An electrical and infrared controllable color emission quantum dot light-emitting diode Open Access

Through years of research, infrared radiation (IR) has been studied and has affected almost every aspect of technology. In the last two decades, infrared-to-visible upconversion (IVU) has attracted a great deal of attention and has been studied quite intensively, for it can lead to a wide range of applications, ranging from infrared sensing, night vision, bio-imaging, to anti-counterfeiting.^1–8 With materials capable of achieving up-conversion, several types of devices based on quantum wells (QWs), quantum dots (QDs), graphene, and triplet–triplet annihilation (TTA) have been designed.^9–13 For example, the Sun Group has designed an ultrahigh responsive IR photodetector based on hybrid QDs and graphene heterostructures.¹⁴ Dai’s research demonstrated a stable structure of QD light-emitting diodes (QLEDs) by inserting a layer between the hole transport layer (HTL) and the QD layers.⁹ Yu’s work on an up-conversion phototransistor by integrating an infrared photodetector with an organic light-emitting diode (OLED) demonstrated a considerable improvement in performance.² Yang’s study presented an IVU device based on a type II/type I core–shell QD heterostructure.⁸ In addition, Wu’s article used a micro-cavity to enhance IVU efficiency.⁷ However, according to the published reports, even though great effort has been made, there still exist several shortcomings that prevent the developed devices from being used in practical applications, such as complicated design of the device structure, costly fabrication process, and weak optical absorption. In order to overcome the existing difficulties, in this study, we developed an IVU device based on the integration of IR QDs and a mixture of visible QDs of QLEDs. In addition to circumventing the previous drawbacks, the designed IVU QLED also contains several unexplored features as discussed below.

QD-based devices show great advantages in the fields of display, light harvesting, and many others because of their unique properties, such as size-dependent bandgap, easy processability, stability, color purity, high quantum yield, etc. With the advantages of QDs being used as emission materials, QLEDs are now one of the most intensive research topics.^15–27 However, owing to the restriction of bulky integration and incompatible fabrication of IR-to-visible devices, until now, they have still not been able to meet the requirements of practical application of IVU devices. In this work, to overcome various obstacles, different QD materials, such as IR PbS QDs and a mixture of different sizes of CdSe/ZnS QDs, were integrated together, and with an appropriate design of the device structure, a high-performance IVU-QLED was achieved. PbS QDs have a small bandgap, which can be modulated with different ligands. Quite interestingly, this single device possesses a dual electrical-infrared modulation to control the emission color at a visible range, which has excellent characteristics without a complicated structure and complicated fabrication processes. Notably, an infrared-controllable QLED with a tunable emission color based on varying input IR radiation power density is an unprecedented device. With a pre-designed picture pattern and color index of the emitted light, the IVU-QLED developed here is very promising for visual signal encoding and anti-counterfeiting technology.

In the IVU-QLED device, PbS and a mixture of CdSe/ZnS QDs were selected as the carrier generation layer (CGL) and emission layer (EML), respectively, which were fabricated by all-solution processes. PbS QDs were used to absorb input IR radiation and generate electron–hole pairs. CdSe/ZnS core–shell QDs with two different emission wavelengths (540 and 630 nm) were selected as the emission material because of their high electroluminescence efficiency. Based on a well-designed band alignment among the light emitting layer and electron and hole transporting layers, this IVU-QLED device has excellent control of the emission behavior under varied IR illumination. An electron-to-photon (e-to-p) conversion efficiency of 4.77%, photon-to-photon (p-to-p) conversion efficiency of 0.2%, and responsivity of 0.43 (A/W) were achieved when the device was operated under a bias voltage of 4 V with a low turn-on voltage of 2 V. The proposed methodology shown here can be extended to many other combinations of QD systems, even with a mixture of more than two different kinds of light emitting QDs. Therefore, it is foreseeable that our study can pave a key step for the development of many other not-yet realized optoelectronic devices.

The IVU-QLED devices were made by all-solution processes. The pre-patterned ITO glass substrates were sonicated using soap water for 30 min, deionized water twice for 10 min, acetone for 30 min, ethanol for 10 min, and isopropanol for 15 min followed by oxygen plasma treatment for 15 min to obtain a better film formation. 10 mg ml⁻¹ of PbS QDs (NNCrystal US, with a size of 2.7–5.7 nm) in toluene solution was spin-coated at 500 rpm for 10 s (first step) and 2000 rpm for 60 s (second step). Next, 20 µl of 1,3-benzenedithiol (Alfa Aesar™) mixed with methanol in 10 mM was dropped on the PbS QD film with a waiting time for 60 s to exchange the oleic acid ligand with the BDT ligand. The same spin-coating step as described in the PbS spin-process was carried out immediately after the 60 s. Finally, methanol was spin-coated using the same spin-process of PbS QDs to clean the un-exchanged ligands on the surface. The thickness of the PbS QD film is about 45 nm.

