Ternary I–III–VI quantum dots (QDs) have been regarded as an alternative to Cd- and Pb-based QDs because of their appealing optoelectronic properties and their deficiency in highly toxic components. In this paper, we present the synthesis of highly luminous and emission-tunable CuInS2/ZnS core/shell QDs and their application to the fabrication of highly efficient electroluminescent QD light-emitting diodes (QLEDs). To evaluate the possibility of applying CuInS2/ZnS QDs to photodetectors (PDs), the CuInS2/ZnS-graphene hybrid PDs with a high-responsivity were successfully constructed for the first time. In this study, QLEDs based on the as-prepared CuInS2/ZnS core/shell QDs exhibited a high external quantum efficiency of 3.36% at a forward current of 2.8 V, which is higher than that of CuInS2-based electroluminescent QLEDs reported previously. The resulting PD exhibited a surprisingly high responsivity of 35 A/W. CuInS2/ZnS QDs provide new opportunities for the fabrication of non-toxic QLEDs and PDs exhibiting high performances and are promising for future applications in the field of optoelectronic devices.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Quantum dots (QDs) have attracted increasing attention because of their advantages and unique properties such as a size-dependent band gap, tunable emission wavelength, and narrow full-width at half-maximum (FWHM) [1, 2]. They have been employed for the construction of optoelectronic devices including QD light-emitting diodes (QLEDs) [3–5], photodetectors (PDs) [6, 7], and solar cells [8, 9]. During the past few decades, the synthesis and application of II–VI and IV–VI QDs have been extensively studied [10, 11]. However, heavy metals such as cadmium and lead are harmful to the human body and environment, so their commercial applicability is limited. Ternary I–III–VI QDs are of great interest for fundamental studies and technical applications because of their parallel optoelectronic properties and deficiency in highly toxic elements [12–22]. Thereinto, CuInS2 QDs do not contain any Class A element (Cd, Pb, and Hg) or Class B element (Se and As), so that they are eco-friendly and more acceptable for commercial application. Besides, they have advantages such as a wide emission spectrum ranging from the visible to near-infrared (NIR), high absorption coefficients, and radiation stability, which make them fundamental to solar energy conversion and bio application [19, 21, 22]. Most LEDs based on CuInS2 QDs have been constructed by integrating a mixture of QDs of different emission wavelengths or a mixture of QDs and rare-earth phosphors on a blue or violet chip, realizing white LEDs with high color rendering index values [16, 23]. Few reports focus on electroluminescence light-emitting devices based on CuInS2 QDs, and their performances lagged much behind those devices based on II-VI QDs. Kim et al. synthesized multilayered CuInS2/ZnS colloidal QDs and employed them as active emission layer to fabricate QLEDs. By optimizing the thickness of QD layer, their devices exhibit a peak luminance of 1564 cd/m2, current efficiency of 2.52 cd/A, and ηEXE of 1.1% . Shen et al. fabricated hydroxyl-terminated CuInS2/ZnS QDs with a simple and in situ ligand exchange strategy, and their optimized QLED device with seven layers based on these hydroxyl-terminated CuInS2/ZnS QDs exhibited a maximum luminance of 8,735 cd/m2 and external quantum efficiency (ηEXE) of 3.22% . Therefore, there is a great need and growing interests to develop high performance QLEDs based on CuInS2/ZnS QDs. In order to achieve high performance devices, high fluorescent core/shell QDs are inevitable. ZnS is typically chosen as a shell candidate for CuInS2 core passivation. References have reported that the photoluminescence quantum yield of CuInS2 QDs is dramatically enhanced typically upon ZnS overcoating [25, 26]. Generally, on one hand, the absorption and emission ranges of CuInS2 system can be tuned by changing the Cu:In ratio [21, 23, 25]. On the other hand, upon ZnS shell overcoating, the