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Solution-Processed Conducting Polymer/Metal Oxide Charge Generation Layer: Preparation, Electrical Properties and Charge Generation Mechanism Yong Lei, Zhen Liu, Changjun Fan, Xuefeng Peng, Xiaxia Ji, Guoqing Li, Zuhong Xiong, and Xiaohui Yang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b11838 • Publication Date (Web): 27 Dec 2016 Downloaded from http://pubs.acs.org on December 29, 2016

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The Journal of Physical Chemistry

Solution-Processed Conducting Polymer/Metal Oxide Charge Generation Layer: Preparation, Electrical Properties and Charge Generation Mechanism

Yong Lei，Zhen Liu, Chang-Jun Fan, Xue-Feng Peng, Xia-Xia Ji, Guo-Qing Li, Zu-Hong Xiong, and Xiao-Hui Yang*

School of Physical Science and Technology, Southwest University Chongqing 400715, China

CONTACT AUTHOR:

E-mail: [email protected]

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ACS Paragon Plus Environment


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ABSTRACT: Conducting polymer/metal oxide hetero-structures have been commonly used in solution-processed tandem organic optoelectronic devices as charge generation layer (CGL) or charge recombination layer. However, the underlying working mechanism remains unexplored. We optimize the preparation of poly(3,4-ethylenedioxythiophene) -poly(styrenesulfonate) (PEDOT:PSS)/zinc oxide (ZnO) CGL and report that the configurations and compositions of the interconnects significantly affect the performance of light emitting devices. Two-unit and in particular the firstly reported three-unit solution-processed tandem polymer light emitting devices show the luminance efficiency matching the total luminance efficiency of the constituent two and three light emitting units, respectively. Current-voltage and capacitance-voltage measurements on the devices with various interconnects indicate that charges are generated at the PEDOT:PSS/ZnO interface. CGL-generated current can be described with the Richardson-Schottky thermal emission model, yielding the barrier height close to the “effective band gap” of the PEDOT:PSS/ZnO hetero-structure.

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1. INTRODUCTION

Organic light emitting devices (OLEDs) are regarded as one of the most promising display and lighting technologies in the 21st century due to their advantages such as self-emitting, low voltage, fast response, high efficiency and flexibility1, 2. OLEDs are current-driven devices, in other words, emission intensity is approximately proportional to the drive current. Large drive current can shorten the operating life-time of devices3, 4. To solve the paradox of high luminance and long-term stability, tandem organic light emitting devices consisting of vertically stacked light emitting units interconnected by charge generation layer (CGL) are proposed. In such devices, charges generated by the CGL can be injected into the adjacent light emitting units, resulting in multiple photon emission from one injected hole-electron pair. As such, high luminance can be realized at relatively low drive current, thereby providing an elegant solution to prolong the operating life-time of devices. In 2003, Kido et al.5 for the first time reported tandem light emitting devices using the CGL with the structure of cesium (Cs):2,9-dimethyl-4,7-biphenyl-1,10 -phenanthroline (BCP)/indium tin oxide (ITO), vanadiumpentoxide (V2O5) or 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ). Further studies show that the manifold structures such as n-/p-doped organic layer6-8, n-doped organic/organic layer9, 10, n-doped organic layer/transition metal oxide11-13, organic p-n junction14,

15

and different work-function metal stacks16,

17

can function as

effective CGLs. Meanwhile, the charge generation mechanisms for various CGLs have been studied. Kröger et al.18 reported that charges were generated in the 3



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n-/p-doped

organic

layer-based

CGL

via

temperature-independent

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electric

field-induced electron tunneling from the Highest Occupied Molecular Orbital (HOMO) level of p-doped organic layer to the Lowest Unoccupied Molecular Orbital (LUMO) level of n-doped organic layer through a thin depletion layer. For the n-doped organic layer/transition metal oxide-based CGLs, charge generation is reported to occur at the hole transport layer/transition metal oxide interface19, inside the transition metal oxide layer20 or at the both locations21. Most studies on tandem light emitting devices are focusing on vacuum-deposited small-molecule devices having complicated structures. Solution-processed devices on the other hand have the advantages of low-cost, large-area and printable production22-24. To construct such devices, the orthogonal solubility of materials in the interconnect is required, which is rather challenging. Höfle et al.22 described the stacked light emitting devices in inverted device architecture with solution-processed tungsten trioxide (WO3)/poly(3,4-ethy-lenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS)/zinc oxide (ZnO) CGL, taking the advantage that the CGL can be prepared from alcohol and aqueous solution. Luminance efficiency (LE) of the yellow devices with two polymer light emitting units was 18 cd/A22, matching the total LE of the reference single devices. It is worth noting that the PEDOT:PSS/titanium dioxide (TiO2) hetero-structure has also been applied as carrier recombination layer in high-efficiency tandem solar cells25, 26. Nevertheless, how the individual layer in such CGL affects the properties of light emitting devices and more importantly, the charge generation mechanism underlying the commonly-adopted and versatile conducting

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polymer/metal oxide interconnects has yet to be further studied. In this paper, we discuss the influence of the configurations and compositions of the interconnects on their charge generation and injection properties. The charge generation mechanism is carefully explored by current density-voltage and capacitance-voltage measurements of various interconnects-based devices. Our results indicate that charge generation occurs at the PEDOT:PSS/ZnO interface. CGL-generated current can be described with the Richardson-Schottky thermal emission model, yielding the barrier height close to the “effective band-gap” of the PEDOT:PSS/ZnO hetero-structure. Our findings provide deep insights into the charge generation and injection mechanism underlying the PEDOT:PSS/ZnO CGL and are helpful for future design of solution-processable CGL with enhanced properties.

