Understanding how coordination complexes couple to metal electrodes is central to interfacial electron-transfer theory. Although the Fe(CN)₆3−/4− redox couple is nearly reversible at the Pt(111) single crystal surface in pH 3 sulfate media, it becomes quasi-reversible in 0.1 M H₂SO₄, reflecting pronounced interfacial restructuring under strongly acidic conditions. We show that exposure to ferri/ferrocyanide does not result in adsorption of intact Fe(CN)₆ species; instead, it induces ligand-selective modification of Pt(111), forming a CN-terminated, stratified interface. XPS reveals a dominant N 1 s feature at 397.36 eV, characteristic of surface-bound CN, accompanied by weak Fe 2p intensity, indicating minimal direct coupling of the Fe center to the electrode. In situ STM resolves ordered Pt(111) – (3 × 2√3)rect domains in pH 3 sulfate that deteriorate at positive potentials. In contrast, under pH 1 conditions, only short-range order is observed together with formation of an additional ∼2.5 Å interfacial layer above ∼0.2 V. This outer layer increases the effective electron-transfer distance and weakens coupling, thereby retarding kinetics. Collectively, these results establish that Fe(CN)₆3−/4− probes a ligand-defined interface in which a CN-terminated inner layer and a dynamically structured outer layer govern outer-sphere electron-transfer behavior, with kinetics controlled primarily by interfacial organization rather than direct molecular adsorption.

Electrochemical energy and substance conversion devices involve complex electrode processes characterized by multiple charge transfer steps, competing pathways, and various intermediates. Such complexity makes it challenging to enhance the electrocatalytic activity. The prevailing strategy typically focuses on optimizing the geometric and electronic structures of the electrocatalysts to align the adsorption energies of reaction intermediates with the peak of the activity Volcano curve. In this study, we demonstrate that surface decoration can effectively shape the micro reaction environment for the model system of the oxygen reduction reaction (ORR) on Pt electrodes. By applying a partial hydrophobic I* adlayer on the Pt surface, we can shift the equilibrium of OH* reduction and weaken H2O* adsorption, which significantly enhances ORR kinetics. With in situ scan tunneling microscopy (STM) and theoretical calculations, our study reveals the formation of isolated Pt-2 surface units situated in a hydrophobic valley surrounded by adsorbed iodine atoms. This minimalist Pt-2 active unit exhibits significantly greater activity for the ORR compared to an extended Pt surface. Our results overturn the previous consensus that the I* adlayer always poisons the electrocatalytic reaction. Our systematic studies also reveal that whether I* acts as a poison or a promoter, as well as the extent of its promotional effect, depends on its coverage. This finding could pave the way for developing highly efficient catalysts with potential applications in fuel cell technology and metal air batteries and extension to other electrochemical conversion reactions such as ammonia synthesis and CO2 reduction.

Electrode potential provides a powerful external parameter for directing molecular organization at electrified interfaces, yet how molecular backbone chemistry converts this control into distinct assembly pathways remains poorly understood. Here we investigate the potential-dependent adsorption of two pi-conjugated brominated molecules, PPr-4Br and DTPPr-4Br, on Au(111) using in situ electrochemical scanning tunneling microscopy together with voltammetry. Single-crystal X-ray diffraction confirms the planar geometry of the pyrrolopyrrole core in both molecules, enabling extended pi-conjugation. Within a low-potential window (0.2-0.4 V vs Ag/AgCl), both molecules adsorb in predominantly flat-lying configurations aligned along the < 110 > direction of Au(111), forming sparse chain-like adlattices characteristic of pi-Au interactions. Upon positive polarization, however, their structural evolution diverges markedly. PPr-4Br retains substantial lateral mobility and reorganizes reversibly into denser coincidence structures with increasing coverage. In contrast, incorporation of thiophene units in DTPPr-4Br strengthens molecule-substrate coupling, producing coverage-insensitive low-density arrangements and inducing premature lifting of the Au(111) reconstruction, ultimately limiting long-range order. These results demonstrate that electrode potential governs electrochemical self-assembly through a competition between admolecular mobility and substrate pinning, and that subtle variations in pi-conjugated backbone structure strongly shift this balance. The findings provide direct atomic-scale insight into how molecular design and electrochemical control cooperate to determine interfacial organization of pi-conjugated systems on metal surfaces.

