Optimizing π-conjugated system of spiro-based HTMs; structures and concept towards boosting efficiency of PSCs

Abstract

Perovskite solar cells (PSCs) have attracted significant attention due to their rapidly increasing power conversion efficiencies (PCEs), now exceeding 25.8%, along with low-cost fabrication and versatile material tunability. Among the core components of PSCs, hole transport materials (HTMs) play a pivotal role in enhancing charge extraction, suppressing recombination losses, and improving overall device stability. This review highlights the significant progress made from 2020 to 2025 in the development of new π-conjugated organic HTMs for perovskite solar cells (PSCs), with a particular focus on spiro-based structures. Traditional organic HTMs such as spiro-OMeTAD remain widely used, but their reliance on dopants and high cost has prompted the exploration of cost-effective, dopant-free alternatives. Notably, small molecules like TPE-NPD and polymeric HTMs such as PTAA have achieved PCEs exceeding 21%, offering enhanced thermal and chemical stability. Recent advancements in molecular engineering, such as π-conjugation expansion, donor–acceptor design, and the introduction of heteroatoms, have significantly improved hole mobility, film uniformity, and energy level alignment. This review not only summarizes these material developments but also analyzes charge transport mechanisms, interfacial optimization strategies, and stability trade-offs, highlighting promising design concepts for next-generation, efficient, and durable spiro-based HTMs in PSCs.

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Introduction

Perovskite solar cells (PSCs) have attracted significant attention owing to their low fabrication cost, tunable photophysical properties, and excellent defect tolerance. Over the past decade, these third-generation solar cells have become highly promising photovoltaic devices because of their superior light absorption and outstanding optoelectronic performance.^1–4 PSCs offer great research value and development potential, as evidenced by their power conversion efficiency (PCE) increasing from 3.8% in 2009 to 26.14% in recent reports.⁵ The PSCs can be constructed with either an inverted (p-i-n) or a conventional (n-i-p) shape. The framework of the device is based on the layer of perovskite positioned between the electron transport layer (ETL) and hole transport layer (HTL), as depicted in Fig. 1. Photogeneration is the mechanism by which the perovskite material produces charge carriers, functioning as a layer that absorbs light.⁶ High efficiency depends on both defect-free absorber layers and effective charge carrier extraction.^7,8 They exhibit a 1.53–1.56 eV bandgap and a high-power conversion efficiency, surpassing 26% in labs, with a standard formula of ABX₃.^9–11 Despite their potential in photovoltaic applications, challenges like limited stability and flexibility hinder commercialization.^12–14

Fig. 1 Schematic layer structures of p-i-n (left) and n-i-p (right) perovskite solar cell architectures. Transparent conducting oxides such as fluorine-doped tin oxide (FTO) or indium tin oxide (ITO) act as the front electrodes, while gold (Au), silver (Ag), or carbon layers serve as back electrodes. Arrows indicate the direction of electron and hole transport within the device.

