Carrier Transport in Zinc Phthalocyanine Doped ... - ACS Publications

For transport mechanism study, we have focused on both interface and bulk effects ... (c) Three layered device structure for carrier injection investi...
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Carrier Transport in Zinc Phthalocyanine Doped with a Fluorinated Perylene Derivative: Bulk Conductivity versus Interfacial Injection Lin Chen,† Ligong Yang,*,‡ Zhisheng Yang,† Minmin Shi,† Mang Wang,† Hongzheng Chen,*,†,‡ Wenhua Zhang,§ and Faqiang Xu§ Department of Polymer Science and Engineering, Key Laboratory of Macromolecule Synthesis and Functionalization of Ministry of Education, State Key Lab of Silicon Materials, Zhejiang UniVersity, Hangzhou 310027, People’s Republic of China, Zhejiang-California International Nanosystems Institute, Zhejiang UniVersity, Hangzhou 310027, People’s Republic of China, and National Synchrotron Radiation Laboratory, UniVersity of Science and Technology of China, Hefei 230029, People’s Republic of China ReceiVed: April 13, 2009; ReVised Manuscript ReceiVed: July 28, 2009

P-doping of zinc phthalocyanine (ZnPc) with a perylene derivative of N,N′-di-(2,4-difluorophenyl)-3,4,9,10perylenetetracarboxylic diimide (D24DFPP) is investigated for wide range of doping ratios. I-V measurements in air for the three-layered devices [Au(40 nm)/organic(400 nm)/Au(35 nm)] show a 4 orders of magnitude increment in current density after doping. Ultraviolet and X-ray photoemission spectroscopy (XPS and UPS) studies confirm that the efficient host-to-dopant electron transfer is the primary reason for this increment. Carrier transport is found to be more efficient in the lightly doped film than in the heavily doped film for our D24DFPP-doped ZnPc system. Morphology study and the detailed modeling analysis attribute this to both bulk conductivity and interfacial injection effect. The lower injection barrier and better energy level alignment with the electrode in the lightly doped ZnPc film facilitate the field-emission tunneling process, leading to a more efficient carrier injection compared with the heavily doped film. For the heavily doped film, the inferior transporting performance is also correlated with its disrupted crystalline structure and more severe phase separation. Introduction Organic semiconductors have attracted increasing attention because of their unique physical properties such as the extremely high absorption coefficients, the excellent compatibility with flexible substrates, and the much lower processing cost for largearea production. So far, organic semiconductors have been successfully applied in commercially available organic lightemitting devices (OLED).1,2 Their applications in photovoltaic devices (OPV) and thin film transistors (OTFT) have also gained great progress.3-5 However, owing to the lower cohesive energy of organic molecules, carrier motion is usually carried out using an inefficient hopping mode. Therefore, poor carrier transport is always a primary limit for fabricating high performance organic optoelectronic devices.6 Various methods have been tried to solve this problem, including molecular tailoring and ordered nanostructure fabrication.7-9 Among these methods, electrical doping, which is a crucial technique in silicon based semiconductor industry, has been proven to be an efficient route to improve carrier transport in organic semiconductors. By doping with strong π-electron donors or acceptors, the conductivity of the matrix n-type or p-type organic materials has been found to increase dramatically with respect to their intrinsic states.10-13 Leo and his co-workers have systematically studied the influence of doping on the Fermi level and carrier density * Corresponding authors. E-mail: [email protected] (H.C.) and [email protected] (L.Y.). † Department of Polymer Science and Engineering, Key Laboratory of Macromolecule Synthesis and Functionalization of Ministry of Education, State Key Lab of Silicon Materials, Zhejiang University. ‡ Zhejiang-California International Nanosystems Institute, Zhejiang University. § University of Science and Technology of China.

