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New Class of Organic Hole-Transporting Materials Based on Xanthene Derivatives for Organic Electronic Applications Jefferson S. Martins,*,†,‡ Aloisio A. Bartolomeu,§ Willian Henrique dos Santos,§ Luiz Carlos da Silva Filho,§ Eliezer Fernando de Oliveira,∥ Francisco Carlos Lavarda,∥ Alexandre Cuin,‡,⊥ Cristiano Legnani,†,‡ Indhira O. Maciel,†,‡ Benjamin Fragneaud,†,‡ and Welber G. Quirino*,†,‡ †

Nano − Grupo de Nanociências e Nanotecnologia, ‡Centro de Estudos em Materiais, Instituto de Ciências Exatas, and ⊥Laboratório de Pesquisa em Química Bioinorgânica, Departamento de Química, Universidade Federal de Juiz de Fora, 36036-900 Juiz de Fora, Minas Gerais Brazil § Departamento de Química and ∥Departamento de Física, Universidade Estadual Paulista (UNESP), Faculdade de Ciências, 17033-360, Bauru, São Paulo Brazil S Supporting Information *

ABSTRACT: In this work, we investigate the influence of three novel 14-aril-14H-dibenzo[a,j]xanthene derivatives (XDs) modified with different functional groups as very promising hole-transporting materials for organic optoelectronic devices. Optical, electronic, and structural properties were analyzed by UV−vis absorption spectrum, cyclic voltammetry, and powder X-ray powder diffraction (XRPD). We investigated the influence of these XD as hole-transporting layers (HTL) on the performance of a simple stack bilayer OLED built with commercial aluminum tris(8-hydroxyquinoline) Alq3 acting as an electron-transporting and emissive layer (EML). As a proof-of-principle the XD devices were compared to reference devices fabricated with one of the most common hole-transporting materials, the N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)-2,2′-dimethylbenzidine (α-NPD). The structure of the devices was ITO/HTL (50 nm)/Alq3 (50 nm)/Al (120 nm) without encapsulation. Under the same conditions, the devices using XD as HTL exhibited high performance and significant durability when compared to the reference ones. These results are also supported by a theoretical study using density functional theory (DFT) showing that this set of XD presents a higher hole mobility than α-NPD. Thus, we demonstrated that this class of molecules are very promising when used as hole-transport material in organic electronic devices.

1. INTRODUCTION

transparent substrates, vibrating screens that make it pump out sound, and all kind of smart devices, making this technology even more innovative, accessible, and sustainable. Therefore, the continuous designing of novel and efficient materials with spectral, thermal, and morphological stability is a need to overtake this demand. In this context, xanthene derivatives (XD) have generated considerable interest as hole injectors or transporting injected holes for organic devices (OLEDs, OPVs, and transistors)18−22 due to its low cost, simple synthetic methodology, easy chemical modification, and good thermomolecular stability.23 Moreover, XD can also be employed as blue-emitting materials due to their high photoluminescence (PL) efficiency and wide band gap.24−27

1

Since the pioneering work of Tang and Van Slyke confirming that the development of high-performance OLEDs is dependent on the high efficiency of charge injection and adequate mobility, various molecules and conjugated polymers have been developed for charge-transporting hosts.2−8 In the last decades several hole-transporting materials (HTM) were developed and mostly consist of triaryl amines and benzidines in many forms, for instance, (i) amine family (TcTa, TPT1),9−11 (ii) benzidine family (α-NPD, NPB, TPD),12−14 and (iii) spiro-linked or spirocyclic fluorine (Spiro-TPD, Spiro-2NPB, SpiroTAD).15−17 Even though several hole-transporting materials with better hole mobility than NPB derivatives have been reported in recent years, the NPB are still considered the most successful HTM and are still largely used in organic devices. Organic electronics is still seen as a growing area since the research and development of novel electronic devices has been pushed toward all-plastic optoelectronic applications. Organic electronics also includes printed devices, flexible electronics, © 2017 American Chemical Society

