Passivation of Metal Oxide Surfaces for High-Performance Organic

Article pubs.acs.org/cm

Passivation of Metal Oxide Surfaces for High-Performance Organic and Hybrid Optoelectronic Devices Shuyi Liu, Szuheng Ho, Ying Chen, and Franky So* Department of Materials Science and Engineering, University of Florida, Gainesville, Florida 32611, United States S Supporting Information *

ABSTRACT: The exciton quenching properties of solutionprocessed nickel oxide (NiOx) and vanadium oxide (VOx) are studied by measuring the photoluminescence (PL) of a thin emitting layer (EML) deposited on top of the metal oxides. Strong exciton quenching is evidenced at the metal oxide/EML interface, which is proved to be detrimental to the performance of optoelectronic devices. With a thin polyvinylpyrrolidone (PVP) passivation polymer adsorbed on top of metal oxides, the PL quenching is found to be effectively suppressed. A short UV−O3 treatment on top of the PVP-passivated metal oxides turns out to be a key procedure to trigger the chemical binding between the PVP passivation polymer and the metal oxide surface species, which turns out to be necessary for efficient hole injection and extraction for organic light emitting diodes (OLEDs) and solar cell devices, respectively. With the PVP passivation layer followed by UV−O3 treatment, the OLEDs incorporating NiOx as a hole transport layer (HTL) shows a record current efficiency of 90.8 ± 2.1 Cd A−1 with significantly suppressed efficiency rolloff, the OLEDs incorporating VOx as a hole injection layer (HIL) also shows higher current efficiencies at higher luminescence. Both perovskite solar cells and polymer solar cells incorporating NiOx HTLs show a 60% enhancement in power conversion efficiency (PCE) with PVP passivation polymer.

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active layer and the metal oxide layer have been used.18,20 However, instead of addressing the quenching problem directly, these strategies were to keep the exciton-forming zone away from the metal oxide surface, which complicates the device architecture and adds limitation to its application. Therefore, a more favorable and straightforward way to suppress exciton quenching is to passivate the surface of the metal oxides. One strategy is to use self-assembly monolayers (SAMs) on top of the oxides.24−28 However, most SAMs are not designed for device applications, and they are especially unfavorable for solution-processed optoelectronic devices due to the hydrophobic surface of the SAM layer that is problematic for the wetting process.26−28 Therefore, to passivate the metal oxide surface for the solution process, a hydrophilic polymer is required and polyvinylpyrrolidone (PVP) is a potential candidate.29−32 The advantages of PVP polymer are (i) good complexion ability with transitional metal ions,31,33,34 (ii) good solubility in both organic and polar solvents due to its amphipathic properties; and (iii) large band gap energy (Eg ∼ 5.6 eV31). However, the challenge of using PVP as a passivation layer is that it is an insulating polymer prohibiting carrier transport and injection/extraction, which is inappropriate for optoelectronic device applications. As a result, there is hardly any report of using PVP as a passivation layer to make efficient

INTRODUCTION Recently, metal oxide semiconductors have been used as charge transport layers in organic light-emitting diodes (OLEDs), photovoltaic cells (OPVs), and quantum dot (QD) devices.1−17 On the basis of their charge transport properties and energy band alignment, they can be used as electron transport layers (ETLs), electron injection layers (EILs), hole transport layers (HTLs), or hole injection layers (HILs). Compared to their organic counterparts, metal oxides have several advantages such as high carrier mobility, tunable energy level alignment, and good air-stability. Furthermore, as most metal oxides are insoluble in organic solvents, they are compatible with solution processing for multilayer devices. Several metal oxides have already been shown to be solution-processable, making them compatible with roll-to-roll processing for OLED and OPV fabrications.1,5,7,11−17 However, for most of the metal oxides synthesized in air, their surfaces are known to be rich of hydroxyl species which act as exciton quenching sites affecting the device performance.18−20 In our previous work, while efficient solution-processed OLEDs incorporating solutionprocessed NiOx can be demonstrated,1 the efficiency roll-off is very strong when NiOx is used as an HTL. Contradictory to other reports that the efficiency roll-off is due to the poor charge balance,21−23 we found that it is actually due to strong quenching at the NiOx HIL/HTL interface. To alleviate the quenching problem, strategies such as changing the carrier profile in the active layers by modifying the injection layers19 and inserting an exciton blocking layer to spatially separate the © XXXX American Chemical Society

Received: January 12, 2015 Revised: February 18, 2015

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DOI: 10.1021/acs.chemmater.5b00129 Chem. Mater. XXXX, XXX, XXX−XXX

