Synergistic experimental and theoretical investigation of carbazole–cyanopyridine-based hole-transporting materials

Abstract

This work highlights the design, synthesis, and characterization of three new hole-transporting materials (DJ01-alkyl, PR01-alkyl, and PM01-alkyl) based on donor–acceptor–donor (D–A–D) and acceptor–acceptor–donor (A–A–D) concepts. Crystals of two compounds, DJ01-alkyl and PR01-alkyl, were obtained under similar crystallization conditions. The molecular structures were thoroughly examined using DFT, photophysical, electrochemical, and thermal methods. The UV-vis absorption spectrum of DJ01-alkyl displayed a significant bathochromic effect compared to its counterparts PR01-alkyl and PM01-alkyl. This could be because the thiophene units enhance conjugation and lead to a bathochromic shift. Compared to DJ01-alkyl (1.7 × 10⁻⁵ cm² V⁻¹ s⁻¹) and PM01-alkyl (1.6 × 10⁻⁵ cm² V⁻¹ s⁻¹), PR01-alkyl was found to have a higher hole mobility value of 2.1 × 10⁻⁵ cm² V⁻¹ s⁻¹. To further explain and complement the experimental data, DFT calculations of the geometry, electronic structure, absorption, reorganization energy, transition density matrix, and density of states of compounds were performed. These characteristics make it abundantly evident that compounds based on carbazoles and cyanopyridines are very attractive materials for use as hole-transporting materials in perovskite solar cells.

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1. Introduction

Research on renewable energy is gaining significance and is increasingly crucial to meet future energy security requirements.¹ Since clean energy is abundant in nature, different types of solar cells have gained international interest as renewable energy sources. Perovskite solar cells (PSCs) have garnered a lot of attention from researchers in recent years due to their low cost and sharp rise in efficiency, which has increased from 3.8% in 2009 to 27.0% to date.^2–5 A perovskite absorber is sandwiched between an electron-transporting material (ETM) and a hole-transporting material (HTM) in a typical PSC.^6,7 HTMs play an important role in PSCs in extracting holes from the perovskite layer to the metal electrode and also help in device stability.^2,8 A desirable HTM should be able to form films, have high hole mobility, good solubility, and the right frontier molecular orbitals. Hence, 2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenylamine)9,9′-spirobifluorene (Spiro-OMeTAD) is the most widely used HTM in PSCs, despite its low charge carrier mobility and low-yield production, which severely limit its widespread use.^9–11 The benefits of Spiro-OMeTAD include the capacity to sustain a stable amorphous state and an excellent energy level match between the perovskite absorber and this HTM's highest occupied molecular orbital (HOMO).¹² Due to the poor conductivity and mobility of pristine Spiro-OMeTAD films, dopants like 4-tert-butylpyridine (TBP) and lithium bis(trifluoromethylsulfonyl)imide (Li(TFSI)) are used. These dopants make PSCs unstable because of their hygroscopic properties.^13–15 Therefore, dopant-free HTMs, such as metal complexes,^16,17 conjugated polymers,^18,19 small organic molecules,^20,21 and inorganic compounds,²² have been investigated and used in PSCs in place of Spiro-OMeTAD. In general, the HOMO level of an HTM needs to be well aligned with the valence band of perovskites in order to enable an efficient hole injection from the perovskite to the HTM.^23–27

The advantages of carbazole-based HTMs include the low cost and availability of the 9H-carbazole precursor and its derivatives, strong electron-donating ability, suitable energy level alignment, and excellent chemical and environmental stability arising from the fully aromatic framework. Moreover, the structural versatility of carbazole allows for facile modification through the incorporation of various alkyl or functional groups at the nitrogen atom or on the outer benzene ring, enabling precise tuning of optoelectronic properties, solubility, and molecular packing behavior.^28–31