Hole transport materials (HTMs) play a crucial role in n-i-p PSCs and remain a key factor limiting further improvements in efficiency and stability. Since the development of spiro-OMeTAD, it has achieved notable power conversion efficiency (PCE) in n-i-p PSCs because of its suitable energy levels and uniform film formation.¹⁵ In PSCs, HTMs are crucial because they effectively extract holes from the perovskite layer and move them to the back electrode while preventing degradation and moisture.^16–18 The stiff frameworks of spiro-based conjugated small molecules, which offer rapid hole mobility, appropriate energy levels, and good solubility, make them favorable candidates for HTMs in high-performance PSCs. Their amorphous morphology generally provides moderate thermal stability, although long-term durability under operational conditions remains an area of ongoing improvement.¹⁹ It works best in a typical (n-i-p) framework, where the perovskite layer must be covered by a thick layer of 200–300 nm.^20–23 The performance of PSCs decreases due to photogenerated charge carrier recombination caused by direct contact between the electrodes and the perovskite layer. In order to minimize this, the perovskite layer is positioned between ETL and the HTL.^24–26 Ensuring appropriate energy level alignment for efficient charge separation, the HTL collects these carriers and transfers them to electrodes, as shown in Fig. 2. The HOMO of the HTM should be higher than the valence band maxima (VBM) of the perovskite, and the LUMO of the perovskite should also have a high energy level, to enhance charge carrier transit at the perovskite–HTM interface.^27–29 Commercialization depends on constructing high-performing, low-cost HTMs. During construction, a highly hydrophobic HTM shields the perovskite layer in n-i-p type designs from moisture.^30–32 On the other hand, hydrophobic HTM with low hydrophobicity is needed for p-i-n designs in order to prevent HTM removal during perovskite layer deposition, ensure HTM stability, and prevent wetting and crystallization problems.³³ Electron-deficient groups can be introduced to produce donor–acceptor structures, which facilitate charge transport and enable energy level fine-tuning for ideal alignment with the perovskite layer, hence increasing hole mobility.^34,35 Dopants improve conductivity in HTMs, which also include polymers, tiny organic molecules, and phthalocyanine compounds.^36,37 The goal of strategies like π-conjugation expansion and donor–acceptor alternation is to produce dopant-free HTMs. By improving hole mobility and stability, these methods hope to produce PCE in perovskite solar cells that are equivalent or maybe even higher than those of conventional doped HTMs.^38,39 Perovskite solar cell's stability and efficiency are also increased when passivating groups are added to organic HTMs.⁴⁰

Fig. 2 Energy level diagram of n-i-p type device configuration.

The developments in spiro-based organic compounds that improve hole transport in PSCs are reviewed in this article. The HTMs are crucial for both removing and moving holes as well as preventing charge recombination. Furthermore, HTMs have a significant impact on PSC stability.⁴¹ Researchers have enhanced the structural design of HTMs, leading to better charge transport and overall device performance, by extending the π-conjugation system through the addition of different organic components. The influence of these alterations on PSC efficiency is discussed, along with other ways for optimizing HTMs, such as modifying the molecule structure to enhance charge mobility and stability. In general, research on organic HTMs is a vital field that aims to improve perovskite solar cells' longevity and performance.

Optimizing π-conjugated system of spiro HTMs

In this section, the C1–C22 series of spiro-based HTMs are arranged in a logical order to highlight the gradual improvement in π-conjugation and charge transport characteristics. The discussion begins with the traditional spiro-OMeTAD (R1) and its modified analogues (C1–C2), where conjugation was extended through coplanar and aromatic linkages. The following molecules (C3–C8) are based on spiro-fluorene and SFX cores, focusing on π-extension and heteroatom substitution to enhance hole mobility and film stability. The next group (C9–C13) includes carbazole and dithienometallole derivatives, which explore donor–acceptor interactions and dopant-free design. Finally, advanced spiro frameworks (C14–C22) are presented, incorporating linear or multi-spiro configurations to further improve molecular stacking, energy level alignment, and device stability. This sequential arrangement reflects the progressive development of molecular engineering strategies used to optimize π-conjugation in spiro-based HTMs. A conjugated planar structure, as opposed to the non-conjugated sp₃-hybridized carbon in spiro-OMeTAD (R1), improves intermolecular π–π stacking, increasing hole mobility in HTMs.^42–44 The central core and peripheral arylamine groups can be joined by conjugated and coplanar units such as double bonds,⁴⁵ thiophene,⁴⁶ and benzothiadiazole to broaden conjugation, enhance molecular stacking, and thus increase hole mobility.^47,48 The researcher presented FTPE-ST (C1), a novel material with a 3D structure made possible by coplanar 3,4-ethylenedioxythiophene (EDOT) units that grow like arms and enlarge the conjugated system (Fig. 3). Improved multidirectional conductivity and superior solubility in non-halogenated solvents, including 2-methylanisole (2-MA), were two benefits of this design. In addition to prolonged conjugation, the sulphur atom in EDOT units on the perovskite surface also efficiently interacts with lead ions that were not well-coordinated. Therefore, devices with C1 have a remarkable power conversion efficiency (PCE) of 25.21%, which was higher than the traditional R1's 24.77% PCE. This enhancement emphasizes how well-performing C1 is and how much more it can do to advance solar technology.⁴⁹ Xuepeng Liu et al., replaced four anisole units in spiro R1 with 4-methoxybiphenyl groups for synthesizing a novel spiro-HTM known as “C2.” This design increases hole mobility and improves the glass transition temperature by extending the π-conjugation and better matching the HOMO level with the perovskite valence band. Because C2 dissolves easily in chlorobenzene, it has been blended more easily with dopants, resulting in homogeneous HTL architectures. Perovskite solar modules (PSMs) showed a verified efficiency of 21.78% on a 27.86 cm² area, while small-area devices (0.06 cm²) using C2 obtained an impressive efficiency of 25.24%. C2-based devices' improved mobility of charge carriers and charge extraction capacity were the reasons behind C2's high fill factor (84.9%) in comparison to R1.⁵⁰ The performance improvement highlighted how well the new HTM-optimized device functions. In addition, it was found that encapsulated C2-based devices had exceptional stability and high performance, maintaining 87.1% of their initial efficiency after 600 h under ISOS-L-3 (encapsulated large-sized devices operation under specific conditions) circumstances and 95.1% after 2560 h under ISOS-L-1 (encapsulated small-sized devices operation under ambient conditions) settings.⁵⁰