Understanding how electrochemical parameters-such as electrode potential, electrolyte composition, and pretreatment conditions-influence surface reconstruction and the resulting electrocatalytic activity is essential for elucidating the intrinsic structure-activity relationship. We investigate the reversible surface reconstruction between the hexagonal (hex) and (1 & times; 1) phases of Au(100) in alkaline media w/o O-2. Using cyclic voltammetry, we systematically investigated the effects of potential scan rate, temperature, electrochemical pretreatment, and atmosphere on the kinetics of the potential-induced (hex) <-> (1 & times; 1) transition. The (hex) -> (1 & times; 1) de-reconstruction occurs only at potentials >= 1.0 VRHE and is tightly coupled to OH-adsorption. The resulting (1 & times; 1) structure remains metastable at lower potentials, exhibiting a structural memory effect. The (1 & times; 1) -> (hex) reconstruction is kinetically sluggish and occurs only at potentials < 0.85 VRHE. The (hex) -> (1 & times; 1) de-reconstruction process is accelerated by higher temperature and modulated by O-2 via interaction with ORR intermediates. This dynamic surface reconstruction correlates directly with the anomalous ORR behavior. The higher activity observed during the reverse scan is attributed to a surface enriched in (1 & times; 1) domains formed at high potentials. Conversely, the switch from the 4e-to the 2e-ORR pathway at E <0.7 VRHE arises, in part, from the enrichment of the (hex) structure. Our results highlight surface structural information under reaction conditions as a key factor in understanding the mechanism and kinetics in electrocatalysis.

Efficient, low-cost electrocatalysts are essential for sustainable hydrogen production in alkaline media, where the hydrogen evolution reaction (HER) is limited on platinum (Pt) by sluggish water dissociation. Surface modification of Pt with 3d metals such as nickel (Ni) can create a Pt-Ni bifunctional interface, in which Ni and Pt atoms interact with the O and H ends of H2O, respectively, to facilitate O-H bond cleavage. In this work, we systematically investigated how Ni loading on a well-defined Pt(111) single crystal governs HER activity, using cyclic voltammetry coupled with in situ scanning tunneling microscopy (STM). Three Ni coverages were investigated: 0.71, 1.8, and 6.6 monolayers (ML), where 1 ML represents the amount of Ni required to form a complete atomic layer on Pt(111). These are referred to as Samples A, B, and C, respectively. Among them, Sample B exhibits the highest HER activity, reaching 10 mA cm-2 (corresponding to a H2 production rate of 52 nmol s-1 cm-2) at an overpotential of only 50 mV, compared to 150 mV and 200 mV required for Samples A and C, respectively. High-resolution STM reveals distinct potential-dependent interfacial structures: triangular fractal Ni hydroxide domains at pre-HER potentials and moire-type metallic Ni overlayers under HER conditions. These findings demonstrate that both the oxidation state and areal coverage of Ni critically determine HER performance, underscoring the importance of precisely tuned Pt-Ni interfacial architectures for optimizing alkaline HER catalysis.