emissions of CuInS2/ZnS QDs are relatively widely tunable [16, 17]. In this work, CuInS2/ZnS core/shell QDs were synthesized with the hot injection method. We applied a combination of methods including size tuning, composition tuning and surface tuning, the emission wavelength of CuInS2/ZnS QDs ranging from 580 to 800 nm was finely tuned. Surface ligands usually influence the charge injection because of their poor conductivity. CuInS2/ZnS QDs with two different surface ligands were synthesized. Electroluminescent (EL) QLEDs with a high external quantum efficiency based on the prepared CuInS2/ZnS QDs were fabricated using the solution method. In another hand, QDs have been employed as light absorption materials for the construction of PDs because of their advantages such as direct band gaps, large absorption coefficients, and high carrier mobilities [27, 28]. CuInS2/ZnS QDs have been verified to be perfect light harvesters [19, 22]. Therefore, hybrid PDs with high responsivities based on these CuInS2/ZnS QDs were also constructed. In this preliminary work, the fabricated QLEDs based on octanethiol-capped CuInS2/ZnS QDs exhibited satisfactory luminescence performance with a maximum ηEXE of 3.36% at 2.8 V, maximum current density of 0.48 cd/A at 3.2 V, and maximum luminescence of 113.83 cd/m2 at 7.6 V. Besides, graphene has been used intensively in photodectors (PDs) owing to their high carrier mobility and their light absorption in a broad wavelength range [29, 30]. References reported that the gain of a photoconductor based on QDs can be increased dramatically due to the long lifetime of carriers in the QDs [31, 32]. In our work, the hybrid graphene PDs based on CuInS2/ZnS QDs were successfully constructed for the first time with a high-responsivity 35 A/W. The properties of the QLEDs and PDs based on CuInS2/ZnS QDs are expected to improve with further studies on the QDs and device structures. These results suggest that CuInS2/ZnS QDs show great promise for next-generation photoelectric devices.
2. Experimental details
Cuprous iodide (CuI, 99.99%, powder), indium acetate (In(Ac)3, 99.99%, powder), Zinc acetate dehydrate (Zn(OA)2, 97%, powder), 1-dodecanethiol (DDT, 98%), 1-octadecene (1-ODE, 90%), 1-octanethiol (OTT, 98%), oleic acid (OA, 90%), oleylamine (OAm, 90%), chlorobenzene (99%), octane (96%), ethanol (99%), toluene (99%), and hexane (99.7%) were purchased from Aldrich. Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS), Poly[bis(4-phenyl)(4-butylphenyl)amine] (Poly-TPD) were purchased from Xi’an polymer Light Technology Corp. All reagents were used as received without further experimental purification.
2.2. Synthesis of CuInS2 cores
In brief, a mixture of 0.25 mmol CuI, 1 mmol In(AC)3, 5 mL DDT, 10 mL OAm, and 10 mL 1-ODE loaded in a 50-mL three-neck flask were degassed at 80 °C for 40 min and heated to 150 °C under an Ar flow. Then, the solution was maintained at this temperature for 15 min and heated to 230 °C at which CuInS2 cores began to grow. The growth of CuInS2 cores lasted for about 10 min at 230 °C, and the solution turned from yellowish to orange and then to red and dark brown at the end. Higher reaction temperatures and/or longer reaction times will lead to larger CuInS2 cores with longer emission wavelengths. Purified CuInS2 QDs were obtained by repeating the precipitation/dispersion method and then dispersed in hexane for characterization.
2.3. Synthesis of CuInS2/ZnS QDs
For the preparation of CuInS2/ZnS QDs, a mixture of 2 mmol Zn(Ac)2, 1 mL DDT, 2 mL OA, and 3 mL ODE loaded in a 25-mL flask were heated to 100 °C and maintained at this temperature for 30 min so Zn(Ac)2 dissolved and the Zn precursor was prepared. The Zn precursor was dropped into the CuInS2 solution when the growth of the core completed. The mixed solution was heated to 240 °C, which took 20 min, and CuInS2/ZnS QDs were synthesized. Purified CuInS2/ZnS QDs were obtained by repeating the precipitation/dispersion method and were dispersed in hexane for characterization and device fabrication.
