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Quantum Dot Light-Emitting Devices: Beyond Alignment of Energy Levels Gary Zaiats,† Shingo Ikeda,†,‡ Sachin Kinge,§ and Prashant V. Kamat*,† †
Notre Dame Radiation Laboratory, Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, United States ‡ Osaka Municipal Technical Research Institute, Osaka 536-8553, Japan § Advanced Technology Division Toyota Motor Europe, Zaventem B-1930, Belgium S Supporting Information *
ABSTRACT: Multinary semiconductor nanoparticles such as CuInS2, AgInS2, and the corresponding alloys with ZnS hold promise for designing future quantum dot light-emitting devices (QLED). The QLED architectures require matching of energy levels between the different electron and hole transport layers. In addition to energy level alignment, conductivity and charge transfer interactions within these layers determine the overall efficiency of QLED. By employing CuInS2−ZnS QDs we succeeded in fabricating red-emitting QLED using two different hole-transporting materials, polyvinylcarbazole and poly(4butylphenyldiphenylamine). Despite the similarity of the HOMO−LUMO energy levels of these two hole transport materials, the QLED devices exhibit distinctly different voltage dependence. The difference in onset voltage and excited state interactions shows the complexity involved in selecting the hole transport materials for display devices. KEYWORDS: quantum dots, LED, light-emitting devices, hole-transferring material, electroluminescence, CIZS
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INTRODUCTION Colloidal semiconductor quantum dot-based light-emitting devices (QLEDs) offer the advantage of spectral tunability, vivid colors, and high CRI (color rendering index) values.1−6 Moreover, their solution processability makes them good candidates for printed electronics and flexible display devices.7 Many recently published papers focus on QLEDs based on CdSe/CdS and their heterostructures with ZnS.8−11 However, growing public awareness of the health impact of Cd substances requires development of devices that exclude heavy metals.12 An alternative QLED design strategy could employ multinary semiconductor nanoparticles, such as CuInS2 or AgInS2 and their alloys with ZnS (CIZS and AIZS, respectively).13−19 These nonstoichiometric compounds offer high structural and compositional flexibility.16,20,21 The interstitial and surface defects play an important role in defining the optical properties of such multinary semiconductor QDs.22,23 The semiconductor QD films become electroluminescent when subjected to an external electrical field. Charges injected through the opposite sides of the QD layer recombine to produce emission that is characteristic to band-gap or trap sites. In order to achieve high emission efficiency one needs to ensure charge transfer in the preferred direction with minimal short-circuit current losses. The typical architecture of a QLED device involves a light-emitting layer sandwiched between the layers of hole transport material (HTM) and an electron © 2017 American Chemical Society
transport material (ETM). Sequential deposition of multiple layers (usually two or three layers of different ETM or HTM) rectifies the flow of charge carriers and thus minimizes the loss of charge carriers.24 The selection of the ETM and HTM layers is guided by the energy levels of valence and conduction band edges (usually presented with reference to vacuum). On the basis of the energy level diagrams, one attempts to strategize the highest probability of charge carrier recombination within the QD layer. However, such diagrams focus mainly on energy levels and ignore other factors such as charge density and mobility of the hole or electron conductive layer.5,25,26 Such factors can have a significant effect on the QLED performance. In this work, we examine two common hole-conductive materials, poly(vinylcarbazole) (PVK) and poly(4-butylphenyldiphenylamine) (TPD) (Scheme 1), which possess similar HOMO and LUMO energy levels but differ in charge transport properties.27,28 The effects of different conductivities and interactions of HTMs with QDs on the performance of QLEDs are discussed. Received: June 2, 2017 Accepted: August 25, 2017 Published: August 25, 2017 30741
DOI: 10.1021/acsami.7b07893 ACS Appl. Mater. Interfaces 2017, 9, 30741−30745
Research Article
ACS Applied Materials & Interfaces
The existence of such interactions is also supported by the shorter lifetime of QD films on glass substrate relative to the nanoparticles in chloroform solution (Figure 2).
Scheme 1. Schematic Representation of Chemical Structure for (A) Poly(vinylcarbazole) and (B) Poly(4butylphenyldiphenylamine)
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RESULTS AND DISCUSSION Optical Properties of the Nanoparticles. Colloidal CuInS2−ZnS with its band gap around 2.5 eV exhibits an absorption feature below 560 nm. The tail absorption which can extend to 700 nm is attributed to intraband-gap transitions arising from the defect states.21,22 Figure 1A shows the
Figure 2. (a−d) Photoluminescence decay curves of CIZS nanoparticle in (a) chloroform, (b) on glass substrate, (c) in PVK, and (d) TPD. Samples were excited at 458 nm, and emission was monitored at 630 (a) and 660 nm (b−d).