While the S-Au-S motif has been established for thiol molecules adsorbed on a Au(111) substrate, the current in situ scanning tunneling microscopy (STM) study on the adsorption of 2-mercaptopyridine (2-MPY) on a (111)-oriented Au single crystal bead revealed a different adsorption mode under potential control in 0.1 M H2SO4. 2-MPY molecules were adsorbed and self-assembled rapidly into ordered 2D arrays on this Au(111) electrode immersed in 50 mu M 2-MPY ethanol and aqueous dosing solutions for 1 min. The modified Au(111) surface was atomically smooth without vacancy islands (VI). A series of ordered 2-MPY structures were identified by molecular resolution STM imaging, which were stable against the potential modulation between 0.2 and 0.9 V (versus Ag/AgCl). However, the ordered 2-MPY adlayer dissolved rapidly and irreversibly when the potential was made negative to E < 0.1 V, which is presumed to stem from protonation and desorption 2-MPY admolecule from the Au(111) electrode. The 2-MPY spatial structures found in the current study are different from the typical (p x root 3) structures reported for thiols on Au(111). High-quality STM imaging discerned the internal structures of 2-MPY admolecules, from which their binding modes with the Au electrode are inferred. The surface coverage, spatial structure, and degree of ordering for the 2-MPY adlayer changed with the dosage and media.

The adsorption behavior of bromobenzene (BrB) on an Au(111) electrode was investigated using in situ scanning tunneling microscopy (STM) under potential control in 0.1 M sulfuric and perchloric acid solutions. Real-time STM imaging revealed that BrB adsorption induced immediate structural changes on the Au(111) surface, including the formation of 2.3 & Aring; deep vacancy islands (VIs) and predominantly disordered BrB structures. The preferential interaction of BrB with Au adatoms, rather than the Au(111) terrace, likely drove VI formation. As the potential increased, BrB coverage expanded, leading to a structural transition from 1D molecular chains to 2D arrays, and eventually to a well-ordered 3D multilayer film. Concurrently, the BrB molecular orientation shifted from a flat-lying to an upright configuration. At positive potentials, the multilayer BrB film remained structurally stable but dissolved upon a negative potential shift and irreversibly decomposed at more negative potentials. Additionally, BrB assembled differently in perchloric acid, highlighting the critical role of anions in interfacial organization. Local linear BrB structures preferentially aligned along the (121) direction of Au(111), forming triangular fractal patterns and an increasingly disordered 3D film at positive potentials.

Electrochemical energy and substance conversion devices involve complex electrode processes, characterized by multiple charge transfer steps, competing pathways, and various intermediates. Such complexity makes it challenging to enhance electrocatalytic activity. The prevailing strategy typically focuses on optimizing the geometric and electronic structures of the electrocatalysts to align the adsorption energies of reaction intermediates with the peak of the activity Volcano curve. In this study, we demonstrate that surface decoration can effectively shape the micro reaction environment for the model system of oxygen reduction reaction (ORR) on Pt electrodes. By applying a partial hydrophobic I* adlayer on the Pt surface, we can shift the equilibrium of OH* reduction and weaken H2O* adsorption, which significantly enhances ORR kinetics. With in situ scan tunneling microscopy (STM) and theoretical calculations, our study reveals the formation of isolated Pt2 surface units situated in a hydrophobic valley surrounded by adsorbed iodine atoms. This minimalist Pt2 active unit exhibits significantly greater activity for ORR compared to an extended Pt surface. This strategy could pave the way for developing highly efficient catalysts with potential applications in fuel cell technology and metal air batteries and extension to other electrochemical conversion reactions such as ammonia synthesis and CO2 reduction.

We report the anion-modulated self-assembly and electrochemical polymerization of 2-bromothiophene (BrT) on Au(111), probed by cyclic voltammetry and in situ scanning tunneling microscopy (STM). BrT forms ordered adlayers with distinct unit cells-(3 x 3), (3 x 2/3), (4 x /31), and (/7 x /13)-whose packing density depends on the applied potential and electrolyte anion. In H2SO4, bisulfate coadsorption stabilizes open, dynamic BrT structures, whereas in HClO4, weak perchlorate adsorption enables compact packing at higher potentials. At anodic bias (>= 0.9 V verse Ag/AgCl), BrT undergoes irreversible oxidative coupling, yielding surface-bound oligothiophenes aligned with Au(111) crystallography. The dual S/Br anchoring of BrT ensures stable adsorption, preferential alignment, and robust film growth. These results reveal a critical role of anion-molecule interactions in directing surface assembly and demonstrate a tunable pathway for potential-controlled nanostructure fabrication.

