayer.
The ionization potential of iso-PATAT adsorbed on ITO was estimated to be 5.47 eV, which is shallower than that of its isomer counterpart (PATAT, 5.53 eV). In contrast, the ionization potential of tribromo-substituted iso-PATAT-Br was found to be deeper at 5.61 eV.
Inverted PSC devices using these iso-PATAT molecules as hole-collecting monolayers were fabricated and evaluated. All devices showed PCEs over 22%. In particular, iso-PATAT-Br-based PSCs exhibited superior performances with PCEs up to 24.1% (Figure 1b), which is higher than the devices fabricated with our previously reported PATAT (PCE = 23.5%) under the same condition.
In this presentation, molecular design, characterization, and device evaluation will be discussed in detail
1) A. Al-Ashouri, V. Getautis, S. Albrecht et al., Science
2020, 370, 1300.
2) M. A. Truong, A. Wakamiya et al., J. Am. Chem. Soc.
2023, 145, 7528.

Figure 1