The average photoluminescence lifetime was calculated by fitting the time-correlated single-photon counting measurements with a biexponential decay kinetics. The fitting parameters of the emission decay are summarized in Table S1. The chloroform-dissolved nanoparticles have an average lifetime of 174 ns compared to 142 ns for the same sample spin coated on a glass slide. The mixtures of CIZS nanoparticles with PVK or TPD exhibit a further reduction in the decay lifetime (τave = 131 or 116 ns, respectively) as compared to the decay process of pristine CIZS. The shorter lifetimes of CIZS QDs embedded in polymers are indicative of the additional energy or charge transfer deactivation pathways. It is likely that hole transport properties of TPD and PVK play an important role in the faster disappearance of the photogenerated charge carriers. Light-Emitting Devices. In order to optimize the QLED design strategy it is important to know the relationship between the electronic structure of the ETL and HTL materials and the performance of the electro-optical device.29,30 A QLED based on CIZS was constructed, the configuration of which is shown in Figure 3A. The active layer of CIZS QD is sandwiched between the electron transport layer (in this work ZnO) and a hole transport layer (in this work TPD or PVK).2,4 The ITO and Al contacts enable application of external electric bias. As the externally applied voltage is turned on, the electrons are injected from the aluminum anode and transferred to the QD layer with mediation through the ZnO layer.11,31 At the same time the holes are injected from ITO, transported through the hole transport layers, and arrive at the QD layer. This architecture ensures the greater probability of achieving electron−hole recombination in the QD layer to generate the emissive state (Figure 3B). The electroluminescence spectra of the representative devices containing CIZS QDs with PVK or TPD as HTMs at different voltages are shown in Figure 3C and 3D, respectively. An image of a typical working device is shown in Figure 3E. As the applied voltage is increased above the threshold value, we can see an increased emission from these devices. They exhibit emission characteristics similar to the one seen in the photoluminescence spectra. A close look at the spectra, however, indicates a blue shift of ∼20 nm with increasing applied voltage. As shown earlier, both band-gap excitation and
Figure 1. (A) Absorbance spectra of CIZS quantum dots (QDs) (a) in chloroform, (b) on glass substrate, (c) mixed with PVK, and (d) mixed with TPD. (B) Photoluminescence of (a) CIZS quantum dots dissolved in chloroform, (b) spin coated on glass substrate, (c) mixed with PVK, and (d) mixed with TPD. Mixtures of QDs with TPD and PVK were spin coated on glass substrate for optical measurement. ̀ Photoluminescence spectra were obtained from excitation with light at a wavelength of 450 nm.
absorption spectra of CIZS QDs in chloroform solution (spectrum a), on glass substrate (spectrum b) and mixed with polymers (spectra c and d). The QD−polymer mixtures were spin coated on glass substrate for the optical measurements. The CIZS nanoparticles suspended in chloroform exhibit a broad absorption spectrum, without exhibiting any clear excitonic peak. These features of the absorption spectra are typical for multinary nanoparticles and originate from interstitial and surface defects.16,20,21,23 The spectral characteristics of the CIZS absorption are not altered as a result of spin coating of the nanoparticles on glass substrate or from interaction between the nanoparticles and PVK. However, the films containing the mixture of CIZS and TPD exhibit an additional broad absorption feature in the UV region with a maximum at 370 nm. This new absorption band originates from TPD (Figure S1). The photoluminescence (PL) spectra of the QD samples in chloroform and on glass slides (Figure 1B, the relative intensities are exhibited in Figure S1) were recorded using an excitation wavelength of 450 nm. The PL spectra of spin-coated samples on glass slides are slightly red shifted relative to the emission spectrum of the nanoparticles in chloroform. The red shift observed in these films may arise from the interparticle interactions and/or change in the dielectric coefficient of the medium. For example, a recent study has shown interparticle energy transfer as a result of donor−acceptor interactions.23 30742
DOI: 10.1021/acsami.7b07893 ACS Appl. Mater. Interfaces 2017, 9, 30741−30745
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Figure 3. (A) Schematic diagram of quantum dot light-emitting devices. (B) Energy levels of the materials that were mentioned in A. Electroluminescence spectra of the quantum dot devices with PVK or TPD as hole conductive layer at different operating voltages (as indicated by a−e in the inset) are exhibited in C and D correspondingly. (E) Typical image of electroluminescence from a device with TPD as hole-transferring material.