11-acryloylamino undecanoic acid (AAUA) is a versatile polymerizable surfactant that has been applied to coat medical devices, and these applications can benefit from a fundamental understanding of its interaction with a metal substrate. Cyclic voltammetry and in situ scanning tunneling microscopy (STM) were used to examine the adsorption configuration of AAUA molecules on an ordered Au(111) electrode and their mutual interactions, as AAUA was adsorbed from a methanol dosing solution. In addition to the van der Waals force between the aliphatic groups, the hydrogen bonding between the carboxylic acid and acrylamide groups was also important to guide the spatial arrangement of AAUA admolecules on the Au electrode. The -COOH group of AAUA admolecule likely dissociated in neutral media to -COO-, which formed hydrogen bonds with H2PO4- in phosphate buffer solution (PBS). This interaction between the AAUA admolecules and ions in the electrolyte resulted in different electrochemical characteristics observed in phosphate buffer solution (PBS) and potassium sulfate (K2SO4). Molecular-resolution STM imaging revealed distinctly different AAUA spatial structures on the Au electrode in PBS and K2SO4. Shifting the potential positively to 0.5 V (versus Ag/AgCl) led to lifting of the reconstructed Au(111) to the (1 x 1) phase and the dissolution of the ordered AAUA film, suggesting that the orientation of the AAUA admolecule was altered. The ordered AAUA adlayer could be partially recovered by shifting the potential negatively.

The adsorption of pyridazine (PD) on a gold electrode served as a model to study the orientation and arrangement of an organic adsorbate bearing a heterocyclic molecular structure at an electrified interface. Because the two N-ends of PD could interact with a Au electrode, the acid-base equilibrium of PD at an electrified interface can be different from that in a solution phase. Unprotonated and protonated PD interacted with the Au electrode differently, leading to dissimilar spatial structures on the Au(1 11) electrode. Moreover, having a large dipole moment of 4.22 D, PD adopted different adsorption configurations, as the Au potential was modulated. The indispensable anion present in the supporting electrolyte can compete with PD for surface sites on the Au electrode. Since the potential of a charged conductor is established by the physical contact with an electrolyte, the study of the adsorption of PD on a Au(1 11) electrode calls for the use of an in situ tool, such as a scanning tunneling microscope (STM), which provides sub-nanometer information of the interface in real-time and real- space. Different phases of adsorbed PD molecules on an ordered Au(1 11) electrode were revealed as a function of potential, pH and anion. The most notable event for PD adsorbed on Au(1 11) is attributed to the sharp transition from a disordered state to highly organized structures in 0.1 M H2SO4 2 SO 4 and HClO4. 4 . The obtained STM images enabled a direct measurement of the spacing between PD admolecules, from which the molecular reorientations on the Au electrode were inferred. PD admolecules flipped from the horizontal to the upright configuration, tethered to Au substrate via the two N-ends, at positive potentials. Different ordered PD structures seen in H2SO4 2 SO 4 and HClO4 4 imply anions were coadsorbed with PD molecule on the Au electrode. The acid-base equilibrium of PD at the Au electrode was examined with STM in pH 1 and 3 media.