2.4. Device fabrication
2.4.1. EL-QLEDs based on CuInS2/ZnS QDs
Briefly, QLEDs were fabricated by the following procedures. The indium–tin oxide (ITO) glass substrates were cleaned by ultrasonic treatment successively in detergent, deionized water, acetone, and isopropanol for 15 min each; dried under a N2 flow; treated in a UV-ozone for 20 min; and transferred to an argon-filled glove box with oxygen and water levels below 20 ppm for the following layer deposition. The hole-injection layer of PEDOT:PSS aqueous solution (4 wt.%, 100 μL) was spin-coated onto the substrates at 3500 rpm for 50 s and baked at 150 °C for 30 min. The hole-transport layer of Poly-TPD (in chlorobenzene, 8 mg/mL) was deposited followed by baking at 150 °C for 30 min. Next, CuInS2/ZnS QDs (in octane, 15 mg/mL) were deposited by spin-coating at 2,000 rpm for 50 s to form an emission layer, and colloidal ZnO:Mg nanocrystals (in ethanol, 20 mg/ml) were then deposited by spin-coating at 2,000 rpm for 40 s and baked at 60 °C for 30 min to form an electron-injection layer. Finally, the Al anodes were then deposited via vacuum thermal evaporation at a rate ~0.1 nm/s under a pressure below 5 × 10−6 Torr.
2.4.2 Hybrid PDs based on CuInS2/ZnS QDs
In brief, PDs were fabricated by the following procedures. A 300-nm-thick SiO2 layer as a dielectric was thermally grown on a heavily doped Si wafer, which was employed as the gate electrode. Next, single-layer graphene was deposited onto the SiO2 film by means of chemical vapor deposition and Au electrodes were then deposited on the single-layer graphene surface via vacuum thermal evaporation with a designed metal mask to form a 40-μm-long channel. The CuInS2/ZnS QDs (in toluene) with an emission wavelength of 730 nm was spin-coated on the channel surface at 2,000 rpm for 50 s.
The material properties were characterized using a high-resolution transmission electron microscope (TEM; JEM-2010, JEOL) working at 200 kV, X-ray diffractometer (MiniFlex II X-ray diffractometer, Rigaku) using Cu Kα radiation (λ = 0.154 nm) at 50 kV and 250 mA at room temperature, steady-state photoluminescence (PL) spectrophotometer (Cary Eclipse, Varian) utilizing a xenon discharge light source and R-928 photomultiplier tube, and UV-vis spectrophotometer (Cary 300, Varian) with spectral resolution set to 5 nm. Time-resolved photoluminescence (PL) were carried out by using a femtosecond pump setup at 400 nm wavelength, the fluorescent light was directed to a spectrometer (Bruker Optics 250IS/SM) and detected by an intensified charge coupled device detector (Andor, IStar740) with time resolution of ∼60 ps. Absolute PL quantum yields (QYs) of the prepared QDs were tested by an absolute QY test system (Quantaurus-QY C11347-11, Hamamatsu Photonics) with spectral resolution set to 2 nm at room temperature. The luminescent and visual performances of the fabricated QLEDs were measured on a spectrometer (Maya 2000 Pro, Ocean Optics) coupling with an integrating sphere (3P-GPS-033-SL, Labsphere) every 0.2 V with voltage increasing from 0 to 10 V. All current–voltage (I–V) characteristics of the QD-graphene hybrid PDs were measured by a Keithley 2400 current and voltage source meter unit.
3. Results and discussion
3.1. Structural and optical properties of CuInS2/ZnS QDs
CuInS2 is a direct semiconductor with a bandgap of 1.5 eV (~830 nm). Considering the quantum confinement effects, this implies that the emission of the CuInS2 alloy system can be tuned in the visible to NIR range. There are primarily three strategies to tune the emission properties of QDs: size tuning, composition tuning, and surface tuning. Generally, the luminescence properties including emission spectra and PL QYs can be finely tuned by applying a combination of these strategies [16, 33]. With a given In:Cu ratio, size control of the CuInS2 QDs can be achieved by adjusting the temperature and time during the synthesis process. However, the CuInS2 QDs were found to be substantially sensitive to oxygen and amine because catalytic agents will accelerate the oxidation of CuInS2. By introducing a ZnS shell onto the CuInS2 core, not only was this stability problem addressed, but also the obtained CuInS2/ZnS core/shell QDs exhibited greatly improved PL QYs up to 30% . Previous papers have reported that higher In:Cu ratios tend to lead to a peak-wavelength blue-shift of the CuInS2 QDs, and a ZnS shell tends to result in further blue-shift [23, 25].