Received: March 2, 2017 Revised: May 11, 2017 Published: May 22, 2017 12999

DOI: 10.1021/acs.jpcc.7b02034 J. Phys. Chem. C 2017, 121, 12999−13007

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The Journal of Physical Chemistry C Scheme 1. Synthesis of 14-Aryl-14H-dibenzo[a,j]xanthene Derivatives

In this work, we investigated a novel class of 14-aril-14Hdibenzo[a,j]xanthene complex derivatives modified with different functional groups. We show that these molecules are very promising HTMs for organic devices since they exhibit excellent carrier injection/hole-transporting characteristics as well as good thermomolecular stability. Conventional bilayer devices using XD as an HTL were fabricated and compared to the reference device fabricated with N,N′-bis(naphthalen-1-yl)N,N′-bis(phenyl)-2,2′-dimethylbenzidine (α-NPD). Electroluminescence, density−voltage (J−V), and luminance−voltage (L−V) characteristics of the devices were obtained in detail and showed enhanced optoelectrical performances. On the other hand, we demonstrate that this set of XD presents a higher hole mobility than α-NPD by comparing the thermal, electrochemical, optical, and structural characteristics to a density functional theory (DFT) study.

Figure 1. Structure of 14-aryl-14H-dibenzo[a,j]xanthene derivatives used in this work.

model, in organic materials, one of the key parameters that govern the behavior of the charge transfer rate is the reorganization energy (λ) due to the geometric relaxation in the charge transfer.30−32 The charge transfer rate (KCT) may be expressed as follows32,34

2. EXPERIMENTAL SECTION 2.1. General Procedure for One-Pot Synthesis of 14Aryl-14H-dibenzo[a,j]xanthene Derivatives (XD-01, XD02, and XD-03). To a solution of NbCl5 (25 mol %) in anhydrous CH2Cl2 (2.0 mL), maintained at ambient temperature under N2 atmosphere, we added a solution of 2-naphthol (1.0 mmol) and the respective aldehyde (0.5 mmol) in anhydrous CH 2Cl2 (4.0 mL). Once the addition was completed, magnetic stirring was maintained at the same temperature for 24 or 48 h. The reaction mixture was quenched with H2O addition (3.0 mL), and the product was extracted with CH2Cl2 (10.0 mL). The organic layer was separated and washed with saturated aqueous NaHCO3 (3 × 10.0 mL) and brine (2 × 10.0 mL), dried over anhydrous MgSO4, and filtered, and the residual solvent was evaporated under vacuum. The residue was recrystallized in ethanol to ensure high-purity products (Scheme 1). The full experimental details and spectroscopic characterization (1H and 13C NMR, infrared, UV−vis, and mass spectrometry) of these compounds can be found in the full paper recently published.28,29 Among the synthesized materials, three derivatives named XD-01, XD-02, and XD-03 have been chosen as HTM candidates, and their molecular structures are shown in Figure 1. 2.2. Computational Details. Charge transport in organic materials rises primarily via a hopping mechanism, in which the mobility (μ) of the charge carriers (electrons or holes) is directly proportional to the transfer rate (KCT) of charge carriers (hopping probability per unit time) described by the Einstein relationship30−33 μ=

(eA2 2) K CT (KBT )

⎛ π ⎞1/2 ⎛ 2π Hab 2 ⎞ ⎛ −λ ⎞ ⎟e ⎜ K CT = ⎜ ⎟ ⎜ ⎟ h ⎝ λKBT ⎠ ⎝ ⎠ ⎝ 4KBT ⎠

(2)

in which h and ⟨Hab⟩ are Planck’s constant and the electronic coupling matrix element between neighboring interacting molecules, respectively. As it can be seen, λ has a double contribution and is inversely proportional to the charge transfer rate. As KCT is directly proportional to the charge mobility,32,35 the smaller λ the better the charge mobility. Assuming that the reorganization energy is dominated by the internal contribution,32,36−38 we can calculate the reorganization energy related to the transport of holes (λhole) through λhole = [E(1)(M) − E(0)(M)] + [E(1)(M+) − E(0)(M+)] (3)

