Polymer charge-transfer complexes for opto-electronic applications

Synthetic Metals 159 (2009) 1438–1442

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Polymer charge-transfer complexes for opto-electronic applications Sanchao Liu ∗ , Jianmin Shi, Eric W. Forsythe, Steven M. Blomquist, Dave Chiu U.S. Army Research Laboratory, 2800 Powder Mill Road, Adelphi, MD 20783, United States

a r t i c l e

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Article history: Received 18 December 2008 Received in revised form 17 March 2009 Accepted 25 March 2009 Available online 15 May 2009 Keywords: Charge-transfer complexes Polymer/TCNQ complexes Polycarbazole

a b s t r a c t The formation of charge-transfer (CT) complex to increase the conductivity has been the subject of intense research activity for the past decades. Those CT complexes have been used as organic semiconductors in field effect transistors (FETs), charge injection and transport materials in organic light-emitting diodes (OLEDs) and organic photovoltaic (OPV) cells. In this paper, a serials of new CT complexes with polymers as donor and TCNQ as acceptor were prepared. The polymers are polycarbazoles with various content of carbazole moiety in the back chain. The X-ray crystal structure of the model compound 4,4 -bis (N-carbazolyl)-1,1 -biphenyl(CBP)/TCNQ complex showed the formation of 2:1 stack structure (with 1:1 carbazole moiety: TCNQ ratio). The polycarbazole/TCNQ complexes form uniform films by spin-coating. Devices with the structure of ITO/polycarbazole:TCNQ complex/Mg:Ag were fabricated. The current–voltage characteristics showed that the devices exhibit much higher conductivity compared to their analogy ITO/polycarbazole/Mg:Ag structure devices. Devices with different polycarbazole:TCNQ ratios were fabricated and the current–voltage results showed that the conductivity increases as the ratio of polycarbazole:TCNQ increases. The conductivity reaches the maximum at the ratio of 1:1. These polymer complexes can be low-temperature processed on large area flexible substrates and are of potential use for low-cost printed electronics. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Since the first synthesis of metallic charge-transfer (CT) complex of tetrathiafulvalene (TTF)/tetracyanoquinodimethane (TCNQ), many researchers have focused on the study of highly conducting organic CT complexes [1–4]. In the past decades, these organic charge-transfer materials have been the subject of intense research activity. They have been used as organic semiconductors in field effect transistors (FETs), charge injection and transport materials in organic light-emitting diodes (OLEDs) and organic photovoltaic (OPV) cells [5–9]. Takahashi’s group [10–11] and Shibata’s group [12] reported the use of TTF/TCNQ derivative CT complexes as source and drain electrodes in their organic FETs to increase the efficiencies in the carrier injections and to reduce the contact resistance. In OLED, a light doping of organic layer can increase the charge mobility, decrease the operation voltage and increase the device efficiency [13]. Blochwitz et al. [14] and Zhou et al. [15] used a tetrafluoro-TCNQ doped hole transporting layer composed of arylamine in an OLED and found that the device resulted in excellent electroluminescence performance with stable hole injection, reduced driving voltage and enhanced device stability. However, in all the above studies, the CT complexes layers were formed by vac-

∗ Corresponding author. E-mail addresses: [email protected], [email protected] (S. Liu). 0379-6779/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2009.03.030

uum deposition, and it is very difficult for the vapor co-doped CT complex film formation to be controlled precisely in device fabrication due to the high vapor pressure of acceptor TCNQs caused by its relatively small molecular size. This drawback has greatly limited its use in commercial applications. To accommodate future request for low-cost, large area electronic devices, it is desirable to fabricate organic electronic devices using simple deposition techniques such as spin-coating and ink-jet printing. Polymer CT complexes have advantages over traditional small molecule CT complexes in that they are easily dissolved in most organic solvents and thus make them suitable for solventbased processes, making large area fabrication feasible with less processing steps and at lower cost. Polymer CT complexes have been studied widely [16–19]. Among them, polyvinylcarbazole (PVK) was the most studied polymer as electron donor host materials. With good photoconductivity properties, PVK forms CT complexes with many electron acceptors that are characterized by high photo-electric sensitivity in the visible region and are used in electrophotography, memory device and potentially solar cells [20–22]. PVK is also used as host polymer for hole-transporting molecules in OLED to enhance the native hole-transporting properties of the matrix. Studies show that the formation of PVK CT complex increases the conductivity largely [23–24]. However, in the case of the formation of PVK complex with TCNQ, the most widely used electron acceptor, the highest ratio of TCNQ:PVK in the complex formed in solution is only 1:6 mole ratio due to insuffi-