A biocompatible and antifouling polymeric medical coating was developed through rational design for anchoring pendant groups for the modification of stainless steel. Zwitterionic 2-methacryloyloxyethyl phosphorylcholine (MPC) was copolymerized individually with three anchoring monomers of carboxyl acrylamides with different alkyl spacers, including acryloylglycine (2-AE), 6-acrylamidohexanoic acid (6-AH), and 11-acrylamidoundecanoic acid (11-AU). The carboxylic acid groups are responsible for the stable grafting of copolymers onto stainless steel via a coordinative interaction with metal oxides. Due to hydrophobic interaction and hydrogen bonding, the anchoring monomers enable the formation of self-assembling structures in solution and at a metallic interface, which can play an important role in the thin film formation and functionality of the coatings. Therefore, surface characterizations of anchoring monomers on stainless steel were conducted to analyze the packing density and strength of the intermolecular hydrogen bonds. The corresponding copolymers were synthesized, and their aggregate structures were assessed, showing micelle aggregation for copolymers with higher hydrophobic compositions. The synergistic effects of inter/intramolecular interactions and hydrophobicity of the anchoring monomers result in the diversity of the thickness, surface coverage, wettability, and friction of the polymeric coatings on stainless steel. More importantly, the antifouling properties of the coatings against bacteria and proteins were strongly correlated to thin film formation. Ultimately, the key lies in deciphering the molecular structure of the anchoring pendants in thin film formation and assessing the effectiveness of the coatings, which led to the development of medical coatings through the graft-onto approach.

The electrodeposition of foreign metals on a platinum (Pt) electrode can be used to alter the electrochemical properties of the modified electrode. The current study employed in situ scanning tunneling microscopy (STM) to characterize the spatial structures of a monolayer and multilayer lead (Pb) electrodeposited on an ordered Pt(100) electrode in 0.1 M perchloric acid (HClO4) with and without formic acid under potential control. The Pt(100) bead crystal used in this study was pretreated by a modified annealing and quenching method, producing a long-range ordered, unreconstructed (1 x 1) surface. The underpotential deposition (UPD) of Pb on the Pt(100) electrode resulted in a pair of sharp and reversible peak at 0.5 V (vs. Ag/AgCl), prior to the bulk deposition at E < -0.45 V in 0.1 M HClO4 + 1 mM Pb(ClO4)(2). Cyclic potential sweeping in the UPD regimes resulted in a stable voltammogram, but bulk Pb deposition led to varying morphologies of the profiles. Atomic resolution STM images were obtained to reveal a well-ordered Pt(100)-(root 2 x root 2)R45 degrees-Pb structure and elongated rows of Pb aggregates and local (root 2 x 2 root 2) R45 degrees in the initial and final stages of Pb UPD, respectively. Bulk Pb deposition at E < -0.5 V led to a crystalline Pb film, but a roughened Pt(100) surface was imaged after the Pb deposit was anodically stripped. The activity of the Pt(100) electrode toward formic acid oxidation was nearly tripled after being modified with a sub-monolayer of Pb, and increased by another 20 % when it was loaded with multilayer Pb.

Molecules with a fused pyrrole and thiophene backbone can be harnessed into organic semiconductors and optoelectronics devices. Closely related to these applications is contact with a frequently used gold electrode. This organic/metal interface has been examined with scanning tunneling microscopy (STM) at the molecular level, revealing the adsorption orientation and spatial structure of the Au(111) electrode. The current study focused on the adsorption of 1,4-bis(4H-dithieno[3,2-b:2 ',3 '-d]pyrrol-4-yl)benzene (BDTP) on an ordered Au(111) electrode. The BDTP adlayer was adsorbed onto a Au crystal by immersion in 0.01 mM BDTP/benzene dosing solution for 2 min after the Au electrode was pretreated with the annealing-and-quenching process. The STM experiment was performed in 0.1 M HClO4 or H2SO4 under potential control. The cyclic voltammetric results indicate that the BDTP adlayer was stable between -0.2 and 0.7 V (versus Ag/AgCl) in both acids before desorption and oxidation commenced at negative and positive potentials. High-quality STM imaging was achieved to provide insights into the interactions between BDTP admolecules and with the Au support. The van der Waals interaction between BDTP and Au(111) was strong enough to partially lift the Au(111)-(root 3 x 22) reconstructed phase. The three-dimensional adsorption configuration of the BDTP admolecule on the Au(111) electrode is inferred from the corrugated features in the STM image. Potential control affected the interfacial interactions, the coadsorption of anions, and finally the spatial molecular arrangement on the Au electrode.

