Photophysical and charge transport properties of pyrazolines

Abstract

Pyrazoline, an intense green emitting molecule both in solution and solid state, with extended π-conjugation has been synthesized via simple two-step reactions in high yields. Having the electron rich pyrazoline moiety with good redox behavior, pyrazolines can be potential candidates for charge transport material in organic electronic devices. UV-Visible absorption spectra of pyrazolines exhibit peaks below 400 nm, which is a desired feature for charge transport materials because it avoids interference with donor absorption that falls in the visible to NIR region. Electrochemical and theoretical studies show that the HOMO energy level lies at around −4.8 to 5.2 eV depending on the substituents, which is in fact compatible with the PEDOT:PSS/P3HT and work function of the ITO electrode. The experimental hole transport value, measured using the hole only device and space charge limited current (SCLC) method, was found to be in the range of 10⁻⁵ to 10⁻⁶ cm² V⁻¹ s⁻¹, depending on the substituents. The maximum hole mobility calculated by theoretical methods for the pyrazolines is 0.75 cm² V⁻¹ s⁻¹.

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Introduction

Electron rich, π-conjugated organic molecules play a crucial role in optoelectronic devices such as donor molecules in organic photovoltaics (OPV), sensitizers in dye-sensitized solar cells (DSSCs), emitters in organic light-emitting diodes (OLEDs), charge transporting materials in OPV, DSSC, and OLEDs. This is due to the flexibility in designing these molecules with the desired properties, established standard protocols, and relatively low cost large-scale synthesis. Nonetheless, the critical aspect that limits faster progress in organic photovoltaics is the inherent difficulty in generating and transporting charge carriers.^1,2 The stacked structure of organic photovoltaic devices not only involves the charge transport within the material, but even more competition in the charge transfer is experienced at the interface between different materials, namely, electrode/organic,³ inorganic/organic⁴ or organic/organic heterojunctions.⁵ To overcome this problem, charge carriers for transporting either holes or electrons or both (ambipolar) across the interfaces become crucial. They decide the overall efficiency of the devices for many reasons such as smoothening of the electrode surface, protection of the active layer from the electrode, and blocking of excitons.⁶ Significant progress has been made by several research groups in developing the charge carriers with high carrier mobility, stability, and durability. The most efficient and commonly used polymer based hole transport material is poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) due to its smooth anode surface, reduced current leakage, and higher device stability. However, hygroscopicity,⁷ unstable morphology,⁸ electrical inhomogeneity,⁹ intrinsic acidity, weak electron blocking,¹⁰ and low LUMO level¹¹ of PEDOT:PSS restrict its application as an ideal hole transport material. Several attempts have been made to find alternative hole transport materials to PEDOT:PSS, which include metal oxides such as molybdenum oxide (MoO₃),¹² nickel oxide (NiO_x),¹³ and vanadium oxide (V₂O₅).¹⁴ The metal oxides have the capability to align with a wide range of energy levels, enabling them to form low-resistance ohmic contacts with the organic materials, high transparency, desirable band structure and excellent stability in ambient conditions, which can extend the lifetime of organic electronics.¹⁵ However, harsh experimental conditions, such as high-temperature annealing¹⁶ and vacuum deposition processes, required for metal oxides make them incompatible with the solution processes, and thus limit their practical applications. It is therefore desirable to develop suitable alternatives having better air- and thermal-stability, which are compatible with the solution-processable fabrication of solar cells. In this regard, a variety of small molecules have been developed as charge carriers, such as triarylamine derivatives,^17,18 spirothioxanthene,¹⁹ phenothiazine²⁰ and spiro-OMeTAD.^21–23 Among them, triphenylamine (TPA) derivatives have been widely used as HTMs because of their efficient hole mobilities, high electron donating properties and excellent stability.^24,25 The charge carrier mobility of spiro-OMeTAD was found to be in the range of 10⁻⁵ cm² V⁻¹ s⁻¹ to 10⁻³ cm² V⁻¹ s⁻¹.^26,27 However, one of the suggested limitations of spiro-OMeTAD is low hole mobility, which causes excessive interfacial recombination losses.^28,29 Nonetheless, spiro-OMeTAD continues to be the best performing candidate for HTM, but its higher cost associated with difficulties in its synthesis slow down the growth and advancement of high efficiency solar cells (Chart 1).

Chart 1 Molecular structures of different HTMs.