S. Liu et al. / Synthetic Metals 159 (2009) 1438–1442

cient space between the carbazole side chains on the PVK for the TCNQ molecules to be inserted into them [23]. Furthermore, in most of the device applications using this class of the polymer CT complexes, the percentage of the electron acceptors such as TCNQ is typically below 10% [13–15,17–19]. The optimized acceptor concentration in charge complex was determined empirically, but no clear reason was given based on molecular structure information. In designing these polymer charge complexes, it is critical to have the charge complex structure information, which will provide guidance to design more efficient organic semiconductors to be used in many types of electronic device applications. In this paper, we would like to report our studies on carbazole/TCNQ class of CT molecular structure by synthesis of 4,4 -bis(9-carbazolyl)-1,1 -biphenyl (CBP)/TCNQ charge complex. Based on the CT molecular structure obtained from its X-ray spectra, a series of novel conducting organic materials based on polycarbazole/TCNQ complexes were synthesized. A device with an ITO/polycarbazole:TCNQ/Mg:Ag sandwich-like structure was fabricated and the device was characterized by current–voltage characteristics. 2. Experimental 2.1. Materials TCNQ was purchased from Aldrich and sublimated at 145 ◦ C. All the other materials are purchased from Aldrich without further purification. The structures of the molecules used are shown in Fig. 1.

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solvent. One example is as follow: PCEA solution (132 mg/10 ml) in DCE was added into hot TCNQ solution (100 mg/30 ml) according to 1:1 molar ratio of repeating units on PCEA:TCNQ. The mixture immediately changed to dark green color. The mixture was stirred at refluxing temperature for one hour and then slowly cooled down to room temperature. 2.3. Device fabrication Polymer/TCNQ complexes in DCE (10 mg/ml) were spin-coated on ITO patterned glass substrates at 1000 rpm for 60 s. The films were baked at 70 ◦ C/15 min at atmosphere. The Mg:Ag cathode was deposited in a bell-jar vacuum coater (10−6 Torr). The deposition rate was 10 Å/s with an Mg to Ag ratio of 10:1. 2.4. Crystal structure determination of CT complex CBP/TCNQ complex is obtained by mixing solution of CBP and TCNQ in dichloromethane. After slow evaporation of solvent, shiny black needle crystals appeared. The X-ray diffraction measurements were performed on a Bruker SMART 1000 CCD diffractometer (graphite monochromator, MoK␣ radiation,  = 0.71073 Å) at Chemistry and Biochemistry Department, University of Maryland. [CCDC 713342 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/ data request/cif.]

2.2. Synthesis of CT complexes

2.5. Elemental analyses

The TCNQ complexes were made by mixing hot solutions of the polymer and TCNQ in dichloroethane (DCE), and evaporating the

The elemental analyses of the CT complexes were measured by Schwarzkopf Microanalytical Laboratory, Woodside, NY.

Fig. 1. The molecular structures of the polymer donors. PCEA: poly(9H-carbazole-9-ethyl acrylate); PCEMA: poly(9H-carbazole-9-ethyl methacrylate); cPCEA: poly[(methyl methacrylate)-co-(9-H-carbazole-9-ethyl acrylate)], 25% PCEA; cPCEMA: poly[(methyl methacrylate)-co-(9-H-carbazole-9-ethyl methacrylate)], 15% PCEMA.

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2.6. The current density–voltage measurements The current–voltage curves of the device were measured with a Keithley 2400 sourcemeter. 3. Results and discussion

Fig. 2. X-ray crystal structure of CBP/TCNQ complex showing (a) the chemical structures of CBP and TCNQ; (b) the packing projected along the b axis; (c) the packing projected along the a axis.