in which E(0)(M) and E(0)(M+) represent the energy of neutral and cationic states calculated with the respective lower energy geometries and E(1)(M+) and E(1)(M) represent the energy of the cation in the geometry of the neutral molecule and the energy of neutral molecule in the geometry of the cationic molecule, respectively. λ hole for α-NPD and xanthene derivatives were calculated using eq 3. The three structures studied in this paper, neutral and charged, were completely optimized by density functional theory (DFT), employing the Becke three-parameter Lee−Yang−Parr exchange-correlation hybrid functional (B3LYP)39,40 and 6-31G(d,p) basis set.41 The calculations were run with the GAUSSIAN09 software.42 This methodology was chosen due its good results and satisfactory accuracy in other studies done with similar materials.37,43−45 The equilibrium geometries were confirmed by vibrational spectrum calculations since no imaginary frequencies were found. 2.3. Structural Analysis. X-ray powder diffraction (XRPD) analysis was obtained from a ground polycrystalline powder of the XD compounds. Diffraction data were collected by

(1)

in which kB, T, e, and A are, respectively, the Boltzmann constant, temperature, elementary charge, and the hopping transport distance. According to the Marcus semiclassical 13000


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The Journal of Physical Chemistry C

such as degradation temperature (Td), glass transition (Tg), crystallization (Tc), and melting temperature (Tm) of the compounds were analyzed. The redox potential of the XD was measured by a cyclic voltammeter (CV) with the Ivium Potentiostat system model CompactState using a threeelectrode cell equipped with a carbon working electrode, auxiliary platinum wire, and Ag/AgCl pseudoreference electrodes in 0.1 mol·L−1 KCl solution. The electrochemical response of XD was evaluated on modified working electrode53 in which the XD materials were thermally deposited on the surface of the glassy carbon electrode. 2.5. Device Fabrication. Electroluminescent devices were fabricated onto an indium−tin−oxide (ITO) coated glass substrate with 15Ω/□ supplied by LUMTEC. Glass/ITO substrates were previously etched and cleaned with acetone and washed with alkaline detergent. After that they were sonicated with isopropyl alcohol. Finally, the substrates were treated in an ultraviolet−ozone cleaner chamber. The organic layers were deposited through thermal evaporation in an inert glovebox environment vacuum deposition system at 3 × 10−4 Pa with a deposition rate of 0.5 and 1.0 Å s−1. The layer thicknesses were controlled in situ through a quartz crystal monitor and confirmed with profilometric measurements. In order to measure the hole mobility of the different HTMs studied in this work, we fabricated devices where the HTL was sandwiched between both electrodes: ITO/HTL (50 nm)/Al (120 nm). We also fabricated some double-layer OLED devices with the following architecture: ITO/HTL(50 nm)/Alq3(50 nm)/Al(120 nm), where the xanthene derivatives XD-01, XD02, and XD-03 and the α-NPD were used as HTL. Commercially available (bis(2-methyl-8-quinolinolate)-4(phenylphenolato)aluminum) Alq3 was used as an electrontransporting and emissive layer (EML), and aluminum, deposited under same vacuum conditions, was used as cathode. At the same time, a double-layer standard device using α-NPD was produced as reference. The fabricated devices had an active area of 4 mm2 and operated in forward bias voltage, with ITO as the positive electrode and Al as the negative electrode. 2.6. Device Characterization. The electrical and optical properties of the devices were simultaneously recorded with a LabView-based program using a Keithley 2240 current−voltage source and a calibrated radiometer/photometer (Newport Power Meter, model 1936-c). The electroluminescence spectra were obtained with an Ocean-optics USB2000+UV−vis spectrometer at room temperature without encapsulation.

