Quantum Dot Light-Emitting Devices: Beyond Alignment of Energy

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Quantum Dot Light Emitting Devices: Beyond Alignment of Energy Levels Gary Zaiats, Shingo Ikeda, Sachin Kinge, and Prashant V. Kamat ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07893 • Publication Date (Web): 25 Aug 2017 Downloaded from http://pubs.acs.org on August 27, 2017

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ACS Applied Materials & Interfaces

Quantum Dot Light Emitting Devices: Beyond Alignment of Energy Levels Gary Zaiats, Shingo Ikeda,1,, 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

*

Corresponding author E-mail: [email protected].

Website: www3.nd.edu/~kamatlab/ Keywords: quantum dots, LED, light emitting devices, hole transferring material, electroluminescence, CIZS

ACS Paragon Plus Environment


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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 overall efficiency of QLED. By employing CuInS 2-ZnS QDs we have succeeded in fabricating red-emitting QLED using two different hole transporting materials, polyvinylcarbazole and poly(4-butylphenyldiphenylamine). 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 show the complexity involved in selecting the hole transport materials for display devices.


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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 make 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 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 non-stoichiometric 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 bandgap 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. A typical architecture of QLED device involves a light emitting layer sandwiched between the layers of hole transport material (HTM) and an electron 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). Based on the energy level diagrams, one attempts to strategize highest probability of charge carrier recombination wiithin 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 significant effect on the QLED performance.



4 In this work, we examine two (A)

(B) N

common

hole

conductive

materials: n

N

polyvynilcarbazole

(PVK)

and

poly(4n

butylphenyldiphenylamine) (TPD) (Scheme 1) which possess similar HOMO and LUMO energy levels but differ in charge transport properties.27,28

C4 H9

Scheme 1. Schematic representation of chemical structure for (A) poly(vinylcarbazole) and (B) poly(4butylphenyldiphenylamine).

The effects of different

conductivities and interactions of HTMs with QDs on the performance of QLEDs are discussed.

Results and Discussion Optical Properties of the Nanoparticles Colloidal CuInS2-ZnS with its bandgap around 2.5 eV exhibits absorption feature below 560 nm. The tail absorption which can extend to 700 nm is attributed to intrabandgap transitions arising from the defect states.21,22 Figure 1(A) shows the absorption spectra of CIZS QDs in chloroform

QDs+Chloroform QDs+Glass QDs+PVK QDs+TPD

1.0 0.8 0.6 0.4

(A)

0.2 300

400

500

600

Wavelength [nm]

700

Intensity [arb. units]

solution (spectrum a), on glass substrate (spectrum b) and mixed with polymers (spectra c,d). The

Absorbance

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QDs+Chloroform QDs+Glass QDs+PVK QDs+TPD

1.0 0.8 0.6 0.4 0.2

(B) 500

600

700

800

Wavelength [nm]

900

Figure 1. (A) Absorbance spectra of CIZS quantum dots (QDs) (a) in chloroform (b) on glass substrate, (b) mixed with PVK (d) mixed with TPD. (B) Photoluminescence of (a) CIZS quantum dots dissolved in chloroform, (b) spincoated on glass substrate, (c) mixed with PVK (d) mixed with TPD. The mixtures of QDs with TPD and PVK were spincoated on glass substrate for the optical measurement. The photoluminescence spectra were obtained `from excitation with light at wavelength 450 nm.

QD-polymer mixtures were spin-coated on glass substrate for the optical measurements. The CIZS nanoparticles suspended in chloroform exhibit broad absorption spectrum, without exhibiting any clear excitonic peak. These features of the absorption spectra are typical for multinary


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5 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 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 1(B), 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 redshifted 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 The existence of such interactions is also supported by shorter lifetime of QD films on glass substrate relative to the nanoparticles in chloroform solution (Figure 2). The average photoluminescence lifetime was calculated by fitting the time correlated single photon counting measurements with a bi-exponential 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 spincoated on a glass slide. The mixtures of CIZS nanoparticles with PVK or TPD exhibit a further reduction in the decay lifetime (ave=131 or 116 nsec respectively) as compared to the decay process of pristine

Figure 2. (a)-(d) Photoluminescence decay curves of CIZS nanoparticle in (a) chloroform, (b) on glass substrate, (c), in PVK, and (d) TPD. The samples were exited at 458 nm and emission was monitored at 630 nm for (a) and 660 nm for (b-d)

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.