2. METHODS ZnO layer modified by PEIE was used to promote electron injection from indium tin oxide (ITO) cathode27, 28. Poly[{2,5-di(3,7-dimethyloctyloxy)-1,4-phenyleneviny lene-co-{3-(4‫׳‬- (3‫׳‬,7‫׳׳‬-dimethyloctyloxy)phenyl)-1,4-phenylenevinylene}-co-{3-(3‫׳‬-( 3‫׳‬,7-dimethyloctyloxy)phenyl)-1,4-phenylenevinylene}] (SY-PPV) and poly[2-meth‫׳‬ oxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) were employed as the light emitting polymers. PEDOT:PSS/ZnO, MoO3 and Al were used as the CGL, hole injection layer and anode, respectively. A 30 nm ZnO layer was prepared on ITO substrates with the sol-gel method using zinc acetate (Zn(OAc)2) precursor stabilized with 5 vol.% ethanolamine according to

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the process described by Sun et al29. Subsequently, 10 nm PEIE and 70 nm SY-PPV layer were deposited from the 2-methoxyethanol and chlorobenzene solution. For some devices, a V2O5 layer was constructed by spin-coating the isopropanol solution of vanadium triisoproxide30 and annealing the precursor film at 150 ℃ for 20 min under the ambient conditions. PVP AI 4083 or PH-1000 PEDOT:PSS water dispersion with 1 vol.% surfactant Zonyl FS-300 was deposited on top of V2O5 or SY-PPV layer under the ambient conditions, afterwards the samples were annealed at 120 ℃ for 10 minutes to remove the moisture. A ZnO layer was prepared with or without ethanolamine additive onto PEDOT:PSS layer prior to the deposition of a 10 nm PEIE and 70 nm MEH-PPV layer. Finally, 10 nm MoO3 and 100 nm Al were thermally evaporated under 10-4 Pa. For three-unit tandem devices, except that the component light emitting units adopted SY-PPV, other layers were prepared in the same way as described above for two-unit tandem devices. Various interconnects were prepared in the same way as aforementioned onto PEDOT:PSS

covered

ITO

substrates.

For

some

devices,

a

60

nm

poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-(4,4'-(N-(4-butylphenyl) (TFB) layer was deposited from the chlorobenzene solution onto PEDOT:PSS layer, which was followed by the preparation of PEDOT:PSS or PEDOT:PSS/ZnO interconnect. 50 nm BCP or BCP:10% CsF, 1 nm CsF and 100 nm Al were successively vacuum-deposited. And the deposition rates for BCP, CsF and Al were 0.2, 0.02 and 0.4 nm/s, respectively. For double-insulating structure, a 50 nm polyacrylonitrile (PAN) layer was spin coated from the dimethyl sulphoxide solution onto ITO substrates. In some

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cases, a 60 nm TFB layer was applied on top of PAN layer. Various interconnects and a 30 nm SY-PPV layer were then prepared as stated above, which was followed by thermal deposition of 30 nm CsF and 100 nm Al. Current density-voltage-luminance (V-I-L) characteristics were measured by a Keithley 2400 Source-Measure unit and a Konica-Minolta Chroma Meter CS-100A. Electroluminescent spectra was obtained by an Ocean Optics USB4000-UV-VIS spectrometer. Temperature dependent I-V characteristics of devices were measured with a cryogenic chamber. Surface morphology was recorded by a Hitachi atomic force microscope. Capacitance-voltage characteristics were obtained by a Keithley 4200CUV semiconductor characterization system, using the AC voltage amplitude of 50 mV and frequency of 1 KHz.

3. RESULTS AND DISCUSSION

Scheme 1 shows the configurations of bottom-LEU, top-LEU and two-unit TOLED (c) as well as the energy level diagram of the two-unit TOLED. The CGL comprises PEDOT:PSS and ZnO layer for charge generation and injection into the adjacent light emitting units. For the two-unit tandem devices, the bottom and top light emitting unit consist of a SY-PPV and MEH-PPV emission layer (EML). For the three-unit tandem devices, SY-PPV is used exclusively as the EML in the subelements.

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Al

MoO3

PEIE

MEH-PPV

Bottom-LEU

ZnO

ITO

Al

PEDOT:PSS

SY-PPV

PEIE

ZnO

ITO

Top-LEU

(a)

(b)

Al

MoO3

MEH-PPV

ZnO

PEIE

PEDOT:PSS

SY-PPV

PEIE

ZnO

ITO

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Two-unit TOLED (c)

(d)

Scheme 1. Configurations of bottom-LEU (a), top-LEU (b) and two-unit TOLED (c) as well as the energy level diagram of the two-unit TOLED (d).

3.1. Configuration and Composition of the CGL. Due to the large surface energy difference between SY-PPV and PEDOT:PSS, PEDOT:PSS tends to aggregate when deposited onto SY-PPV layer. A surfactant Zonyl FS-300 (1 vol.%) is blended with PEDOT:PSS to improve wetting. Figure 1 show the surface morphology images of a PEDOT:PSS layer and bilayer of PEDOT:PSS and ZnO on top of SY-PPV layer. The surface of PEDOT:PSS film is relatively uniform and pin-hole free (Figure 1a). The RMS roughness value for the PEDOT:PSS layer is ca. 1.7 nm, larger than the reported value of 0.28 nm for the PEDOT:PSS layer on top of glass substrates31. The RMS roughness value slightly increases to 2.0 nm upon the further deposition of a

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ZnO layer (Figure 1b). Smooth, full-coverage and pin-hole free PEDOT:PSS/ZnO CGL is obtained, which enables efficient charge generation and injection, the further deposition of high-quality light emitting polymer layers and the protection of the bottom light emitting unit from the solvent used for the preparation of the overlying light emitting unit.