overnight scans (about 14 h) in the 2θ range of 7−105° with steps of 0.02° using a Bruker AXS D8 da Vinci diffractometer, equipped with Ni-filtered Cu Kα radiation (λ = 1.5418 Å) and a Lynxeye linear position-sensitive detector. The optics were set up as primary beam Soller slits (2.94°), fixed divergence slit (0.3°), receiving slit (10 mm), and generator at 40 kV and 40 mA. Former unit cell parameters were found using 21 first standard peaks followed by indexing through the single-value decomposition approach46 implemented in TOPAS.47 XD-01 XRPD has been already reported,48 and it shows a different molecular packing when compared to XD-02 and XD-03. However, the basic organic compound remains the same in the three compounds. Cell parameters of the three compounds were refined using the 7−55° 2θ range by the Pawley method.49 Then the structure solution processes were performed by the simulated annealing technique,50 also implemented in TOPAS. Each organic molecule was idealized as a rigid body model built using the Z matrix formalism based on data from single crystals of −H analogous compounds as shown in our previous work.51 Besides the standard distance/ angle for aromatic CC and C−H bonds, rotation, translation, and torsion angles (see Figure 2) were left free for all rigid

Figure 2. Sketch of a generic XD compound, where R stands for the −C(CH3)3 group for XD-02 and the −phenyl group for XD-03. τ1 describes the torsion angle between O−C−C−C. τ2 and τ3 illustrate the free rotation about C−C bonds. H atoms were omitted for better visualization.

bodies in the simulated annealing step. Final refinements were carried out by the Rietveld method.52 It is important to note that the rigid body models introduced at the solution stage were maintained in the final refinement. An isotropic thermal parameter was assigned to all atoms, set at the Biso 2.4(0.1) Å2 thermal parameter. Crystal data, fractional atomic coordinates, and displacement parameters of XD-02 and XD-03 structures are supplied in standard CIFs deposited in the Cambridge Crystallographic Data Centre (CCDC 1529653−1529654). The data can be obtained free of charge at http://www.ccdc. cam.ac.uk/conts/retrieving.html [or from Cambridge Crystallographic Data Centre (CCDC), 12 Union Road, Cambridge CB2 1EZ, UK; fax: + 44 (0) 1223−336033; e-mail: deposit@ ccdc.cam.ac.uk]. 2.4. Optoelectronic and Thermal Properties. Optical absorption spectra in the UV−vis range of thermally deposited XD thin films (50 nm thick) were obtained with a UV−vis spectrophotometer from SHIMADZU, model UV-1800, using a clean glass substrate as reference. Also, a Synergy H1 (BioTek Instruments) luminescence spectrometer was used to obtain photoluminescence (PL) spectroscopy of the molecules diluted in ethanol. To investigate the thermal properties of the XDs organic compounds, thermogravimetric analysis (TGA) was carried out using a DTG-60 (SHIMADZU). Also, differential scanning calorimetry (DSC) measurements were obtained under nitrogen atmosphere using a DSCQ1000 (TA Instruments). On the basis of these experiments, thermal parameters

3. RESULTS AND DISCUSSION 3.1. Computational and Experimental Study of the Hole-Transport Properties in XD Molecules. Figure 3 presents the final conformation obtained after geometry optimization from DFT/B3LYP/6-31G(d,p). For the molecule α-NPD (Figure 3a), all angles between the planes of adjacent rings have values near 50°, due to the repulsion among the hydrogen atoms. The pristine xanthene plane of XD-01 (Figure 3b), XD-02 (Figure 3c), and XD-03 (Figure 3d) slightly bent due to the attached radicals, as observed in Figure 3b, in which a front view of the XD-01 molecule is presented. Concerning the XD-01 molecule, the aromatic ring bonded to the xanthene stays practically perpendicular to the xanthene plane, while in molecule XD-03 the two aromatic rings linked to xanthene have dihedral angles of about 40° between their planes, due to repulsion between the nearest hydrogen atoms. 13001


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Table 2. First Set of Data Shows the Hole Mobility Experimentally Evaluated Based on a SCLC Model for αNPD, XD-01, XD-02, and XD-03; Other Data Are Published Work Results That Allow a Good Bibliographic Overview of the Hole Mobility of HTMs As Compared to the Benzidine Family (NBP derivatives) material α-NPD XD-01 XD-02 XD-03 α-NPD

Figure 3. Optimized geometries obtained for (a) α-NPD, (b) XD-01 (left, perspective view; right, front view), (c) XD-02, and (d) XD-03.