Numerous investigations have also demonstrated that pyridine units can efficiently passivate interface defects and improve the capacity to extract and transport holes.³² The pyridine moiety can function as a Lewis base, which helps with the π-conjugation and therefore prevents the defect passivation of the perovskite.^33–36 It was reported by Xu et al.³⁵ that compared to Spiro-OMeTAD-based solar cells with TBP added, PSCs with pyridine functionalized HTMs demonstrated significantly higher long-term stability. Also, Duan et al.³⁷ have reported that the cyano (CN) group has been used extensively as a stronger acceptor in D–A type conjugated small molecules as dopant-free HTMs for high-performing PSCs because of its high hole-mobility and structural variety. Numerous other organic electronics applications, including OSCs, OLEDs, electrochromic and photochromic windows, and sensors, have had success with thiophene derivatives. Thiophene derivatives have been used as HTMs in PSCs due to their many potential uses, high chemical stability, superior electrical configuration, and remarkable synthetic diversity. In actuality, thiophene-based HTMs benefit from their electron-rich thiophene cores, which raise the rate at which charges self-exchange.^38,39 Additionally, organic HTMs carrying nitro and bromo groups have been employed in PSCs.^40,41 The tunable optical and electrical features of D–A–D type conjugated molecules make them some of the most intriguing systems.^42,43 Furthermore, the A–A–D system often shows a strong dipole–dipole interaction, which may help with molecular packing and enhance the hole transport capacity.^44,45 Additionally, alkyl chains, which are often introduced to enhance the solubility of HTMs, also play a crucial role in improving device performance by influencing molecular packing and film morphology.⁴⁵ Long alkyl chains are also known to create an insulating layer during interface engineering and help to reduce electron recombination and increase the V_oc.⁴⁶ Thus, it has been found that the solubility of HTMs in organic solvents and, consequently, the molecular ordering during the spin coating process are influenced by the length of the alkyl chains as reported by Zhang et al.⁴⁷ Also, addition of a hexyl chain into a carbazole molecule enhances the carbazole moiety's spatial hindrance, making it suitable for producing thin and amorphous layers with a suitable morphology on substrates.^48,49

In this work, we have synthesized three new carbazole and cyanopyridine based HTMs termed DJ01-alkyl (thiophenyl), PR01-alkyl (4-bromophenyl), and PM01-alkyl (4-nitrophenyl), new simple carbazole-based HTMs containing cyanopyridine as a core moiety (Scheme 1 illustrates their chemical structures). Among them, DJ01-alkyl possesses a D–A–D type molecular architecture, whereas PR01-alkyl and PM01-alkyl possess a A–A–D configuration. In recent years, thiophene-based derivatives have gained significant attention as HTMs in PSCs owing to their excellent performance, low cost, and high reliability.³⁹ Moreover, organic HTMs bearing bromo and nitro substituents have also been explored in PSCs, demonstrating promising performance and tunable electronic properties.^40,41 Thus, our research sheds light on the viability of modifying the donor units to achieve dopant-free HTMs in addition to offering logical design guidelines for effective HTMs.

Scheme 1 The synthesis of DJ01-alkyl, PR01-alkyl, and PM01-alkyl.

2. Results and discussion

2.1 Materials synthesis

Scheme 1 gives the detailed synthesis of the three new molecules, DJ01-alkyl, PR01-alkyl, and PM01-alkyl, with yields of 64–72%. DJ01, PR01 and PM01 were synthesized using a mixture of N-ethylcarbazole-3-carboxyaldehyde, which was kept constant, and other reactants that were varied, like 2-acetylthiophene (DJ01), 4-bromoacetophenone (PR01), and 4-nitroacetophenone (PM01), which was already reported in our previous work.⁵⁰ DJ01, PR01, and PM01 exhibit limited solubility in common organic solvents, which poses challenges for solution processing and detailed characterization. Therefore, alkylation was intentionally carried out as a molecular design strategy to enhance solubility. The introduction of alkyl substituents increases the hydrophobic character and conformational flexibility of the molecules, weakens strong intermolecular interactions such as π–π stacking, and reduces molecular planarity. As a result, the alkylated derivatives show improved solubility in conventional organic solvents (Table S5), facilitating good hole mobility. ¹H NMR spectroscopy shows the formation of alkylated products by typical chemical shifts of the alkyl group of about 1–4.5 ppm, were used to correlate the chemical structures of all three compounds. The SI provides a detailed synthesis process (S4–S13). The compounds exhibit good solubility in a variety of common organic solvents, including dichloromethane, chloroform, tetrahydrofuran, chlorobenzene, and toluene. The SI provides a thorough cost estimate and synthesis processes for the production of compounds. The costs for synthesizing DJ01-alkyl, PR01-alkyl, and PM01-alkyl (including reagents, solvents, and other consumables) were calculated to be just $11.41, $11.90, and $11.79 per g (Tables S1, S2, and S3), respectively, which are markedly lower than that of the benchmark HTM, Spiro-OMeTAD ($400 per g).⁵¹ A summary of the cost calculations of various carbazole based HTMs is shown in Table S4. This suggests a viable approach for commercial production scale-up.