By applying a combination of size tuning, composition tuning, and surface tuning, CuInS2/ZnS QDs with emission wavelengths ranging from 580 to 800 nm were prepared. For comparison, replacing DDT with OTT, CuInS2/ZnS QDs with OTT ligands were also prepared and employed for device fabrication. The as-prepared CuInS2/ZnS QDs are DDT-capped unless otherwise stated. The absorption and PL spectra of the prepared CuInS2 and CuInS2/ZnS QDs are presented in Fig. 1(a). The symmetric PL spectra with a FWHM in the range of 45–67 nm indicates that the particle sizes are almost uniform. The time-resolved PL decay curves of the CuInS2 and CuInS2/ZnS QDs are presented in Fig. 1(b). All decay curves can be fitted well by a single exponent written as , where a is the amplitude, τ is the decay time constant, and F(t) is the pulse response function. The obtained average lifetimes are 88.09 ns for the CuInS2 QDs with an emission wavelength of 680 nm, and 198.76, 131.79, and 157.71 ns for the CuInS2/ZnS core/shell QDs with emission wavelengths of 610, 630, and 760 nm, respectively, as listed in the inset of Fig. 1(b). The prepared CuInS2/ZnS QDs exhibit much longer PL lifetimes than those of the CuInS2 cores. The most probable transition mechanism in CuInS2 QDs is the donor-acceptor pair recombination, where the photo-excited electron and hole are trapped in respective deep donor and acceptor levels inside the band gap, followed by their radiative recombination . Being a ternary compound, CuInS2 naturally contains more lattice imperfections than, for example, II-IV semiconductors. These defects act as potential fluctuations to localize carriers, consequently causing long lifetimes for the relevant transitions . After introducing the ZnS shell onto the CuInS2 core, the rate of the defected-related donor-acceptor transition increases as the surfaces-state varies, hence the PL lifetimes enhances . These results demonstrate that the ZnS shell passivated the CuInS2 core effectively. TEM images of the CuInS2/ZnS QDs are shown in Fig. 1(c). The inset of Fig. 1(c) shows the corresponding high-resolution TEM image, from which no clear boundary can be observed between the core and shell. This indicates a continuous composition gradient inside the QDs. To further characterize the structures of the CuInS2 and CuInS2/ZnS QDs, their crystallographic properties were determined by the X-ray diffraction method. The XRD patterns for bulk CuInS2 and ZnS are also shown for reference. According to the XRD patterns shown in Fig. 1(d), the characteristic peaks (27.86°, 46.91°, and 55.08°) of the CuInS2 QDs agree well with those of the bulk CuInS2 (JCPDS No. 65-2732). This indicates a compositional homogeneity of CuInS2 rather than a mixture of CuS2 and In2S3. However, after overcoating ZnS onto the CuInS2 cores by dropping a S precursor, the characteristic peaks of the CuInS2/ZnS QDs become slightly different from the standard zinc blend XRD pattern (JCPDS No. 65-0309). This is because ZnS exists in both wurtzite and zinc blende phases during QD growth, which has been widely reported [23, 35, 36]. In short, it is suggested that ZnS tends to coat the surface of the CuInS2 core and form a core/shell heterostructure rather than an alloy compound.
3.2. Luminescence properties of the QLEDs based on CuInS2/ZnS QDs
The prepared CuInS2/ZnS QDs are employed as a light-emitting layer to fabricate the QLEDs. As shown in the schematic in Fig. 2(a), the devices have a multilayer structure of ITO/ PEDOT:PSS/Poly-TPD/QDs/ZnO:Mg/Al. Except for Al that is deposited by thermal vacuum deposition, all the other layers are sequentially spin-coated from solution on the glass substrate with a pre-patterned ITO transparent anode.