To study the formation of CT complex between polycarbazole and TCNQ, CBP was used as the model compound. Single crystals of CBP/TCNQ complex were grown by slow evaporation of solvent. X-ray structure analysis was carried out on the CBP/TCNQ complex and the results were shown in Fig. 2. According to X-ray structure determination, the donor molecule is not planar. The plane of two carbazole moieties is perpendicular to the plane of the two central phenyl rings. The donor molecules are stacked in a plane-to-plane fashion in which the adjacent layer forms interleaved parallel stacks with the carbazole moieties parallel to each other. The acceptor (TCNQ) molecules form sandwiched stacking between two carbazole moieties. It is clearly indicated from the X-ray structure that the CT complex formed has a 1:1 donor (carbazole moiety) to acceptor (TCNQ) mole ratio. Based on the X-ray results, we chose polycarbazoles as the donor molecules for our study because it is capable of forming 1:1 mole ratio complex with TCNQ. The structures of the donor molecules are shown in Fig. 1. Those molecules have electron-donor groups, i.e., carbazole moieties, on the side chains. Comparing to PVK, these donors are with extra ethyl groups and ester bonds to introduce flexibility to the side chain to stabilize the CT complexes. The TCNQ acceptor molecule can then be inserted between two adjacent donor molecules for increased complex stability. Previous study by Litt et al. showed that the polymer side chain complexes have up to 50 times higher equilibrium constants than those of the corresponding small molecule model complexes [25]. Thus, polycarbazole/TCNQ complexes can be formed at high ratio with high stability. The polymer:TCNQ complexes were synthesized by mixing correspondence equivalent of polymer and TCNQ in dichloroethane. The results of elementary analysis of the CT complexes are listed in Table 1. Elemental analysis confirms the formation of the polymer/TCNQ complexes at the expected mole ratio. To study the conductivity of the polymer CT complexes formed, the complexes were spin-coated on a clean glass substrate pre-coated with indium tin oxide (ITO). The thickness of the organic film is around 50–70 nm. Then, a layer of 2000 Å Mg/Ag was co-deposited as the cathode to form the ITO/polymer:TCNQ/Mg:Ag device structure.

Table 1 Elemental analysis of polymer/TCNQ complexes. Sample abbreviation

Donor:TCNQ

Formula

Elemental analysis (%) C

H

N

CBP/TCNQ

2:1

C60 H32 N10

Calcd. Found

80.70 80.21

3.61 3.61

15.68 15.87

PCEA/TCNQ

1:1

C29 H19 N5 O2

2:1

C46 H34 N6 O4

5:1

C97 H79 N9 O10

Calcd. Found Calcd. Found Calcd. Found

74.18 74.06 75.19 73.87 76.11 74.11

4.08 4.29 4.66 4.84 5.20 5.31

14.92 14.08 11.44 10.94 8.23 8.07

PCEMA/TCNQ

1:1

C30 H21 N5 O2

Calcd. Found

74.52 73.43

4.38 4.59

14.48 14.01

cPCEA/TCNQ

1:1

C44 H43 N5 O8

Calcd. Found

68.64 66.06

5.63 5.99

9.10 8.16

cPCEMA/TCNQ

1:1

C58.3 H66.3 N5 O13.3

Calcd. Found

66.66 63.69

6.36 6.40

6.66 6.66

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Table 2 Current density of polymers and their 1:1 polymer:TCNQ complexes at 20 V.

Fig. 3. J–V characteristics of ITO/cPCEA:TCNQ/Mg:Ag devices with various cPCEA:TCNQ ratios: cPCEA:TCNQ = 1:0, 10:1, 5:1, 2:1, 1:1.