Fig. 3 Structural representation of spiro-based C1 and C2 HTMs.

Yan Zhang et al., were added conjugated groups (benzene, naphthalene, and anthracene) into the diphenylamine (DPA) and spiro[fluorene-9,9′-xanthene] (SFX) groups of the base molecule R2 for synthesizing three dopant-free HTMs C3–C5, respectively. Inductive doping was used in this design in an effort to maintain electrical neutrality while improving hole mobility. By adding conjugated groups to HTMs (C3–C5), self-doping was produced for increased hole mobility without the need for external dopants. The distribution of the HOMO and LUMO in R2 is observed throughout the molecule. On the other hand, the new molecules C3–C5 have HOMOs mostly on the central core, which facilitates hole transit through Intramolecular charge transfer (ICT).⁵¹ Owing to their advantageous molecular planarity and effective ICT states, C3 and C5 function very well as dopant-free HTMs in PSCs. On the other hand, structural distortion of two diphenyl amine groups due to the insertion naphthalene group that prevents intermolecular stacking is responsible for C4's decreased hole mobility. Having an excellent PCE of 16.29%, the PSC device with C3 outperformed as compared to devices based on R2 (14.64%), confirming the efficacy of the conjugated group method for dopant-free HTMs.⁵¹ SFX-based HTMs R3, similar to spiro-fluorene R2 but with an extra oxygen atom, prevent intermolecular π–π interactions and enhance solubility, forming a stable amorphous layer that efficiently extracts holes from perovskite layers. Adding a pyridine unit further improves PSC stability by enhancing binding, surface passivation, and reducing charge recombination, outperforming doped spiro-OMeTAD.^52,53 For use in dopant-free HTMs for PSCs, researchers synthesized pyridine-functionalized SFX moieties (C6) for research purposes. As illustrated in Fig. 4, the synthesized molecule C6, has para-position pyridines connected to an SFX core and TPA groups at both ends. With HOMO and LUMO values of −5.08 eV and −2.19 eV, respectively, C6 displays great, broad absorption in the visible region and advantageous electrochemical characteristics. The PSCs with C6 HTMs exhibited remarkable repeatability, retaining 95% of their initial power consumption after 600 h of tracking at ambient temperature and even after soaking in light for 270 h straight.⁵⁴

Fig. 4 Structural representation of spiro-based C3–C5 HTMs.