PEDOT:PSS solution (Heraeus Clevios TM PH1000) was spin-coated on the PbS QD film at 5000 rpm for 60 seconds and heated on a hot plate at 150 °C for 10 min. Poly-TPD (FMPV, 8 mg ml⁻¹ in chlorobenzene) and PVK (Sigma-Aldrich, 1.5 mg ml⁻¹ in toluene) were spin-coated at 3000 rpm for 60 s and heated at 150 °C and 120 °C, respectively. Two different emission wavelengths of CdSe/ZnS core–shell QDs were mixed in toluene (UniMat, 5 mg ml⁻¹ for λ = 630 nm and 25 mg ml⁻¹ for λ = 540 nm) and spin-coated at 700 rpm for 120 s. ZnO nanoparticles (15 mg ml⁻¹ in isopropanol) were spin-coated at 800 rpm for 60 s. After all transport layers were spun on the indium tin oxide (ITO) substrate, 100 nm of silver film was thermally evaporated on the top as the electrode. Because the size of the sample in our study is designed as 2 × 2 mm², we can fabricate a series of IVU-OLED arrays with many different samples on the same substrate for testing the reproducibility of our measurements.

The cross section image of the IVU-QLED was recorded by using a field emission scanning electron microscope (SEM) system (JEOL JSM-6500F) under an acceleration voltage of 15 kV. The electrical, electroluminescence (EL), and photoresponse measurements of the device were measured with a probe station at room temperature, consisting of a source measure unit (Keithley 2400) as the power supply, a 980 nm laser system, and a high-resolution spectrometer (HORIBA Jobin Yvon iHR550) to record the EL spectra.

Figure 1(a) presents the schematic illustration of the IVU-QLED structure. This IVU-QLED is an all-solution-process device with a working area of 0.04 cm², which is defined by the overlapping area across indium tin oxide (ITO)-glass and the silver electrode. The IR-photoactive gate comprises PbS QDs, which were infrared-sensitive and served as the CGL. The relative band structure of each constituent material is shown in Fig. 1(b). During the operation, the device was exposed to IR irradiation with a wavelength of 980 nm. The infrared photons can pass through the transparent ITO-glass substrate (sheet resistance ≈15 $Ω$ cm⁻¹) and be absorbed by the PbS QDs, which exhibit an absorbance peak at 1000 nm, and thus can be used to generate photocarriers. The carriers will transport through the hole transport layers (HTLs) including poly (3,4-ethylenedioxythiophene):poly (styrene sulfonate) (PEDOT:PSS), poly (N,N′-bis-4-butylphenyl-N,N′-bisphenyl) benzidine (poly-TPD) and poly (N-vinylcarbazole) (PVK). These holes were gradually guided by HTLs to the light emitting layer (EML) (mixture of CdSe/ZnS QDs) and be confined in the EML by the large valence band offset arising from the zinc oxide (ZnO) layer, preventing the recombination of electrons and holes outside of the EML. ZnO nanoparticles serve as the electron transport layer (ETL) for its appropriate energy band. The electrons injected from the Ag cathode were transferred through the ETL (ZnO nanoparticles) and confined in the EML by the large conduction band offset of PVK. With the well-designed band alignment, the recombination in the mixture of CdSe/ZnS QDs was dominated; hence, the corresponding bright visible light emission is obtained. The cross-sectional scanning electron microscopy (SEM) image of PbS QDs with a thickness of about 45 nm and the mixture of the CdSe/ZnS QD layer with a thickness of 100 nm are shown in Figs. 1(c) and 1(d), respectively. The cross-sectional SEM image of the full QLED devices has been shown in our previous report.²⁶ 

To compare the IR-sensing performance, QLEDs were made and measured with and without PbS QDs. Figure 2(a) shows the current density–voltage (J–V) characteristic of the reference device with and without the PbS QD layer (ITO/PEDOT:PSS/poly-TPD/PVK/mixed CdSe/ZnS core–shell QDs/ZnO/Ag). For the device without PbS QDs, the current density is being cut off when the voltage is less than 1.5 V. Note that the turn-on voltage required for the QLED to emit light is about 2.0 V. The injected current increases rapidly when the applied voltage exceeds 2.0 V. Both the results of Fig. 2(a) show that the QLEDs with and without PbS QDs can work at a relatively low voltage. For the device without PbS QDs, generally a turn on voltage of about 2 V is required for it to possess a diode behavior. If the applied voltage is larger than the turn on voltage, the current increases rapidly and shows a steeper slope. With the insertion of the PbS QD layer, the J–V curve will deviate far away from the ideal diode behavior because of the existence of the additional resistance layer. In addition, the J–V curve will not exhibit a clear cut-off and turn on voltage. For a pure LED, the diode-like curve is certainly preferred based on the conventional working principle of the LED. For LED devices with PbS QDs, the turn on voltage is not as obvious as pure LED devices, which can also be found in the previous reports.²⁸ 