of organic semiconductors and discovered that the improved conductivity is related to the Fermi level movement toward the transport state after doping.14 An enhancement of carrier injection via tunneling through space charge regions was also pointed out by Kahn et al. as one of the reasons to account for the improved carrier transport.15 Apart from these investigations on the doping mechanism, electrically doped organic transport layers have also been successfully applied in both OLED and OPV devices.16-18 Most of the inspiring work as mentioned above is carried out in lightly doped films ranging from 0.01% to ∼3% (molar ratio). However, since the molecular interaction between organic matrix and dopant is too weak for complete dopant ionization, which is the case for the inorganic atom doping system, a higher level of doping ratio is often needed to improve the doping effect for device application.19 Moreover, film morphology and crystalline structure vary sensitively with doping levels, making the transport behavior of organic materials much more complicated.20 Therefore, a systematic study of transport mechanism in organic semiconductors over a large scaled doping ratio is an urgent requirement both practically and theoretically. In addition, the organic materials which could be used as dopant are still very limited. For example, orthochloranil, dicyanodichloroquinone (DDQ) and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) are hitherto the only reported acceptor materials.6 Hence, the search for new kinds of efficient and low cost dopant material is necessary. Here, we report our investigation on the transport mechanism of zinc phthalocyanine (ZnPc), a well-known p-type semiconductor widely used in optoelectronic devices,21,22 doped by a new type of acceptor, N,N′-di-(2,4-difluorophenyl)-3,4,9,10perylenetetracarboxylic diimide (D24DFPP). The fluorinated

10.1021/jp903381n CCC: $40.75  2009 American Chemical Society Published on Web 09/02/2009

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Figure 1. (a) Molecular structures of the matrix and the dopant materials. (b) Coplanar device structure for bulk conductivity measurement. (c) Three layered device structure for carrier injection investigation.

perylene derivative D24DFPP has recently been found to be a typical n-type semiconductor with fairly good mobility and airstability.23 Moreover, D24DFPP shows a lower LUMO level than its unfluorinated precursor, which renders it better electron attracting characteristic as a potential p-type dopant. For transport mechanism study, we have focused on both interface and bulk effects corresponding to an extended doping ratio (from lower to higher doping levels), which is seldom reported in former researches. Experimental Section The molecular structures of our matrix and dopant materials are shown in Figure 1a. ZnPc was purchased from SigmaAldrich. D24DFPP was synthesized in our lab using a reported method.23 ZnPc and D24DFPP were purified via three cycles of gradient sublimation before use. For conductivity measurements and aggregate structure characterizations, films were grown in a high vacuum chamber (about 10-6 Torr). ZnPc and D24DFPP compounds were coevaporated from fused silica crucibles onto precleaned fused silica substrates with the growth rates of 10-40 Å/min for ZnPc and 1-10 Å/min for D24DFPP to obtain the molar doping ratios of 3%, 10%, 20%, and 30%. The thickness of the organic layer was kept constant at 150 nm. Two pieces of evaporated gold contacts with a 3 mm distance were predeposited onto the substrates for conductivity measurements. The device structure is shown in Figure 1b. Because of the large contact distance, the observed currents are bulk limited and therefore show an ohmic behavior. We have used several methods to get insight into the morphology of the doped films namely UV-vis absorption spectrum (UV-vis, Varian Cary bio100 spectrometer), atomic force microscopy (AFM, DI instrument Nanoscope 3D Multimode SPM, Contact Mode) and X-ray diffraction (XRD, Rigaku D/max diffractometer with Cu KR radiation). The photoemission spectroscopy (PES) experiments, including ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS), were carried out using the synchrotron radiation light sources. All experiments were performed in an ultrahigh vacuum (UHV) system consisting of three interconnected chambers equipped for film growth (10-7 Torr), sample delivery (10-9 Torr), and surface/interface analysis (10-10 Torr). The substrates for all samples were highly doped Si (100) wafers precoated with 60 nm polycrystalline Au films. The organic layer was deposited step by step in order to