In this report, we have studied the photophysical and hole transport properties of a new class of compounds based on the pyrazoline skeleton. We have synthesized four pyrazolines with methoxy and allyloxy substituents at different positions. Pyrazolines, well-known brightening agents,²⁷ are widely used in biological systems, synthetic fibers, fluorescent probes, chemosensors, hole transport materials,^30,31 electrophotography, and electroluminescence,^32,33 owing to their strong fluorescence and blue emitting properties. Pyrazoline derivatives are synthetically accessible, thermally and environmentally stable and absorb at considerably shorter wavelengths. The pyrazoline derivatives with high glass transition temperatures (T_g) have been known as excellent hole transport materials in organic electroluminescent devices.^34,35 Higher hole mobilities in pyrazoline doped polymers have also been reported.³⁶ Polymer-based hole transport materials have advantages like chemical and thermal stability, but these molecules have disadvantages of poor solubility and lower power conversion efficiency. Different electron donating groups have been selected to unravel the hole transport properties in organic solar cells. Their unique properties, such as rich electron density, ease of modulation of redox behavior, excellent spectral properties along with the judicious choice of substituents, have been the criteria for efficient hole transporting materials. In particular, their influences on their electronic energy levels and ionization potentials have been investigated to develop better hole-transport materials. Both experimental and theoretical approaches have been employed to characterize the synthesized compounds and to understand the structure–property relationships (Chart 2).

Chart 2 Synthesized molecules.

Results and discussion

Synthesis of the pyrazolines was accomplished by a simple, efficient, two-step process outlined in Scheme 1. The intermediate compound 1,5-diphenylpenta-1,4-dien-3-one derivatives (B) and pyrazolines were synthesized according to a reported procedure.^37,38 Briefly, Claisen–Schmidt condensation of 1 mole of acetone with 2 moles of substituted aromatic aldehyde (C) in 30 mL of ethanol affords the corresponding 1,4-dien-3-one, derivative ‘B’, in excellent yields (90–95%). Further treatment of ‘B’ with excess phenyl hydrazine in acetic acid produces pyrazoline (A) as a yellow solid in 80–85% yield, and the crude product was purified by column chromatography over silica gel using hexane and ethylacetate (90 : 10, v/v) as eluent.

Scheme 1 Synthesis of 1,5-diphenyl-3-styryl-4,5-dihydro-1H-pyrazole derivatives.

The IR spectra of dibenzylideneacetones exhibit characteristic absorption bands corresponding to α,β-unsaturated ketones (1650–1680 cm⁻¹) and trans C=C (970–990 cm⁻¹). Pyrazolines show characteristic absorption bands in FTIR spectra at 1590–1600 cm⁻¹ and 1310–1330 cm⁻¹, due to C=N and C–N moieties, respectively. The compounds were also characterized using ¹H and ¹³C NMR spectra (ESI†).

The thermal stabilities of the pyrazolines 1a–1d were determined by DSC and TGA measurements (Fig. 1) and the data are given in Table 1. The glass transition temperatures of all the compounds were found to be in the range from ∼78 to 95 °C, with the exception of 1a. A second heating scan was performed to detect the decomposition temperature (T_d) by TGA. Decomposition temperatures of pyrazolines measured by TGA were found to be above 290 °C, which would enable these materials to be processed at elevated temperatures during device fabrication. An endothermic peak observed at 88 °C for 1b in DSC has been ascribed to the melting point of the system, and the broad exothermic peak at 263 °C indicates the thermal polymerization of the allyloxy group.

Fig. 1 TGA and DSC thermogram of pyrazolines 1a–1d at scan rate of 10 °C min⁻¹ in N₂ atmosphere.

Table 1 Thermal characteristics of compounds 1a–1d

Compound	Tg^a (°C)	Tm^a (°C)	TID (°C)
a Determined by DSC, scan rate 10 °C min^−1, N2 atmosphere. Tm, temperature at melting; Tg, glass-transition temperature; and TID, thermal decomposition onset.
1a	79	151	267
1b	NA	88	271
1c	95	142	286
1d	80	183	309

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Systematic UV-Visible and fluorescence spectral studies were carried out to understand the electronic properties of pyrazolines. The UV-Visible absorption spectra of 1a–1d in acetonitrile, as shown in Fig. 2, along with the results summarized in Table 2 indicate that there are no significant changes in the absorption spectra, except the molar extinction coefficient for the different substituents. The lowest energy peak observed at 381 nm has been assigned to the transition from singlet ground (S₀) to the first singlet excited state (S₁) and the band at 275 nm is ascribed to S₀ → S₂ transition.