Among the three synthesized molecules, we were able to get single crystals for DJ01-alkyl and PR01-alkyl. Single crystals were grown from chlorobenzene solvent at room temperature. The chemical structures of DJ01-alkyl and PR01-alkyl were confirmed by X-ray diffraction analysis as displayed in Fig. 1a and b. Furthermore, a single-crystal study reveals that DJ01-alkyl crystallises in the monoclinic space group P2₁/c with cell parameters a = 20.0534(5) Å, b = 11.8621(3) Å, c = 10.8047(2) Å, V = 2567.70(10) ų, and Z = 4, whereas PR01-alkyl crystallises in the monoclinic space group C2/c with cell parameters a = 42.0517(6) Å, b = 7.42350(10) Å, c = 18.4826(3) Å, V = 5737.19(15) ų, and Z = 8 (Table S6). Furthermore, single crystal structure analysis reveals that for both the molecules the centre moiety (cyanopyridine), the carbazole moiety and the remaining moieties thiophenyl (DJ01-alkyl) and 4-bromophenyl (PR01-alkyl) do not lie in the same plane. In the case of the PR01-alkyl moiety the dihedral angle between 4-bromophenyl and cyanopyridine is 23.40°, while the dihedral angle between the carbazole and cyanopyridine is 48.94°. On the other hand, for the DJ01-alkyl moiety the dihedral angle between cyanopyridine and the thiophenyl ring is 16.64° and the dihedral angle between carbazole and cyanopyridine is 42.83°. Thus, the thiophenyl ring is almost planar with the cyanopyridine ring in the DJ01-alkyl moiety, which helps it to transfer the charge through the ring, and this results in a bathochromic shift in the emission.^52,53 Also, in the crystal structure we observed N⋯H interactions (2.772 Å) which help this charge transfer (Fig. 1c). Furthermore, we can observe π–π interactions in the PR01-alkyl moiety (Fig. 1d), where the π–π interaction distance in the crystal structure is 3.690 Å, which is well within the range for π–π interactions.⁵⁴ These π–π interactions play a crucial role in the hole mobility of the PR01-alkyl moiety.⁵⁵ Therefore, we calculated the interaction energy of the π–π conjugated molecular pair of the PR01-alkyl moiety by using Crystal Explorer (CE) software.⁵⁶ Furthermore, the interaction energy calculated from CE also indicates the presence of π–π interactions, which results in high dispersion energy in the PR01-alkyl moiety pair (−87.3 kJ mol⁻¹).

Fig. 1 Single-crystal structures of (a) PR01-alkyl and (b) DJ01-alkyl; and (c) N⋯H interactions in DJ01-alkyl and (d) π–π interactions with distances in PR01-alkyl.

2.2 Photophysical and electrochemical properties

Fig. 2a displays the UV-vis absorption and fluorescence emission spectra of DJ01-alkyl, PR01-alkyl, and PM01-alkyl, and Table 1 summarizes the characteristic data. The absorption spectra show that DJ01-alkyl has an absorption maximum peak at 350 nm, and PR01-alkyl has a strong absorption maximum peak at 335 nm with a weak shoulder peak at 365 nm, whereas PM01-alkyl has an absorption maximum peak at 341 nm. Compared to its counterparts PR01-alkyl and PM01-alkyl, DJ01-alkyl shows a significant bathochromic impact in its UV-vis absorption spectrum. This could be because the thiophene unit enhances the overall conjugation in DJ01-alkyl and often leads to a bathochromic shift.⁵⁷ The emission maxima (λ^max_em) of DJ01-alkyl, PR01-alkyl, and PM01-alkyl from the photoluminescence (PL) analysis are 496 nm, 500 nm, and 488 nm, respectively. Additionally, thin-film absorption and emission spectra were recorded, which are displayed in Fig. 2b. It is observed that these molecules in thin films show slightly red shifted spectra in the lower energy absorption band compared to those in solution, indicating that intermolecular π–π interactions exist in these thin films.^50,58,59 Based on the corresponding intersection values of DJ01-alkyl (418 nm), PR01-alkyl (424 nm), and PM01-alkyl (416 nm), the optical band-gap energies were found to be 2.96 eV for DJ01-alkyl, 2.92 eV for PR01-alkyl, and 2.98 eV for PM01-alkyl, respectively. Additionally, Fig. 3 and Fig. S14 display photographs of the synthesised compounds in DMSO solution (1 × 10⁻⁵ M) and powders in normal light, short UV light, and long UV light. In addition, quinine sulfate solution in 0.1 N H₂SO₄ was used as a standard to determine the relative fluorescence quantum yields (Φ_f) of the compounds in DMSO. The obtained Φ_f values were 52.90% (DJ01-alkyl), 49.05% (PR01-alkyl) and 3.50% (PM01-alkyl), respectively.

