Charge transport in electrically doped amorphous organic semiconductors.

Macromolecular Rapid Communications

Review

Charge Transport in Electrically Doped Amorphous Organic Semiconductors Seung-Jun Yoo, Jang-Joo Kim* This article reviews recent progress on charge generation by doping and its influence on the carrier mobility in organic semiconductors (OSs). The doping induced charge generation efficiency is generally low in OSs which was explained by the integer charge transfer model and the hybrid charge transfer model. The ionized dopants formed by charge transfer between hosts and dopants can act as Coulomb traps for mobile charges, and the presence of Coulomb traps in OSs broadens the density of states (DOS) in doped organic films. The Coulomb traps strongly reduce the carrier hopping rate and thereby change the carrier mobility, which was confirmed by experiments in recent years. In order to fully understand the doping mechanism in OSs, further quantitative and systematic analyses of charge transport characteristics must be accomplished.

1. Introduction Electrical doping is one of the most important and fundamental technologies in organic semiconductors (OSs) as well as inorganic semiconductors to control the electrical properties of materials. It can enhance the conductivity of bulk organic films by increasing the carrier density and also reduce the contact resistance between electrodes and organic layers.[1–4] Electrical doping allows the formation of charge generation and recombination layers in tandem organic devices. Research on doping in OSs goes back to the discovery in the year of 1977 that the conductivity of polyacetylene can be enhanced over many orders of magnitude by doping it with iodine.[5] Since then, a large number of materials have been proposed as dopant materials.[6–48] Despite of wide use of dopants in OSs, the charge generation S.-J. Yoo, Prof. J.-J. Kim Department of Materials Science and Engineering Seoul National University Seoul 151–744 , South Korea E-mail: [email protected]

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efficiency (CGE) is still quite low compared to inorganic semiconductors.[24,26,49–55] The binding energy of the holes (electrons) to the negatively (positively) ionized dopant in OSs is in the order of ≈0.5−0.8 eV which is much higher than that of inorganic semiconductors due to the low dielectric permittivity of about 3.[56,57] Therefore, charge transfer from a host molecule to a dopant generates a strongly Coulomb-bound charge pair rather than a free carrier in disordered OSs, because the thermal excitation by the lattice vibrations at room temperature is unsufficient to fully dissociate the charge pairs into free carriers. In order to understand the doping mechanism in OSs, it is necessary to systematically analyze the charge transfer process and then grasp how it affects the conductivity or carrier density and mobility in doped organic layers. This article reviews recent progress on charge generation by doping and its influence on the carrier mobility in OSs. We begin with introducing the dopant materials and their CGE. Then the charge transfer process will be described based on two different models, namely the integer charge transfer (ICT) model and the hybrid charge transfer complex (CTC) model. The ionized dopants

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DOI: 10.1002/marc.201500026


Charge Transport in Electrically Doped Amorphous Organic Semiconductors

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formed by the charge transfer process between hosts and dopants can act as Coulomb traps for mobile charges, and the broadening of the Gaussian density of states (DOS) upon doping induced by Coulomb traps associated with ionized dopants will be discussed. Finally, we review the experimental results of charge mobility in doped OSs.

Seung-Jun Yoo obtained the BS degree from the Department of Material Science and Engineering, Seoul National University, Korea, in 2010. He is currently a PhD student under the guidance of Prof. Jang-Joo Kim in the Organic Photonics Laboratory, Seoul National University. His general research interest is charge transport in doped organic semiconductors.