Fig. 2 UV-Visible absorption and fluorescence emission of pyrazoline derivatives 1a–1d in acetonitrile solvent.

Table 2 Photophysical properties of pyrazoline derivatives in acetonitrile solvent

Cmpd	λabs nm (ε 10^4 M^−1 cm^−1)	λfl nm	Stokes shift (cm^−1)	τfl (ns)	Φfl	Kr [10^8 s^−1]	Knr [10^8 s^−1]
1a	225 (2.14), 275 (2.13), 381 (3.30)	472	5060	2.95	0.19	0.56	2.77
1b	227 (2.49), 285 (3.14), 381 (3.91)	474	5150	3.65	0.18	0.49	2.24
1c	265 (2.38), 385 (2.86)	480	5270	2.84	0.17	0.53	2.63
1d	304 (3.90), 383 (4.80)	481	5320	2.83	0.19	0.61	2.62

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The absorption maxima of 1a–1d were found to be red-shifted when compared to the structurally similar chromophores based on pyrazolinopiperidines, wherein the five membered pyrazoline ring is fused with the piperidine moiety.³⁸ This difference can be attributed to the enhanced planarity of the pyrazolines studied here compared to the reported compound, which eventually increases the π-conjugation throughout the molecule and reduces the energy gap between the ground and first excited singlet state. Nevertheless, the absorption onset for the derivatives lies below 400 nm; thus, it is not expected to interfere with the absorption of the donor materials of organic solar cells, which generally appears in the visible-NIR region. Further, 1a–1d exhibit intense yellowish green fluorescence with a maximum at 475 ± 5 nm in both solution and solid state.

Interestingly, 1a–1d have negligible overlap in absorption and fluorescence spectra, as can be seen from Fig. 2, with large Stokes shift values over 5000 cm⁻¹.

Non-overlapping absorption and emission spectra are highly desirable for molecules to be used as fluorescent probes in microscopic applications. The fluorescence quantum yield was determined to be 0.19 ± 0.2 with respect to quinine sulphate (0.05 M H₂SO₄) in water.³⁹ The fluorescence lifetimes were measured by exciting the sample at 375 nm with 100 ps pulsed light, and the decay profiles monitored at respective maximum emission wavelengths are given in Fig. S2.† The decay profiles were fitted satisfactorily with single exponential function (ψ² = 1 to 1.2) and fluorescence lifetimes of 2.95, 3.65, 2.84, 2.83 ns were obtained for 1a, 1b, 1c, and 1d, respectively. The radiative and non-radiative rate constants calculated from fluorescence quantum yields and lifetimes were found to be insensitive to the nature of the substituents (Table 2).

The possibility of the intramolecular charge transfer interactions of these compounds was studied by measuring the steady state fluorescence spectra in solvents of different polarities. While the absorption spectra of pyrazolines 1a–1d were found to be insensitive, the fluorescence spectra show solvent polarity induced, red-shifted emission spectra characteristic of intramolecular charge transfer interactions (ESI, Fig. S1 & Table S1†). In this case, the pyrazoline and styrene moieties are expected to act as electron donor and acceptor components, respectively. This is also supported by the molecular orbital pictures of highest occupied (HOMO) and lowest unoccupied (LUMO) molecular orbitals, wherein electron density redistribution occurs during the HOMO → LUMO transition (vide infra). The data summarized in Table S1† indicate that the number of methyl groups does not have any significant effect on the intramolecular charge transfer interactions.

The density functional theory (DFT) based calculations were carried out to obtain insight into the structure property relationship of these pyrazoline materials. The calculated frontier molecular orbital (FMO) distributions of molecules are shown in Fig. 3.

Fig. 3 Contour plots (isosurface value = 0.025 au) of the HOMO and LUMO levels of designed hole transport molecules at the B3LYP/6-31G* level. The hydrogen atoms are omitted here for clarity.

From Fig. 3, it can be observed that both HOMO and LUMO are localized on the whole molecules. However, close analysis of these orbitals reveals that HOMO is predominantly localized on the pyrazoline moiety and LUMO is localized on the styrene unit. As a result, there is a possibility for the intramolecular charge transfer transition between the pyrazoline to styrene unit, which can also be understood from solvent polarity dependent fluorescence studies.

