Functionalized Zinc Porphyrins with Various Peripheral Groups for Interfacial Electron Injection Barrier Control in Organic Light Emitting Diodes.

Here, we use a simple and effective method to accomplish energy level alignment and thus electron injection barrier control in organic light emitting diodes (OLEDs) with a conventional architecture based on a green emissive copolymer. In particular, a series of functionalized zinc porphyrin compounds bearing π-delocalized triazine electron withdrawing spacers for efficient intramolecular electron transfer and different terminal groups such as glycine moieties in their peripheral substitutes are employed as thin interlayers at the emissive layer/Al (cathode) interface to realize efficient electron injection/transport. The effects of spatial (i.e., assembly) configuration, molecular dipole moment and type of peripheral group termination on the optical properties and energy level tuning are investigated by steady-state and time-resolved photoluminescence spectroscopy in F8BT/porphyrin films, by photovoltage measurements in OLED devices and by surface work function measurements in Al electrodes modified with the functionalized zinc porphyrins. The performance of OLEDs is significantly improved upon using the functionalized porphyrin interlayers with the recorded luminance of the devices to reach values 1 order of magnitude higher than that of the reference diode without any electron injection/transport interlayer.

Introduction

Three decades after the first publications on electroluminescence obtained in organic semiconductor heterojunctions,[1,2] several products based on organic light emitting diodes (OLEDs) have already entered the market. Although OLEDs represent a mature technology, they continue to attract wide scientific attention because of their potential to find more applications especially as next generation flat panel lighting sources, having the advantages of low voltage, high brightness and efficiency, solution processing, low fabrication cost and flexibility.[3−5] The efficiency of OLEDs strongly depends on the balanced charge injection, transport, and recombination of both electrons and holes, as well as on efficient exciton decay and light extraction processes. Because low work function (WF) metals such as calcium (Ca), barium (Ba), and magnesium (Mg) strongly degrade in the presence of oxygen and moisture,[6] aluminum (Al), despite its relatively high WF of ∼4.1–4.3 eV, is widely applied as a cathode electrode material because of its high ambient stability and ease of processing.[7] However, the relatively large WF of Al in combination with the generally low electron affinity of most organic semiconductors results in a significant electron injection barrier formed between the Al cathode and organic semiconductors commonly used as the emissive layer (EML) of OLEDs. A vast number of approaches have been applied to reduce the electron injection barrier, thus improving the charge balance and enhancing the OLED efficiency. In particular, interfacial materials such as dipolar molecules,[8−10] sulfonium salts,[11] polyoxometalates,[12,13] ionic liquids,[14] conjugated polyelectrolytes,[15−19] and self-assembled monolayers[20−22] are inserted between the Al cathode and the EML in the form of thin interlayers to facilitate electron injection toward the lowest unoccupied molecular orbital (LUMO) of the organic semiconductor used in the EML.

On the other hand, porphyrins, among other materials, are considered as ideal building blocks for future electronic devices because they exhibit several desirable functionalities such as strong visible absorption and high chemical stability as well as ultrafast charge transport and long exciton diffusion length due to their extended conjugation and π-electron delocalization.[23−28] Metallated and free-base porphyrin compounds have been applied as visible light absorbers in dye-sensitized and organic solar cells.[29−31] They have been also proven promising as  charge transport materials in the emerging technology of perovskite solar cells.[32−35] Recently, our group and others reported the utilization of thin porphyrin layers in organic solar cells with both the conventional and the inverted architectures to promote electron extraction toward the cathode.[36−38] However, in the field of OLEDs, works on the electron injection capability of porphyrins are limited to that previously reported by our group, which demonstrated that appropriately oriented aggregated nanostructures of the free-base and zinc-metallated meso-tetrakis (1-methylpyridinium-4-yl) porphyrin chloride can improve the energy level alignment at the cathode interface, increase electron injection and transport rates, and enhance the efficiency of OLEDs.[39]