(a)

(b)

Figure 1. Surface morphology images of a PEDOT:PSS layer (a) and bilayer of PEDOT:PSS and ZnO (b) on top of SY-PPV layer.

In order to study the influence of the electrical conductance of PEDOT:PSS layer on device properties, we have prepared the devices using the PEDOT:PSS/Al or CGL/Al hole injection contact with two kinds of PEDOT:PSS, namely PVP AI 4083 and PH-1000 PEDOT:PSS with the conductivity of 7.0×10-5 and 0.006 S/cm32, 33. The V-I-L and luminance efficiency-current density (LE-I) characteristics of the devices are

shown

in

Figure

S1,

Supporting

Information.

The

PVP AI

4083

PEDOT:PSS-based devices show the LE of ca. 5.7 cd/A, more than two times higher than that of the PH-1000 PEDOT:PSS-based devices (2.3 cd/A). The diminution of

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the LE in the devices with a PH-1000 PEDOT:PSS layer may be ascribed to strong luminescence quenching effect of PH-1000 PEDOT:PSS layer34. On this basis, we choose PVP AI 4083 PEDOT:PSS in the remainder of this work. Subsequently, we investigate the role of PEDOT:PSS layer in the charge generation and injection process of the PEDOT:PSS/ZnO CGL. For this purpose, the devices having the CGL/Al hole injection contact with varying PEDOT:PSS layer thickness i.e. device A with the structure of ITO/ZnO(30 nm)/PEIE(10 nm)/SY-PPV(70 nm)/PEDOT:PSS(0, 30 or 60 nm)/ZnO(30 nm)/PEIE(10 nm)/Al are prepared. The summary of device structures in section 3.1 and 3.2 is shown in Table 1. The devices with a 30 or 60 nm PEDOT:PSS layer show the similar light emission on-set voltage (ca. 0.1 cd/m2) of 4.0 V, in contrast, the devices without a PEDOT:PSS layer show significantly lower current density at a given voltage and very dim light emission (Figure 2a). The LE of the former devices is 5.7 cd/A (Figure 2b), which is approximately three orders of magnitude higher than that of the latter device (ca. 0.001 cd/A). The results indicate that PEDOT:PSS layer plays a vital role in the charge generation and injection process of the PEDOT:PSS/ZnO CGL. Table 1. Summaries of the Device Structures in Section 3.1 and 3.2 Devices Structures A B C D E F EL1 EL2 CGL

ITO/ZnO(30 nm)/EL1/PEDOT:PSS(0,30,60 nm)/ZnO(30 nm)/PEIE(10 nm)Al ITO/PEDOT:PSS(60 nm)/ZnO(0,10,30 nm)/EL2/MoO3(10 nm)/Al ITO/ZnO(30 nm)/EL1/CGL /EL2/MoO3(10 nm)/Al ITO/ZnO(30 nm)/EL1/PEDOT:PSS(60 nm)/Al ITO/ZnO(30 nm)/EL2/MoO3(10 nm)/Al ITO/ZnO(30 nm)/EL1/V2O5(10 nm)/CGL /EL2/MoO3(10 nm)/Al PEIE(10 nm)/SY-PPV(70 nm) PEIE(10 nm)/MEH-PPV(70 nm) PVP AI 4083 PEDOT:PSS(60 nm)/ZnO(30 nm)

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300

60 nm 30 nm 0 nm

10000

200 100 10 100 1 0.1 0 0.01 0

1

2

1000

Luminance (cd/m ) Luminance efficiency (Cd/A)

2

Current density (mA/cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4

8

0.1

0.01

1E-3

12

10

100 2


Voltage (V)

(a)

(b)

Figure 2. Properties of the devices using the CGL/Al hole injection contact with varying PEDOT:PSS layer thickness i.e. with the structure of ITO/ZnO(30 nm)/PEIE(10 nm)/SY-PPV ( 7 0 n m ) / P E D O T: P S S ( 0 , 3 0 o r 6 0 n m ) / Z n O ( 3 0 n m ) / P E I E ( 1 0 n m ) / A l : V- I L (a) and LE-I characteristics (b).

A sol-gel method with Zn(OAc)2 as the precursor is widely used to prepare ZnO layer, in many cases, a small amount of ethanolamine is added into the precursor solution as a stabilizer35. We report the significant impact of ehtanolamine additive on the properties of the devices with the CGL/Al or PEDOT:PSS/Al hole injection contact. Figure S2 compares the V-I-L and LE-I characteristics of the devices. The devices processed with ethanolamine show significantly decreased current density and LE compared to the counterparts processed without ethanolamine. Fabiano et al36 reported the N-based materials can reduce PEDOT+, thereby changing the conductivity and work-function of PEDOT:PSS as indicated by the comparison of current density of the hole-only devices (Figure S3, Supporting Information). In order to study the function of ZnO layer in the CGL, the devices using the ITO/CGL electron injection contact with varying ZnO layer thickness i.e. device B

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with the structure of ITO/PEDOT:PSS(60 nm)/ZnO(0, 10 or 30 nm)/PEIE(10 nm)/MEH-PPV(70 nm)/MoO3(10 nm)/Al have been prepared and their V-I-L and LE-I properties are shown in Figure 3. The device with a 10 or 30 nm ZnO layer shows the LE of 2.3-2.5 cd/A, which is similar to that of the analogous device using the ITO/ZnO/PEIE electron injection contact27. In contrast, the device without a ZnO layer fails to emit light, manifesting that ZnO layer is crucially important for the charge generation and injection process of the CGL. The above results show that only the PEDOT:PSS/ZnO hetero-structure enables efficient charge generation and implying

that

it

serves

400

200 100 100 10

2

Luminance efficiency (Cd/A)

2

1000

a

fully

functional

CGL.