Fig. 2 (a) Normalized UV-vis absorption and fluorescence emission spectra of DJ01-alkyl, PR01-alkyl, and PM01-alkyl in DMSO solution; (b) normalized absorption and emission spectra in thin films; (c) cyclic voltammograms in THF-TBAPF₆ (0.1 M), scan speed: 100 mV s⁻¹, potentials vs. Fc/Fc⁺ and (d) energy level diagram.

Fig. 3 Photographic images of DJ01-alkyl, PR01-alkyl, and PM01-alkyl in DMSO solution (1 × 10⁻⁵ M): (a) normal light; (b) short UV light and (c) long UV light.

Table 1 Optical, thermal, electrochemical and electrical properties of the HTMs

HTMs	λ abs (nm)	λ ^maxem (nm)	λ ^solint (nm)	E g  ^solopt (eV)	λ ^filmabs (nm)	λ ^filmem (nm)	λ ^filmint (nm)	T d (°C)	T g (°C)	T m (°C)	HOMO (eV)	LUMO (eV)	µ (cm^2 V^−1 s^−1)
DJ01-alkyl	350 (max)	496	418	2.96	359	455	424	357	—	116	−5.37	−2.40	1.7 × 10^−5
PR01-alkyl	335 (max), 365	500	424	2.92	338	462	426	326	—	118	−5.38	−2.46	2.1 × 10^−5
PM01-alkyl	341 (max)	488	415	2.98	352	457	436	314	122	158	−5.37	−2.38	1.6 × 10^−5

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The energy levels of the HTMs were experimentally determined via cyclic voltammetry (CV) measurements. The CVs of the HTMs in THF solution are shown in Fig. 2c (Table 1 provides a summary of the data). This figure shows that DJ01-alkyl, PR01-alkyl, and PM01-alkyl all had one oxidation peak. The three compounds showed first oxidation potentials of 1.001, 1.011, and 1.005 eV, respectively. From the onset oxidation (E^onset_ox), the HOMOs of these compounds were found to be −5.37 eV for DJ01-alkyl, −5.38 eV for PR01-alkyl and −5.37 eV for PM01-alkyl, which were deeper than that of Spiro-OMeTAD (−5.2 eV).⁶⁰ The device's higher open-circuit voltage (V_oc) may be enhanced by the deeper HOMO levels of the HTMs.⁶¹ The optical band-gap energies were added to the lowest unoccupied molecular orbital (LUMO) energies to determine the HOMO energy level.⁶² Furthermore, the LUMO energy levels were calculated to be −2.40 eV, −2.46 eV and −2.38 eV, respectively. Hence, the higher LUMO energy levels may be able to more successfully stop electron transport into the metal electrode.

2.3 Thermal and hydrophobic properties

The thermal properties of these HTMs were investigated using differential scanning calorimetry (DSC) measurements and thermogravimetric analysis (TGA) (Fig. 4a and b). The temperatures of decomposition (T_d or 5% weight loss temperature) for DJ01-alkyl, PR01-alkyl, and PM01-alkyl are approximately 357 °C, 326 °C, and 314 °C, respectively, indicating the molecules’ strong thermal stability. Using DSC under nitrogen at a heating rate of 10 °C min⁻¹, the phase transition behaviour of the compound was examined. Although neither the DJ01-alkyl nor the PR01-alkyl molecules displayed a glass transition temperature (T_g), their respective melting points (T_m) were found to be 116 °C and 118 °C, respectively. However, 122 °C was identified as the T_g and 158 °C as the T_m of PM01-alkyl. The conjugation between the nitro group and the phenyl ring restricts the rotational freedom along the main chain, thereby increasing the rigidity of PM01-alkyl. Also, the strong polarity of the nitro group assists in the enhancement of intermolecular forces and restriction of molecular mobility, which resulted in the observed T_g.⁶³ Furthermore, it is evident that the T_g of PM01-alkyl is marginally comparable to that of Spiro-OMeTAD (120 °C), which is better for the cell encapsulation process.⁶⁴

Fig. 4 (a) TGA profiles; (b) DSC profiles and (c) steady state PL spectra of pristine, DJ01-alkyl, PR01-alkyl and PM01-alkyl.