sequentially vary the layer thickness, with transferring to the analyzer chamber at each growth step for UPS and XPS characterizations. The deposition rate was in accordance with the above-mentioned experiments. For the doped organic layers, the doping ratios of 3% and 20% were chosen as light and heavy doping cases to be studied. The electronic structure of the organic/Au interface was measured by UPS using 32 eV synchrotron radiation source with -7 V bias voltage. Vacuum level shift was determined from the low-kinetic energy onset (secondary electron cutoff), which was obtained by the energy-axis intersection of a straight line fitted to the slope of the spectra. The Fermi energy positions EF of the conductive substrates were obtained as the center of the Fermi edge slopes. Sample work function (Φ) was calculated using the equation of Φ ) hν - W, where W is the spectrum width (the energy difference between the substrate Fermi level and the low kinetic energy onset).24 XPS measurements were carried out with a 1253.7 eV Mg KR radiation at the typical pass energy of 50 eV, the peak positions and widths were determined by fitting the data with Gaussian/Lorentzian (70:30) line shapes. The binding energies of all PES spectra were normalized to the Fermi level of the electron energy analyzer. The intensities of all spectra were normalized to the total incoming photon flux measured with an amperemeter. For PES data, both compounds displayed excellent stability under photon and electron irradiation in the testing environment at room temperature. To investigate the interfacial effect on charge injection into the doped ZnPc films, three-layered devices [Au(40 nm)/ organic(400 nm)/Au(35 nm)] were also prepared (Figure 1c). The top Au electrode was evaporated through a shadow mask. The entire thickness was monitored during the deposition with quartz crystal microbalance and corrected by ellipsometry. All I-V measurements were carried out in atmosphere at room temperature right after the devices were prepared, using Agilent Technologies 4155C semiconductor parameter analyzer. Results and Discussion Conductivity of D24DFPP-Doped ZnPc Films. Aggregate Structure Controlled Bulk Transport. Bulk conductivity (σ) of pure and D24DFPP-doped ZnPc films was calculated from I-V measurements of the coplanar devices (structure shown in Figure 1b). Figure 2a shows variation with increasing the doping ratio. It is obvious from Figure 2a that a more than 2 orders of magnitude increment of σ is obtained after the introduction of

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Figure 3. UV-vis absorption spectra of pure and D24DFPP-doped ZnPc films with different molar doping ratios. Inset: absorption spectrum of pure D24DFPP film.

Figure 2. (a) Bulk conductivity of D24DFPP-doped ZnPc films with different doing ratios. (b) The conductivity of pure ZnPc films with increasing time exposed to air during the test. (c) The conductivity of doped ZnPc films with increasing time exposed to air during the test.

10% D24DFPP. Since the conductivity of pure D24DFPP is much poorer than that of ZnPc (pure D24DFPP film shows almost no current signal in our testing limit of 1 nA), there is no reason that the increased σ has resulted from the linear superimposition of the two component materials. Therefore, the increased σ is indeed due to the doping effect: the dopantionization induced increment of free carrier density.6 Here, we should point out that the calculated σ (9.96 × 10-7 S/cm tested in air) of pure ZnPc in our experiment is much higher than the reported value (10-10 S/cm tested in vacuum).20 Considering the atmosphere environment for our I-V measurements, we attribute this discrepancy to oxygen doping effect,14 which has been proved by the fact that the conductivity of pure ZnPc film increases with increasing the time exposed to air during the test (see Figure 2b). This discrepancy might cause some underestimation of the doping efficiency since the conductivity of the doped films is found to be much less affected by the oxygen (Figure 2c). This is probably due to the higher electron capture ability of our dopant material. All of the contrast experiments were done under the same conditions to help us understand and explain the transport mechanism. From Figure 2a, it is also found that the conductivity of D24DFPP-doped ZnPc films show a “peak” behavior, unlike the consecutive increment for F4-TCNQ doped ZnPc.12 The σ value keeps increasing in a lightly doped region, while it falls down in a heavily doped region. The maximum σ is observed around 10% dopant concentration. In order to get a compre-

