Investigation of Exciton Recombination Zone in ... - ACS Publications

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Investigation of exciton recombination zone in quantum dot light-emitting diodes using a fluorescent probe Xiaoyu Huang, Heng Zhang, Dingxin Xu, Feng Wen, and Shuming Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08574 • Publication Date (Web): 21 Jul 2017 Downloaded from http://pubs.acs.org on July 24, 2017

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ACS Applied Materials & Interfaces

Investigation of exciton recombination zone in quantum dot light-emitting diodes using a fluorescent probe

Xiaoyu Huang1, 2, Heng Zhang1, Dingxin Xu1, Feng Wen2, Shuming Chen1* 1

Department of Electrical and Electronic Engineering, Southern University of Science and Technology, Shenzhen, 518055, P. R. China [email protected]

2

Key Lab. of Advanced Material of Tropical Island Resources, Ministry of Education, Department of Materials and Chemical Engineering, Hainan University, Haikou, 570228, P. R. China

Abstract Exciton recombination zone, where the photons are generated, can greatly affect the performance such as the efficiency, the color purity of the quantum dot (QD) light-emitting diodes (QLEDs). To probe the exciton recombination zone, 4-(dicyanomethylene)-2-t-butyl-6(1,1,7,7

-tetramethyljulolidyl-9-enyl)-4H-pyran

(DCJTB) is doped into the charge transport layer as a fluorescent sensor; by monitoring the Förster resonant energy transfer (FRET) between QD and DCJTB, the location of the recombination zone can be determined. It is found that the electron transport layer (ETL) has a great impact on the recombination zone. For example, in QLEDs with ZnMgO ETL, the recombination zone is near to the interface of QD/hole transport layer (HTL) and is shifted to the interface of QD/ETL as the driving voltage is increased, while in devices with 1,3,5-tris(2-N-phenylbenzimidazolyl) benzene (TPBi) ETL, the recombination zone is close to the interface of QD/ETL and is moved to the interface of QD/HTL with the increasing of the driving voltage. Our results can also clarify the light emission mechanism in QLEDs. In devices with ZnMgO ETL, the emission is dominated by direct charge recombination, while in devices with TPBi ETL, the emission is contributed by both FRET and direct charge recombination. Our studies suggest that fluorescent probe can be a powerful tool for 1 / 20

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investigation of exciton recombination zone, light emission mechanism and other fundamental processes in QLEDs.

Keywords: quantum dot; light-emitting diodes; exciton recombination zone; fluorescent probe; Förster resonant energy transfer; direct charge recombination 1. Introduction Recently, colloidal quantum dot light-emitting diodes (QLEDs) have been widely investigated because of their advantages such as tunable emission color, high color saturation, inherent high stability and low-cost solution processability, which make them promising candidates for next-generation displays

1-10

. In recent years,

researchers have made significant progress in improving the performance of QLEDs. For example, by engineering the device architectures and improving the material quality, QLEDs with external quantum efficiency (EQE) exceeding 20.5% 11, 23% and 15.1%

13

12

for red-, green- and blue-QLEDs, respectively, have been reported,

making them close to the practical applications. Though the performance of QLEDs have been rapidly enhanced, some fundamental processes such as the light emitting mechanism, the exciton recombination zone remain unclear, which might hinder the further improvement of device performance. Generally speaking, there are two explanations for the light emission in QLEDs, namely direct charge recombination 14 and Förster resonant energy transfer (FRET) 15. In the case of direct charge recombination, electrons and holes are injected from charge transport layer (CTL) into QD layer, forming excitons that subsequently recombines via emission of photons

5,9,16,17

. As for FRET mechanism, excitons are

formed on CTL firstly, then the excitonic energy is transferred to QD via dipole-dipole coupling

17,18

. In earlier device architectures with organic electron

transport layer (ETL) like TPBi, FRET is considered as the main mechanism that accounts for the generation of photons