To understand the suitability of these molecules as charge carriers, electrochemical properties were studied using cyclic voltammetry and differential pulse voltammetry (DPV), using glassy carbon as the working electrode, platinum wire as the counter electrode and tetrabutylammonium hexafluorophosphate as the supporting electrolyte. The energy of the HOMO was calibrated using the half wave potential of the ferrocene/ferrocenium redox couple, which is −4.5 eV below vacuum.⁴⁰ The DPV of pyrazolines in homogeneous solution is given in Fig. 4 and the findings are summarized in Table 3. The first oxidation potentials of 1a, 1b, 1c, and 1d in acetonitrile solution were measured to be 0.74, 0.62, 0.44, and 0.67 V, respectively, with reference to the Fc/Fc⁺ redox couple. The first oxidation peak might have originated from the loss of electrons from pyrazoline nitrogens. The HOMO values calculated from the electrochemical data based on the empirical formula HOMO = −(E_{ox. onset} + 4.5) (eV) corresponds to −5.24, −4.88, −5.02, −5.07 eV for 1a, 1b, 1c, and 1d, respectively.

Fig. 4 Differential pulse voltammograms (dpv) of pyrazolines 1a–1d (10⁻³ M solutions, scan rate of 100 mV s⁻¹ vs. Ag/Ag⁺) in 0.1 M solution of tetrabutylammonium hexafluorophosphate in acetonitrile.

Table 3 Electrochemical, theoretical HOMO–LUMO and free energy changes for the electron transfer processes of pyrazolines

Compd	Eox^a (V)	Ered^a (V)	EHOMO^b (eV)	ELUMO^c (eV)	Eg^c (eV)	EHOMO^d (eV)	ELUMO^d (eV)	Eg^e (eV)	Eg^f (eV)
a The redox potential of compounds obtained from differential pulse voltammetry using glassy carbon as the working electrode with reference to the Fc/Fc^+ couple. 0.1 M tetrabutylmmonium hexaflurophosphate was used as a supporting electrolyte.b EHOMO/LUMO = HOMO and LUMO energy levels calculated from the redox potentials.c Eg = electrochemical HOMO–LUMO energy gap.d HOMO and LUMO energy calculated using Gaussian 09 programme at B3LYP/6-31G level.e Computed HOMO–LUMO energy gap.f Optical HOMO–LUMO energy gap (zero–zero transition energy estimated from the point of intersection of normalized absorption and emission spectra in acetonitrile).
1a	0.74	−0.92	−5.24	−3.58	−1.6	−4.59	−1.38	−3.21	−2.75
1b	0.38	−1.43	−4.88	−3.07	−1.8	−4.57	−1.37	−3.20	−2.74
1c	0.62	−0.93	−5.02	−3.47	−1.5	−4.54	−1.33	−3.21	−2.71
1d	0.67	−2.12	−5.07	−2.28	−2.7	−5.47	−2.23	−3.24	−2.77

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These values are in accordance with the work function of the ITO electrode (4.5–4.7 eV), and are also comparable to the HOMO of the widely used hole transport material PEDOT:PSS (5.1–5.4 eV).⁴¹ Furthermore, the significantly higher HOMO energy levels of 1a–1d than the acceptor PCBM (HOMO = −6.10 eV; LUMO = −3.70 eV)⁴² are expected to facilitate the electron transfer process between the hole transport layer and acceptor layer. The electrochemical stability of the components was understood from the constant redox behavior upon repeated reduction–oxidation cycles. HOMO–LUMO band gaps for 1a, 1b, 1c, and 1d calculated from electrochemical methods were found to be 1.66, 1.81, 1.50, and 2.79 eV, respectively, which are in close agreement with those obtained by the optical method. The energy level diagram of the components is shown in Fig. 5.

Fig. 5 Energy level diagram of the corresponding materials in BHJ OSCs.