In the current work, we use four different zinc porphyrin compounds as thin interlayers between the EML and the Al cathode in OLED devices with the conventional configuration. These porphyrins are appropriately functionalized in order to exhibit high molecular dipole moments and controllable assembly configuration compared to the reference compound with no peripheral groups. The insertion of these functionalized porphyrin interlayers induces the formation of large negative interfacial dipoles, thus reducing the electron injection barrier at the EML/Al cathode interface. As a result, OLEDs using poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(1,4-benzo-2,1′,3-thiadiazole)] (F8BT, with a 9:1 F8:BT ratio) as the EML and the functionalized porphyrins as thin cathode interlayers exhibited up to a 10-fold improvement in the device luminance compared to the reference device with an Al cathode.

Results and Discussion

Design Rules for Porphyrin Compounds

The design of functionalized porphyrin compounds was accomplished under the requirement of facilitating efficient electron transfer toward the EML (F8BT). We, therefore, started by adding to the simple reference compound [5,10,15,20-tetraphenyl-porphyrinato] zinc(II) (ZnTPP) an electron withdrawing group such as a triazine moiety,[40] in order to facilitate intramolecular electron transfer to the porphyrin ring through the polarizable π-electron density of triazine.[41] We thus obtained the {5-[(4-(4,6-dichloro-1,3,5-triazin-2-yl)amino)phenyl]-10,15,20-triphenyl-porphyrinato}zinc(II) (ZnTPP-cc). Next, the triazine moiety was substituted either by a glycine group (NH2–CH2–COOH) leading to {5-[4-((carboxymethyl)amino)-6-chloro-1,3,5-triazin-2-yl)amino)phenyl]-10,15,20-triphenyl-porphyrinato} zinc(II) (ZnTPP-cc-gly) or by two glycine groups leading to {5-[4,6-(bis-(carboxymethyl)amino-1,3,5-triazin-2-yl)amino)phenyl]-10,15,20-triphenyl-porphyrinato} zinc(II) (ZnTPP-cc-gly). The addition of the hydrophilic glycine moieties was performed in order to allow controlled spatial configuration of the porphyrin molecules having −COOH end groups which are expected to orient with their hydrophilic side away from the hydrophobic organic semiconducting underlayer. The chemical structures of the porphyrin compounds are shown in Figure (upper part), while their synthetic procedure (Figure S1) along with details on the synthesis is presented in Supporting Information.

Figure 1

Chemical structures, optimized ground state geometries, and the corresponding dipole moments (absolute values and direction of the corresponding vectors) of (a) ZnTPP, (b) ZnTPP-cc, (c) ZnTPP-cc-gly, and (d) ZnTPP-cc-gly.

Theoretical Study

We next performed theoretical calculations using the Gaussian 09 computational package.[42] The optimized ground state geometries and the corresponding dipole moments of the four porphyrin compounds are presented in Figure . It is observed that the porphyrin ring remains planar without any distortion in the ground state, while the four phenyl rings orient themselves perpendicular to the porphyrin ring plane. The molecular dipole moment μ(D) of the parent compound ZnTPP was found to be as low as 0.07 D because of its highly symmetric structure. The functionalized porphyrin with the triazine moiety (ZnTPP-cc) exhibits a higher μ(D) = 4.4698 D, which is further increased to 5.2429 D and 4.9865 D for ZnTPP-cc-gly and ZnTPP-cc-gly, respectively, bearing the glycine group(s). The significant magnitude of μ(D) exhibited by the triazine-functionalized porphyrins combined with its favorable orientation could beneficially affect the interfacial electron injection barrier by lowering the WF at the EML/porphyrin/Al interface.[43]

The electron density plots of the two highest occupied (HOMOs) and the two lowest unoccupied (LUMOs) molecular orbitals (MOs) involved in the first allowed transitions of each porphyrin molecule are given in Figure . The relevant excited states in all porphyrin-type systems are multireference and four MOs, two HOMOs (HOMO - 1, HOMO) and two LUMOs (LUMO, LUMO + 1), are involved for the description of the lowest excited states and transitions between them. It is observed that, although HOMO – 1 plots are localized on the porphyrin ring, HOMO plots have a significant contribution from the Zn atom, which is in accordance with results reported by others.[44] LUMO plots have significant density on the porphyrin rings with no contribution from Zn atoms. The energies of the MOs of the designed porphyrins compounds were calculated and are also included in Figure . From these values, it becomes evident that glycine substitution has no profound effect on the optical properties of porphyrin compounds. The same conclusion can be drawn from the theoretically calculated excited states and transition dipole moments of those compounds (Figures S2–S5).