2. Dopants in Organic Semiconductors Charge generation by doping takes place through charge transfer between the host and the dopant molecules. Therefore, materials with large work function or deep lowest unoccupied molecular orbital (LUMO) level are preferred as p-dopants for efficient charge transfer from host molecules to the dopant. In turn, materials with small work functions or high lying highest occupied molecular orbital (HOMO) levels have been developed as n-dopants to easily transfer an electron from the dopants to host molecules. Halogen compounds such as iodine or bromine[6] and alkali metals[30–33] were used as p- and n-dopants, respectively, at the early stage of research on conducting polymers. Since then a large number of p- and n-dopants have been developed to improve the CGE and thermal stability with low diffusivity. The molecular structures and the energy levels of representative p-dopants are shown in Figure 1.[1,11,15,28,58–61] Hole transporting materials used as host materials in this Review are shown in Figure 2 including their energy levels.[62–66] The p-dopants can be classified into allorganic molecules, transition-metal-oxides (TMOs), and organo-metallic complexes. Organic p-dopants include tetrafluoro-tetracyano-quinodimethane (F4-TCNQ), 1,3,4,5, 7,8,-hexafluoro-tetracyano-naphthoquinodimethane (F6TCNNQ), and fluorinated fullerene (C60F36).[10–18] These molecules contain a large number of electron-withdrawing groups such as –F or –CN to lower the LUMO levels. TMOs have been widely used as p-dopants, since the work function of TMOs can be easily tuned between extremely low (3 eV for ZrO2) and high (7 eV for V2O5) by controlling the core metal and the cation oxidation state.[67] Representative TMOs for p-type dopants include WO3, V2O5, MoO3, and ReO3 which are known for their deep-lying conduction band edges or high work functions.[19–27,50,51,54] Recently organo-metallic complexes such as tris[1,2-bis(trifluoromethyl)ethane-1,2-dithiolene] (Mo(tfd)3) have been introduced as efficient p-dopants.[26,28,29] On the other hand, alkali metals such as Li, Cs, Rb, Mg, K, and Na were used as n-dopants in early years, since their work functions are small.[30–33] In order to overcome the diffusion issues of alkali metals, alkali metal compounds have been developed as n-type dopants.

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Jang-Joo Kim received his BS and MS degrees from the Department of Chemical Engineering, Seoul National University, Korea, in 1977 and 1980, respectively, and his PhD degree from the Department of Materials Science and Engineering, Stanford University, USA, in 1987. After one and a half years as a post-doctoral fellow in SRI International, USA (1986–1987), he returned to Korea to join the Electronics and Telecommunications Research Institute as a senior member and a principal member of technical staff from 1987 to 1996. He was a professor at Gwangju Institute of Science and Technology, Korea, from 1997 to 2003 and since 2003 he has been a professor in the Department of Materials Science and Engineering at Seoul National University. He is also currently a member of the Korea Academy of Science and Technology. His research areas include electronic processes in organic semiconductors and organic electronic devices.

Examples include Li compounds (LiF, 8-quinolinolato lithium (Liq), dipivaloylmethanato lithium (Lidpm), acetylacetonato lithium (Liacac), lithium 2-(2-pyridyl) phenolate (LiPP), lithium 2-(2′,2″-bipyridine-6′-yl)phenolate (LiBPP), lithium 2-(isoquinoline-1′-yl)phenolate (LiIQP)),[34–37] and alkali metal carbonate (Rb2CO3, Cs2CO3).[38–40] However, these alkali metal compounds can decompose into metal during evaporation or be reduced by a subsequent metal contact.[36,37,68] Molecular compounds have also been synthesized as n-type dopants, including bis(ethylenedithio)-tetrathiafulvalene (BEDT-TTF), tetrathianaphthacene (TTN), bis(cyclopentadienyl)cobalt(II) (CoCp2), 2,2′:6′,2″-terpyridine ruthenium (Ru(terpy)2), 2,2′-bipyridine chromium (Cr(bpy)3), 4,4′,5,5′-tetramethyl-2,2′-bipyridine chromium (Cr(TMB)3), and chromium or tungsten with the anion of 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine (Cr2(hpp)4, W2(hpp)4).[41–48] Their work functions or HOMO levels are displayed in Figure 3.[38,42,46,69]


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Figure 1. a) Energy level of the representative p-type dopants.[1,11,15,28,58–61] In case of ReO3, the conduction band edge is not observed, but it is expected to be located near the work function. Molecular structure of b) tetrafluoro-tetracyano-quinodimethane (F4-TCNQ), c) 1,3,4,5,7,8,-hexafluoro-tetracyano-naphthoquinodimethane (F6-TCNNQ),d) fluorinated fullerene (C60F36),and e) tris[1,2-bis(trifluoromethyl)ethane-1,2-dithiolene] (Mo(tfd)3).