The charge-carrier (hole and electron) mobility is a key factor in deciding the efficiency of organic solar cells. It has been evident from the previous studies that the charge mobility of organic molecules can be described well by the incoherent hopping model,^1,43–45 where charge transport is viewed as a charge exchange reaction:

Mⁿ + Mⁱ → Mⁱ + Mⁿ (1)

where Mⁿ represents the neutral state of the system and Mⁱ represents the ionic state of the system. The hole/electron transfer rate can be expressed by the standard Marcus–Hush, expressed as eqn (2).^46–48where T is the temperature, λ is the reorganization energy, V is the electronic coupling matrix element (transfer integral) between neighboring molecules in an organic single crystal, h is the Planck constant and k_B is the Boltzmann constant. It is evident from eqn (2) that there are two key parameters that determine the charge transfer rate, λ and V. Theoretical investigations on the charge transport properties of several organic materials imply that both V and λ are equally responsible for the charge transfer properties. Thus, these two parameters need to be properly tuned to obtain the desired properties. Molecules having maximum transfer integral with minimum V_ab are expected to show optimal transport properties. The total reorganization energy of the molecule is given by the sum of internal and external reorganization energies. The former term involves the contributions from intramolecular geometry vibrations and the latter term describes the change in electronic polarization of the surrounding medium. The external reorganization energy can be neglected because most of the organic materials have low polarity, and therefore the main contribution to the total reorganization energy would be from internal reorganization. In this study, the internal reorganization energy (for holes and electrons) is calculated by using eqn (3) and (4), and is schematically depicted in Scheme 2.

λ₊ = λ₁ + λ₂ (3)

λ₋ = λ₃ + λ₄ (4)

λ₁ = E⁺(M₀) − E⁺(M⁺); λ₁ = E⁰(M⁺) − E⁰(M₀)λ₃ = E⁻(M₀) − E⁻(M⁺); λ₁ = E⁰(M⁺) − E⁰(M₀)

where E⁰(M₀), E⁺(M⁺), and E⁻(M⁻), respectively, denote the total energy of neutral, cationic, and anionic species at their optimized geometries. E⁺(M₀) and E⁰(M⁺) denote the total energy of the cationic and neutral species at their optimized geometry of M₀ and M⁺, respectively. Similarly, E⁻(M₀) and E⁰(M⁻) represent the total energy of the anionic and neutral species at the optimized geometry of M₀ and M⁻, respectively. The reorganization energies for electrons (λ₋) and holes (λ₊) of the molecules were computed at the B3LYP/6-31G* level using the Gaussian 09 program.^49,50 It has been evident from the previous studies that the B3LYP functional with 6-31G* basis set provides the most reliable results on the reorganization of organic molecules.^{25,48,51–53}

Scheme 2 Internal reorganization energy for holes (λ_h) in arbitrary units.

The transfer integral V_ab represents the strength of electronic coupling between the two adjacent neighboring molecules a and b. V_ab can be calculated based on Koopmans' theorem⁵⁴ by assuming that the interacting molecules are identical, symmetrically equivalent and also have the same site energies. This method has been most frequently used in the literature for the estimation of transfer integrals in organic semiconductors.¹

where E_H/L+1 and E_H−1/L are the energies of HOMO/LUMO + 1 and HOMO − 1/LUMO, respectively, obtained from the closed shell configuration of the neutral states of two stacked pyrazoline molecules. Once the charge transfer rate is obtained by eqn (2), the hopping mobility can be estimated by the Einstein relation, expressed as eqn (6).where T is temperature, k_B is Boltzmann constant, and D is the diffusion coefficient. The relation between D and k_ct for a one dimensional system is given by D = l²k_ct, where l is the space distance between two interacting molecules.

Reorganization energy (λ) is one of the important factors that govern the charge mobilities of the organic semiconductors. According to Marcus–Hush theory, lower λ value is expected to yield higher charge mobility. Thus, understanding the geometric deformation becomes vital because it has significant influence on the reorganization energy of organic molecules. The calculated reorganization energies of pyrazoline molecules are listed in Table 4. The electron reorganization energy (λ₋) is calculated to be 0.49, 0.54, 0.50, and 0.54 eV for 1a, 1b, 1c, and 1d, respectively. Similarly, the hole reorganization energy (λ₊) is found to be 0.32, 0.33, 0.35, and 0.43 eV for 1a, 1b, 1c, and 1d, respectively. For all the molecules studied here, the magnitude of the electron reorganization energy (λ₋) is found to be profoundly larger than its corresponding hole reorganization energy (λ₊). This observation indicates that pyrazolines could be good hole transport materials rather than electron transport materials. We also examined the effect of substituents on λ₊ values. It can be seen that 1a and 1b have comparatively lower hole reorganization energies than 1c and 1d. The slightly higher reorganization energies for 1d than 1a are due to the slight deviation from planar geometry by the addition of more substituents on R₁, R₂ and R₃ positions.