The adsorption of pyridazine (PD) on a gold electrode served as a model to study the orientation and arrangement of an organic adsorbate bearing a heterocyclic molecular structure at an electrified interface. Because the two N-ends of PD could interact with a Au electrode, the acid-base equilibrium of PD at an electrified interface can be different from that in a solution phase. Unprotonated and protonated PD interacted with the Au electrode differently, leading to dissimilar spatial structures on the Au(111) electrode. Moreover, having a large dipole moment of 4.22 D, PD adopted different adsorption configurations, as the Au potential was modulated. The indispensable anion present in the supporting electrolyte can compete with PD for surface sites on the Au electrode. Since the potential of a charged conductor is established by the physical contact with an electrolyte the study of the adsorption of PD on a Au(111) electrode calls for the use of an in situ tool, such as a scanning tunneling microscope (STM), which provides sub-nanometer information of the interface in real-time and real-space. Different phases of adsorbed PD molecules on an ordered Au(111) electrode were revealed as a function of potential, pH and anion. The most notable event for PD adsorbed on Au(111) is attributed to the sharp transition from a disordered state to highly organized structures in 0.1 M H2SO4 and HClO4. The obtained STM images enabled a direct measurement of the spacing between PD admolecules, from which the molecular reorientations on the Au electrode were inferred. PD admolecules flipped from the horizontal to the upright configuration, tethered to Au substrate via the two N-ends, at positive potentials. Different ordered PD structures seen in H2SO4 and HClO4 imply anions were coadsorbed with PD molecule on the Au electrode. The acid-base equilibrium of PD at the Au electrode was examined with STM in pH 1 and 3 media.

A nickel film electroplated onto a metal substrate can be used as a catalyst for water splitting and a magnetic material for spin valves. Although the nucleation and growth of Ni on Au(111) have already been examined with in situ scanning tunneling microscopy (STM), the current study provides new insights of the structure of the first layer of Ni on an ordered Au(111) electrode in 0.1 M KSO4 + 1 mM H2SO4 + 10 mM NiSO4 (pH 3). Prolonged STM scanning of the Ni monolayer on a Au(111) electrode revealed interfacial mixing to produce a surface alloy, initially assuming segregated Ni domains and later transforming them to a homogeneous Ni/Au phase. The formation of the Ni/Au(111) surface alloy affected the structure of the subsequent bulk Ni deposition. The inclusion of 2-mercapto-1-methylimidazole (MMI) in the deposition bath incurred Ni deposition at a less negative potential and a faster rate, resulting in an overall 5.3 times more Ni deposited on the Au electrode in potentiodynamic experiments. MMI molecules were adsorbed on the Ni deposit to prevent Ni dissolution in the Au(111) electrode. MMI could catalyze the presumed rate-determining step from Ni2+ to Ni+ en route to the metallic Ni. The resultant Ni film with MMI had a 3D texture without a preferred crystal orientation on the Au electrode, as opposed to a layer type growth of Ni on Au(111) without MMI.

The adsorption of organic molecules on gold electrodes serves as a model to understand the organic/inorganic electrified interface, which is relevant to the study of molecular electronics and organic thin film semiconduc-tors. Our previous study on terthiophene (TT) adsorption on an Au(1 1 1) electrode shows that immersing Au (111) crystals in a TT ethanol dosing solution installs an ordered TT adlayer on the sample. The current study addresses the adsorption of 3',4'-bis(hexylthio)-2,2':5',2''-terthiophene (DTDST), a molecule with a TT back-bone attached with two thiolhexyl chains, on an ordered Au(1 1 1) electrode. High-quality STM images were obtained to reveal the internal and 2D spatial structures of DTDST admolecules. The potential greatly influ-enced the organization of DTDST on the ordered Au(1 1 1) electrode. Although the pristine DTDST adlayer was disordered, it transformed into ordered Au(1 1 1) -(3 & RADIC;3 x 9) and (5 & RADIC;3 x 26) structures after applying a potential more negative than 0 V (vs. Ag/AgCl) in 0.1 M H2SO4 and HClO4, respectively. Shifting the poten-tial more positive than 0.25 V resulted in coadsorption of bisulfate anions and restructuring of the DTDST adlayer. High-quality molecular resolution STM images were collected to reveal the azimuthal orientation of the DTDST admolecule on the Au(1 1 1) electrode. The thiolhexyl chains of DTDST admolecules could arrange in such a way that allowed intermolecular van der Waals interactions. Oxidation of adsorbed DTDST molecules to yield oligomers was also revealed by in situ STM.