Figure 2

Plots of the two HOMOs (H - 1, H) and two LUMOs (L, L+1) of (a) ZnTPP, (b) ZnTPP-cc, (c) ZnTPP-cc-gly, and (d) ZnTPP-cc-gly. The energies of the corresponding MOs are also shown.

Optical Properties and Assembly Configurations

The absorption spectra of porphyrin compounds in methanol solutions (10–5 M) are presented in Figure a. All compounds display a qualitatively similar spectral shape with three well-resolved spectral bands. The short wavelength intense Soret band appears at 422 nm and is attributed to the S0 to S2 transition, while the two weaker Q bands appear at 558 and 598 nm.[45] The latter are characteristic absorption bands of zinc porphyrin compounds and correspond to S0 to S1 transitions.[46,47] The inset in Figure a shows a magnification of the Q-band absorption region. Passing from solution to solid state, the Soret and Q absorption bands of porphyrin films obtained via spin coating 1 mg mL–1 methanol solutions on quartz substrates are red-shifted with respect to those of their solution spectra (Figure b). The red shift of the Soret band of ZnTPP is quite modest, while the overall intensity of the peaks is low compared to the functionalized compounds. Furthermore, the spectra of the functionalized porphyrins (especially of ZnTPP-cc) reveal a pronounced broadening of the Soret absorption band, exhibiting a tail toward long wavelengths. This is an indication for the formation of J-aggregates in the solid state;[48] such aggregates are considered beneficial for electron transport.[49] Moreover, the emission of porphyrin compounds in methanol solutions and in films was investigated by recording steady-state photoluminescence (PL) spectra. In solutions (Figure c), the two characteristics bands of zinc porphyrins appear in all cases at 605 and 655 nm.[50] However, a significant quenching of the PL intensity, especially in the case of the functionalized porphyrins, is observed in the solid state (Figure d). These findings indicate efficient π–π stacking of the functionalized porphyrins upon self-assembly in the solid state. In the functionalized derivatives, coordination of Zn metal center of one molecule with carbonyl oxygen (as illustrated in Figure e in the representative example of the possible assembly configuration of ZnTPP-cc-gly)[50] or triazine nitrogen of another molecule may contribute to π–π stacking. In addition, chlorine and carboxylic acid of the peripheral groups may play a vital role in the assembly configuration by forming halogen or hydrogen bonds, respectively.[51,52] In this manner, a continuous network (thin film) is formed upon the π–π stacking of individual molecules of the functionalized compounds. Note that such π–π stacking results in a significant degree of electron delocalization through extended π-orbital overlap, which is beneficial for electron injection/transport.[53]

Figure 3

Absorption spectra of porphyrin (a) solutions in methanol (10–5 M) (the inset shows the magnified Q-bands region) and (b) thin films on quartz substrates. Steady-state PL spectra of porphyrin (c) solutions in methanol (10–5 M), (d) thin films on quartz substrate. (e) A possible network formed via self-assembly of ZnTPP-cc-gly molecules in the solid state. (f) Absorption spectra of F8BT films (on ITO/glass substrates) without and with porphyrin layers atop. The PL spectra of the same films are shown as inset.