A cross-sectional TEM image of the prepared device is shown in Fig. 2(b). All the interfaces in the device, except for that between PEDOT:PSS and Poly-TDP, can be clearly distinguished. This is because either PEDOT:PSS or Poly-TPD with a small thickness here is a transparent organic polymer film. The electroluminescence spectra of the prepared QLED measured from 4 to 10 V are presented in Fig. 2(c). As the voltage increases, the relative intensity of the EL spectra increases steadily with the FWHM (~75 nm) remaining almost unchanged. Furthermore, unlike some reported EL-QLEDs whose peak emission shifts to longer wavelengths with a temperature rise resulting from an increasing voltage or current, the EL peak wavelength of the prepared device has a slightly perceptible shift. This may indicate that the prepared CuInS2/ZnS QDs have a good thermal stability to some extent.
The charge injection problem, including the low efficiency of charge injection from the charge transport layers (CTLs) to the QD emitting layer, and the exciton quenching caused by accumulated space charges near the interface between the CTLs and QDs, is one of the main factors hindering the development of high-efficiency CuInS2 QD-based EL-QLEDs. Organic ligands surrounding the QDs stabilize the growth of QDs, prevent the aggregation and precipitation of QDs, and passivate the QD surface. They also generate a large barrier, making the charge injection difficult because of its poor conductivity . Figure 2(d) presents the current density (J) and luminance (L) of the devices based on the CuInS2/ZnS QDs with OTT (CH3(CH2)6CH2SH) and DDT (CH3(CH2)10CH2SH) ligands as a function of driving voltage (V), while Fig. 2(e) presents the external quantum efficiency ηEXE and current efficiency ηA of the same devices as a function of V. The PL QYs of the OTT- and DDT-capped CuInS2/ZnS QDs are measured to be 45% and 60%, respectively. As can be seen from Fig. 2(d), both the devices based on OTT- and DDT-capped QDs exhibit a low turn-on voltage of approximately 2.8 V. At a given voltage, the device based on DDT-capped QDs always exhibits a higher luminance than that based on the OTT-capped QDs, this result may attribute to the higher PL QY of the DDT-capped QDs than that of the OTT-capped ones. The current densities for both devices are almost the same when the voltage is below 5.6 V. With the voltage beyond 5.6 V, however, the current density for the device based on OTT-capped QDs increases more rapidly than that based on the DDT-capped QDs. Highly efficient QLEDs not noly require QDs with bright emission but also need to maintain the superior emission property when they are assembled to QD solids. First, the PL QY of the DDT-capped QD solution (60%) is higher than its OTT counterpart (45%). Besides, the chain of DDT is longer than that of OTT, which can suppress the inter-dot interactions between the QDs. Although the mechanism of the inter-dot interaction is still under debate, the long chain has proved an efficient way to reduce those undesired inter-dot interactions . Therefore, it is not surprised that the luminance of the QLED with DDT-capped QDs is higher than its OTT-capped counterpart, because the long chain capped QDs have high PL QY but also helps in suppressing the harmful inter-dot interactions and preserving the bright emission of the QDs in solid state. On the other hand, the long chain DDT will also increase the charge injection barrier into the QD emitter because of the decreased inter-dot interactions . A DDT ligand contains more CH2 groups that an OTT one, hence charge carrier should overcome higher physical barrier to inject into the QD emitter. Therefore, the current density of the device based on OTT-capped QDs increases more rapidly than that based on DDT-capped QDs. The device based on OTT-capped QDs exhibits a maximum current efficiency of 0.48 cd/A at 3.2 V, approximately 1.4 times that of the device based on the DDT-capped QDs with 0.34 cd/A at 3.4 V. The maximum ηEXE is achieved at 3.36% at 2.6 V for the device based on OTT-capped QDs, which is more than 20% higher than that of the device based on DDT-capped QDs with a peak ηEXE of 2.77% at 2.2 V. It appears that the device based on the CuInS2/ZnS QDs with OTT performs better than that with the DDT. To some extent, the performance differences among these devices indicate that organic ligands with suitable chain length will favor the performance improvement of electroluminescence QLEDs.