As in Fig. 5, the chemical structure of R4, which shows that 9.9′-spirobifluorene core containing four p,p′-dimethoxydiphenylamine units, is present in R1. By changing the OMe substituents or switching them out for the SMe unit, which lowers the HOMO level and increases charge extraction due to which its optical and electrical characteristics can be adjusted.^55,56 Spiro R4 has been the focus of research as a more affordable substitute.^57–59R4 and other SFX-based materials have outperformed R1 in terms of PCE, with 20.8% as opposed to 20.4%. However, R4 HTMs keep depending on dopants such as t-BP and LiTFSI, which may compromise their long-term stability.⁶⁰ In order to optimize charge transport capabilities, chemical structure and intermolecular interactions must be carefully considered during the synthesis of new organic HTMs. Engineered as low-cost, dopant-free HTMs for stable and effective PSCs, Vinay Kumar and his colleagues synthesized two novel SFX-core-based compounds, SP-Naph (C7) and SP-SMe (C8). Due to the bigger naphthyl rings than phenyl groups, dimethoxyphenylnaphthylamines in C7's asymmetrically modified SFX-core were expected to improve the molecular interactions, π-conjugation length, and electrical characteristics of the salt. In contrast, C8 partially substitutes methylsulfanyl (SMe) groups for methoxy groups in the diphenylamine units. The improved band alignment with the perovskite layer and better interactions with undercoordinated Pb²⁺ ions were expected to boost hole extraction through the SMe groups passivating surface imperfections. The impact of both molecular designs on charge-extraction capacities, recombination kinetics, and intermolecular interactions were assessed both experimentally and theoretically. When considering R4, conductivity was enhanced with a lower HOMO value in C7 because the HOMO density is mostly on the phenylnaphthylamine unit. In case of C8, LUMO density is dispersed among the fluorene and –SMe groups, while HOMO density is distributed throughout the entire core unit. The increased donor capacity of S atoms and better HOMO–LUMO overlap, according to DFT calculations, suggested that C8 provides improved hole transport.⁶¹ Excellent PCE of 20.51% and 21.95%, respectively, were attained by devices using C7 and C8 as HTMs in planar n-i-p PSCs, outperforming the 19.23% PCE of devices using doped spiro-OMeTAD. Furthermore, as compared to spiro-OMeTAD-based cells, the novel devices demonstrated superior illumination, temperature, and environmental stability.⁶¹ For PSCs, spiro C9, a low-cost fluorene-based HTM, was developed as a more cost-effective substitute for spiro-OMeTAD (R1). C9, which was synthesized with a yield of more than 78% from low-cost resources, has a small molecular weight, a strong electron-donating capacity, appropriate energy levels, and good environmental stability. Because of its structural modification potential, optoelectronic characteristics and solubility can be tuned.^62–64 In comparison to Li-doped R1, which has a PCE of 15.93%, the PSC using dopant-free C9 as the HTM was able to obtain a PCE of 15.66%. Furthermore, dopant-free R1 (PCE 9.34%) performed much worse than C9. This depicted how C9 was a good substitute for improving PV performance in PSCs. Excellent optoelectronic qualities and performance are achieved by the inclusion of –OH groups in C9, which improves charge mobility, reduces the ground state oxidation potential (GSOP), decreases the energy gap (E_0–0), and increases electron donation.⁶⁵

Fig. 5 Molecular structure representations of C7–C9.