Next, we verified that the functionalized porphyrins can form a continuous film when deposited on top of F8BT used as the EML in OLEDs. In Figure f, the UV–vis absorption spectra of F8BT without and with porphyrins spin-coated from 0.5 mg mL–1 solutions are presented. In all spectra, a pronounced peak at 376 nm is evident, which is attributed to F8BT absorption.[53] Upon deposition of the functionalized porphyrin compounds, a small but clearly visible peak appears at 436 nm corresponding to the Soret band of the porphyrins. This peak is absent in the case of the ZnTPP, which indicates the failure to obtain a homogeneous film upon effective π–π stacking of individual molecules when using the compound with no peripheral groups. The optical microscopy image of a ZnTPP layer reveals the tendency of the specific porphyrin compound to form large clusters rather than a continuous film (Figure S6), which is not the case for that is ZnTPP-cc-gly (which is also shown in Figure S6 for comparison). This observation suggests that porphyrin functionalization with appropriate moieties is imperative in order to obtain homogeneous and continuous networks formed via π–π stacking of individual porphyrin molecules atop of organic semiconductors to allow for efficient electron injection/transport from the cathode electrode. The formation of a thin film of functionalized porphyrins atop F8BT was also indicated by the change in topography and decrease of the surface root-mean-square roughness as verified by atomic force microscopy (AFM) images shown in Figure S7. Note that the PL spectra of the same films (Figure f, inset) exhibited one broad peak at 530 nm which is assigned to F8BT’s emission with no contribution from porphyrin thin overlayers, which clearly indicates that the primary role of porphyrins in our devices is to transfer electrons and not to emit (or absorb) light.

Control of the Electron Injection Barrier

Figure a shows the  structure of the fabricated OLEDs, which consist of indium tin oxide (ITO) as the transparent anode electrode, poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT–PSS) as the hole transport layer, and F8BT as the EML. In some devices, an Al contact was directly deposited on top of F8BT to serve as the top cathode electrode (reference devices), while in others a thin porphyrin interlayer was inserted between F8BT and Al to modify the cathode interface and facilitate electron injection/transport.

Figure 4

(a) Schematic of the OLED device structure and (b) measured Voc under 1.5 AM illumination of the F8BT-based OLEDs using various porphyrin interlayers. (c) Work function as derived from CPD of pristine and porphyrin-coated aluminum samples and (d) illustration of energy diagram at the cathode interface of OLEDs upon porphyrin modification.

To support the argument that the insertion of porphyrin interlayer at the cathode side of the device may result in the alteration of the energetics at the polymer/Al interface, we performed photovoltaic measurements in order to directly probe the open circuit voltage (Voc) of OLEDs without and with porphyrin modification. The value of Voc in a diode is generally related to the built-in potential generated upon contact of the organic emissive layer with the modified electrodes.[54] Current density–voltage (J–V) characteristics were recorded under illumination with AM 1.5 simulated light (1 sun or ∼100 mW cm–2). The results are shown in Figure b. It is observed that the measured Voc values present a pronounced enhancement upon device modification with the functionalized porphyrins. In particular, the reference device with an Al cathode shows a Voc of ∼0.55 V, which is modestly increased to 0.60 and 0.70 V upon the insertion of ZnTPP and ZnTPP-cc, respectively. This increase becomes more pronounced upon modification with ZnTPP-cc-gly (∼1.15 V) and ZnTPP-cc-gly (∼1.10 V), which is attributed to an enhancement in the device built-in potential and is suggestive of a decrease in the effective WF of the porphyrin-modified Al electrode, thus resulting in a significant reduction of the electron injection barrier up to ∼0.60 eV.