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reference sample, which is prepared by depositing the same EML on top of a 4,4′-cyclohexylidenebis[N,N-bis(4-methylphenyl)benzenamine] (TAPC) thin film. Because of its large Eg and high triplet energy (ET), TAPC can effectively block the singlet and triplet excitons, forming a nonquenching interface with the phosphorescent emitter.38−40 To investigate the exciton quenching effects in OLED performance, phosphorescent green OLEDs were fabricated on different samples. A dual-EML was used for easy tuning of the charge balance and confining of the emitting zone and was composed of a 20 nm thick layer of 5 wt % Ir(ppy)3 doped 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP) and a 20 nm thick layer of 5 wt % Ir(ppy)3 doped TCTA. Tris[3-(3-pyridyl)-mesityl]borane (3TPYMB) and LiF/Al were vacuum deposited as the ETL and cathode, respectively. To investigate the role of UV−O3 treatment on PVP-passivated metal oxides, atomic force microscopy (AFM) was used to examine the phase presence on PVP-passivated NiOx and XPS measurements were carried out to study the surface chemistry. To demonstrate that the PVP passivation is also beneficial to metal oxides for other optoelectronic applications, perovskite solar cells with planar heterojunction (PHJ) structure were prepared on different NiOx samples. The fabrication details of the perovskite solar cells are given in the Supporting Information.

optoelectronics. In this work, we demonstrate a novel strategy of using PVP as passivation layer on top of metal oxides to suppress exciton quenching and make highly efficient OLED and solar cell devices. While PVP itself is an insulating polymer, by further treating the PVP passivation layer with UV-ozone (UV−O3), carrier injection from the PVP passivation layer can be drastically improved, resulting in substantially enhanced device performance. Here, we chose NiOx (as an HTL) and vanadium oxide (VOx) (as an HIL) to study the PVP passivation effect and found that PVP is effective to passivate both metal oxide surfaces to suppress the exciton quenching. To facilitate charge injection, the PVP-passivated metal oxides are treated with UV−O3, resulting in the formation of a strong chemical binding between PVP and the metal oxides surface species which enables charge injection. As a result, phosphorescence green OLEDs incorporating the PVP-passivated NiOx HTLs followed by subsequent UV−O3 treatment show a high current efficiency of 90.8 ± 2.1 Cd A−1, which, to the best of our knowledge, is the highest in all OLEDs incorporating solution-processed metal oxides as a carrier transport layer. Furthermore, OLEDs using NiOx as an HTL or VOx as an HIL show significantly reduced efficiency roll-off with a PVP passivation layer. In addition to OLEDs, this approach can also be used to improve the performance of perovskite solar cells incorporating solution-processed NiOx HTLs, yielding a significantly enhanced power conversion efficiency (PCE) of 10.9 ± 0.3%. Since this passivation technique is fully compatible with solution processing, we believe that this approach is ubiquitous for metal oxides used in solution-processed optoelectronic devices.

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RESULTS AND DISCUSSION PVP Passivation on NiOx HTLs and Its Application in OLEDs Devices. To study the exciton quenching effect of NiOx HTL, EMLs composed of a 20 nm-thick TCTA doped with 5 wt % iridium Ir(ppy)3 were deposited onto as-prepared NiOx, PVP-passivated NiOx before and after UV−O3 treatment, and TAPC as a control HTL. Figure 1a shows the PL spectra of all four samples with the data normalized to the PL intensity on top of the TAPC control sample. The as-prepared NiOx sample shows the lowest PL intensity, indicating the strong luminescence quenching nature of the metal oxide. With a thin PVP passivation layer, this quenching effect is effectively suppressed as evidenced by the similar PL intensity compared with the control sample. With subsequent UV−O3 treatment on PVP-passivated NiOx, the PL intensity is also very strong, indicating that the suppression of luminescence quenching is not significantly affected by the treatment. There are two possible exciton quenching mechanisms due to NiOx: (i) deep trap states located at the NiOx surface act as the nonradiative recombination sites and (ii) energy/charge transfer from the emitter to the NiOx surface species. Since exciton quenching via mechanism (i) could only take place when the excitons reach the NiOx surface, inserting a thin exciton blocking layer between the NiOx and EML should efficiently suppress this quenching effect. On the basis of this assumption, we fabricated samples with 10, 20, and 30 nm thick TAPC films as exciton blocking layers between the NiOx film and the EML. The PL intensities of these samples are shown in Figure 1b. Interestingly, with a 10 nm TAPC exciton blocking layer, the luminescence quenching is still very strong. By increasing the TAPC thickness, the luminescence quenching is gradually suppressed and the PL intensity is completely recovered with a TAPC thickness of 30 nm. These results indicate that there is a long-range interaction between the quenching species at the NiOx surface and the excitons, leading to a quenching distance of ∼30 nm. One possible candidate for such a quenching species is nickel oxy-hydroxide (NiOOH), which has been reported to be a strong luminescence quencher.15,41 The presence of the dipolar NiOOH species on NiOx surface is evidenced from our previous work.1 The strong NiOOH dipoles could facilitate the nonradiative decay leading to a high exciton quenching rate at long distances.42