Although carbazole derivatives have demonstrated superior charge transport, thermal stability, and morphological stability, there is limited application for them as HTMs in PSCs.^66,67 The derivatives of N³,N⁶-bis(di-4-anisylamino)-9H-carbazole (DAnCBZ) have garnered interest lately due to their ease of synthesis, affordability, and excellent photovoltaic performance (PCE ≥18%).⁶⁸ Different central aromatic cores, such as bipyridine, phenyl, fluorine, phenothiazine, and pyrene, are present in distinct DAnCBZ derivatives.^69–73 A new carbazole-based SFX derivative called SFXDAnCBZ (C10) has been put forward by researchers; it has excellent efficiency as an HTM for PSCs because it has SFX as the central core unit and DAnCBZ as the end-cap unit, respectively. When C10 was at its best concentration of 17.5 mM, it obtained a PCE of 20.87%; this was much less than when spiro-OMeTAD was at its ideal concentration of 70 mM, which attains a PCE of 21.13%. As a result of the thinner HTMs produced by this decreased concentration, holes can travel more quickly and be captured by the anode more effectively, which also suggests cost savings from reduced material utilization. When compared to the conventional spiro-OMeTAD, the DAnCBZ unit in C10 improves molecular conjugation, charge transfer, and overall efficiency in PSCs. Better electrical interaction inside the material is made possible by this enhanced conjugation, which in C10 leads to better photovoltaic performance and higher power conversion efficiency. For small molecule HTMs based on DAnCBZ, this was the greatest PCE that has been documented till the year of 2020.⁷⁴ Since the electronic properties of element-bridged biaryl systems, such as dithienometallole derivatives, are dependent on the bridging element, they are extensively studied for organic electronic materials.⁷⁵ Organic light-emitting diodes, field-effect transistors, and solar cells used dithienometalloles, particularly those that contain group 14 heavy elements like silicon, germanium, and tin.^76–78 These materials were useful in developing cutting-edge organic electronic devices because of their special qualities, which include increased conjugation and improved interactions between molecules because of the heavy components.⁷⁹ A linearly conjugated molecule based on dithienosilole has been synthesized recently as an HTM for PSCs.⁸⁰ The molecule's versatility in high-performance photovoltaic applications was highlighted by this innovation. Similarly, as R1, chemical changes in R5 improve the hole transport and encourage the development of affordable, dopant-free HTMs.⁸¹ In recognition of their ideal planarity and spiro conjugation, investigators developed spiro(dithienogermole)s with increased conjugation. Among them, triphenylamine-substituted spiro(dithienogermole) (C11), was investigated as an HTM for PSCs and showed better stability and efficiency than R1 as well as R5. Better performance was achieved with smoother film architecture on the perovskite layer, indicating more effective charge separation, especially towards the increased hole mobility of C11. Furthermore, C11 outperformed spiro-OMeTAD in hole extraction. These findings highlighted the potential of molecules based on spirobi(dithienogermole), in particular C11, as potent dopant-free HTMs. Moreover, when compared to spiro-OMeTAD, C11 has a higher lying HOMO, which is probably due to the thiophene rings' electron-donating properties and their increased conjugation with aminophenyl units. Although some spiro(dithienogermole) derivatives performed better than others, the reasons for this are not entirely evident and call for additional research and improvement of the manufacturing process.⁸² Lewis base groups were added to the spiro-OMeTAD structure using spiro-type HTMs without changing the structure's conformation. The goal of this modification was to improve passivation effects while maintaining the benefits of spiro-OMeTAD.^83,84 Spiro-CN and spiro-PS, abbreviated as C12 and C13, respectively, are the two HTMs that were synthesized. The dicyano group, a potent electron-withdrawing center found in C12, reduces the HOMO energy level and helps passivate the perovskite layer. C13 interacted with undercoordinated Pb²⁺ cations through its thiocarbonyl group to provide a similar passivation effect. C12 outperformed the others in terms of surface passivation, film-forming ability, hole mobility, and HOMO band alignment with perovskite. This resulted in a planar n-i-p structured PSC with low hysteresis and a maximum efficiency of 19.90%. On the other hand, because of lower film-forming quality, C13 demonstrated an efficiency of 18.07% while having better hole mobility. The new spiro-type HTMs C12–C13 have conjugated backbones bonded by a single sp₃ carbon atom, just like spiro-OMeTAD.⁸⁵ Since C12's conjugation is longer than C13's as shown in Fig. 6, it has an extended conjugation, which accounts for its increased efficiency and minor red shift in the electronic spectrum.⁸⁶

Fig. 6 Molecular structure representations of C10–C13.