To directly characterize the change in WF of Al upon porphyrin modification, we evaluated the surface WF of Al and porphyrin-modified Al films by measuring the contact potential difference (CPD) (Figure c). In agreement with the enhanced Voc in the porphyrin-modified OLEDs, porphyrin deposition on Al gradually reduced the surface WF to 4.01, 3.89, 3.74, and 3.71 eV (WF of Al = 4.04 eV) upon coating with ZnTPP, ZnTPP-cc, ZnTPP-cc-gly, and ZnTPP-cc-gly, respectively. This WF decrease is attributed to a downward shift of the vacuum level upon inserting a thin porphyrin interlayer on Al film because of the generation of a negative dipole and a local electric field as illustrated in Figure d. This shift is more pronounced in the samples with the glycine-bearing functionalized porphyrins and can be attributed to one or to a combination of the following mechanisms/processes: (i) a rearrangement of charge in the metal underneath the porphyrin adsorbate resulting in a local net charge induced electric field.[55] (ii) A synergistic effect arising from the presence of the molecular (i.e., porphyrin) dipole along the direction perpendicular to the surface and an interface dipole formed at the interface between the porphyrin and Al that is partially attributed to spontaneous interfacial electron transfer. (iii) Dipolar polarization due to the spontaneous adsorption of the carboxylic acid groups onto the metal surface. (iv) In the case of carboxylic acid-bearing porphyrins, oxidation of Al to Al2O3 at the interface can occur, thus reducing the WF in the oxidized interface.[56,57] (v) Similarly, for the compounds bearing chlorine ions, spatial separation of these ions probably induces a chemical reaction with Al and reducing the WF at the interface as in the case of LiF/Al interfaces.[58,59] We also note that a critical parameter for the effective contribution of some the aforementioned mechanisms (mechanisms ii–iv) in the reduction of the porphyrin-modified Al interface work function is the orientation of the porphyrin molecules on the surface upon stacking. For example, in our previous work, we found a stronger electric field interface dipole when molecules adopt the face-on rather than the edge-on orientation.[39] Herein, we propose that the hydrophilic carboxylic acid groups are likely oriented toward the surface of Al, whereas the hydrophobic porphyrin ring is oriented toward F8BT resulting in a higher hydrophilicity of the F8BT/functionalized porphyrin surfaces relative to pristine F8BT and F8BT/ZnTPP ones (Figure S8 and Table S1). Therefore, mechanisms (ii) and (iii) may act cooperatively and contribute to the observed work function reduction. In any case, to unravel the exact mechanism for the formation of a negative dipole at the porphyrin/Al interface is beyond the scope of this work, which emphasizes that with appropriate functionalization of porphyrin compounds fine tuning of the interfacial electron injection barrier of OLEDs can be successfully achieved.

Emission Quenching Studies

It is well known that in the case of OLEDs the emission efficiency is limited by exciton quenching at charge injection interfaces.[60] Steady-state and transient PL decay traces for thin F8BT films (∼10 nm, of the order of the exciton diffusion length) deposited on nonquenching (glass) and quenching (Al) substrates without and with thick porphyrin interlayers deposited via drop-casting 2 mg mL–1 solutions are shown in Figure . It is evident in Figure a that the PL emission of F8BT on glass is suppressed upon the insertion of porphyrin interlayers due to a reduction of the exciton density decaying in the F8BT layer as excitons diffuse toward the porphyrin-modified interface. The same conclusion can be drawn from time-resolved PL (TRPL) traces shown in Figure c. Upon fitting the results with multi-exponential curves, we obtain the average lifetime ⟨τ⟩ in each case (Table ). A pronounced decrease in ⟨τ⟩ witnesses the electron accepting character of porphyrins. This is particularly evident for the glycine substituted porphyrins, where a decrease of the F8BT PL lifetime by a factor of 2 is observed because of their stronger electron accepting nature. Moreover, a new decay channel is observed with the glycine-substituted porphyrins, as manifested by the appearance of a third, very short, (∼0.3 ns) lifetime component with an almost 30% contribution to the overall lifetime, which is not present in the reference and the other two porphyrin-modified glass/F8BT interfaces. We attribute this component to the emergence of a fast F8BT exciton decay/dissociation process, likely assisted by exciton diffusion/transfer toward the modified interface. This exciton lifetime reduction combined with the PL spectra suppression is rather surprising because the estimated LUMO of these compounds lies above that of F8BT (∼3.3 eV).[61] However, our previous studies demonstrated that a large vacuum level shift toward lower values occurs upon depositing a thin porphyrin layer on top of F8BT.[39] This should result in a significant lowering of their LUMO relative to that of F8BT and could offer a viable explanation for the exciton quenching occurring at the porphyrin/F8BT interface but further investigation is needed, which, however, is beyond the scope of the present work. Exciton quenching due to direct energy transfer to the porphyrins would not be expected to play a role in the devices as there is a minimal overlap between the emission spectrum of F8BT (530 nm) and the Soret (about 430 nm in the solid state) bands of porphyrins, while their Q-bands have negligible intensity in the solid state. For the samples with the ZnTPP and ZnTPP-cc films, weak PL originating by the porphyrins is observed (Figure a). However, this cannot be attributed to energy transfer from F8BT to the porphyrins but to direct excitation, because if energy transfer occurred, then the quenching of F8BT and the reduction of its lifetime would be more pronounced in the samples with ZnTPP and ZnTPP-cc porphyrins.