As shown in Fig. S15, the water contact angles on DJ01-alkyl, PR01-alkyl, and PM01-alkyl films are found to be 78.9°, 81.2°, and 73.9°, respectively, indicating their hydrophobic nature. Such hydrophobicity enables these HTMs to effectively inhibit moisture penetration into the perovskite layer. In contrast, the dopant free Spiro-OMeTAD-based HTM film exhibits a lower contact angle of 75°, signifying a higher affinity toward ambient moisture.⁶⁵ These results clearly demonstrate that DJ01-alkyl and PR01-alkyl possess enhanced hydrophobic characteristics compared to Spiro-OMeTAD, thereby providing a more efficient moisture barrier and potentially improving the long-term stability of PSCs. Furthermore, PR01-alkyl exhibits the highest water contact angle among the three, suggesting its superior moisture resistance relative to DJ01-alkyl and PM01-alkyl.

2.4 Steady state photoluminescence

The steady-state photoluminescence (PL) was measured for the synthesized HTMs in order to better understand their hole extraction efficiency. As a reference, the steady-state PL spectrum of the pristine perovskite layer without any HTL was also measured (Fig. 4c). In agreement with the emission peak of MAPbI₃, the bare perovskite films exhibited an emission peak at 797 nm. Additionally, a dampening of the PL intensity was seen in all perovskite thin films with HTLs. A more effective hole extraction procedure at the perovskite–HTM interface is shown by the higher PL quenching value of the HTM coated perovskite layer when compared to the bare perovskite layer. These findings show that, in accordance with their hole mobility, DJ01-alkyl and PR01-alkyl have higher hole extraction efficiency than PM01-alkyl.

3. DFT study

3.1 Computational methods

The B3LYP/6-311G(d) level of theory was used to optimize the geometries of all molecules because of its high accuracy in energy and ability to take into account additional parameters, such as atomic orbitals. This basis set was chosen in order to forecast the optical absorption characteristics of the planned HTMs and to optimize the conductor-like polarizable continuum model (CPCM) in DMSO solvent. Frontier molecular orbitals (FMOs) are crucial for understanding and forecasting how charges flow in molecules because they provide information on carrier injection and transportation. The chosen DFT functional was used to compute electron affinity (EA), ionization potential (IP), chemical hardness (η), electrostatic potential (ESP), reorganization energy (RE), transition density matrix (TDM), and density of states (DOS).

3.2 Structural analysis and frontier molecular orbitals

The dihedral angles of the molecules being studied are shown in Fig. 5. They were examined to obtain information about the planarity of the molecules, which is essential for simple charge mobility. On the other hand, all three compounds have about the same dihedral angles between the carbazole and cyanopyridine units. Three segments make up compound DJ01-alkyl: carbazole, cyanopyridine, and thiophenyl. The cyanopyridine unit attached to the carbazole is severely distorted out-of-plane, with a dihedral angle of 44.9° on one side and a slight twist of −2.6° with the thiophenyl portion on the other. PR01-alkyl has a highly distorted angle between the cyanopyridine and carbazole (44.3°) and the cyanopyridine and 4-bromophenyl (−22.9°) units. Also, PM01-alkyl has a highly distorted angle between the cyanopyridine and carbazole (44.2°) and the cyanopyridine and 4-nitrophenyl (−24.3°) units. Among the three synthesized compounds, DJ01-alkyl is more planar than PR01-alkyl and PM01-alkyl.

Fig. 5 Optimised structures and frontier molecular orbital contour plots for the studied molecules of (a) DJ01-alkyl, (b) PR01-alkyl, and (c) PM01-alkyl.

The frontier molecular orbitals (FMOs) consist of the HOMO and the LUMO. Analysis of the electronic distribution within these orbitals provides valuable insight into the intramolecular charge transfer (ICT) characteristics of a molecule.⁶⁶ As shown in Fig. 5, the HOMO is distributed mainly at carbazole units, and the LUMO is located at the core and some part of the end group in the case of DJ01-alkyl and PR01-alkyl, whereas in the case of PM01-alkyl the electron is mainly located at the nitrophenyl group and some part of the core. The computational energy level diagram is shown in Fig. 6d. The synthesized molecules exhibit HOMO energy levels between −5.83 eV and −5.84 eV and LUMO energy levels in the range of −2.31 eV to −3.06 eV. From the present design strategy of D–A–D and A–A–D architectures, it can be inferred that there is no significant alteration in their HOMO energy levels in the case of experimentally and theoretically obtained values.