hensive explanation for this unique conducting behavior, we made investigations on both stacking mode and morphology of the organic layer. UV-vis spectra of pure and D24DFPP-doped ZnPc films are shown in Figure 3. In combination with the inset absorption spectrum of pure D24DFPP film, we could clearly discern that the two main peaks centered at 620 and 700 nm are attributed to ZnPc Q-band absorption while the two subordinate peaks centered at 500 and 540 nm are attributed to D24DFPP. The dopant material shows almost no absorption at the position of these two Q-band peaks of ZnPc. With increasing the doping ratio of D24DFPP, apparent variation of ZnPc Q-band peaks was observed. First, the two separated peaks in ZnPc Q-band shift toward each other, corresponding to a gradually weakened π-π interaction of ZnPc molecules caused by the introduction of a heterogeneous material.25 Second, there is an intensity reversion between these two peaks: the peak at around 700 nm goes up while the other peak at around 620 nm goes down. As the former peak is related to ZnPc monomer while the latter is due to dimers or higher aggregates,26 this phenomenon further proves the interruption of host ZnPc polycrystalline structure by the guest material.27 Surface morphology of D24DFPP-doped ZnPc films with different doping ratios were observed by AFM in tapping mode (Figure 4). It is found that thermal deposited ZnPc film without dopant shows homogeneous single phase morphology (Figure 4a). A compact pileup of ZnPc grains is observed distinctively in the microscope. When 3% content of dopant was added by coevaporation, D24DFPP domains appear, dispersing uniformly in ZnPc matrix. Further increase of the dopant content induces more D24DFPP aggregates. They connect each other like discontinuous nanoparticle strings, as shown in Figure 4c. The host grains also get larger and blurry in boundary. Nevertheless, the ZnPc matrix phase is still continuous throughout the film with no sharp phase separation visible. At the heavily doped level, the situation is quite different: a much rougher film surface with large D24DFPP aggregates could be observed for 20% doped film (Figure 4d). These aggregates are cross-linked throughout the matrix film, indicating severe phase segregation between the two component materials. Surface roughness and grain sizes of the films are also obtained from AFM measurements and listed in Table 1, to get a deeper understanding of the morphology evolution. From the table, we could clearly see that the film roughness gets larger and larger with increasing the doping ratio. Meanwhile, the grain sizes of the aggregates are also enlarged until they finally get linked or cross-linked with each other. All of these results point to a dopant induced phase segregation process. This is not the same as the classical

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Figure 4. AFM images for the morphology of D24DFPP-doped ZnPc films with different doping ratios: (a) 0%, (b) 3%, (c) 10%, and (d) 20%.

TABLE 1: Roughness and Grain Size of Three Different Doping Systemsa grain size (nm) dopant concentration (%)

ZnPc

D24DFPP

film surface roughness (nm)

0 3 10 20

40-50 60-80 60-80 (conglutinated) cross-linked

80-120 80-120 (stringed)

4.21 4.70 6.10 9.38

a Film roughness is evaluated using Rq (root-mean-square roughness) as obtained directly from AFM data; grain size of matrix and dopant material are measured from AFM images.

Figure 5. X-ray diffraction patterns of D24DFPP-doped ZnPc films with various dopant contents at room temperature.

ZnPc/F4-TCNQ system, which showed better phase homogeneity at similar doping levels.20 It is probably due to the larger and less planar molecular configuration of D24DFPP compared with F4-TCNQ. The dopant induced structure disruption and phase segregation of the host material is further confirmed by XRD study on the crystal structure of the D24DFPP-doped ZnPc films. As shown in Figure 5, the intensity of the predominant peak of ZnPc (200) lattice plane at 2θ ) 6.86° decreases with increasing the doping ratio, referring to a structure transformation of the ZnPc matrix from polycrystalline to amorphous state.28,29 A tiny shift of this main peak toward the small angle direction is also observed, indicating a gradually increased interplanar distance. Submerged in the broad substrate diffraction peak (2θ ) 16°-26°), it is also noticed that a small peak at 2θ ) 18.8°, which is attributed to D24DFPP aggregates,30 appears at the higher doping level (more than 10%). This agrees very well with the AFM images in which the aggregation and the linkage of D24DFPP particles become obvious after the 10% dopant concentration. Interestingly, further increment of D24DFPP does not increase this small peak as expected but shows some decrement. Similar phenomenon was also found in other perylene based composite films, in which different diffraction peaks dominate at different perylene contents.31 This observation indicates that the crystalline structure and growth preference of the dopant material is

Figure 6. Schematic illustration of bulk transport trend for D24DFPPdoped ZnPc films. Dark gray domains of aggregated dopant particles (ionized or un-ionized) are distributed within the lighter matrix of ZnPc. Spatial distribution of these domains affects both the density and the mobility of the holes (white crosses) traveling through the films.