3

10000

30 nm 10 nm 0 nm

300

as

Luminance (cd/m )

injection,


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2

1

0

0 0

2

4

0

6

100

200

300 2


Voltage (V)

(a)

(b)

Figure 3. Properties of the devices using the ITO/CGL electron injection contact with varying ZnO layer thickness i.e. devices with the structure of ITO/PEDOT:PSS(60 nm)/ZnO(0, 10 or 30 nm)/PEIE(10 nm)/MEH-PPV(70 nm)/MoO3(10 nm)/Al: V-I-L (a) and LE-I characteristics (b). 3.2. The Tandem Light Emitting Devices. The two-unit tandem devices (device C) consisting of the bottom SY-PPV (device D) and top MEH-PPV light emitting units (device E) interconnected by the PEDOT:PSS/ZnO CGL have been prepared.

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We also prepare the tandem devices (device F) with a 10 nm V2O5 layer incorporated between SY-PPV layer and PEDOT:PSS/ZnO CGL to make a comparison with the previously reported tandem devices22. The properties of device C-F, including the I-V and LE-I characteristics as well as the EL spectra measured at 50 mA/cm2 are shown in Figure 4. As shown in Figure 4a, the I-V characteristics of device E are shifted toward lower voltage direction compared to those of device D, which is related to different hole mobility of SY-PPV and MEH-PPV37 and the utilization of different hole injection layers, i.e. PEDOT:PSS and MoO3 hole injection layer for device D and E. At a given current density, the drive voltage of device C roughly matches the sum of the drive voltage of device D and E, e.g. at 100 mA/cm2, the drive voltage values of device C, D and E are 11.9, 7.6 and 3.8 V, respectively, indicating a rather small voltage drop across the CGL. The implementation of a V2O5 layer only slightly affects the I-V characteristics of the devices, which can be rationalized by the fact that PEDOT:PSS and V2O5 possess similar hole injection property (Figure S4, Supporting Information). Meanwhile, it can be inferred that V2O5 layer plays a minor role in the charge generation process of the CGL. As shown in Figure 4b, device C, D and E show the respective maximum LE of 7.5, 5.3 and 2.3 cd/A. The addition of a V2O5 layer impacts the LE of the device very slightly (Figure 4b). In this regard, the utilization of the PEDOT:PSS/ZnO CGL enables the simplification of CGL structure without compromising device performance. Device C, D, E and F show the respective maximum power efficiency (PE) of 2.2, 2.5, 2.1 and 2.1 lm/W. The PEs of device C and F lie in between those of device D and E, which can be mainly attributed to

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different LEs of light emitting units using various light emitting polymers（SY-PPV or MEH-PPV）and hole injection layers (PEDOT: PSS or MoO3), but not to the large voltage drop across the CGL as discussed above. Further discussion about the PE of the tandem devices is presented in Figure S5 Supporting Information. Figure 4c shows the EL spectra of device C-F. The EL spectrum of device C contains both SY-PPV and MEH-PPV emission coming from the bottom and top light emitting unit, indicating simultaneous operation of both subelements.

200

100

0

-1

2

Luminance efficiency (Cd/A)

300

4 4 0 2 -4

-8

0

3

6

9

12

15

0

100

200

300

0 400

2

Voltage (V)


(a)

(b) 1.0 0.8

Intensity (a.u)

Power efficiency (lm W )

6

8


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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bottom top tandem tandem-V2O5

0.6 0.4 0.2 0.0 300

400

500 600 Wavelength (nm)

700

(c)

Figure 4. The characteristics of the two-unit tandem devices (ITO/ZnO(30 nm)/PEIE(10

nm)/SY-PPV(70

nm)/PEDOT:PSS(60

nm)/ZnO(30

nm)/PEIE(10

nm)/MEH-PPV (70 nm)/MoO3(10 nm)/Al) and constituent bottom and top light emitting units (ITO/ZnO(30 nm)/PEIE(10 nm)/SY-PPV(70 nm)/PEDOT:PSS (60 14


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nm)/Al,and ITO/ZnO(30 nm)/PEIE(10 nm)/MEH-PPV(70 nm)/MoO3(10 nm) /Al): V-I plots (a), LE-I-PE curves (b) and EL spectra at 50 mA/cm2 (c).