First, we used one polymer, cPCEA, and checked its complex with TCNQ at different ratio. Fig. 3 shows the current density–voltage (J–V) characteristic of cPCEA/TCNQ complexes at various complexing ratio, namely, 1:0, 10:1, 5:1, 2:1 and 1:1. The CT complexes at all the ratios formed uniform films. However, when a cPCEA:TCNQ ratio of 1:2 was tried, the spin-coated layer was opaque after being oven-baked and yellow particles appeared on the surface, resulting in a poor J–V curve. The results from Fig. 3 indicated that the formation of CT complexes increases the electrical conductivity of the devices, and as the ratio of TCNQ increases, i.e., as the ratio of CT complex formed inside the polymer increases, the current density increases and reaches maximum at the 1:1 ratio where the donor and acceptor formed a 1:1 CT complex. For example, the current density at 20 V for polymer cPCEA is only 3 mA/cm2 . However, for the 1:1 complex with TCNQ, it reaches 154 mA/cm2 , with more than 50 times increase. Fig. 3 also shows that the J–V characteristic of device with 1:1 CT complexes is nearly symmetric and the current was observed in both forward and reverse bias mode. To further confirm our conclusion on the effect of the mole content of the CT complex on the conductivity, three more poly-

Polymer

Mole % of carbazole moiety

PCEA PCEMA cPCEA cPCEMA

100 97 15 7

Current density (mA/cm2 ) Polymer

Complex

23 22 2.7 2.6

279 246 154 49

mers with different percentage of carbazole moiety on the polymer chain were chosen for the comparative study. PCEA and PCEMA are homopolymers of 9H-carbazole-9-ethyl acrylate and 9H-carbazole9-ethyl methacrylate, which possess one carbazole donor on each repeating unit, and thus can form the maximum 1:1 CT complexes on the polymer chains. On the other hand, cPCEA is a copolymer of PCEA with poly(methyl methacrylate) (PMMA), which has only 25 mol% of PCEA inside the polymer chain and thus can only form a maximum of 25 mol% CT complex on each polymer chain. For cPCEMA, a copolymer of 15 mol% PCEMA and PMMA, the least amount of CT complexes is formed inside the polymer. We used the fixed weight percentage of polymer in the device, but the polymers have different molecular weight. The mole ratio of the polymer molecule is inversely proportional to the molecular weight. So when we consider the mole content of the carbazole moiety in each polymer, the mole ratio of the carbazole moiety in the polymers ended up being 100% for PCEA, 97% for PCEMA, 15% for cPCEA, and 7% for cPCEMA. The J–V characteristic of devices with different polymer:TCNQ complexes, all at the1:1 ratio is showed in Fig. 4. To clearly show the change of conductivity before and after formation of CT complexes and the effect of the mole content of the CT complexes on conductivity, the current density of polymers with different mole content of the donor and their 1:1 polymer:TCNQ complexes at 20 V was listed in Table 2. From Table 2 we can see that there is a big increase of current density (up to 50 times) after formation of CT complexes for all polymers investigated. As the mole percentage of carbazole moiety inside the polymer increases, i.e. the mole percentage of CT complexes formed increases, the current density increases. Both Table 2 and Fig. 4 clearly show that the conductivity of the device increases with the increase of the carbazole-containing comonomer in the copolymer, and is the highest with the homopolymer. These results draw the conclusion that the best method to increase the conductivity of polymer CT complexes is to increase the percentage of CT complexes formed inside a polymer. It is noticed that the annealing of the devices after fabrication would further increase the conductivity of the device significantly through changing the microstructure of the CT complexes inside the device. Results of this detail study will be reported in the near future. 4. Summary

Fig. 4. J–V characteristics of ITO/polymer:TCNQ (1:1)/Mg:Ag devices for different polymers with various percentage of donor inside the polymers.

It is demonstrated in our study that polycarbazoles can form stable CT complexes with TCNQ and the complexes have good film forming properties. X-ray crystal study on model compound CBP/TCNQ showed the formation of 1:1 donor:acceptor CT complex. Characterization of devices with an ITO/polymer:TCNQ/Mg:Ag structure indicates that the CT complexes containing devices have much higher conductivity comparing to their analogs with polymers only. The conductivity of the device increases as the amount of CT complex formed inside the polymer increases and reaches maximum at the 1:1 donor:acceptor ratio. The combination of the CT complex structure information, relationship between conductivity of polymer CT complex and ratio of CT complex in polymer matrix obtained from this research could provide a guideline for

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