also deeply affected by the content of host material, especially when they are in a comparable proportion. Based on the mentioned experimental results, an aggregate structure controlled variation of carrier mobility and carrier concentration with different dopant content is proposed to explain the conductivity behavior in D24DFPP-doped ZnPc films. The generalized mechanism is depicted in Figure 6. For pure ZnPc film, the low conductivity relies mainly on its extremely low carrier concentration, since the thermal-emission process is very weak at room temperature. When the ZnPc film is doped with a small amount of D24DFPP (no more than 10% in our experiment), hole concentration increases rapidly due to the ionized acceptor (Figure 6b). Concomitantly, the hole mobility is also supposed to increase because the energy distribution of the mobile majority carriers shifts toward the shallower sites, reducing the energy offset between Fermi level and the effective transport level.32 As a result, in the light doping region, conductivity increases dramatically with a superliner behavior as reflected from Figure 2a. Further increment of dopant content, however, turns the variation of both carrier mobility and carrier concentration to a different situation. From Figure 6c, we could see, although the dark gray dopant domain increases with increasing dopant content, the ionization induced free carriers are actually decreased. This is due to the reduced specific contact area between dopant and matrix materials caused by the dopant aggregation. (The detailed principle is confirmed

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Figure 7. Charge interaction between host and dopant materials. (a) Energy level matching between ZnPc and D24DFPP deduced from UPS and UV-vis spectra. (b) UPS spectra (low binding energy cutoff) of 10 nm thick film of pure ZnPc and D24DFPP on Au substrate. Inset: high binding energy cutoff. (c) XPS core level spectra of F1s during the deposition of D24DFPP on ZnPc.

and discussed in section on electronic structure). On the other hand, dopant aggregation also has a destructive effect on the matrix crystalline structure (proved by XRD investigation), which would lead to spatial and energetic disorder of the composite material and finally cause the reduction of hole mobility.33-35 In addition, the more severe carrier trapping and scattering due to the increased dopant concentration could impair the hole mobility as well. As shown in Figure 6c, many of the generated holes are bounded by the ionized dopant aggregation as accumulated negative charge centers; the free holes might also suffer from neutral dopant scattering.36 In considering all of these factors, a decreased conductivity from a low doping level to a phase separated hybrid situation would be expected, just in accordance with our conductivity measurement results (Figure 2). Electronic Structure of D24DFPP-Doped ZnPc Films. Principle of Doping Induced Transport ImproWement. The energy levels of ZnPc and D24DFPP are given in Figure 7a. The ionization potential (IP) of ZnPc and D24DFPP are

Chen et al. determined by UPS measurements (Figure 7b) to be 5.22 and 7.12 eV, respectively (IP ) hν - EHOMO + EHBECutoff).37 From their absorption onsets in Figure 3, the optical band gaps of ZnPc and D24DFPP are estimated as 1.56 and 2.07 eV, respectively. Thus, the electron affinity of ZnPc and D24DFPP are calculated as 3.66 and 5.05 eV, respectively (assuming a band gap equal to the photon energy at the onset of optical absorption).38 The calculated energy offset between the HOMO level of ZnPc and the LUMO level of D24DFPP is 0.17 eV. Such small barrier value provides a possibility for electron transfer from ZnPc to D24DFPP, which is further demonstrated by XPS measurements. Representative XPS spectra of F1s are shown in Figure 7c as a function of D24DFPP thickness on ZnPc. F is chosen as the detected element because it is one of the distinguishing elements between the two molecules. Zn is ruled out for its weak signal intensity. After the deposition of 0.3 nm D24DFPP, F1s was dominated by the component peak (1) centered at a binding energy of 683 eV. Upon increasing the D24DFPP thickness to 1.0 nm, a new component peak (2) at higher binding energy appears. Further deposition leads to an intensity increment in peak (2) and a gradual attenuation of peak (1). As has been demonstrated in other interface systems, such two types of peaks are generally attributed to the different states of the deposited molecule: surface state and bulk state.39 In our case, obviously, the initially detected peak (1) is attributed to the surface state while the next detected peak (2) refers to the bulk state. Therefore, the lower binding energy of peak (1) compared to peak (2) for bulk stated F indicates the formation of negatively charged D24DFPP at the direct contact with ZnPc where significant charge transfer occurs. The gradually lost peak (1) and the intensified peak (2) also agreed well with the less detected surface region after continuous deposition. In addition, we observe a chemical shift of both peaks (1) and (2) toward higher binding energy as well as the broadening of fwhm (full width at half maximum) for peak (2) with increasing the thickness of D24DFPP. These observations demonstrate that there is a series of transition-stated F between the surface state and the bulk state, perhaps caused by partial charge transfer between host and dopant materials. Peak (3) is possibly caused by shakeup processes corresponding to π-π* transition of aromatic molecules.40 The evolution of F1s spectra (Figure 7c) during the incremental deposition process provides evidence from a molecular point of view that doping induced transport improvement in our system is mainly a result from the ionization of dopant material by charge interaction with the host material. Charge Injection in ZnPc Film: Effect of Dopant Concentration. Three-layered Au/ (D24DFPP-doped) ZnPc/Au devices with different doping ratios are fabricated to study the injection behavior of our doping system. J-V results are shown in Figure 8, with increasing current density for 0%, 20%, 3%, and 10% device. This trend is generally in agreement with our conductivity results, except for two differences. First, the current density of a 3% doped device is much higher than that of a 20% doped device, whereas the conductivity is similar for both of them. Second, a 3-4 orders of magnitude increment of current density for 3% doped ZnPc than the undoped ZnPc is observed, which is much higher than the corresponding conductivity increment. These discrepancies demonstrate that, with much shorter electrode distance, charge transport is no longer bulk dominated; charge injection through the metal/organic interface is playing an important role. Moreover, as reflected in Figure 8 inset, the Log J-Log V behavior is completely different for nondoped, lightly doped (3%, 10%), and heavily doped (20%) cases, indicating distinct transport or injection