The three-unit tandem devices consisting of the PEDOT:PSS/ZnO CGL and bottom, middle and top SY-PPV light emitting units have been prepared and their characteristics are shown in Figure S6 Supporting Information. The LE of the three-unit tandem devices approximately matches the sum of the LE of the component light emitting units, e.g. the LEs at 5000 cd/m2 for the tandem device, bottom, middle and top light emitting unit are 20.5, 5.7, 5.2 and 10.5 cd/A, respectively. This is the first report of the three-unit tandem devices employing solution-processed CGL and polymer light emitting units. It is apparent that the increase in the number of light emitting units results in almost no performance losses, reflecting the robustness of the PEDOT:PSS/ZnO CGL. The stability measurements of the tandem devices and constituent light emitting units are made and the results are shown in Figure S7 Supporting Information. The operating life-time of the tandem devices is short and lies in between those of the constituent light emitting units. Pu et al. 38 reported that the life-time of the tandem devices was intermediate between those of the subelements and correlated the degradation of light emitting devices with unstable carrier injection layers such as PEDOT:PSS and PEIE layer. 3.3. The Working Mechanism of the PEDOT:PSS/ZnO CGL. In order to gain deep insights into the working mechanism of the PEDOT:PSS/ ZnO CGL, various interconnects including PEDOT:PSS, PEDOT:PSS/ZnO and ZnO 15



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layer are incorporated into the devices with injection-blocking contacts denoted as device G, H and I, respectively i.e. the devices with the general structure of ITO/PEDOT:PSS(50 nm)/TFB(60 nm)/interconnect/BCP(50 nm)/CsF(1 nm)/Al to examine whether or not charges are generated.39 TFB layer is not inserted into device J and K with the PEDOT:PSS/ZnO and PEDOT:PSS interconnect to study the role of the TFB/PEDOT:PSS interface in the charge generation process. Device L without the addition of any interconnect has also been made for comparison. The summary of device structures in section 3.3 is presented in Table 2. Table 2. Summaries of the Device Structures in Section 3.3 Devices Structures G

ITO/PEDOT:PSS(50 nm)/TFB(60 nm)/PEDOT:PSS(60 nm)/PEIE(10 nm)/BCP(50 nm)/ CsF(1 nm)/Al H ITO/PEDOT:PSS(50 nm)/TFB(60 nm)/CGL/PEIE(10 nm)/BCP(50 nm)/CsF(1 nm)/Al I ITO/PEDOT:PSS(60 nm)/TFB(60 nm)/ZnO(30 nm)/PEIE(10 nm)/BCP(50 nm)/CsF(1 nm)/Al J ITO/CGL/PEIE(10 nm)/BCP(50 nm)/CsF(1 nm)/Al K ITO/PEDOT:PSS(60 nm)/PEIE(10 nm)/BCP(50 nm)/CsF(1 nm)/Al L ITO/PEDOT:PSS(50 nm)/TFB(60 nm)/BCP(50 nm)/Al M ITO/CGL/PEIE(10 nm)/BCP:10%CsF(50 nm)/CsF(1 nm)/Al CGL PEDOT:PSS(60 nm)/ZnO(30 nm)

As charge injection and transport in the devices under the reverse voltage (ITO electrode negatively biased) are frustrated, large current can be only originated from charge generation in the interconnects. Figure 5a shows the I-V characteristics of device G-L under the reverse voltage. Rather a small current (ca. 52 µA at 10 V) is measured for device L, proving that charge injection from the external electrodes is indeed greatly suppressed. In contrast, device H and J show ca. 200 times larger current (ca. 12.9 and 11.1 mA at 10 V for device H and J), whereas current of device G, I and K remains almost unaltered regardless of the presence of a TFB layer or not,

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indicating that charge generation only occurs in the PEDOT:PSS/ZnO hetero-structure. To ascertain the role of the TFB/PEDOT:PSS interface in the charge generation process, we have carried out the capacitance-voltage measurements of the double-insulating devices (Figure S8 Supporting Information). The results indicate that there are charge accumulation and displacement in the devices with the PEDOT:PSS/ZnO CGL regardless of whether a TFB layer is present or not.

0.1 0.01 1E-3

10

0.73 eV -12

0.1 0.01

2

1

Ln(J 0/T )

1

100

2

10


2

1000

TFB/CGL CGL TFB/PEDOT:PSS PEDOT:PSS TFB/ZnO none

100 Current density (mA/cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


110 K 170 K 230 K 288 K

-14

-16

4

6

8

10

-1

0

4

8

1E-3 0.8

12

1000/T(K )

1.0

1.2

1.4

1.6

1/2

Voltage (V )

Voltage (V)

(a)

(b)

Figure 5. I-V characteristics of the various interconnectors-based devices (ITO/PEDOT:PSS(50 50 nm)/TFB(60 nm)/PEDOT:PSS(60 nm)/PEIE(10 nm)/BCP(50 nm)/CsF(1 nm)/Al, ITO/PEDOT :PSS(50 nm)/TFB(60 nm)/PEDOT:PSS(60 nm)/ZnO(30 nm)/PEIE(10 nm)/BCP(50 nm)/CsF(1 nm )/Al, ITO/PEDOT:PSS(50 nm)/TFB(60 nm)/ZnO(30 nm)/PEIE(10 nm)/BCP(50 nm)/CsF(1 nm) /Al, ITO/PEDOT:PSS(60 nm)/ZnO(30 nm)/PEIE(10 nm)/BCP(50 nm)/CsF(1 nm)/Al, ITO/PEDOT:PSS(50 nm)/PEIE(10 nm)/BCP(50 nm)/CsF(1 nm)/Al) and devices without the addition of any interconnector (ITO/PEDOT:PSS(50 nm)/TFB(60 nm)/BCP(50 nm)/CsF(1nm) /Al) under the reverse voltage (a); J-V1/2 plots of device M (ITO/PEDOT:PSS(60 nm)/ZnO(30 nm) /PEIE(10 nm)/BCP:10%CsF(50 nm)/CsF(1 nm)/Al) (b) and lnJ0-1000/T curve in the inset of (b).