Fig. 6 Absorption data measured in DMSO of (a) DJ01-alkyl, (b) PR01-alkyl, and (c) PM01-alkyl along with their oscillator strength at the TD-DFT/B3LYP/CAM-B3LYP/ωB97XD/CPCM (DMSO) level of theory and (d) band gap plots of the designed molecules plotted using Origin software.

The available experimental data and the computed maximum absorption wavelengths (λ^abs_max) of the DJ01-alkyl, PR01-alkyl, and PM01-alkyl HTMs were contrasted (refer to Fig. 6a–c). All things considered, the vertical excitation of these HTMs is better described by the long-range corrected functional approximations of B3LYP, which also show excellent agreement with experimental data. Since B3LYP agrees with the experimental data the best out of the three functional approximations, it is used in this study.

3.3 Reorganization energy, electron affinity, ionization potential and stability

Reorganization energy is a metric used to quantify charge carrier mobilities and approximate charge transfer (CT) characteristics. The excitons must go in the direction of the corresponding electrodes after splitting into electrons and holes in order for them to recombine. The two forms of reorganization energies are external λ_ext, which changes with external parameters like the polarization of the surrounding environment, and internal λ_int, which depends on the interior geometry of the molecule.⁶⁶ According to Marcus theory, this energy's characterisation reveals the transfer rate; a larger charge-transport rate is indicated by a lower k value.^67,68Table 2 shows that for the reorganization energies, the λ_h values of DJ01-alkyl, PR01-alkyl and PM01-alkyl are 0.10 eV, 0.09 eV and 0.10 eV. It can be observed that these compounds are expected to exhibit excellent hole transport properties.

Table 2 Computed optical, electrochemical, reorganization energies and stability values

HTMs	λ abs (nm)	HOMO (eV)	LUMO (eV)	E g (eV)	IP (eV)	EA (eV)	λ h (eV)	λ e (eV)	η (eV)
DJ01-alkyl	350	−5.83	−2.32	3.51	5.63	2.18	0.10	0.435	1.39
PR01-alkyl	335, 365	−5.84	−2.31	3.53	5.63	2.34	0.09	0.367	1.64
PM01-alkyl	341	−5.85	−3.06	2.79	5.65	2.75	0.10	0.70	1.45

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Also, we calculated the ionization potential (IP) and electron affinity (EA) of the synthesized compounds from their optimized cationic, anionic, and neutral geometries (see Fig. 7). Further, Table 2 shows the IP and EA values that were computed. In comparison with Spiro-OMeTAD (4.47 and 1.07 eV, respectively), the systems under study exhibited comparatively higher IP (5.42–5.65 eV) and EA (2.34–2.75 eV) values,³ proving the stability of the hole transport materials in terms of the compounds’ ability to withstand oxidation. Chemical hardness is a measure of how difficult it is for a molecule to share electrons with the surrounding medium.

η ≅ 1/2 (IP − EA) (1)

Fig. 7 Schematic representations of the optimized neutral, cationic, and anionic geometries of the synthesized compounds along with their ionization potential (IP) and electron affinity (EA) values.

In this case, electron affinity and ionization potential are reciprocally connected to IP and EA. The more stability a system has, the higher its hardness value is. The chemical hardness values are as follows: DJ01-alkyl – 1.39 eV, PR01-alkyl – 1.64 eV, and PM01-alkyl – 1.45 eV. Among the three synthesized compounds PR01-alkyl shows a comparable hardness value to spiro-OMeTAD (1.62 eV).

3.4 Electrostatic surface potential (ESP)

The ESP, which characterizes the charge density distribution on a system backbone and is essential for forecasting a molecule's behaviour, is a crucial metric for understanding the stability of molecules.² The electrophilic and nucleophilic sites, respectively, are shown by the positive and negative ESP areas, suggesting possible intermolecular interactions.² The negative sites are mainly found at the cyano (CN) group due to their electron withdrawing nature in contrast to the positive charge located all over the molecules in the case of DJ01-alkyl and PR01-alkyl (Fig. 8). On the other hand, the negative charge is also located at the nitro group in the case of PM01-alkyl, and it is also believed that the nitro (–NO₂) group present in a molecular architecture can interact with Pb²⁺ defect sites of a perovskite, and therefore, it significantly reduces the surface defects of the perovskite, as reported by Subramani et al.⁶⁹ However, the ESP map of molecules evidently shows that the cyano group is the major negative charge bearing site in all three molecules and the cyano group can interact effectively with the Pb²⁺ of the perovskite and thus passivate the defects.⁵⁸

Fig. 8 Electrostatic surface potentials of (a) DJ01-alkyl, (b) PR01-alkyl and (c) PM01-alkyl.