(N,N′-di(naphthalene-1-yl)-N,N′di(phenanthrene-9-yl)biphenyl4,4′-diamine) α-NPD

Table 1 presents the calculated λhole for α-NPD, XD-01, XD02, and XD-03 using eq 3. As we can see, all xanthene

H = tetraarylbenzo[1,2-b:4,5-b′] difurans Me = tetraarylbenzo[1,2-b:4,5-b′] difurans Ph2N−C6H4 = tetraarylbenzo[1,2b:4,5-b′]difurans (p-tol)2N−C6H4 = tetraarylbenzo [1,2-b:4,5-b′]difurans NPB N1,N1,N3,N3-tetra([1,1′-biphenyl]4-yl)-N5,N5-diphenylbenzene1,3,5-triamine) NPB 2,2′,7,7′-tetrakis(N,N-carbazole)spiro(fluorene-9,9′-xanthene) 2,3′,6′,7-tetrakis(N,N-carbazole)spiro(fluorene-9,9′-xanthene)

Table 1. Reorganization Energy for Hole Transfer (λhole) Obtained for α-NPD, XD-01, XD-02, and XD-03 structure

α-NPD

XD-01

XD-02

XD-03

λhole (meV)

9.083

7.277

7.040

6.723

derivatives have a lower λhole than α-NPD. This suggests that they will have a higher hole mobility than α-NPD. According to our calculations, XD-03 seems to be the best material among the other molecules presented in this work. Indeed, its λhole is the lowest of all cases studied in the present work. As mentioned previously, we also experimentally evaluated the hole mobility of each xanthene molecule. The hole mobility (μ0) was determined by fitting the J−V curves to the model of a single carrier space charge-limited current (SCLC), which was described by the Mott−Gurney equation54−58 J=

⎛ V2 Vint ⎞ 9 μ0 ε0ε int exp 0.891 γ ⎟ ⎜ 8 d ⎠ d3 ⎝

μhole, cm2 V−1 s−1 1.1 × 10

SCLC

TOF/0.2 MV cm−1

1.7 2.0 1.9 5.7

× × × ×

−4

10−4 10−4 10−4 10−4

ref this work

59

2.2 × 10−4

TOF/0.25 MV cm−1

3.6 × 10−4

8

6.4 × 10−4 8.0 × 10−5 2.8 × 10−3 5.6 × 10−4 SCLC

2.15 × 10−4 2.09 × 10−3

6

SCLC

2.15 × 10−4 9.63 × 10−4

60

1.57 × 10−3

experimental conditions. On the other hand, the precision of such experimental measurements should fluctuate depending upon the kind of technique used (i.e., time-of-flight, space charge-limited current, etc.). This means that a reliable study between the hole-transport properties of each organic material should be done through a relative comparison of a given molecule to a well-referenced material in the exact same experimental conditions. The bibliographic review shown in Table 2 clearly shows that some HTMs families present hole mobility slightly higher than that of NPB derivatives whenever others tend to decrease depending on the radical attached to the main molecule. Also, few HTMs presented in Table 2 show significant enhancement of the hole mobility by an order of magnitude. This set of data confirms our belief that the type and position of the radicals attached to the main molecule should govern the HTM properties. 3.2. Structural Analysis. Crystallographic information on XD-02 and XD-03 compounds are described in Table 3. XD-02 and XD-03 compounds have the same crystal system/space group: monoclinic and C2/c. Even if they belong to a different crystal system than XD-01, the asymmetric units remain the same. The crystallographic model of XD-03 obtained based on X-ray diffraction experiments is shown in Figure 4. The schematics of XD-01 and XD-02 were omitted here because their structure is very similar to XD-03 where the phenyl group is changed by −H or −C(CH3)3 groups. X-ray diffraction experiments show that the naphthalene groups in XD-01, -02, and -03 molecules are nearly coplanar, as we predicted by DFT