trap-induced emission dictate the overall emission in the photoluminescence spectra. Varying the contribution from these recombination pathways (band gap versus trap state) can influence the shape of the emission spectra. A similar scenario of activation of both of these states seems to occur when the emission is induced by the externally applied voltage. The broad emission peak can thus be deconvoluted by considering two emitting states that exist in CIZS nanoparticles (see Figure S4 for deconvolution).21,22 It is interesting to note that the blue shift observed in the emission spectrum at higher applied voltage is indicative of the increased contribution from the band edge recombination. Thus, one can conclude that the trap sites which get saturated at lower applied voltages have a lesser influence on the emission observed at higher applied voltages. On the basis of the oxidation onset (Figure S7) and the band-gap values from absorbance measurements (Figure S3) similar HOMO and LUMO levels of TPD and PVK are expected. Thus, one would expect to see a similar behavior between the two devices to generate electroluminescence, regardless the choice of HTM. However, the difference between two HTMs becomes evident when we compare the minimum required voltage for inducing emission. The emission onset of QLED with TPD occurs at 2 V, whereas the one with PVK occurs at 5 V. The devices with PVK require more than two times higher voltage to obtain the same emission intensity from a QLED device (Figure 4A). The corresponding I−V curves of the devices are exhibited in Figure S5). To compare the performance of the QLED devices, we measured the emission power of QLEDs by employing a calibrated silicone photodiode. The power conversion efficiency (PCE) was obtained as the ratio of the emission power output to the electrical power input. The QLED employing TPD shows the highest PCE at an applied voltage of 2.25 V. Since the emission intensity does not change significantly with increasing voltage above 3 V (Figure 4A), we see a drop in the efficiency at applied voltages greater than 2.5 V. The scenario is slightly different for QLEDs with PVK as the HTM. We see almost steady PCE with changes in the applied voltage greater than 5 V. The difference in the behavior of the QLED with two HTM shows that the energy alignment cannot be used as a sole design parameter. Charge Transfer Interaction and Conductivity of HTM Layers. Another important consideration is the interaction
Figure 4. (A) Integrated intensity of electroluminescence with TPD or PVK as hole-transferring material. (B) Power conversion efficiency (PCE) as a function of voltage for devices with TPD or PVK as holetransferring material.
between the polymer used as HTM and the CIZS QDs. The absorption and emission spectra of the two polymers are presented in Figures S1A and S1B, respectively. In order to probe the influence of polymers on the emissive behavior of CIZS QDs we recorded the excitation spectra of three different films (Figure 5): QDs on glass slide and mixed with PVK or TPD. All three samples show a very broad peak at 500−550 nm, in agreement with the absorption features (Figure 1A). However, the mixture with TPD (spectrum c in Figure 5) exhibits an additional peak at 410 nm. The peak indicates possible charge transfer or energy interaction between TPD and CIZS QDs. This emission does not directly arise from TPD since it does not emit at wavelengths greater than 600 nm (Figure S3). Hence, we attribute the emission response seen with excitation at 410 nm to the charge transfer or energy interaction between the CIZS and the TPD. Such a charge interaction is expected to facilitate better hole transport in the QLED. 30743
DOI: 10.1021/acsami.7b07893 ACS Appl. Mater. Interfaces 2017, 9, 30741−30745
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more than 50% and improve the relative efficiency of the device by more than five times.
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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/acsami.7b07893. Results of XPS analysis, absorbance, and photoluminescence of the polymers, cyclic voltammetry analysis of the polymers, time-resolved photoluminescence decay traces, summary of fitting parameter for photoluminescence decay traces, and deconvolution of photoluminescence peaks (PDF)
Figure 5. Excitation spectra (emission 660 nm) of CIZS QD films on different substrates: (a) on glass, (b) mixed with PVK, on glass substrate, and (c) mixed with TPD, on glass substrate.
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In order to probe the resistivity of the HTM layers we also examined the devices by excluding the active CIZS QD layer. The device configuration was ITO/PEDOT/HTM/Al. We monitored the current−voltage characteristics to probe the resistivity of the two HTM layers. Figure 6 exhibits the
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Prashant V. Kamat: 0000-0002-2465-6819 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The research described here was supported by a grant from Toyota Motors Europe. G.Z. thanks ND Energy for the partial postdoctoral fellowship. S.I. thanks Osaka Municipal Technical Research Institute for the financial support to conduct research at the University of Notre Dame. This is document no. NDRL 5168 from the Notre Dame Radiation Laboratory which is supported by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy through award DE-FC02-04ER15533.
Figure 6. Current−voltage characterization of (a) PVK and (b) TPD. Lines represent 3 cycles at a scan rate 100 mV/s. Inset exhibits the same data on a linear scale.
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current−voltage dependence of PVK and TPD on a semilogarithmic scale (for convenience, the inset represents the same results on a linear scale). Both polymers show close values for the “conductivity onset” (for convent representation current values higher than 0.1 mA are shown), as expected from materials with similar electronic structure. However, different current−voltage dependencies can be observed, in qualitative agreement with electroluminescence behavior (compare the inset of Figure 6 to Figure 4A). This behavior suggests that the difference in the emission onset seen in Figure 4A originates from the shape of I−V curves of the films. PVK exhibits almost logarithmic I−V behavior indicating the complexity of the resistive behavior of the HTM layer. On the other hand, for TPD it shows a linear dependence between current and voltage (inset in Figure 6). Clearly the difference in the resistive behavior of the two HTM layers is contributing to the overall performance of QLED.
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CONCLUSION CIZS alloyed nanoparticles are promising candidates as a heavy-metal-free alternative for QD-LEDs. Fulfillment of their potential requires development of particles with higher photoluminescence efficiency and selection of proper electron or hole conductive layers. The energy levels alignment between materials is a very important factor while considering materials for the selective contacts. However, it cannot be treated as a sole indicator for the performance of the device. Our results show that the higher conductivity and existence of efficient energy or charge transfer paths between the layers have a very significant impact. They can reduce the turn-on voltage by 30744
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