The molecules based on spiro-[fluorene-9,9′-xanthene] (SFX) core were well-known for their excellent optoelectronic properties and ease of synthesis, which makes them perfect for use as HTMs.^57,87 Xu and Bi et al., presented the SFX-based HTMs X60 and X59, which achieved power conversion efficiencies of 19.84% and 20.8%, respectively.⁸⁸ These outcomes were more economical than spiro-OMeTAD and were competitive with it. Furthermore, in order to show the flexibility of this design strategy, Seok et al., synthesized fluorene-terminated HTMs DM and SFX-DM (R6).⁸⁹ By expanding the conjugated system, Haoxin Wang et al., developed SFX-DM-DPA (C14), a novel organic HTM with a peripheral 4,4′-dimethoxydiphenylamine structure and an SFX core structure. Expanding the π-conjugated system, this improvement improved hole mobility and stereoscopic spatial structure over the prior HTM, R6. Utilizing C14, PSCs outperformed R6 with an amazing PCE of 22.7%. Furthermore, at 65 °C and 45% relative humidity, C14-based devices retained 82% of their initial efficiency. The presented research provides a useful framework for future HTM development by demonstrating the substantial influence of molecular design on HTM performance in PSCs.⁹⁰ Since linear molecules are π-conjugated, they generally exhibit greater intermolecular interactions and higher intrinsic hole mobility when contrasted with spiro-type HTMs. In dopant-free perovskite solar cells, this structural alteration can further enhance hole transport properties.^91–93 By integrating a linear structure with fluorinated SFX⁹⁴ as shown in Fig. 7, Bingxue Wu et al., have developed a new class of linear-spiro HTMs (C15–C17) that show extended π-conjugation, presenting favorable features associated with linear organic HTMs while retaining the desirable features of spiro compounds, which include excellent film-forming properties. The addition of DTP units improves hole extraction and transport, attaining hole mobility values up to 10⁻⁴ cm² V⁻¹ s⁻¹ in dopant-free M6-F (C17). C17 is superior to C15 and C16 in terms of hole transport, defect passivation, and hydrophobicity. The LUMO is lowered and the HOMO is somewhat raised upon the introduction of thiophene units in C16, whereas the HOMO is raised to −4.99 eV by incorporating DTP units in C17. The PCEs of small-area and large-area PSCs using C17 were 22.17% and 21.21%, respectively, while the PCEs of large-area PSCs delivered PCEs of 20.31% and 19.33%. Notably, dopant-free PSCs maintained 88% efficiency after 1000 hours at 30% relative humidity, showing enhanced stability and performance.⁹⁵

Fig. 7 Molecular structure representations of C14–C17.

There are still issues with spiro-type HTMs despite their advancements, such as the requirement for high concentrations of solutions and doping levels that might compromise the stability of the device.^20,96 Current isotropic regulatory approaches limit molecular design freedom, as most spirobifluorene sites are consistently functionalized, limiting structural variation.^89,97,98 This shows the demand for more flexible methods in the design of spiro-type HTMs for enhanced performance, as it limits the capacity to fine-tune electrical and film properties. In order to improve HTM design, researchers suggested an anisotropic regulatory technique that takes advantage of the unique reactivities of halogens in C–N coupling to alter spiro-skeletons separately.^99,100 The new HTM C18 was synthesized by adding carbazole and diphenylamine moieties as evidence of concept, as shown in Fig. 8. This strategy produced notable gains in hole mobility, thermal stability, and overall performance. It also outperformed spiro-OMeTAD with a PCE of 24.53% and improved stability over extended periods of time. When compared to spiro-OMeTAD, C18 crystals that were formed by the solvent diffusion process and examined using single-crystal X-ray diffraction exhibit different stacking tendencies. Both are members of the triclinic space group; however, C18 has four molecules per unit cell, whereas spiro-OMeTAD has two. Spiro-OMeTAD's steric hindrance limits hole carrier delocalization by limiting π–π stacking.¹⁰¹ On the other hand, the bis(4-methoxyphenyl)amine group at the terminal of C18 permits several quick interactions, which enhance intermolecular hole hopping and boost transport capacity.¹⁰² As shown in Fig. 9, the HTM of tailored spiro-type with dibenzofuran edge groups has been developed for use in PSCs. Higher glass transition temperature, enhanced hole mobility, better film-forming ability, and ideal energy level alignment were all displayed by molecule C19. Beyond spiro-OMeTAD and other dibenzofuran equivalents, C19 demonstrated superior stability and reached a power conversion efficiency of 23.38%, paving the way for effective, stable PSCs.¹⁰³Table 1 shows the common characterization techniques for π-Conjugated spiro-based HTMs in PSCs.

Fig. 8 Molecular structure of C18 and basic properties. (A) Unit cell of the single structure of C18. (B) Dimeric crystal structures within the unit cell. (C) DSC curves for C18 and spiro-OMeTAD. The glass transition temperature is shown by the vertical dashed lines. (D) Spiro-OMeTAD and C18's normalized UV-vis absorption spectra in solution and solid state. (E) Cyclic voltammograms of spiro-OMeTAD and C18. Reproduced with permission from ref. 102. Copyright © 2024 Xuran Wang et al.