19

. For example, Chen et al. used

magneto-electroluminescence (MEL) as an in situ tool to investigate the emission 2 / 20


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mechanism and concluded that ET is the main mechanism for light emission in the hybrid QLEDs with TPBi ETL

20

. However, in modern device structures with ZnO

nanoparticles ETL, direct charge recombination is speculated to be the dominant emission mechanism

5,21

, but there is no direct evidence to support this speculation

and it is still not clear which mechanism dominates the light emission. The emission mechanism is closely related with the exciton recombination zone. If the exciton recombination zone is confined within the QD layer, then the emission is dominated by direct charge recombination. If the recombination zone is extended to the CTL, then the emission is contributed by both FRET and direct charge recombination. In other words, if the recombination zone can be precisely located, then the emission mechanism can be clarified 6. In addition, the recombination zone can greatly affect the performances such as the color purity, the efficiency of the devices and thus its position should be precisely located and tuned. For example, Ji et al. enhanced the device performance by separating the recombination zone from the charge carrier accumulation interface, which suppresses exciton quenching caused by accumulated carriers

22

. Kim et al. achieved a bicolored QLED by effectively

extending the recombination zone throughout both QD and CTL layers 23. Mutlugun et al. boosted up the efficiency while maintaining the color purity of the devices by carefully controlling the exciton recombination zone

24

. However, no systematic

investigations on the recombination zone have been reported. To deeply understand the light emitting mechanism and further improve the device performance, it is necessary to investigate the recombination zone. In this work, a fluorescent dye 4-(dicyanomethylene)-2-t-butyl-6(1,1,7,7tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) is doped into CTL as a sensor to probe the recombination zone; by monitoring the FRET process between QD and DCJTB, the location of the recombination zone can be determined. It is found that the ETL has a great impact on the recombination zone. For example, in QLEDs with ZnMgO ETL, the recombination zone is confined within the QD layer and is near to the interface of QD/hole transport layer (HTL), which thus indicates that the photons 3 / 20



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are mainly generated by the direct charge recombination. However, in devices with TPBi ETL, the recombination zone is extended from the QD to the ETL, and thus the emission is contributed by both FRET and direct recombination. Our work demonstrates that with DCJTB sensor, the exciton recombination zone can be probed, the emission mechanism can be clarified and the majority excess carriers can be identified.

2. Results and discussion

Inverted QLEDs with structure glass/ITO/ZnMgO 40 nm/QDs 15 nm/TcTa Rd nm/TcTa:DCJTB 5 nm /TcTa 25-Rd nm/NPB 20 nm/HAT-CN 10 nm/Al 100 nm were fabricated. To probe the recombination zone, DCJTB was doped into TcTa as a fluorescent sensor. The distance between DCJTB and QD is defined as Rd and was varied from 0 to 10 nm. DCJTB was chosen as a sensor because its absorption spectrum is well overlapped with the emission spectra of blue (B) and green (G) QDs, while its emission spectrum can be easily differentiated from those of QDs, as shown in Figure 1 (a). In other words, the excitonic energy of QDs can be effectively transferred to the DCJTB via the FRET process if Rd is smaller than the Förster transfer radius R0. By varying the Rd and detecting the emission signal of DCJTB, the recombination zone of QDs can be determined. To set an appropriate Rd, the critical transfer distance R0 (in unit of Å) was calculated first using the formula 15,25:

/

=0.2108[ ]

in which κ2 is the orientation factor; Φ0 is the quantum yield of QDs; n is the refractive index of the intervening medium, and J (in units of M-1cm-1nm4) is the integral that indicates the degree of spectral overlap between QD emission spectrum and DCJTB absorption spectrum, which can be calculated using formula 16:

=

in which is the emission spectrum of QD normalized to its area, and is the molar extinction coefficient (in units of M-1cm-1) of DCJTB. By using a refractive 4 / 20


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index of 1.7, κ2 of 2/3 (for random donor and acceptor transition dipole orientations) and Φ0 of 80%, the calculated R0 for the G-QDs and the B-QDs to DCJTB is 6.0 nm and 5.7 nm, respectively. The energy transfer efficiency is determined by 26: / According to the formula, at Rd=R0, the transfer efficiency is 50%, and at RdR0, the transfer efficiency drops rapidly and approaches zero at Rd=10 nm, as displayed in Figure 1 (b). So to probe the recombination zone of QD, the Rd was set between 0 to 10 nm.