Bulk Conductivity versus Interfacial Injection

Figure 8. J-V characteristic of Au/organic/Au devices with different doping ratios. Inset: the Log-Log plots.

mechanisms. Here, we conducted the UPS measurements, in combination with the modeling analysis, to further investigate the injection behavior at Au/ZnPc interface under light (3% as representative) and heavy doping (20%). UPS spectrum of pure ZnPc film grown on Au as a function of layer thickness is shown in Figure 9a with the high binding energy (HBE) cutoff shown on the left side and the Fermi edge cutoff on the right side. Before the deposition of ZnPc, Au substrate shows its Fermi level and low kinetic energy onset at 26.35 and -0.41 eV, respectively. Therefore, Au work function is calculated to be 5.24 eV using the method mentioned in the experimental section. The abrupt shift of HBE cutoff from Au to 0.3 nm ZnPc toward the lower kinetic energy direction indicates a downward vacuum level, which is caused by the formation of a 0.4 eV interface dipole barrier. This dipole is attributed to electron transfer from the organic material to the metal.15 EF is near the midgap at 0.91 eV above the leading edge of ZnPc HOMO, very similar to the value reported in ref 41. The HOMO peak of ZnPc has a very slight shift toward higher binding energy. For intrinsic semiconductor of pure ZnPc, this is due to the change in polarization screening from the organic-metal interface to the surface of the thick organic film.42 When the film is 3% doped, as shown in Figure 9b, the interface dipole is reduced by 0.17 eV with respect to the undoped case. This is due to the strong electron attraction of D24DFPP which tends to reduce the electron transfer from the organic layer to the metal substrate. Meanwhile, we got a gradual shift of the HOMO toward lower binding energy with increasing the layer thickness, indicating an upward molecular level bending away from the interface. The bulk position of the leading edge of the HOMO is 0.32 eV below EF, much closer than that of the undoped ZnPc film, which demonstrates that hole injection barrier is reduced. The UPS spectra for 20% D24DFPP-doped ZnPc (Figure 9c), however, does not show a further improved hole injection interface. Quite oppositely, the interface dipole increases back to 0.42 eV, indicating a reduced p-doping effect, in accordance with our previous conclusion in bulk conductivity. What’s more, the HOMO peak of ZnPc appears to shift away from EF with increasing the layer thickness, showing a slight downward bending behavior near the metal/organic interface. In considering that many of the metal/n-type semiconductor interfaces show such a downward band bending behavior,43,44 we presume that the UPS result for the heavily doped films is a coreflection of several interfaces including Au/ZnPc, Au/ D24DFPP, or even ZnPc/D24DFPP. Unfortunately, a distinct D24DFPP HOMO peak is not observed in Figure 9c as a direct evidence for our hypothesis. It might be submerged by ZnPc