The color change behavior observed above under different carrier injections can be understood as follows: The 630 nm CdSe/ZnS QDs have a bigger size and a larger probability to emit light due to the charge transfer from the smaller size of CdSe/ZnS QDs with a larger bandgap. The simultaneous emission of two kinds of CdSe/ZnS QDs leads to double peaks on electroluminescence (EL) at both 540 and 630 nm. During the operation, when a small voltage is applied on the device, the emission wavelength at 630 nm of CdSe/ZnS QDs is dominated, and hence, the red color can be observed first. As the applied voltage was gradually increased, more carriers reached the CdSe/ZnS QDs with an emission wavelength of 540 nm, and the probability of the green light emission was increased. Thus, the color-changing behavior of the IVU-QLED varied gradually from red to orange to yellow. Based on the mixture ratio of 1:5 for the 630 and 540 nm CdSe/ZnS QDs, the green light eventually becomes the dominant emission of the PbS-QLED device under voltage and infrared tuning. As the applied voltage is gradually increased, it generates more carriers. The electrons in the conduction band of 630 nm CdSe/ZnS QDs are saturated; then the excessive electrons move to the higher conduction band of 540 nm CdSe/ZnS QDs. Thus, the emission intensity of 540 nm increases and causes the color change. Similarly, as the illumination of the 980 nm laser increases, it can enhance the injection current and generate more carriers. When the carriers in the energy band of 630 nm CdSe/ZnS QDs are saturated, the injected carriers will move to the energy band of 540 nm CdSe/ZnS QDs and lead to a tunable emission property. The device works in a saturation region when the IR illumination exceeds 15 mW cm⁻².

As the emission spectrum of the device can be manipulated by IR illuminations, the corresponding International Commission on Illumination (CIE) color coordinates can also be defined and sketched. Figure 5(a) shows the CIE color coordinates of the device from the emission results shown in Fig. 3, demonstrating that the IVU-QLED device can be used to recognize input IR signals with an unprecedented feature of tunable color emission. This unexplored characteristic should be very useful for anti-counterfeiting applications.

According to the results in Fig. 4, the pronounced responses of the different intensities of IR illumination can be clearly seen. In front of the IR illumination, one can apply different patterns to obtain specific visual responses as shown in Fig. 5(b). Any other 2D patterns and different ratios of color-changing can also be demonstrated by using the IVU-QLED device with an appropriate IR source to achieve the realization of anti-counterfeiting and visual signal encoding. Note that only when the color ratio and picture pattern fit the pre-designed encoded signal, the information can be decoded and read. This characteristic can greatly enhance the security for the development of information technology in the future.

Combing several advantages as shown above, including infrared-to-visible upconversion, dual function modulation, tunable color emission, fast response time, and cost-effective solution processes, the developed IVU-QLED device should be very valuable for the exploration of optoelectronic devices with great potential application, in particular for anti-counterfeiting information technology and bio-imaging.

In conclusion, an all-solution-processed IVU-QLED device possessing the ability of IR-detection and visual response was designed, fabricated, and demonstrated. By using the integration of IR QDs and the mixture of different sizes of CdSe/ZnS QDs as both the CGL and EML, respectively, in the designed configuration, the current through the IVU-QLED can be dual-modulated by both incident IR and external bias. The device can serve as an infrared sensor as well as a visible display, and it also possesses the unexplored feature of the tunable color emission characteristics. Furthermore, extra messages or patterns of visualized data can be added to the input signal to obtain a recognizable and visual response, which is very useful for the perfect security of anti-counterfeiting. Due to the versatility of semiconductor QDs, the functionalities of the IVU-QLED device, such as absorption and emission wavelength, can be manipulated by simply using different sizes or materials of QDs. Compared with the previous studies, our designed IVU-QLED device, which possesses several advantageous features, including cost-effectiveness, dual functional modulation, tunable color emission, patterning of visible display, infrared-visible upconversion, and fast response time, should be very useful and timely for the development of future generation optoelectronic devices with applications ranging from bio-imaging to anti-counterfeiting.

This work was financially supported by the “Advanced Research Centre for Green Materials Science and Technology” from the Featured Area Research Centre Program within the framework of the Higher Education Sprout Project by the Ministry of Education (Grant No. 111L9006) and the Ministry of Science and Technology in Taiwan (Grant Nos. MOST110-2634-F-002-043 and MOST 110-2112-M-002-044).

The authors have no conflicts to disclose.

Zun-Hong Jiang and Hsia Yu Lin contributed equally to this work.

Zun-Hong Jiang: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Writing – original draft (equal). Hsia Yu Lin: Conceptualization (equal); Methodology (equal); Resources (equal); Writing – review & editing (equal). Yang Fang Chen: Conceptualization (equal); Project administration (equal); Resources (equal); Supervision (equal); Writing – review & editing (equal).

The data that support the findings of this study are available from the corresponding author upon reasonable request.