Table 4 Calculated hole (λ₊) and electron (λ₋) reorganization energies of hole transporting materials at B3LYP/6-31G* level (energies in eV)

Compound	Hole			Electron
	λ1	λ2	λ+	λ3	λ4	λ−
1a	0.16	0.15	0.32	0.25	0.24	0.49
1b	0.17	0.16	0.33	0.25	0.28	0.54
1c	0.19	0.16	0.35	0.25	0.25	0.50
1d	0.23	0.20	0.43	0.26	0.28	0.54

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The charge transfer integral (V_ab), a parameter that defines the electronic coupling between the two molecules, is influenced because of intermolecular stacking. Although the molecular stacking can generate a large number of conformations, having extended π-conjugation, the intermolecular stacking is expected to be dominantly influenced by the π–π interactions.

Indeed, face-to-face π-stacking with a large orbital overlap facilitates the enhanced electronic coupling between the two adjacent molecules and consequently aids the inter-chain charge transport of an organic molecule. In the absence of any experimental results, two different methodologies have been used to compute the transfer integral (V_ab). In one method, the initial geometry of the two monomers is co-facially stacked at the distance of ∼3.5 Å, and then the structures are further optimized. The orbital energies required for charge transfer integral calculation are obtained from the optimized geometries. In the second case, the single point calculations of the two monomers are co-facially stacked at the distance of ∼3.5 Å. Electronic coupling values of all the dimers are calculated for the optimized structures of two adjacent dimers obtained at the M05-2X/6-31G* level of theory. Optimized structures of the dimers are shown in Fig. 6.

Fig. 6 Optimized geometry of dimers using M05-2X/6-31 method.

Charge transfer integrals (V_ab), charge transfer rates (K), and hopping mobilities (μ) are calculated using eqn (2), (5), and (6), respectively, for all the dimers and results are given in Table 5. The abovementioned calculations are performed at room temperature (298 K). As shown in Table 5, both the charge transfer rate and the mobility have the same trend. Both are larger for holes compared to electrons, except for system 1c. Thus, these molecules might be good hole transport materials. The considerably larger K and μ values for holes compared to electrons is due to small hole reorganization energies and larger electronic coupling. The calculated carrier mobility is quite consistent with the experimental values (vide infra). However, theoretical values provide an upper limit of the charge mobility values compared with experimentally reported one, although the trends remains the same (Table 6).

Table 5 The charge transfer integrals (V_ab in eV) and the average intermolecular distances (in Å) for the M05-2X/6-31G* geometries optimized for the lowest energy dimer, reorganization energy (λ), maximum transfer integral (t), maximum rate of charge hopping (K), and drift mobility (μ) for holes and electrons of hole transporting materials

	λ+ (eV)	λ− (eV)	D (Å)	Vab (eV)		K (s^−1)		μ (cm^2 V^−1 s^−1)
				th	te	h^+ × 10^12	e^− × 10^11	h^+	e^−
1a	0.32	0.49	3.39	0.096	0.09	12.1	16.5	0.54	0.07
1b	0.33	0.54	2.73	0.148	0.14	25.8	23.3	0.75	0.06
1c	0.35	0.50	3.98	0.006	0.03	0.03	0.16	0.002	0.01
1d	0.43	0.54	4.33	0.107	0.06	4.46	0.43	0.33	0.03

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Table 6 Charge transport parameters in the samples generated by theoretical fitting of experimental curves

Compd	μ (cm^2 V^−1 s^−1)	Nν (cm^−3)	Hb (cm^−3)	d (nm)
1b	1.5 × 10^−6	—	—	400
1c	3 × 10^−6	1 × 10^18	6 × 10^17	390
1d	4 × 10^−5	3 × 10^18	1.7 × 10^17	580

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To understand the influence of charge carrier transport on photovoltaic performance, we used the space charge limited current (SCLC) method to measure the hole mobility in the blend. Fig. 7 shows the J–V curves for hole only device configuration. The hole transport properties of pyrazolines 1b–1d were evaluated by measuring the charge carrier mobilities using hole only device configuration, viz., indium tin oxide (ITO)/poly(3,4-ethylendioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS)/hole transport layer (1b, 1c and 1d)/Au. PEDOT:PSS was spin-coated onto the cleaned ITO substrate and dried at 140 °C for 30 min in a vacuum.

Fig. 7 J–V characteristics of devices, experimental (symbols) and theoretical fits (solid lines are obtained using eqn (3) and (4)).