Fig. 9 Chemical structure of C19 with steady state maximum power output of C19 and spiro-OMeTAD. Reproduced with permission from ref. 103. Copyright © 2024 Xuepeng Liu et al.

Table 1 Summary of common characterization techniques for π-conjugated spiro-based HTMs in PSCs

Techniques	Parameter studied	Key findings
UV-vis absorption spectroscopy	Determines optical band gap (Eg) and π–π* transitions	Confirms conjugation length and optical properties; correlates with energy level alignment
Photoluminescence (PL) & Time-Resolved PL (TRPL)	Evaluates charge transfer and recombination at the HTM/perovskite interface	Reduced PL intensity and faster decay indicate efficient hole extraction
Cyclic Voltammetry (CV)	Measures HOMO/LUMO energy levels	Establishes energy alignment between HTM and perovskite valence band
Thermogravimetric analysis (TGA)	Determines thermal stability and decomposition temperature	Higher onset temperature indicates better device stability
Differential Scanning Calorimetry (DSC)	Measures glass transition temperature (Tg)	Higher Tg reflects better morphological stability during operation
X-Ray Diffraction (XRD)	Analyzes crystallinity and molecular packing	Helps identify amorphous or crystalline nature affecting hole mobility
Atomic Force Microscopy (AFM)	Studies surface roughness and film uniformity	Smoother HTM films enhance interfacial contact and device efficiency
Scanning Electron Microscopy (SEM)	Observes HTM layer morphology and coverage	Verifies uniform coating and absence of pinholes
Electrochemical Impedance Spectroscopy (EIS)	Evaluates charge transfer resistance and recombination	Lower resistance correlates with improved hole transport and PCE
Contact angle/wettability tests	Measures surface hydrophobicity	Indicates HTM protection capability against moisture in PSCs

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According to Petrus et al., charge accumulation typically happens on the imine moiety's nitrogen atom, resulting in strong dipoles that increase the HTM layer's packing density. By preventing perovskite from degrading, this increase in charge mobility contributes to improved device stability.^104–106 Three HTMs (C20–C22) with different CPT core designs were developed by Ying-Sheng Lin et.al., for PSCs. The C20 variation obtained a 19.12% PCE, outperforming the spiro-OMeTAD control at 17.69%. C20 devices demonstrated exceptional stability, with 90% and 74.8% of their initial PCEs retained after 240 h and 720 h, respectively. The intermolecular interactions between C20–C22 and Pb²⁺ improve perovskite passivation, leading to improved film quality and solar cell efficiency, according to XPS and GIWAXS studies. Since C20 has two spiro-connected π-chromophores with different polarity, which improve charge separation and prolong absorption wavelengths, it has a wider intermolecular charge transfer (ICT) absorption band. Thus, for HTMs in dichloromethane, the order of ICT transition onset was C20 (469 nm) > C21 (397 nm) > C22 (394 nm) as shown in Fig. 10. The smaller HOMO–LUMO gap in C20 is an outcome of prolonged π-conjugation with four arylamine donors, and this is consistent with the bathochromic shift observed in ICT.¹⁰⁷Table 2 shows the brief information on some optical and photovoltaic properties of the HTMs.

Fig. 10 Chemical structure representations of C20–C22 with absorption and photoluminescence (PL) spectra. Reproduced with permission from ref. 107. Copyright 2023, Elsevier.

Table 2 Brief information on some optical and photovoltaic properties of the HTMs^a