Figure 5

Steady-state PL spectra of F8BT deposited on (a) glass and (b) aluminum substrates without and with porphyrin interlayers inserted between Al and F8BT. PL decay curves of F8BT deposited on (c) glass and (d) aluminum substrates without and with porphyrin interlayers inserted at the substrate/F8BT interface detected at the peaks of the PL spectra.

Table 1

TRPL Fitting Parameters for F8BT Films Deposited Either on Glass or on Al Substrate without and with a Thin Interlayer of Porphyrin Compounds Inserted at the Substrate/F8BT Interface

sample	A1 (%)	τ1 (ns)	A2 (%)	τ2 (ns)	A3 (%)	τ3 (ns)	⟨τ⟩ (ns)	χ2
glass/F8BT			32	1.44	68	2.88	2.42	0.9828
glass/ZnTPP/F8BT			55	1.02	45	2.21	1.56	1.0896
glass/ZnTPP-cc/F8BT			49	1.08	51	2.37	1.74	1.0806
glass/ZnTPP-cc-gly/F8BT	29	0.34	53	1.42	18	2.55	1.31	0.9346
glass/ZnTPP-cc-gly2/F8BT	33	0.27	49	1.21	18	2.25	1.09	0.9321
Al/F8BT	52	0.15	34	0.65	14	1.68	0.53	0.9771
Al/ZnTPP/F8BT	62	0.12	33	0.45	5	1.10	0.28	0.9368
Al/ZnTPP-cc/F8BT	63	0.13	31	0.52	6	1.29	0.32	0.9378
Al/ZnTPP-cc-gly/F8BT	52	0.14	36	0.63	12	1.57	0.49	0.9371
Al/ZnTPP-cc-gly2/F8BT	51	0.15	36	0.65	13	1.56	0.51	0.9533

A different picture is obtained when F8BT is deposited on quenching Al substrates. As expected, the estimated exciton average lifetime ⟨τ⟩ is much lower than that obtained for the glass substrate (0.53 ns for F8BT on Al substrate versus 2.42 ns for F8BT on glass substrate, Figure d and Table ), which we attribute to quenching due to non-radiative energy transfer via long range dipole interaction and decay of F8BT excitons generated close to the interface with Al. However, a very small additional decrease of lifetime is observed upon inserting the functionalized porphyrins and especially those with the glycine group(s), while the decrease was more significant when inserting the symmetric porphyrin compounds (ZnTPP). This phenomenon qualitatively agrees with the steady state spectra detected on Al substrate (Figure b) and implies that the use of functionalized porphyrins (especially of those bearing carboxylic acid groups) even though does not suppress the interaction between the organic semiconductor and the cathode electrode which causes the undesirable quenching and cannot act as effective exciton blocking (i.e., spacer) layers,[62] results in only a slight additional quenching effect due to the porphyrins’ electron accepting nature. We also note that other parameters such as molecular orientation and alignment of the transition dipole moments of the porphyrin molecules may influence the exciton dissociation/charge transfer process when deposited on a metal and a dielectric substrate, as likely suggested by the different F8BT exciton quenching behavior for the various porphyrins on the two substrates.