Figure. 1. (a) Emission and absorption spectra of QDs and DCJTB. The absorption of DCJTB is well overlapped with the emission of QDs, indicating energy transfer from QDs to DCJTB is effective. (b) Energy transfer efficiency as a function of separation distance between QDs and DCJTB.

To examine if the recombination zone is confined within the QD layer, a blue fluorescent sensor BcZVBi (4,4'-bis(9-ethyl-3-carbazovinylene)-1,1'-biphenyl) with emission peak of 475 nm (supporting information Figure S1) was doped into the TcTa. The distance between QD and BcZVBi was varied from 0 to 5 nm. Figure S2 shows the current density-voltage-luminance (J-V-L), current efficiency (CE)-J and EQE-J characteristics of the devices. The introduction of BcZVBi almost does not alter the J-V-L characteristics and the performance of the devices. For example, all QLEDs 5 / 20



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exhibited a CE of 46 cd/A and a EQE of 10.2% at a current density of 100 mA/cm2. In such configuration, the energy of G-QDs cannot up-transfer to the blue BcZVBi sensor, and thus if there is blue emission, it should be resulted from the direct charge recombination. In other words, the blue emission observed in the EL spectra indicates that the recombination zone is extended from the QD layer to the CTL. However, we did not observe any blue emission when all devices were driven from 5 V to a high voltage of 13 V, as shown in Figure 2 (b). This result indicates that the recombination zone is completely confined within the QD layer. This is because both ZnMgO and TcTa can effectively block the charge carriers due to their deep valence band level and low lowest unoccupied molecular orbital (LUMO) level, respectively, which help to confine the excitons within the QD layer.

Figure. 2. (a) Energy level alignment and the proposed emission mechanism. (b) EL spectra of the devices under different driving voltage. The separation distance between QD and BcZVBi was varied from 0 to 5 nm.

Now that the recombination zone is completely confined within the QD layer, the next step is to probe its exact location. The fluorescent sensor DCJTB was doped into the TcTa and the distance between QD and DCJTB was varied from 0 to 10 nm. Figure 3 (c)-(f) show the EL spectra of the devices. The green emission with peak at 528 nm is originated from the QDs, while the red emission centered at 620 nm is generated by DCJTB. Considering that the recombination zone is within the QD layer, 6 / 20


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the emission of DCJTB thus is resulted from the FRET process. Its intensity is very sensitive to the donor-acceptor separation distance because the ET efficiency is reversely proportional to the 6th power of the distance. For example, as shown in Figure 3 (c)-(e), at Rd5 nm, the emission is gradually weakened due to the reduction of the transfer efficiency, and at Rd=10 nm, no DCJTB emission can be observed. The emission of DCJTB is also affected by the driving voltage. As shown in Figure 3 (c) and (d), the intensity is gradually decreased as the increasing of the driving voltage, implying that the recombination zone is shifted away from the DCJTB as the driving voltage is increased. At small driving voltage, the emission of the DCJTB is very strong, indicating that the recombination zone is near to the interface of QD/TcTa, which ensures a small donor-acceptor distance. This result also indicates that the injection of electrons is more efficient than that of holes, which is reasonable because the electron injection barrier is smaller than that of hole. In addition, the electron mobility is higher than that of hole. The excess electrons are accumulated at the interface of QD/TcTa, waiting holes to recombine with. As the driving voltage is increased, more holes can overcome the barrier and inject to the QD, which thus pushes the recombination zone shifting away from the QD/TcTa interface, and thereby increases the donor-acceptor distance and leads to the reduction of the DCJTB emission. At a driving voltage higher than 11 V, no DCJTB emission can be observed, implying that the donor-acceptor distance is longer than 10 nm. Considering that the thickness of the QD layer is ~15 nm (monolayer), the thickness of the recombination zone thus is ~5 nm and is close to the QD/ZnMgO interface. This result also indicates that not the entire dot but only partial of the dot is excited.