HTM	HUMO (eV)	LUMO (eV)	E g (eV)	V OC (V)	Hole mobility (cm^2 V^−1 s^−1)	Stability	FF (%)	PCE (%)
a V OC (V) = open circuit voltage; FF = fill factor; Eg = optical band gap.
C1	Not mentioned	Not mentioned	Not mentioned	1.136	7.2 × 10^−4	1000 h	84.6	25.21
C2	−5.18	−2.25	2.93	1.138	5.19 × 10^−3	2560 h	84.9	25.24
C3	−5.35	−2.46	2.89	1.033	5.20 × 10^−3	500 h	73.86	16.29
C4	−5.39	−2.59	2.80	1.034	2.71 × 10^−4	Not mentioned	66.06	13.73
C5	−5.35	−2.91	2.44	0.955	1.83 × 10^−5	Not mentioned	76.05	15.15
C6	−4.54	−1.16	3.38	1.15	8.28 × 10^−4	600 h	73	17.39
C7	−5.28	−2.31	2.97	1.15	Not mentioned	200 h	75.73	20.51
C8	−5.36	−2.64	2.72	1.16	Not mentioned	200 h	77.65	21.95
C9	−5.24	−2.70	2.54	1.042	Not mentioned	500 h	67.9	15.66
C10	−4.945	−2.165	2.78	1.09	4.28 × 10^−4	620 h	83	20.87
C11	−4.95	−2.62	2.32	1.06	1.5 × 10^−5	480 h	70.34	14.67
C12	−4.41	−2.39	2.95	1.16	1.04 × 10^−3	800 h	78.08	19.90
C13	−4.34	−2.39	3.02	1.07	3.48 × 10^−3	Not mentioned	75.16	18.08
C14	−5.09	−2.24	2.85	1.15	2.52 × 10^−4	1000 h	80.1	22.6
C15	−5.19	−2.32	2.87	1.133	0.50 × 10^−4	—	63.20	17.33
C16	−5.18	−2.58	2.60	1.138	0.88 × 10^−4	—	67.16	18.24
C17	−4.99	−2.55	2.44	1.154	2.10 × 10^−4	—	77.15	21.21
C18	−5.17	−2.29	2.88	1.18	4.5 × 10^−5	500 h	79.22	24.53
C19	−5.10	−5.29	0.19	1.17	Not mentioned		80	23.38
C20	−5.19	−2.92	2.27	1.06	Not mentioned	720 h	78	19.12

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Conclusions and outlook

Research on HTMs is crucial in order to improve power conversion efficiency, open-circuit voltage, and fill factor of PSCs. This review examines the advanced characterization techniques and various synthetic strategies proposed by different research groups for the perfect, affordable, conjugated spiro HTMs for PSCs. These strategies include enhancing molecular planarity and extending the π-conjugation system. It was found that conjugated spiro HTMs showed higher PCE than conventional spiro HTMs. Researchers still face significant challenges in enhancing the efficiency and durability of the PSCs. Innovative structures that can tackle these issues are largely proposed by organic chemistry. Through selective coupling and the introduction of new donor and acceptor materials, the π-conjugation system can be extended to synthesize HTMs with improved efficiency. PSCs exhibit a significant increase in efficiency when thiophene or tetra-thiophene branches are incorporated into the spiro core structure of HTMs. This unit works better than adding other units such as carbazole, dibenzofuran, 4,4′-dimethoxydiphenylamine, DTP, and CPT, to extend the conjugation system, showing the special advantages of these particular branching configurations in improving device performance. It is noteworthy that after 1000 h or more, several reported compounds have kept at least 80% of their original efficiency, suggesting a promising level of stability for possible commercialization. The objective of this research is to evaluate structural modifications and their effect on photovoltaic performance to develop universal strategies for the manufacturing of low-cost, efficient spiro-type HTMs. This emphasizes how important it is to extend π-conjugated systems in order to improve HTM efficiency. The findings will accelerate advances in sustainable energy solutions by improving PSC's scalability and application.

Ethical approval

The researchers sought informed consent from all participants before recruitment for data collection.

Consent for publication

The explicit consent for publication was also obtained from participants.

Author contributions

Zanira Mushtaq: writing – original draft, and investigation. Abdul Ahad: writing – original draft and validation. Sabahat Asghar: formal analysis. Muhammad Sajid Abbas: data curation. Ayesha Zafar: software and data analysis. Adnan Majeed: review & editing, software. Muhammad Adnan Iqbal: conceptualization, resources, supervision. Muhammad Nadeem: visualization. Shahzaib Ali: software. Sana Ejaz: data analysis.

Conflicts of interest

The authors declare that they have no competing interests.

Data availability

Data will be made available on request.

Acknowledgements

The authors are special thankful to the Higher Education Commission (HEC) Islamabad, Pakistan, for providing me research grant under the award letter no.: ASIP/R&D/HEC/2025/09/93507/21 to conduct a smooth research protocol.

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Footnote

† Both are the first authors.

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