Characteristics of OLEDs Using Porphyrin Interlayers

Finally, the influence of the insertion of thin porphyrin films into the device to serve as electron injection and transport interlayers on the current density–voltage (J–V) and luminance–voltage (L–V) characteristics of the OLEDs shown in Figure a is depicted in Figure a,b, respectively. The devices operational characteristics are also summarized in Table . It is observed that the porphyrin-modified devices exhibit a large increase in both current density and luminance and a significant decrease in the turn-on (at L ≈ 10 cd m–2) and operational (at L ≈ 1000 cd m–2) voltages compared to the reference device with an Al cathode. The highest J and L values were obtained for the devices with the ZnTPP-cc-gly (250.0 mA cm–2, 13702.4 cd m–2) and ZnTPP-cc-gly (256.9 mA cm–2, 16693.8 cd m–2). The latter case represents an increase by a factor more than 3 for the current density and 10 for the luminance compared to the reference device (70.7 mA cm–2, 1555.5 cd m–2). Note that the luminance values obtained in the devices using glycine-modified porphyrins are significantly higher compared to the devices with ZnTPP and ZnTPP-cc  due to the superior electron injection properties arising from the reduced electron injection barrier, the likely enhanced electron transport and the negligible additional quenching effect of glycine-based porphyrins, as compared to the other two compounds. As a result, the luminous efficiency (LE) and power efficiency (PE) were significantly increased upon cathode modification with the functionalized porphyrins (Figure c and Table ), strongly suggesting that an improved electron injection/transport primarily due to a decrease of the energy barrier is achieved upon modifying the F8BT/Al interface with a very thin porphyrin layer. It should be also mentioned that upon insertion of porphyrin interlayers at the F8BT/Al interface there was no change in the device electroluminescence (EL) spectra (Figure d). The EL spectra of the reference devices and the porphyrin-modified OLEDs exhibit a broad peak at around 530 nm, characteristic of emission from the F8BT copolymer, suggesting that recombination and emission take place exclusively inside the active layer. We note that the achieved efficiency values (i.e., LE equal to 7.30 cd A–1 for ZnTPP-cc-gly-modified OLED) are among the highest ever reported for similar OLED configurations (i.e., with a conventional architecture using a 70 nm thick F8BT EML and an Al cathode). For instance, when using an ultrathin LiF cathode interlayer[63] or a Ca cathode,[64] luminous efficiencies of the order of 7.0 cd A–1 have also been reported. It is worth noting, however, that Friend et al. have previously reported that optimized OLEDs with an inverted configuration can be made to exhibit LEs of more than 20 cd A–1 by controlling the width of the recombination zone with an approximately 1 μm-thick F8BT layer and a zinc oxide/barium hydroxide electron injection/transport layer deposited at the bottom cathode contact.[65]

Figure 6

OLED device measurements: (a) current density–voltage, (b) luminance–voltage, (c) luminous efficiency–voltage characteristics, and (d) EL spectra of F8BT-based OLEDs without and with porphyrin electron transport interlayers.

Table 2

OLED Performance Characteristics as Derived from J–V–L Curves Shown in Figure

cathode	Vturn-on (V) (at 10 Cd m–2)	Voperational (V) (at 1000 Cd m–2)	Jmax (mA cm–2)	Lmax (cd m–2)	L.E.max (cd A–1)	P.E.max (lm W–1)
Al	6.5	12.0	70.7	1555.5	2.88	0.75
ZnTPP/Al	7.0	10.5	226.4	7194.5	3.55	0.98
ZnTPP-cc/Al	5.2	8.5	256.1	7715.6	4.48	1.30
ZnTPP-cc-gly/Al	4.1	7.2	250.0	13702.4	6.44	2.40
ZnTPP-cc-gly2/Al	5.0	8.2	256.9	16693.8	7.30	1.56