Platinum (Pt) electrodes modified with iron (Fe) have been used as electrocatalysts to split water, to reduce oxygen, and to oxidize methanol. The current study employed cyclic voltammetry and in situ scanning tunneling microscopy (STM) to probe the redox chemistry and spatial structure of an iron hydroxide thin film adsorbed on an ordered Pt(111) electrode in pH 3 sulfate solution (1 mM H2SO4 + 0.1 M K2SO4) containing 10 mM FeSO4. The Pt(111) electrode was pretreated with the conventional annealing-and-quenching method, which resulted in a sub-monolayer native oxide. After this native oxide was stripped off, the adsorption of Fe2+ ions and the reduction of oxygen concurred to yield an ordered FeOH film on the Pt(111) electrode. In situ STM imaging revealed a unique spoke - wheel (SW) structure between 0.1 and -0.1 V (vs. Ag/AgCl). As opposed to the hexagonal FeO(111) bilayer structure observed on Pt(111) in the vacuum, a distorted hexagonal adlattice was seen with this SW pattern. Shifting the potential positively and negatively resulted in transformations of the SW structure into distorted moire & PRIME; patterns. The same STM experiment performed with Fe2(SO4)3 led to a disordered film, indicating that ferrous, not ferric, ions were orderly adsorbed on the Pt(111) electrode. The role of anion in the formation of the FeOH film on the Pt(111) electrode was also conducted in pH 3 chloride solution (1 mM HCl + 0.1 M KCl + FeCl2).

2,2,7,7-tetrakis(N,N-dip-methoxyphenylamine)-9,9-spirobifluorene (spiro-OMeTAD) is a popular hole-transporting material (HTM) in perovskite solar cells (PSCs), However, it suffers from high-cost and stability issues, which need to be overcome for PSC commercial-ization. Here, we report isomers of bithiophene-based HTMs func-tionalized with triarylamine and imidazole for PSCs. The planar 3-ImBT-2D (1) exhibits higher hole mobility than 5-ImBT-2D (2) via modulation of donor-group positions. The PSCs using HTM (1) deliver an excellent power conversion efficiency (PCE) of 21.73% with Li-TFSI doping and 17.79% without dopants. In addition, the Li-TFSI-free device based on 3-ImBT-2D yields a PCE of 21% after HTM surface modification with organic p-dopant dimethylanilinium tetrakis(pentafluorophenyl)borate (DPB). A molecular dynamics study shows that the isomer 3-ImBT-2D (1) folds up after deposition on perovskite films. As a result, the Li-TFSI-free devices exhibit higher stability, retaining 95.9% of the initial PCE after 800 h aging.

A decade of significant research has led to the emergence of photovoltaic solar cells based on perovskites that have achieved an exceptionally high-power conversion efficiency of 26.08%. A key breakthrough in perovskite solar cells (PSCs) occurred when solid hole-transporting materials (HTMs) replaced liquid electrolytes in dye-sensitized solar cells (DSSCs), because HTMs play a crucial role in improving photovoltaic performance as well as cell stability. This review is mainly focused on the HTMs that are responsible for hole transport and extraction in PSCs, which is one of the crucial components for efficient devices. Here, we have reviewed small molecular as well as polymeric HTMs that have been reported in the last two years and discussed their performance based on the analysis of their molecular architectures. Finally, we include a perspective on the molecular engineering of new functional HTMs for highly efficient stable PSCs.