3.5 Transition density matrix and density of states (DOS)

The nature of electrical transitions in a molecule is essentially connected to the features of ICT. Transition density matrix (TDM) analysis was performed for the compounds under investigation in the first excited state (S1), as shown in Fig. 9a–c, in order to assess these kinds of transitions.^70,71 Two prevalent charge transfer types are shown: local excitation (LE) and CT excitation. The excitation mode is LE if the matrix elements are distributed diagonally, whereas CT excitation is the primary excitation mode if the matrix elements are distributed non-diagonally.⁷⁰ Hence, we can observe non-diagonal charge transfer in the case of DJ01-alkyl, PR01-alkyl, and PM01-alkyl, which suggests that CT takes place within the molecules. We also performed DOS studies to confirm our analysis of FMOs (Fig. 9d–f). These computations aid in our comprehension of the roles that various chemical orbitals play in electrical processes.^71–73 The N-ethylcarbazole groups are shown using the blue band in the plots, while the core (cyanopyridine) and the terminal units are shown using the green and purple bands, whereas hexyloxy is shown using the red colour band, respectively.

Fig. 9 Transition density matrixes of (a) DJ01-alkyl, (b) PR01-alkyl, and (c) PM01-alkyl; and densities of states (DOS) of (d) DJ01-alkyl, (e) PR01-alkyl and (f) PM01-alkyl.

The total charge transfer between orbitals is shown using the black band. The donor component, N-ethylcarbazole, has a HOMO density in molecules, as can be seen from the picture. The LUMO is mainly composed of cyanopyridine and thiophenyl and 4-bromophenyl for DJ01-alkyl and PR01-alkyl, whereas, in the case of PM01-alkyl, the LUMO is mainly localised in 4-nitrophenyl and some part of the core.

4. Charge mobility

Furthermore, we examined the hole-transporting capabilities of the synthesized compounds by performing xerographic time-of-flight (XTOF) measurements. Table 3 provides the values of the charge mobility defining parameters, zero field mobility (µ₀), and the mobility under an electric field of 1 × 10⁶ V cm⁻¹. Fig. 10a–c shows the field dependence of compounds’ hole drift mobility. Among the three compounds, the hole mobility value of PR01-alkyl (2.1 × 10⁻⁵ cm² V⁻¹ s⁻¹) is higher than those of DJ01-alkyl (1.7 × 10⁻⁵ cm² V⁻¹ s⁻¹) and PM01-alkyl (1.6 × 10⁻⁵ cm² V⁻¹ s⁻¹), which also corresponds to the theoretically obtained trends of hole transporting rates.

Fig. 10 Field dependence of hole drift mobility for (a) DJ01-alkyl, (b) PR01-alkyl, and (c) PM01-alkyl; and transient photocurrents in the layers of (d) DJ01-alkyl, (e) PR01-alkyl and (f) PM01-alkyl at different sample voltages.

Table 3 Hole mobility values for DJ01-alkyl, PR01-alkyl, PM01-alkyl, Spiro-OMeTAD and some reported HTMs

HTMs	μ 0 (cm^2 V^−1 s^−1)	β ((cm V^−1)^0.5)	μ (cm^2 V^−1 s^−1)	Ref.
DJ01-alkyl	8 × 10^−9	∼0.0077	1.7 × 10^−5	Present work
PR01-alkyl	6 × 10^−9	0.0083	2.1 × 10^−5	Present work
PM01-alkyl	5 × 10^−9	0.0081	1.6 × 10^−5	Present work
Spiro-OMeTAD	1 × 10^−4	—	2.1 × 10^−3	76
DJ01	1.5 × 10^−7	0.0049	1.7 × 10^−5	50
PR01	∼1 × 10^−9	0.0093	1.2 × 10^−5	50
DImP-4D	—	—	∼6 × 10^−6	75
DImF-4D	1.2 × 10^−7	0.0054	8.8 × 10^−6	75
DImT-4D	—	—	∼2 × 10^−6	75
DImBT-4D	∼5 × 10^−8	∼0.0059	5.5 × 10^−6	75
HD1	7.50 × 10^−7	0.0072	1 × 10^−5	77
HD2	5.27 × 10^−7	0.0050	4.4 × 10^−5	77
HD3	1.91 × 10^−6	0.0029	1.1 × 10^−3	77
HD4	—	—	6.1 × 10^−4	77