3.3. Photoresponse characteristics of hybrid PDs based on CuInS2/ZnS QDs
It is noted that QD-based PDs have also received extensive attention. The prepared CuInS2/ZnS QDs were employed to fabricate QD-graphene hybrid PDs as well. Figure 3(a) presents a schematic of a QD-graphene hybrid PD fabricated on a SiO2/Si substrate. For this QD-graphene hybrid PD, graphene provides conductive channels with high carrier mobility, while the QDs act as a strong light absorber. A built-in electric field exists near the interface between the QDs and graphene layer because of the diffusion and drift of the dark holes of the QD film. With light illuminating the top surface of the channel, photon hole–electron pairs are generated. Thereafter, photoholes trapped by the built-in electric field remain in the QD film, and photoelectrons transferred to the graphene channel drift, driven by the drain–source voltage (VDS), enabling the drain–source current (IDS) to be measured. The PD with a channel length of 40 μm was illuminated by a range of incident light wavelengths, and the resulting IDS–VDS characteristics are presented in Fig. 3(b). The measurements were conducted with a fixed incident illumination power of 5 mW. IDS consisted of a dark current Id and photocurrent Iph, thus IDS = Id + Iph. The photocurrent of the PD is high at illumination wavelengths shorter than 660 nm, consistent with the absorption spectrum of the CuInS2/ZnS QDs with an emission wavelength of 730 nm shown in Fig. 1(a). Generally, with small illumination power, the photocurrent of the PD depends linearly on VDS. As shown in Fig. 3(b), for each incident light wavelength, the photocurrent has a good linear relationship with VDS at a zero gate voltage VGS. The inset panel shows a magnified view of the VDS dependence of the PD. As illustrated in Fig. 3(c), under illumination of different light wavelengths, the response of the PD is different. This is because the CuInS2/ZnS QDs exhibit different absorption for photons with different frequencies. The device exhibits high photocurrent; in detail, the maximum values for Iph are 1.96, 1.61, 2.00, 1.26, and 0.22 mA, corresponding to light wavelengths of 405, 450, 520, 630, and 660 nm, respectively. As shown in Fig. 3(d), all the photoresponsivity values corresponding to different illumination wavelengths are above 25 A/W. Importantly, a high photoresponsivity of 35 A/W to light with a wavelength of 660 nm is observed, indicating that our prepared CuInS2/ZnS QDs are suitable as a light absorber for QD-graphene hybrid PDs.
In this study, we synthesized an emission-tunable CuInS2/ZnS for QLED applications with high ηEXE. For comparison, CuInS2/ZnS QDs with OTT and DDT ligands were prepared and used to fabricate QLEDs. QLEDs based on OTT-capped QDs exhibited a maximum current efficiency of 0.48 cd/A at 3.2 V, approximately 1.4 times that of the device based on DDT-capped QDs with 0.34 cd/A at 3.4 V. The maximum ηEXE was achieved at 3.36% at 2.6 V for the device based on OTT-capped QDs, which was about 20% higher than that of the QLED based on DDT-capped QDs with a peak ηEXE of 2.77% at 2.2 V. It is apparent that the QLED based on the OTT-capped QDs performs better than that based on DDT-capped QDs. Short OTT chains may be one of the reasons for the performance difference among these devices. Besides, our synthesized CuInS2/ZnS QDs were also employed as light absorbers to construct QD-graphene hybrid PDs. The constructed PDs exhibited good optoelectronic properties, including a good linear relationship between IDS and VDS under illumination with low light power, a high responsivity of 35 A/W to incident light with a wavelength of 660 nm, and a high responsive photocurrent of 2.0 mA to light with a wavelength of 520 nm. Despite the luminescence properties of EL QLEDs based on CuInS2 QDs still left far behind compared to that of the Cd-based ones, the performances of the QLEDs can be further improved, necessitating much more in-depth studies on the QDs. These results suggest that CuInS2/ZnS QDs show great promise for next-generation photoelectric devices.
Natural Science Foundation of China (61366003, 11564026, 11774141, 61765011); Outstanding Youth Funds of Jiangxi Province (20171BCB23051 and 20171BCB23052); Natural Science Foundation of Jiangxi Province (20151BAB212001, 20151BBE50114, 20171BAB202036 and 20161BAB212035); Science and Technology Project of the Education Department of Jiangxi Province, China (GJJ150727 and GJJ160681).
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