(4)

where J is the current density, μ0 is the hole mobility for electric fields tending to zero, d is the hole-transport layer film thickness, ε0 is the free space permittivity, and ε is the relative dielectric constant of the studied molecule. In this work, the relative dielectric constant was assumed to be 3.5, as it is typically used for organic semiconducting molecules.56 Vint is the internal voltage and was determined as described in eq 5 Vint = Vext − R SI − Vbuilt

characterization method

(5)

where Vext is the external voltage applied to the hole-transport layer, Rs is an estimative of the series resistance, I is the measured current, and Vbuilt is the built-in voltage. The holetransport mobilities obtained for each material are summarized in Table 2. These results indicate that all XD molecules have a slightly higher hole mobility than α-NPD. Moreover, XD-02 and XD03 seem to exhibit greater mobilities when compared to the other molecules presented in this work. These results are in agreement with the DFT results previously discussed. Table 2 allows one to compare the hole mobilities experimentally evaluated for several classes of HTMs. One might keep in mind that the electric properties of organic materials highly depend on the manufacturing process or 13002


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The band gap was calculated from the absorption spectra by measuring the wavelength at which the fundamental absorption occurs (edge of the spectrum) using Tauc plots.62 From these plots we found Eg ≈ 3.5 eV for the three XD compounds. Indeed, all XD compounds do not show any absorption band in the visible light region due its wide band gap, and thus, they are suitable to be employed as a hole injection layer (HIL) and/or hole-transporting layer (HTL). The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels were calculated based on UV−vis and cyclic voltammetry (CV) results. The voltammogram showed similar trends regarding the onset redox potentials and the oxidation cycles (Figure S1). The oxidation onset potential relative to the SCE of the three compounds was found to be about 1.1 eV, which leads to a HOMO ranging from −5.48 to −5.52 eV. These values are very close to the HOMO level of α-NPD (5.4 eV).59,63 As a consequence, the LUMO energy level varies from −1.91 to −2.03 eV and was calculated by subtracting the optical band gap (Eg) from the HOMO value. All values presented in Table 4 are definitive evidence that all three XD can be used as a blue emitting layer (EML) as well as a hole-transporting layer (HTL). 3.4. XD Thermal Stability. TGA experiments were carried out under a nitrogen atmosphere with a heating rate of 10 °C/ min. On the basis of the 5 wt % weight loss (Figure S2) we determined the degradation temperatures of the three XD as follows: 286 °C for XD-01, 328 °C for XD-02, and 340 °C for XD-03. This indicates that these molecules have an excellent thermal stability. DSC measurements were carried out with a first heating/cooling cycle with a scanning speed of 10 and 50 °C/min, respectively, in order to ensure complete amorphization of the samples. Then a heating ramp at 10 °C/min was performed and allowed us to determine the glass temperature transition as it can be seen in Figure S3. Analyzing the corresponding data, we obtained a Tg of about 64 °C for XD01, 93 °C for XD-02, and 84 °C for XD-03. The α-NPD Tg is reported around 95 °C.64,65 This value is higher than the Tg of XD-01. Nonetheless, XD-02 and XD-03 show similar glass temperatures when compared to the α-NPD one. Thus, we believe that these materials could be used in stable and long lifetime organic electronic devices. Table 5 summarizes all thermal parameters for XD compounds including the melting point temperatures. 3.5. Device Performance. Figure 6a shows the electroluminescence spectra of the fabricated devices. The green band centered at 525 nm is typical of Alq3 EL emission. There is a slight enlargement in the electroluminescent emission when XD molecules are used. It is worth noting that the device emission band enlargement should not be attributed to the EL emission of xanthenes, considering that Figure 6a clearly shows that there is no overlap of the Alq3 and xanthene emission. That should be explained by small structural modifications and morphological differences among the HTL used in this work. This should not be understood as a lower performance of the XD-based OLED but as a change in the (X,Y) color coordinates of the CIE chromaticity diagram (Commission Internationale de l’Eclairage) as shown in Table 6. Figure 6b shows the current density−voltage (J−V) curves of all devices, confirming that they have a typical diode behavior once the current increases exponentially as a function of the bias voltage. On the other hand, from the (J−V) curves it is possible to observe that α-NPD reference devices have lower operational voltages when compared to XD ones. Most