J. Phys. Chem. C, Vol. 113, No. 39, 2009 17165 valence band feature due to the much lower dopant content compared with matrix ZnPc. However, the phase separated morphology and large surface roughness of the heavily doped ZnPc film (20%) was observed from AFM images (Figure 4d), which proves that in certain regions a direct contact of Au with D24DFPP is possible. Figure 10 summaries the generalized scheme of interfacial energy level alignment for three types of ZnPc films based on the above analysis. As has been mentioned above, we use a “double phase mixing” schematic diagram to represent the heavily doped situation. It is easy to find from this diagram that the injection barrier is much smaller for the light doping case compared with that of the heavy doping case. We believe it is one of the reasons that cause the transport disadvantage for heavily doped sandwiched devices. As charges are injected into disordered organic solid and transport in it, there are two limiting regimes for device operation: injection limitation and bulk limitation. If at least one contact could supply locally higher charge densities than that in materials in thermal equilibrium, space-charge limited current (SCLC) occurs. For a perfect insulator without intrinsic traps and charges and for charge carrier mobility independent of the electrical field, the SCLC is expressed by Mott-Gurney law.45 In the presence of traps, the current is generally lower, and the quadratic field dependence should be modified by power-law dependence since the current increases fast when the traps are gradually filled with increasing electric field. Injection limitation occurs if the injection barrier is so high that the injected current is insufficient compared with the maximum current possibly supported by this organic material. Traditionally, charge injection into semiconductors is considered either in terms of the Fowler-Nordheim (FN) tunneling model or Richardson-Schottky (RS) thermionic emission model.46 In the FN model, carrier injection is described as a single tunneling jump through a triangular potential barrier into a continuum of states ignoring Coulombic effect, while the RS model is based on the consideration of thermally assisted overbarrier jump for carriers. The unique characteristic of both models is the current has no explicit thickness dependence at constant electric field F. The current density in the FN model as a function of electric field F is given by the equation:47

jFN

(

8π√2m*φB3/2 q3E2 ) exp 8πhφB 3hqE

)

(1)

Where m* is the effective charge carrier mass, ΦB is the zerofield injection barrier, h is the Plank’s constant, and q is the electron charge. On the other hand, the current density in the RS model as a function of the external bias V could be written as

( ) (

jRS ) A*T2 exp

-φB βRSV1/2 exp kBT kBT

)

(2)

where A* is the effective Richardson constant, βRS is a material related constant, kB is the Boltzmann constant, and T is absolute temperature.45 However, both of these injection mechanisms seem to be hardly applied directly for disordered organic semiconductors. The hopping characteristic which results in enhanced backflow and surface recombination of injected carriers at electrode surface should be considered.48 Recent analytic consideration and Monte Carlo simulation of charge

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Figure 9. UPS spectra of (D24DFPP-doped) ZnPc films incrementally deposited on Au: (a) pure ZnPc/Au, (b) 3% doped ZnPc/Au, (c) 20% doped ZnPc/Au; (left) high binding energy cutoff, (right) Fermi edge cutoff.

injection from metal electrode into a disordered hopping system show that quantitative difference exists between the traditional

injection mechanisms and the hopping injection process although the latter type of injection resembles RS model.

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Figure 10. Energy level alignment near the interface between Au and organic layer.