To elucidate the charge generation and injection process, we have measured

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temperature dependent CGL-generated current i.e. the I-V characteristics of device M with the structure of ITO/PEDOT:PSS(60 nm)/ZnO(30 nm)/PEIE(10 nm)/BCP:10% CsF(50 nm)/CsF(1 nm)/Al(100 nm) under different temperatures. In device M, CsF is used as n-type dopant for BCP40 to minimize the effect of BCP layer resistance on the I-V characteristics. The I-V characteristics are fitted with the Richardson-Schottky thermal emission model41 as follows: −‫ ܤ߮(ݍ‬− ඥ‫ܸݍ‬/4ߨεi ݀) J = A∗ T 2 ݁‫ ݌ݔ‬ቈ ቉ ‫ܶܭ‬

(1)

where A* is the effective Richardson constant, T the temperature, ߮‫ ܤ‬the barrier height at the interface, q the electronic charge, V the applied voltage, εi the dielectric permittivity of the organic layer, d the thickness of PEDOT:PSS and ZnO layer and K the Boltzmann constant. In Figure 5b, the ln J vs V1/2 characteristics are presented. Linear relationships in ln J vs V1/2 curves are observed under different temperatures and the current densities J0 at zero voltage are obtained. From the relationship between ln(J0/T2) vs 1000/T presented in the inset of Figure 5b, ߮‫ ܤ‬value can be determined to be 0.73 eV, which is in good agreement with the energetic difference of ca. 0.8 eV between the HOMO level of PEDOT:PSS and conduction band minimum of ZnO measured by ultraviolet photoelectron spectroscopy42. The exacted barrier height equals to the “effective band gap” of the ZnO/PEDOT:PSS hetero-structure, which

strengthens

the

conclusion

that

charge

generation

occurs

at

the

PEDOT:PSS/ZnO interface. We also fit the I-V characteristics with the Nordheim-Flower model43 and determine the barrier height in the range of 0.11-0.29

18


Page 19 of 28

eV, which is decreased by a factor of 2 with increasing temperature from 110 K to 288 K. (Figure S9, Supporting Information). We further investigate the effect of energy barrier height on the charge generation and injection process as shown in Figure S10, Supporting Information. The results exclude the hypothetical CGL operation mechanism that charges are generated inside PEDOT:PSS layer and ZnO works as electron injection layer. On the basis of the above measurements and modelings, the working mechanism for the PEDOT:PSS/ZnO CGL is depicted in Scheme 2. Under an electric field electrons on the HOMO level of PEDOT:PSS thermally hop and reach the conduction band of ZnO. The dissociation of electron-hole pairs leads to the generation of holes and electrons, which can be injected into the bottom and top light emitting unit with high efficiency due to the well-aligned frontier orbitals at the PEDOT:PSS/SY-PPV and ZnO/PEIE/SY-PPV or MEH-PPV interfaces27. ELUMO= -3.3 eV EHOMO= -5.3 eV

-

+

PEDOT:PSS

+ -

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ECB= -4.5eV Ef

ZnO

EVB= -7.8 eV

Scheme 2. The charge generation and injection process of the PEDOT:PSS/ZnO CGL，in which the energy levels of PEDOT:PSS and ZnO are from the reference 42.

4. CONCLUSIONS

In this study, we report that the configurations and compositions of interconnects significantly affect the performance of light emitting devices. Only the PEDOT:PSS/ZnO hetero-structure in combination with top and bottom electrode

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Page 20 of 28

serves as an effective hole and electron injection contact. Two/three-unit tandem organic light emitting devices with the PEDOT:PSS/ZnO CGL show the luminance efficiency matching the total luminance efficiency of the subelements. This is the first report of three-unit tandem organic light emitting devices with solution-processed CGL and polymer light emitting units. Current-voltage and capacitance-voltage measurements on various interconnects-based devices reveal that charge generation occurs at the PEDOT:PSS/ZnO interface. Overall, an appropriate model for charge generation in the PEDOT:PSS/ZnO CGL is presented as thermal emission of electrons from the HOMO level of PEDOT:PSS to the conduction band of ZnO.

ASSOCIATED CONTENT Supporting Information. Supporting Information is available from the Internet at http://pubs.acs.org. Characteristics of devices with different PEDOT: PSS specimen, influence of ethanolamine treatment on hole current, comparison of hole injection properties

of

ITO/PEDOT:PSS

and

V2O5/Al

contact,

capacitance-voltage

characteristics of the double insulating devices comprising different interconnects and the fit of the I-V curve with the Fowler-Nordheim model. (PDF) AUTHOR INFORMATION Corresponding Authors E-mail: [email protected] Notes The authors declare no competing financial interest. Author Contributions 20


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The manuscript was written by contributions of all authors. ACKNOWLEDGEMENT Financial support by the National Natural Science Foundation of China (Grant nos. 61177030 , 11474232 and 11374242) is acknowledged.

REFERENCES (1) Friend, R. H.; Gymer, R. W.; Holmes, A. B.; Burroughes, J. H.; Marks, R. N.; Taliani, C.; Bradley, D. D. C.; Santos, D. A. D.; Das, J. L. B. A.; Gdlund, M. L. E.; et al. Electroluminescence in conjugated polymers. Nature 1999, 394, 121-128. (2) Berner, D.; Houili, H.; Leo, W.; Zuppiroli, L. Insights into OLED functioning through

coordinated

experimental

measurements

and

numerical

model

simulations. Phys. Status. Solidi. A 2005, 202, 9-36. (3) Ishii, M.; Taga, Y. Influence of temperature and drive current on degradation mechanisms in organic light-emitting diodes. Appl. Phys. Lett. 2002, 80, 3430-3432. (4) Ke, L.; Chua, S.-J.; Zhang, K.; Yakovlev, N. Degradation and failure of organic light-emitting devices. Appl. Phys. Lett. 2002, 80, 2195-2197. (5) Kido, J.; Matsumoto, T.; Nakada, T.; Endo, J.; Mori, K.; Kawamura, N.; Yokoi, A. High efficiency organic EL devices having charge generation Layers. SID.Int.