Table 3. Main Crystallographic Characteristics of XD-02 and XD-03 molecular formula fw (g mol−1) cryst syst space group a (Å) b (Å) c (Å) β (deg) V (Å3) Z Rbragg, Rwp

XD-02

XD-03

C31H26O 414.57 monoclinic C2/c 23.441(2) 12.002(6) 19.0495(1) 57.35(5) 4512.6(6) 8 0.075, 0.11

C33H22O 434.55 monoclinic C2/c 21.143(2) 13.077(1) 18.355(1) 64.61(3) 4584.7(6) 8 0.09, 0.12

Figure 4. Crystal structure of XD-03 drawn using SCHAKAL (Keller, 1986). Carbon, hydrogen, and oxygen atoms were colored as gray, light gray, and red, respectively.

calculations. Moreover, the diffraction data are in good agreement with the conformation predicted by DFT geometry optimizations. The angles between the naphthalene groups change slightly when R is −H, −C(CH3)3, and −phenyl, respectively, ranging from 177° for XD-01, 181.7° for XD-02, and 179.1° for XD-03. 3.3. Optoelectronic Properties. The optoelectronic properties of XD compounds are summarized in Table 4. Table 4. Optoelectronic Properties of Xanthene Derivatives material

λabs,max

HOMO (eV)

LUMO (eV)

gap (eV)

XD-01 XD-02 XD-03 α-NPD

222 223 215

−5.48 −5.52 −5.49 −5.40

−1.91 −2.03 −1.98 −2.40

3.57 3.49 3.51

UV−vis absorption of the thermally deposited thin films are shown in Figure 5. The UV−vis spectra showed absorption peaks at 220, 245, and 275 nm. Previous works have shown that the absorption spectra of XD molecules diluted in ethanol28 are slightly shifted as compared to the results we obtained. This should be attributed to the packing effect as reported by Surim et al.,61 indicating that there were no molecular structural changes during thermal evaporation. 13003


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Figure 5. (a) Normalized absorption of xanthene derivatives thin films. (b) Normalized absorption and photoluminescence of xanthene derivatives in ethanol solution (1 × 10−6 mol·L−1).

Table 5. Thermal Properties of DX material XD-01 XD-02 XD-03 α-NPD64,65

Tga

(°C)

64 93 84 95

b

Table 6. Photoelectric Properties of Devices c

Tc (°C)

Tm (°C)

128 126 140

188 298 280

Tdd

(°C)

268 328 340 310

a

Glass transition (Tg). bCrystallization (Tc). cMelting temperatures (Tm). dDegradation temperature.

HTL

Von/offa (V)

ηb (cd/A)

CIE(X,Y)c

XD-01 XD-02 XD-03 α-NPD

6.0/15.2 6.8/30.0 4.5/27.2 3.8/13.3

10 7.2 16 5.7

(0.26,0.58) (0.28,0.57) (0.27,0.58) (0.26,0.29)

a

Turn-on voltage at 1 cd/m2. bCurrent efficiency; at 50 mA/cm2. cCIE coordinates.

transporting layers we choose to use them in a simple stack bilayer OLED. Figure 6c also shows a picture of the XD-03based device working at 18 V bias voltage. From Figure 7 it is clear that the three devices built with XD compounds as a hole-transporting layer showed higher current efficiencies in the range 0−100 mA/cm2 when compared to the reference device. Our theoretical calculations predict that the

Figure 6. (a) Normalized EL spectra of the devices (ITO/HTLs/ Alq3/Al) at room temperature. (b) J−V curves of fabricated devices. (c) Picture of XD-03 device under 18 V bias voltage.

probably this should be attributed to variations of hole injection rates at the interface between the anode (ITO) and XD.66−68 Indeed, several studies have shown that the device performance can be increased by using complex multilayer structures (i.e., hole injection layer, hole blocking, etc.).69 However, to determine how efficient the XDs materials behave as hole-