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In all the cases investigated, the mobility µ is approximated using the formula

where µ₀ is the zero field mobility, β is the field dependence parameter, and E is the electric field strength. The values of mobility defining parameters µ₀ and β as well as the mobility value at the 1 × 10⁶ V cm⁻¹ field strength are obtained from the fitting data shown in Fig. 10d–f. Hole transport in these materials is characterized by high values of coefficient β and this is a consequence of high transport dispersion. This also indicates a high disorder of the energetic distribution of hole transport states.⁷⁴ Joseph et al.⁷⁵ synthesized four HTMs, whose μ values were calculated using the XTOF technique: DImP-4D – ∼6 × 10⁻⁵ cm² V⁻¹ s⁻¹, DImF-4D – 8.8 × 10⁻⁵ cm² V⁻¹ s⁻¹, DImT-4D – ∼2 × 10⁻⁵ cm² V⁻¹ s⁻¹ and DImBT-4D – ∼5.5 × 10⁻⁵ cm² V⁻¹ s⁻¹. Among the four molecules, DImBT-4D achieved the best PCE of 20.11%. Furthermore, when compared with our previously synthesized molecules DJ01 (1.7 × 10⁻⁵ cm² V⁻¹ s⁻¹) and PR01 (1.2 × 10⁻⁵ cm² V⁻¹ s⁻¹), the newly synthesized molecules exhibit improved hole mobility, primarily due to their enhanced solubility, which facilitates the formation of uniform thin films and efficient charge transport. In contrast, the hole mobility of PM01 could not be reliably measured because of its poor and complex solubility and film-forming behavior, whereas PM01-alkyl showed measurable hole mobility, owing to alkylation-induced improvement in solubility and film quality. These reports support and further suggest that our three synthesized molecules can also serve as effective HTMs for the fabrication of PSCs. Furthermore, Table 3 shows that our molecules are better in terms of hole mobility when compared to other HTMs.

5. Conclusion

In summary, three new HTMs, DJ01-alkyl, PR01-alkyl, and PM01-alkyl, were successfully designed and synthesised. Their charge-transporting, photophysical, thermal, and electrochemical properties were systematically examined. All the molecules exhibited good thermal stabilities. Furthermore, the compounds possessed higher LUMO energy levels in the range of −2.38 eV to −2.46 eV and deeper HOMO energy levels in the range of −5.37–5.38 eV. Additionally, these compounds demonstrated improved solubility in widely used organic solvents. Additionally, DFT calculations were conducted to correlate the experimentally observed results. Among the three compounds, PR01-alkyl (2.1 × 10⁻⁵ cm² V⁻¹ s⁻¹) exhibited greater hole mobility than DJ01-alkyl (1.7 × 10⁻⁵ cm² V⁻¹ s⁻¹) and PM01-alkyl (1.6 × 10⁻⁵ cm² V⁻¹ s⁻¹), and these molecular trends coincide well with the molecular trends observed for the reorganization energy that were theoretically obtained. Finally, due to the low cost, these materials show a lot of potential for application as affordable, effective hole transport substances and might be suitable substitutes for the expensive Spiro-OMeTAD.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). The supporting information file contains the synthesis procedure, cost calculation, crystal data, ATR-IR, NMR, Mass spectra, photographic images, contact angle cyclic voltamograms, computational details, etc. See DOI: https://doi.org/10.1039/d6qo00076b.

CCDC 2500277 and 2455342 contain the supplementary crystallographic data for this paper.^78a,b

Acknowledgements

Ahipa is thankful to JAIN (Deemed-to-be University), Bengaluru (Bangalore), Karnataka, India, for support under a Minor Project Grant (Ref. No.: JU/MRP/CNMS/101/2025) and the Science and Engineering Research Board (SERB), Govt. of India, New Delhi, India, for support under a Core Research Grant (Project File No.: CRG/2020/003151). Arijit is grateful to JAIN (Deemed-to-be University), Bangalore, Karnataka, India for a Postdoctoral Fellowship (Ref. No.: JU/APP/CNMS/2025/552). K. R. and V. G. received funding from the Research Council of Lithuania (LMTLT), Agreement No. S-AUEI-23-1 (22-12-2023).

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