21



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Symp .Dig. Tech 2003, 34, 964-965. (6) Liao, L. S.; Klubek, K. P.; Tang, C. W. High-efficiency tandem organic light-emitting diodes. Appl. Phys. Lett. 2004, 84, 167-169. (7) Cho, T.-Y.; Lin, C.-L.; Wu, C.-C. Microcavity two-unit tandem organic light-emitting devices having a high efficiency. Appl. Phys. Lett. 2006, 88, 111106. (8) Schwab, T.; Schubert, S.; Hofmann, S.; Fröbel, M.; Fuchs, C.; Thomschke, M.; Müller-Meskamp, L.; Leo, K.; Gather, M. C. Highly efficient color stable inverted white top-emitting OLEDs with ultra-thin wetting layer top electrodes. Adv. Opt. Mater. 2013, 1, 707-713. (9) Liao, L. S.; Slusarek, W. K.; Hatwar, T. K.; Ricks, M. L.; Comfort, D. L. Tandem organic light-emitting diode using hexaazatriphenylene hexacarbonitrile in the intermediate connector. Adv. Mater. 2008, 20, 324-329. (10) Wu, Y. L.; Chen, C. Y.; Huang, Y. H.; Lu, Y. J.; Chou, C. H.; Wu, C. C. Highly efficient tandem organic light-emitting devices utilizing the connecting structure based on n-doped electron-transport layer/HATCN/hole-transport layer. Appl. Opt. 2014, 53, E1-6. (11) Guo, F. W.; Ma, D. G. White organic light-emitting diodes based on tandem structures. Appl. Phys. Lett. 2005, 87, 173510. (12) Bao, Q. Y.; Yang, J. P.; Li, Y. Q.; Tang, J. X. Electronic structures of MoO3-based charge generation layer for tandem organic light-emitting diodes. Appl. Phys. Lett. 2010, 97, 063303.

22


Page 22 of 28

Page 23 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(13) Hamwi, S.; Meyer, J.; Kröger, M.; Winkler, T.; Witte, M.; Riedl, T.; Kahn, A.; Kowalsky, W. The role of transition metal oxides in charge-generation layers for stacked organic light-emitting diodes. Adv. Funct. Mater. 2010, 20, 1762-1766. (14) Chen, Y. H.; Ma, D. G. Organic semiconductor heterojunctions as charge generation layers and their application in tandem organic light-emitting diodes for high power efficiency. J. Mater. Chem. 2012, 22, 18718-18734. (15) Lai, S. L.; Chan, M. Y.; Fung, M. K.; Lee, C. S.; Lee, S. T. Copper hexadecafluorophthalocyanine and copper phthalocyanine as a pure organic connecting unit in blue tandem organic light-emitting devices. J. Appl. Phys. 2007, 101, 014509. (16) Sun, J. X.; Zhu, X. L.; Peng, H. J.; Wong, M.; Kwok, H. S. Effective intermediate layers for highly efficient stacked organic light-emitting devices. Appl. Phys. Lett. 2005, 87, 093504. (17) Zhang, H. M.; Dai, Y. F.; Ma, D. G. High efficiency tandem organic light-emitting devices with Al/WO3/Au interconnecting layer. Appl. Phys. Lett. 2007, 91, 123504. (18) Kröger, M.; Hamwi, S.; Meyer, J.; Dobbertin, T.; Riedl, T.; Kowalsky, W.; Johannes, H.-H. Temperature-independent field-induced charge separation at doped organic/organic interfaces: Experimental modeling of electrical properties. Phy. Rev. B. 2007, 75, 235321. (19) Hong, K.; Lee, J.-L. Charge generation mechanism of metal oxide interconnection in tandem organic light emitting diodes. J. Phys. Chem. C. 2012,

23



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

116, 6427-6433. (20) Qi, X. F.; Li, N.; Forrest, R. S. Analysis of metal-oxide-based charge generation layers used in stacked organic light-emitting diodes. J. Appl. Phys. 2010, 107, 014514. (21) Yang, J.-P.; Xiao, Y.; Deng, Y.-H.; Duhm, S.; Ueno, N.; Lee, S.-T.; Li, Y.-Q.; Tang, J.-X. Electric-field-assisted charge generation and separation process in transition metal oxide-based interconnectors for tandem organic light-emitting diodes. Adv. Funct. Mater. 2012, 22, 600-608. (22) Höfle, S.; Schienle, A.; Bernhard, C.; Bruns, M.; Lemmer, U.; Colsmann, A. Solution pocessed, white emitting tandem organic light-emitting diodes with inverted device architecture. Adv. Mater. 2014, 26, 5155-5159. (23) Höfle, S.; Bernhard, C.; Bruns, M.; Kubel, C.; Scherer, T.; Lemmer, U.; Colsmann, A. Charge generation layers for solution processed tandem organic light emitting diodes with regular device architecture. ACS. Appl. Mater. Inter. 2015, 7, 8132-8137. (24) Chiba, T.; Pu, Y. J.; Kido, J. Solution-processed white phosphorescent tandem organic light-emitting devices. Adv. Mater. 2015, 27, 4681-4687. (25) Kim, J. Y.; Lee, K.; Coates, N. E.; Moses, D.; Nguyen, T.-Q.; Dante, M.; Heeger, A. J. Efficient tandem polymer solar cells fabricated by all-solution processing. Science 2007, 317, 222-225. (26) Yang, J.; Zhu, R.; Hong, Z.; He, Y.; Kumar, A.; Li, Y.; Yang, Y. A robust inter-connecting layer for achieving high performance tandem polymer solar cells.