Figure 7. Current efficiency as a function of the current density of ITO/HTLs/Alq3/Al devices. 13004



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charge carrier mobility followed the crescent order: α-NPD, XD-01, XD-02, and XD-03. One might expect that this would also be the order of device efficiencies, which is exactly what was obtained in this work. Besides xanthenes good charge mobility, the high LUMO level act as an electron blocking layer (EBL). The ELB effect prevents electron leaks from EML into the HTL; thus, the EML would be the only layer allowed to emit light. XD-03 based devices showed the highest efficiency among all the HTM studied in this work. XD-03 based devices showed a current efficiency about three times higher when compared to α-NPD ones at 50 mA/cm2. All presented arguments lead us to believe that we significantly improved the charge balance by using xanthenes as HTL. The EL performance of the devices is summarized in Table 6. One might question why XD-based OLEDs exhibit a lower durability, since xanthenes have a higher degradation temperature, as shown in Table 6. Indeed, degradation mechanisms in small molecule devices is a complex problem to address. It is mandatory to correlate the impact of the device architecture, material properties, manufacturing processes, or operating conditions at the same time. For instance, the relationship between the glassy temperature (Tg) and the device stability is still not well understood. Even though it is commonly accepted that molecules with a high Tg are desirable for maximizing OLEDs longevity, Adachi et al. demonstrated that materials with low Tg might exhibit long lifetimes whenever others with high Tg exhibited shorter lifetimes.70 Moreover, some works pointed out that one of the factors that could influence the degradation process of OLEDs is the interfacial energy between the anode and HTL.66,70,71 Finally, we believe that the degradation mechanisms of XD-based devices arises from a sum of several effects such as anode/HT interface energy barrier,70 hole injection rates at the interface between the anode and XD,72 or hole accumulation in the Alq3/Al interface.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b02034. Cyclic voltammogram of XD thin films deposited on the surface of a glassy carbon electrode; TGA curves of XD01, XD-02, and XD-03; DSC curves of XD-01, XD-02, and XD-03 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: wgqurino@fisica.ufjf.br. ORCID

Cristiano Legnani: 0000-0002-5234-5487 Welber G. Quirino: 0000-0001-6294-5382 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by Brazilian Agencies CNPq, CAPES, FAPEMIG, FINEP, FAPESP (Procs. 2012/23821-7, 2013/ 08697-0, 2012/21983-0, 2014/20410-1, and 2016/01599-1), ́ INCT/INEO, and Rede Mineira de Quimica (RQ−MG) supported by FAPEMIG (Project REDE-113/10; Project CEXRED-00010-14). We also thank the Thermal Analysis and Particulate Materials Laboratory (Latep) from the National Institute of Metrology, Quality and Technology-Inmetro for DSC analysis. We thank Monica Mélquiades for calculating the hole mobility.

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REFERENCES

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4. CONCLUSIONS In summary, a novel set of three 14-aryl-14H-dibenzo[a,j]xanthene derivative complexes (XD) modified with different functional groups was investigated through the comparison of theoretical calculations and experimental results. Their structures were proposed by theoretical simulations and are in complete agreement with the structures established by stateof-the-art X-ray powder diffraction. DFT results show that xanthene derivatives have a superior hole mobility when compared to the commonly used α-NPD. It appears that XD03 is the best hole-transporting layer among the three materials studied here. Moreover, we observed that these molecules are thermally and chemically stable. Also, we investigated the influence of XD as a holetransporting layer (HTL) on the performance of a simple stack bilayer OLED compared to the reference device fabricated with α-NPD. All XD-based devices have improved current efficiency when compared to reference devices. These new HTMs present a low energy barrier between its HOMO levels and the ITO work function. On the other hand, XDs also presents higher LUMO energy and as a consequence behave as a good EBL. XD compounds are easy to synthesize and exhibit all the qualities required for hole-transporting materials. Finally, we expect that these new molecules will be employed in the design of new types of organic optoelectronic devices. 13005


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