The thickness dependent J-F behavior is conducted to study the dominant transport limitation for the sandwiched devices. From Figure 11, we could see that all three types of doping devices (nondoped, lightly doped, and heavily doped) show uniform thickness independent J-F behavior, indicative of their injection limited transporting mechanism. This seems unusual since the injection barrier has already been reduced to a very low level at least for the light doping case from UPS evaluation (Figure 10). In analogy with the F4-TCNQ doped ZnPc system, we attribute this result to the field limitation effect:15 since the external bias in our experiment is only up to 20 V and the organic films are several hundreds of nanometers thick, bulk limited SCLC behavior relating to high electric field is not likely to be observed. Here, only injection mechanism involved at a low electric field would be responsible for the current enhancement of both the lightly and the heavily doped devices. Figure 12a shows the J-V curves with ln (J/E2) versus (1/E) (FN plots). According to eq 1, the measured data of 3% doped device could be well-fitted by a straight line according to FN tunneling injection model at the high field region. Evidently, it is not the case for either undoped or 20% doped devices. From the slope of the fitted line, a 0.19 eV injection barrier could be calculated, in good agreement with the UPS results. Such a small barrier height further confirms that tunneling through the thin space charge region (about 4 nm from UPS results) near the electrode is facilitated in this 3% doped device. Looking back to the log J - log V curve as plotted in Figure 8 inset, it is interesting to notice that the current density is almost constant before 2 V applied voltage for the light doping cases. That is because tunneling is also not favored at low electric field, as demonstrated by Monte Carlo simulations.49 For bias higher than 2 V, the rapid current increment is due to the field-emission tunneling. Figure 12b presents log J versus V1/2 of Au/organic/Au devices with 400 nm thick organic layer according to the RS model. The slope of this linear fit gives βRS ) 6.29 × 10-22 (cm/V1/2), and an effective RS constant A* ) 1.97 A/cm2 · K2, which are in the same order of the magnitude as those reported by Kahn et al.15 For 20% doped device, the linear relation for log J versus V1/2 indicates that the current transport characteristic of such large-ratio doped device is still injection-limited at least for a film thickness up to 800 nm and fields up to 5 × 105 V/cm. However, the injection process in such an interface is complicated. In view of the energy diagram (Figure 7) of D24DFPP and the HOMO characteristics of heavily doped ZnPc film (Figure 10), the electron injection into D24DFPP and the hole injection into ZnPc might occur at the same time (although it is believed that hole injection is the main process). Therefore, the increment of current density for 20% doped device compared

Figure 11. J-F characteristic of Au/organic/Au devices with different thickness of organic layer. (a) Au/pure ZnPc/Au, (b) Au/3% doped ZnPc/Au, and (c) Au/20% doped ZnPc/Au.

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Chen et al. Acknowledgment. The work was financially supported by the National Natural Science Foundation of China (Grants 20774083 and 50703035) and of the Fund for the Major State Basic Research Development Program (2007CB613400). The authors also would like to thank the developing program of Changjiang Scholar and Innovation Team from Ministry of Education of China (Grant IRT0651) and the National Synchrotron Radiation Laboratory of USTC for financial supports. Thanks also go to Dr. Muddasir Hanif for his kind English review. References and Notes

Figure 12. Modeling analysis of Au/organic/Au devices with a 400 nm thick organic layer. (a) FN plots and (b) log J-V1/2 plots.

with that of the undoped device might be understood from two doping effects: First, part of dopant well dispersed in ZnPc matrix improves the local charge density near the electrode interface so that the injection barrier is effectively lowered. Second, part of the aggregated dopant contacts with the electrode directly and facilitates the injection of electrons as discussed above. On the other hand, however, the severe phase separation (as observed by AFM) itself would also lead to the pile-up of the injected charges near the electrode interface, which finally weakens the doping effect in a 20% doped device as compared with that in a 3% doped device. Conclusion Carrier transport in D24DFPP-doped ZnPc films is investigated at different doping levels from 3% to 30%. J-V measurements of Au/(D24DFPP-doped) ZnPc/Au devices show higher current density for the lightly doped device rather than for the heavily doped case. This result is related to both bulk and interfacial effects: bulk conductivity keeps increasing before reaching its maximum at the doping ratio of 10% and then starts to decrease. Dopant concentration dependent film aggregate structure is proposed to explain this nonmonotonous behavior. Improved interfacial injection for the lightly doped device is attributed to both optimized energy level alignment and fieldemission tunneling effect. Two orders of magnitude increment for conductivity and 3-4 orders of magnitude increment for device current density via a controlled doping prove D24DFPP as an efficient p-type dopant, considering the in-air testing environment. Charge interaction between the matrix and the dopant molecules is confirmed to be the primary reason for this doping effect. Further work is undergoing to optimize the doping ratio. Molecular tailoring is also in plan to synthesize more efficient dopant material with better phase compatibility and energy level matching.

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