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Figure. 3. (a) Schematic structure of the inverted green QLEDs. (b) Energy level alignment of the devices. EL spectra of the device with (c) 0 nm, (d) 5 nm, and (e) 10 nm distance between QD and DCJTB layer, (f) EL spectra of the devices without DCJTB dopant.

We also investigated the recombination zone of the inverted blue QLEDs. Figure 4 (a) shows the energy level alignment of the devices. Similar results have been obtained, i.e., the DCJTB emission is decreased as the separation distance and the driving voltage are increased, as shown in Figure 4 (c)-(e). One difference is that, at a small driving voltage of 6 V, there is a broad red emission even for the devices without the DCJTB dopant, as shown in Figure 4 (e) and (f); however, we do not 8 / 20


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observe any red emission in the PL spectrum of the B-QD. Such broad red emission thus is attributed to the emission of the interfacial exciplex. As shown in Figure 4 (b), at small driving voltage, electrons are accumulated at the interface of QD/TcTa, while holes cannot effectively inject to the QD due to the high injection barrier. Consequently, the accumulated electrons in the conduction band of QD recombine with the holes in the highest occupied molecular orbital (HOMO) of TcTa, leading to the exciplex emission 27-29. The exciplex emission observed here also implies that the hole injection is relatively difficult, especially in blue QLEDs due to the deep valance level (~6.5 ev for blue QD and ~6.1 ev for green QD) of blue QDs 7. As the driving voltage is enhanced, the exciplex emission is reduced because the holes now can overcome the barrier and inject to the QDs. To conclude, it is found that in G-QLEDs and B-QLEDs with ZnMgO ETL, the recombination zone is confined within the QD layer and thus the emission of QD is dominated by the direct charge recombination. Initially, the recombination zone is close to the interface of QD/HTL but is shifted to the QD/ETL interface as the driving voltage is increased. Because the ZnO ETL can significantly quench the excitons 8, the efficiency of the devices would decrease as the increasing of the driving voltage. To solve this problem, it is suggested to use a thin interlayer inserted between ZnO and QD to suppress the quenching, especially at high driving voltage . The thickness of the recombination zone can be as thin as 5 nm. Exciplex emission was also observed in B-QLEDs which is due to the relatively inefficient of the hole injection.

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Figure. 4. (a) The proposed FRET mechanism. (b) Energy diagrams showing the exciplex formation. EL spectra of the device with (c) 0 nm, (d) 5 nm, and (e) 10 nm distance between QD layer and doped layer, (f) without dopant.

To further examine if the recombination zone is affected by the ETL, we replaced the ZnMgO with TPBi, which is a common ETL that is adopted in earlier device architectures. Devices with structure glass/ITO/PEDOT:PSS 50 nm/TFB 16 nm/QDs 15 nm/TPBi Rd nm/TPBi:DCJTB 5 nm/TPBi 35-Rd nm/LiF 1 nm/Al 100 nm were fabricated. DCJTB was doped into the TPBi and the distance between DCJTB and QD was varied from 0 to 10 nm. The schematic device structure and the energy level 10 / 20


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alignment of the devices are shown in Figure 5 (a) and (b), respectively. Figure 5 (c)-(f) display the normalized EL spectra of the devices. It is reasonable to observe the DCJTB emission for the devices with Rd

Investigation of Exciton Recombination Zone in ... - ACS Publications

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