24


Page 24 of 28

Page 25 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Adv. Mater. 2011, 23, 3465-3470. (27) Wang, R. X.; Fan, C. Q.; Xiong, Z. H.; Yang, X. H.; Jabbour, G. E. High-efficiency hybrid organic–inorganic light-emitting devices. Org. Electron. 2015, 19, 105-112. (28) Fan, C. J.; Lei, Y.; Liu, Z.; Wang, R. X.; Lei, Y. L.; Li, G. Q.; Xiong, Z. H.; Yang, X. H. High-efficiency phosphorescent hybrid organic-inorganic light-emitting diodes using a solution-processed small-molecule emissive layer. ACS. Appl. Mater. Inter. 2015, 7, 20769-20778. (29) Sun, Y. M.; Seo, J. H.; Takacs, C. J.; Seifter, J.; Heeger, A. J. Inverted polymer solar cells integrated with a low-temperature-annealed sol-gel-derived ZnO film as an electron transport layer. Adv. Mater. 2011, 23, 1679-1683. (30) Hancox, I.; Rochford, L. A.; Clare, D.; Walker, M.; Mudd, J. J.; Sullivan, P.; Schumann, S.; McConville, C. F.; Jones, T. S. Optimization of a high work function solution processed vanadium oxide hole-extracting layer for small molecule and polymer organic photovoltaic cells. J. Phys. Chem. C. 2013, 117, 49-57. (31) Wang, J.; Zhang, H.; Ji, W.; Zhang, H. Efficient quantum dot light emitting devices with ethanol treated PEDOT: PSS hole injection layer. Synth. Met. 2015, 209, 484-489. (32) Huang, J.; Miller, P. F.; de Mello, J. C.; de Mello, A. J.; Bradley, D. D. C. Influence of thermal treatment on the conductivity and morphology of PEDOT/PSS films. Synth. Met. 2003, 139, 569-572.

25



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 28

(33) X. Crispin; F. L. E. Jakobsson; A. Crispin; P. C. M. Grim; P. Andersson; A. Volodin; C. Van Haesendonck; M. Van der Auweraer; W. R. Salaneck; Berggren, M.

The

origin

of

the

high

conductivity

of

poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT-PSS) plastic electrodes. Chem. Mater. 2006, 18, 4354-4360. (34) Abbaszadeh, D.; Wetzelaer, G. A. H.; Nicolai, H. T.; Blom, P. W. M. Exciton quenching at PEDOT:PSS anode in polymer blue-light-emitting diodes. J. Appl. Phys. 2014, 116, 224508. (35) Liang, Z.; Zhang, Q.; Jiang, L.; Cao, G. ZnO cathode buffer layers for inverted polymer solar cells. Energy Environ. Sci. 2015, 8, 3442-3476. (36) Fabiano, S.; Braun, S.; Liu, X.; Weverberghs, E.; Gerbaux, P.; Fahlman, M.; Berggren, M.; Crispin, X. Poly(ethylene imine) impurities induce n-doping reaction in organic (semi)conductors. Adv. Mater. 2014, 26, 6000-6006. (37) Shi, Q.; Hou, Y.; Lu, J.; Jin, H.; Li, Y.; Li, Y.; Sun, X.; Liu, J. Enhancement of carrier mobility in MEH-PPV film prepared under presence of electric field. Chem. Phys. Lett. 2006, 425, 353-355. (38) Pu, Y. J.; Chiba, T.; Ideta, K.; Takahashi, S.; Aizawa, N.; Hikichi, T.; Kido, J. Fabrication of organic light-emitting devices comprising stacked light-emitting units by solution-based processes. Adv. Mater. 2015, 27, 1327-1332. (39) Tsutsui, T.; Terai, M. Electric field-assisted bipolar charge spouting in organic thin-film diodes. Appl. Phys. Lett. 2003, 84, 440-442. (40) Deng, Y.-H.; Li, Y.-Q.; Ou, Q.-D.; Wang, Q.-K.; Sun, F.-Z.; Chen, X.-Y.; Tang,

26


Page 27 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


J.-X. The doping effect of cesium-based compounds on carrier transport and operational stability in organic light-emitting diodes. Org. Electron. 2014, 15, 1215-1221. (41) Gao, W. Y.; Kahn, A. Controlled p-doping of the hole-transport molecular material N,N ′- diphenyl - N,N ′- bis(1- naphthyl )-1,1′- biphenyl -4,4′- diamine with tetrafluorotetracyanoquinodimethane. J. Appl. Phys. 2003, 94, 359-366. (42) Lin, P.; Yan, X.; Zhang, Z.; Shen, Y.; Zhao, Y.; Bai, Z.; Zhang, Y. Self-powered UV

photosensor

based

on

PEDOT:PSS/ZnO

micro/nanowire

with

strain-modulated photoresponse. ACS. Appl. Mater. Inter. 2013, 5, 3671-3676. (43) Parker, I. D. Carrier tunneling and device characteristics in polymer light-emitting diodes. J. Appl. Phys. 1994, 75, 1656-1666.

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TOC GRAPHIC

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Metal Oxide Charge

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