Conclusions

In conclusion, we synthesized a series of functionalized zinc-metallated porphyrins by adding peripheral groups in order to change the molecular dipole moment and assembly configuration and imlemented them as electron injection/transport interlayers in F8BT-based OLED devices. The interfacial energy level alignment at the cathode interface was investigated in detail, revealing that upon modification with the functionalized porphyrins a significant reduction in the electron injection barrier occurs. With regard to the mechanism for interfacial energy level tuning, the formation of a negative interface dipole probably via spontaneous interfacial electron transfer and dipolar polarization of porphyrin compounds are more likely. In addition, the functionalized compounds have no undesirable effect on the emission quenching at the F8BT/Al interface as revealed by time-resolved spectroscopy. Porphyrin interlayer incorporation significantly improved the OLED performance as manifested by the reduction of turn-on and operational voltage and the increase of the maximum luminous/power efficiency and luminance. This work demonstrates a simple, solution-based strategy to modulate the interfacial  energy level alignment and electron injection/transport in OLEDs by using appropriately functionalized porphyrin compounds, paving the way for their implementation in other organic/inorganic optoelectronic devices.

Experimental Section

Quantum Chemical Calculations

Calculations were performed using the G09 computational package and the DFT and TD-DFT methods. Two functionals were employed, the B3LYP for optimizing geometries and calculating the excited states at the absorption geometries. The 6-31G(d,p) basis set was used for the calculations.

Device Preparation

ITO-coated glass substrates with a sheet resistance of 20 Ω/sq, which served as anode electrodes were used for OLED device fabrication. The substrates were subsequently cleaned in acetone, 2-propanol, and DI water under sonication for 10 min at room temperature and dried with N2 after each bath, respectively. Next, the substrates were treated with O2 plasma for 10 min. Afterward, poly(3,4-ethylenedioxythiophene)–poly(styrenesulfonate) (PEDOT–PSS) purchased from Sigma-Aldrich (1.3 wt % dispersion in H2O) was filtered with a 0.45 μm PVDF filter, spin-coated on the substrates at ambient conditions (at 7000 rpm for 40 s to form a 40 nm thick layer), and annealed for 1 h at 100 °C on a hotplate. The EML was the green-yellow copolymer poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-co-(1,4-benzo-{2,1′,3}-thiadiazole)] (F8BT, with a 9:1 F8:BT ratio), purchased from American Dyes Source (ADS 233 YE). It was spin-coated (at 1200 rpm for 40 s to form a 70 nm thick layer) from a 10 mg mL–1 solution in chloroform (after it was previously filtered through a 0.22 μm pore size PTFE filter) on top of PEDOT–PSS at ambient conditions. After deposition, the F8BT film was annealed at 95 °C for 10 min on a hotplate. Then, in some devices, a thin porphyrin film was deposited on top of a F8BT layer via spin coating (at 2000 rpm for 40 s) from a methanolic solution with a concentration of 0.5 mg mL–1 to serve as the electron injector/transport layer. The devices were completed with a 150 nm aluminum cathode deposited through a shadow mask in a dedicated chamber.

Characterization Techniques

Current density–voltage characteristics were measured with a Keithley 2400 source-measure unit. Luminance and electroluminescence (EL) spectra were recorded with an Ocean Optics USB2000 fiber optic spectrophotometer, assuming a Lambertian emission profile (for luminance measurements). All measurements were carried out in air at room temperature. Absorption spectra were recorded using a PerkinElmer Lambda 40 UV–vis spectrophotometer. The CPD has been evaluated with a single-point Kelvin Probe system (KP010) under ambient conditions. The surface morphology of films was investigated with an NT-MDT AFM system operating in tapping mode. The steady-state PL spectra were recorded by a FluoroMax-4 (Horiba) fluorescence spectrometer. The PL dynamics of the samples were studied under magic angle conditions, by using a time-correlated single-photon counting spectrometer (FluoTime, Picoquant) employing a pulsed diode laser emitting at 470 nm with 60 ps pulse duration as the excitation source (Picoquant). Detection of the fluorescence was realized by a microchannel plate photomultiplier at the peaks of the PL spectra and the instrument’s response function was ∼80 ps. Upon fitting the PL decays with a multi-exponential function, the lifetimes τ and their normalized pre-exponential factors A were obtained. The PL average (mean) lifetime ⟨τ⟩ was calculated by means of