Versatile Hole Selective Molecules Containing a Series of Heteroatoms as Self‐Assembled Monolayers for Efficient p‐i‐n Perovskite and Organic Solar Cells

Abstract

Inverted type perovskite solar cells (PSCs) have recently emerged as a major focus in academic and industrial photovoltaic research. Their multiple advantages over conventional PSCs include easy processing, hysteresis‐free behavior, high stability, and compatibility for tandem applications. However, the maximum power conversion efficiency (PCE) of inverted PSCs still lags behind those of conventional PSCs because suitable charge‐selective materials for inverted PSCs are limited. In this study, excellent hole‐selective materials for inverted PSCs are introduced. A series of tricyclic aromatic rings containing O, S, or Se, respectively, as a core heteroatom, along with a phosphonic acid anchor, form a self‐assembled monolayer (SAM) that directly contacts the perovskite absorber. The influence of heteroatoms in the aromatic structure on the molecular energetics and operating characteristics of the corresponding inverted PSCs is investigated using complementary experimental techniques as well as density functional theory (DFT) calculations. It is found that all of the SAMs formed an energetically well‐aligned interface with the perovskite absorber. The interaction energy between the Se‐containing SAM and perovskite absorber is the strongest among the series and it reduces the interfacial defect density, in turn leading to an extended charge carrier lifetime. As a result, PSCs incorporating the Se‐containing SAM achieves a PCE of 22.73% and retains ≈96% of their initial efficiency after a maximum power point tracking test of 500 h without encapsulation under ambient conditions. All of the SAMs are then employed in organic solar cells (OSCs). Again, the Se‐containing SAM‐based OSCs demonstrates the highest PCE of 17.9% among the three molecular SAM‐based OSCs. This study demonstrates the great potential for precisely engineered SAMs for use in high‐performance solar cells.

Towards cost-efficient and stable perovskite solar cells and modules: utilization of self-assembled monolayers

“…Generally, carbazole-, triphenylamine- and phenothiazine-based molecules display excellent ability for hole transfer. Besides carbazole derivative-based SAMs in PSCs, Ullah et al 41 designed a halogenated phenothiazine-based SAM as the HTL in inverted PSCs (Fig. 4).…”

Self-assembled monolayers as emerging hole-selective layers enable high-performance thin-film solar cells

“…The functional head group endows SAMs with certain properties or functions, and mainstream SAMs are obtained on the basis of carbazole and phenothiazine derivatives for their suitable energy alignment and wide commercial availability. 3 , 4 , 5 Their small conjugation system provides good affinity and low parasitic absorption to OSCs, 3 , 4 but for PSCs, an additional chemical modification on the functional heads is needed to further address the issues in practical situations, like surface wettability and defect passivation. 2 , 5 The flexible alkyl chain is frequently selected as the spacer, and it works synergistically with the head group to optimize the molecular conformation by restricting the movement of the heads, preserving the molecular flexibility and offering interchain van der Waals force.…”

“…In PSCs, the control of nucleation and crystal growth is pivotal for obtaining a high-quality active layer. In order to modulate the interactions with perovskite precursors, it is crucial to graft substituent groups onto the functional heads, 2 , 4 , 5 such as heteroatoms (like S and O) and substituent groups (like amines and halogens). It not only solves the wetting issues, passivates defects at the active layer/SAM interface, and suppresses the interfacial recombination but also facilitates the wide applications of SAMs in PSCs.…”

Tripodal Triazatruxene Derivative as a Face-On Oriented Hole-Collecting Monolayer for Efficient and Stable Inverted Perovskite Solar Cells

“…The viability of this concept was first demonstrated by Magomedov et al They used carbazole derivatives (known as PACz series) with phosphonic acid anchoring groups which spontaneously adsorb onto the surface of metal oxide substrates to form a conformal monolayer. Since then, several molecules with further structural modifications have been reported to function as hole-collecting monolayers. − All these structures are composed of a π-conjugated backbone connected to a single anchoring group. When chemically adsorbed, these monopodal molecules tend to orient with their π-plane perpendicular (edge-on) to the metal oxide surface. , This molecular alignment could result in inefficient orbital overlap with the perovskite surface and/or unsought hole transport in the lateral direction, causing undesired interfacial recombination and interfering with the hole extraction process (Figure a). ,, …”