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6

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 electrooptical 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 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

Figure 3. (A) Schematic diagram of quantum dots light emitting devices. (B) The energy levels of the materials that were mentioned in (A). The 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.

enable application of external electric bias. As the externally applied voltage is turned on, the electrons are injected from aluminum anode and transferred to the QD layer with mediation through 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 emissive state.


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7 The electroluminescence spectra of the representative devices containing CIZS QDs with PVK or TPD respectively as HTMs at different voltages are shown in Figures 3C and D 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 bandgap excitation and trap induced emission dictate the overall emission in the photoluminescence spectra. Varying contribution from these recombination pathways (bandgap versus trap state) can influence the shape of the emission spectra. A similar scenario of

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

Integrated Intensity [arb. units]

activation of both these states seem to occur

60

QDs+PVK QDs+TPD

40 20

(A) 1

spectrum at higher applied voltage is

QDs+PVK QDs+TPD

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 has lesser

PCE [%]

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0.1

(B)

influence on the emission observed at higher 0.01

applied voltages.

2

Based on oxidation onset (Figure S7)

4

6

Voltage [V]

8

Figure 4. (A) Integrated intensity of electroluminescence

and the bandgap values from absorbance with TPD or PVK as hole transferring material. (B) Power conversion efficiency (PCE) as a function of voltage for

measurements (Figure S3) similar HOMO devices with TPD or PVK as hole transferring material. and LUMO levels of TPD and PVK are

expected. Thus, one would expect to see 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



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8 emission onset of QLED with TPD occurs at 2 V whereas the one with PVK occurs at 5 V. The devices with PVK requires more than two times higher voltage to obtain same emission intensity from 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 emission power of QLEDs, by employing a calibrated silicone photodiode. The power conversion efficiency (PCE) was obtained as 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 3V (Figure 4A), we see a drop in the efficiency at applied voltages greater than 2.5V. 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 5V. 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 between the polymer used as HTM and the CIZS QDs. The absorption and emission spectra of the two polymers are presented

in

Figures

S1

A

and

B

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 (a) QDs on glass slide and mixed with PVK or TPD (b, Figure 5. Excitation spectra (Emission 660 nm) of CIZS QD on different substrates: (a) on glass, (b) mixed with c, respectively). All three samples show a films PVK, on glass substrate (c) mixed with TPD, on glass very broad peak at 500-550 nm, in agreement substrate with the absorption features (Figure 1A). However, the mixture with TPD (curve c) exhibit 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


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9 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. In order to probe the resistivity of the HTM layers we also examined the devices by excluding 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 exhibit

current-voltage dependence of PVK (curve a) and TPD (curve b) on semi-logarithmic scale (for convenience, the inset represent same results on linear scale). Both polymers show close values for the “conductivity onset” (for a matter of convention current higher than 0.1 mA), as expected from materials with similar

electronic

structure.

However,

different current-voltage dependencies can be observed, in qualitative agreement with electroluminescence

behavior

(compare

inset of Figure 6 to Figure 4A). This Figure 6. Current-voltage characterization of (a) PVK and (b) TPD. The lines represent 3 cycles at scan rate 100 mV/sec.

behavior suggests that the difference in the The inset exhibits same data on a linear scale. emission onset seen in Figure 4A originates from the shape of I-V curves of the films. PVK exhibit almost logarithmic I-V behavior indicating the complexity of the resistive behavior of the HTM layer. On the other hand, for TPD shows linear dependence between current and voltage (inset in Figure 6). Clearly the difference in the resistive behavior of the two HTM layers are contributing to the overall performance of QLED.

Conclusion CIZS alloyed nanoparticles are promising candidates as 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 higher conductivity and existence of efficient energy or charge transfer paths between



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10 the layers have very significant impact. They can reduce the turn on voltage by more than fifty percent and improve the relative efficiency of the device by more than five times.

Acknowledgments The research described here was supported by a grant from Toyota Motors Europe. GZ thanks ND Energy for the partial postdoctoral fellowship. SI 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 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.

Supporting Information Available: 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 are presented. This material is available free of charge.

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Quantum Dot Light-Emitting Devices: Beyond Alignment of Energy

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