EXPERIMENTAL SECTION

The NiOx solution precursor was synthesized according to our previous publication.1 Nickel acetate tetrahydrate (Ni(CH3COO)2· 4H 2 O) was dissolved in ethanol with monoethanolamine (NH2CH2CH2OH) (0.1−0.2 mol L−1) at a mole ratio of 1:1. The VOx solution precursor was synthesized by mixing vanadium oxytriisopropoxide (VO(O(CH3)2)3) with isopropanol solvent at a volume ratio of 1:50. The metal oxide thin films were prepared by spin-coating the precursor solution onto the appropriate substrates. For optical measurements, quartz substrates were used to prevent UV absorption. For X-ray photoelectron spectroscopy (XPS) measurements, substrates were single crystal Si wafers. For all other measurements, substrates were precleaned indium tin oxide (ITO). After spin-coating, the samples were heated to 500 °C for NiOx and 150 °C for VOx. The as-prepared NiOx film is a p-type semiconductor with a polycrystalline structure confirmed by X-ray diffraction,1,7 and the as-prepared VOx film is an n-type amorphous semiconductor.35−37 The UV−visible transmittance and absorbance spectra of both NiOx and VOx films are given in the Supporting Information. To passivate the metal oxide surface, PVP was dissolved into chloroform and spincasted onto the metal oxide samples. The samples were then spinrinsed with chloroform, yielding an ultrathin (10 nm) quenching process, a 10 nm thick TAPC layer on top of VOx is sufficient to suppress the luminescence quenching. The lower PL intensity from the sample with a PVP-passivated VOx layer is due to the thinner PVP passivation layer (2000 Cd m−2).

the XPS spectrum at high binding energies due to the presence of the ester group with a BE of 288.9 eV. Figure 4d shows the O 1s spectrum of the P-UVO-NiOx film where we observe similar changes in higher binding energies due to the presence of the ester functional group with a BE of 532.9 eV. The carbon atoms from the carbonyl and ester groups makes up 20.9% and 14.8% of the C 1s spectrum, respectively, which are significantly larger than the corresponding values (11.8% and 8.4%) determined from the O 1s spectrum and the C/O atomic ratio. The discrepancy in the carbon composition determined from the C 1s and O 1s signals indicates the presence of additional oxygen atoms due to the formation of ester groups as a result of the chemical reaction between PVP and NiOx, indicating that NiOx shares its oxygen atoms with PVP to form carbonyl/ester groups after UV−O3 treatment. The resulting chemical binding between PVP and the NiOx surface species facilitates hole injection from P-UVO-NiOx into the adjacent EMLs, yielding devices with a higher efficiency. The resolved details of the C 1s, O 1s, and Ni 2p3/2 XPS signals of the PVP passivated NiOx surface before (P-NiOx) and after (P-UVONiOx) UV−O3 treatment are summarized in Table 2. The detailed analysis of the XPS data is described in the Supporting Information. PVP Passivation on VOx HILs and Its Application in OLEDs Devices. To illustrate the applicability of this approach F

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Figure 6b. The device performance data are summarized in Table 3. The integrated Jsc from the EQE spectrum is 20.2 mA

However, the device does not show an enhanced current efficiency at low luminescence intensities. This could be explained by the absence of long-range exciton quenching in the VOx devices. The EL is thus only quenched when the emitting zone is close to the VOx/EML interface, which occurs at relatively high luminescence. PVP Passivation on NiOx HTLs and Its Application in Solar Cell Devices. To demonstrate that the PVP passivation is also beneficial to metal oxides for other optoelectronic applications, we fabricated iodine perovskite (MAPbI3) solar cells with PHJ structures. The active layer of MAPbI3 was synthesized by dipping a 150 nm thick PbI2 film into the MAI solution. The J−V curves (under AM 1.5G 1 sun illumination) of the PHJ perovskite solar cells incorporating the as-prepared NiOx HTL, UVO-NiOx HTL, P-UVO-NiOx HTL and poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as a control device are shown in Figure 6a.49 The external quantum efficiency (EQE) spectrum of the PHJ perovskite solar cell incorporating P-UVO-NiOx HTL is shown in Figure 6b. The device architecture is shown as an insert in