The role of the PEDOT:PSS layer (40 nm) is to reduce the roughness of ITO as well as improve the work function to achieve better hole-only device properties. These active materials 1b, 1c and 1d were spin coated onto PEDOT:PSS from chloroform solution (40 mg mL⁻¹) in a glove box. Finally, Au contacts (200 nm thick) were applied via thermal evaporation through a shadow mask in an 8 × 10⁻⁶ Torr vacuum. The work functions of Au and ITO are close to the HOMO energy level of HTL materials, as well as far below the LUMO energy level. Therefore, the electron injection barrier is considerably high when compared to the hole injection barrier from both the electrodes. As a result, the transport is dominated by holes in the so-called hole only device. The J–V characteristics of these samples were measured with a Keithley 2420 Source Meter unit interfaced with a PC at room temperature.^55–57

At low applied bias, the J–V characteristics follow Ohm's law, [Fig. 7] as the injected holes density p is negligible when compared to the density of thermally generated holes inside the specimen. This is illustrated by the following equation:

where q is the electronic charge, μ is the carrier mobility, d is the thickness of the film, and ε is the permittivity of the material.

With the increase in applied voltage, the injected carrier density increases and a point is reached where it is equal to the thermally generated carrier density and thus the J–V characteristics follow the space charge limited conduction (SCLC) mechanism. The onset voltage (V_Ω), which governs the transition from Ohm's law to SCLC is given as

when the slope of the J–V characteristics is ∼2 and then the Mott–Gurney's equation is obeyed as follows:

The equation does not imply the absence of traps but reveals that up to a particular temperature, thermal energy is sufficient to fill all the traps.

When the slope of the characteristics is >2 and eqn (9) is not followed, it implies that now thermal energy is not sufficient to fill the traps. The J–V characteristics follow the trap limited conduction with traps distributed exponentially in energy and space and is given as follows:

where q is the elementary charge, μ is the charge carrier mobility, N_ν is the effective density of states, d is the film thickness, ε is the dielectric constant of the material, ε₀ is the permittivity of the free space, and H_b is the total trap density at the edge of the valence band.

The J–V characteristics of three samples, viz., 1b, 1c, and 1d, are shown in Fig. 7. The experimental curve of 1b and 1c shows two regions of conduction that are ohmic conduction at a low voltage and trap limited conduction (non ohmic conduction) at a high voltage, and 1d shows trap-free SCLC conduction at the high voltage.

The theoretical curve generated using eqn (10) has been found to fit the experimental curve, as shown in Fig. 7 with charge transport parameters given as follows for all three samples. The hole mobility is measured to be 3 × 10⁻⁶, 4 × 10⁻⁵, and 1.5 × 10⁻⁶ cm² V⁻¹ s⁻¹. 1d is found to show higher hole mobility compared to 1b and 1c.

Conclusion

Photophysical and hole transport properties of pyrazoline derivatives were studied by experimental and theoretical methods. Pyrazolines can be synthesized in good yields by simple, two-step reactions using relatively cheap starting materials. The UV-visible absorption and fluorescence studies show the intense green emission in both solid state and solution state with non-overlapping absorption and emission spectra, which are an important prerequisite for many fluorescence-based applications. Although the electronic properties are not significantly influenced by the substituents, their influence on the charge carrier mobilities can be correlated to their film-forming abilities and reorganization energies. Furthermore, optical, electrochemical, and theoretical measurements show that the HOMO energy levels of pyrazoline lie between −4.8 and −5.2 eV, which are ideal values for hole transport materials. The highest charge carrier mobility measured using the space charge limited current method is found to be in the order of 4 × 10⁻⁵ cm² V⁻¹ s⁻¹ for the dimethoxy substituted phenyl moiety. The smaller reorganization energy for the hole than the electron suggests that pyrazolines can be better hole transport materials. This result may provide a new way to further optimize the hole transport materials based on the pyrazoline skeleton. Furthermore, the pyrazolines as HTM in this study are cost-effective, which can also avoid the complicated synthesis of triarylamine derivatives and significantly reduce the cost of devices.

Author contribution

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Acknowledgements

We acknowledge CSIR-TAPSUN (NWP-54) for funding. We thank Prof. P. Ramamurthy and Dr C. Selvaraju, NCUFP, University of Madras for lifetime measurements.

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Footnote

† Electronic supplementary information (ESI) available: Characterization, UV-visible, fluorescence spectral details. See DOI: 10.1039/c5ra19520a

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