Table 3. Performance of MAPbI3 Perovskite Solar Cells Incorporating PEDOT:PSS, As-Prepared NiOx, UVO-NiOx, and P-UVO-NiOx HTLs devices with diff. HTLs PEDOT:PSS as-prepared NiOx UVO-NiOx P-UVO-NiOx

Voc (V)

Jsc (mA cm−2)

FF (%)

PCE (%)

0.89 ± 0.03 0.92 ± 0.02

8.9 ± 0.5 18.0 ± 0.4

53.7 ± 1.1 49.9 ± 0.8

4.3 ± 0.4 8.3 ± 0.3

1.00 ± 0.03 1.04 ± 0.02

14.4 ± 0.7 20.1 ± 0.4

46.5 ± 0.8 52.1 ± 0.9

6.6 ± 0.4 10.9 ± 0.3

cm−2, which is consistent with the measured value of 20.3 mA cm−2 for the device with P-UVO-NiOx HTL. The PHJ perovskite solar cell with P-UVO-NiOx HTL shows the highest open circuit voltage (Voc) of 1.04 ± 0.02 V, the highest short circuit current density (Jsc) of 20.1 ± 0.4 mA cm−2, and the highest PCE of 10.9 ± 0.3% among all of the four samples, indicating the passivation technique is also beneficial for metal oxides used in solar cells. Similar results were also found in organic bulk heterojunction (BHJ) solar cells incorporating NiOx HTLs. Using poly(N-9′-heptadecanyl-2,7-carbazole-alt5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothidiazole) (PCDTBT) and [6,6]-phenyl-C71 butyric acid methyl ester (PC70BM) as an active layer in the polymer solar cell, we also demonstrated the device power conversion efficiency is enhanced by more than 60% due to passivation of the NiOx layer. More details and analysis are given in the Supporting Information.

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CONCLUSIONS The exciton quenching effect of solution-processed NiOx HTL and VOx HIL were investigated with PL measurements. A longrange quenching effect is evidenced at the NiOx/organic interface while a short-range quenching effect is evidenced at the VOx/organic interface. With a thin PVP passivation polymer, the luminescence quenching of the phosphorescent green emitter on both NiOx HTL and VOx HIL is efficiently suppressed. A short UV−O3 treatment on the PVP-passivated metal oxide surface is required to yield high-performance OLEDs and solar cell devices. XPS analysis revealed that there are chemical bindings between the PVP polymer and the surface species of metal oxides after the UV−O3 treatment, resulting in enhanced charge injection and extraction in OLEDs and solar cell devices, respectively.

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ASSOCIATED CONTENT

S Supporting Information *

UV−visible transmittance and absorbance spectra of both NiOx and VOx films, fabrication details of the perovskite solar cells, electroluminescence spectra of the phosphorescent green OLEDs, photoluminescence measurement of the emitting layer on UV−O3 treated NiOx, AFM phase distribution histogram image of the as-prepared NiOx and PVP-passivated NiOx before and after UV−O3 treatment, Ni 2p3/2 XPS signals of PVP-passivated NiOx before and after UV−O3 treatment, detailed analysis of C 1s and O 1s XPS signals of PVPpassivated NiOx before and after UV−O3 treatment, and PVP passivation on NiOx HTLs for organic bulk heterojunction solar cells application. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 6. (a) The J−V curves of the perovskite solar cells incorporating PEDOT:PSS (gray dash), as-prepared NiOx HTL (black), UVO-NiOx HTL (red), and P-UVO-NiOx HTL (blue) under AM 1.5 G one sun illumination. (b) The EQE spectrum of the perovskite solar cells incorporating P-UVO-NiOx HTLs. Inset: the device architecture of the perovskite solar cells. G

DOI: 10.1021/acs.chemmater.5b00129 Chem. Mater. XXXX, XXX, XXX−XXX

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]fl.edu. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge the support of Wintek Corporation for their funding of this project. The authors would also like to thank Eric Lambers of the University of Florida Major Analytical Instrumentation Center for his help in XPS measurements.

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DOI: 10.1021/acs.chemmater.5b00129 Chem. Mater. XXXX, XXX, XXX−XXX