3. Charge Generation Upon Doping in Organics Semiconductors 3.1. Charge Generation Efficiency in Doped Organic Semiconductors The CGE is defined as the ratio of the number density of carriers generated by doping to that of dopant molecules.[2] charge generation efficiency =

Number density of carriers generated by doping Number density of dopant molecules

(1)

The CGE has been mostly studied for p-doping as shown in Table 1.[16,24,26,49–55] We therefore focus on the CGE of p-doped OSs in this Review. The doping efficiencies

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are different for different combinations of dopants and host molecules and decrease as the doping concentration increases, even in the same dopant/host systems.[16,17] Since the doping in OSs can be explained by electron transfer from host materials to dopants, the energy difference between the HOMO level of the host and the LUMO level (or conduction band edge) of the dopants (∆ECT) is expected to influence the doping efficiency. Lee et al. systematically investigated the effect of the energy difference on the CGE using different combinations of host and dopant molecules.[23–26,61,70] The hole densities in the p-doped OSs were determined by Schottky-Mott analysis of the capacitance−voltage (C−V) characteristics of the metal−insulator−semiconductor (MIS) structure. ReO3 was selected as the p-dopant and 4,4′,4″-tris(N-3-methylphenyl-N-phenylamino)triphenylamine (m-MTDATA), 4,4′,4″-tris(N-2-naphthyl-N-phenylamino)triphenylamine





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Figure 2. a) Energy levels of hole transporting organic materials which are used as host materials and presented in this Review.[62–66] Molecular structure of b) zinc-phthalocyanine (ZnPC), c) poly(2-methoxy-5-(2′-ethylhexyloxy)-p-(phenylene vinylene) MEH-PPV, d) poly(3-hexylthiophene) P3HT, e) 4,4′,4″-tris(N-3-methylphenyl-N-phenylamino)triphenylamine (m-MTDATA), f) 4,4′,4″-tris(N-2-naphthylN-phenylamino)triphenylamine (2-TNATA), g) N,N′-diphenyl-N,N′-bis-1-naphthyl-1–1-biphenyl-4,4-diamine (NPB), h) 1,1-bis-(4-bis(4-methylphenyl)-amino-phenyl)-cyclohexane (TAPC), and i) 4,4′-bis(carbazol-9-yl)-2,2′-biphenyl (CBP).

(2-TNATA), N,N′-diphenyl-N,N′-bis-1-naphthyl-1–1biphenyl-4,4-diamine (NPB), and 1,1-bis-(4-bis(4-methylphenyl)-amino-phenyl)-cyclohexane (TAPC) as hosts


possessing different HOMO levels (m-MTDATA: 5.1 eV, 2-TNATA: 5.1 eV, NPB: 5.4 eV, TAPC: 5.6 eV). The hole carrier densities increased as the doping concentration


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Figure 3. a) Ionization energy levels or work functions of representative n-type dopants.[38,42,46,69] Molecuar structure of b) tungsten with the anion of 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine (W2(hpp)4), c) bis(cyclopentadienyl)cobalt(II) (CoCp2), d) tetrathianaphthacene (TTN), e) chromium with the anion of 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine (Cr2(hpp)4), and f) 8-quinolinolato lithium (Liq).

Table 1. Summary of charge generation efficiency in various doped systems.

Doped systemsa) b)

Doping ratio

ηCG [%]

1 mol%

8.4 × 10−3[49]

c)

n-type dopant /PPEEB

0.5[50]

MoO3/α-NPD or CBP 17.3 mol%

1.9[51]

1:600 (Dopant:Host)

1.0[52]

MoO3/CBP F4-TCNQ/MEH-PPV

≈5[53]

F4-TCNQ/MeO-TPDd)

≈2−4.5[54]

MoO3/S-2CBPe) ReO3/2-TNATA

25 mol%

3.8[24]

ReO3/NPB

25 mol%

0.8[24]

ReO3/TAPC

25 mol%

0.4[24]

ReO3/CBP

25 mol%

0.3[24]

Mo(tfd)3/NPB

2 mol%

3.7[26]

ReO3/NPB

2 mol%

1.9[26]

MoO3/NPB

2 mol%

1.6[26]

F4-TCNQ/NPB

2 mol%

0.9[26]

MRf): 0.00088, 0.3125

36.4 (at MR 0.00088)[16] 4.2 (at MR 0.3125)[16]

C60F36/MeO-TPDd) d)

F6-TCNNQ/ MeO-TPD

f)

MR : 0.0084, 0.1441

20.2 (at MR 0.0084)[16] 7 (at MR 0.1441)[16] ≈5[55]

F4-TCNQ/P3HT

a) left: dopant materials, right: host materials; b)reduced derivate of PPEEB with a covalently attached counterion; c)crystalline perylene diimide (PPEEB); d)N,N,N′,N′-tetrakis(4-methoxyphenyl)-benzidine (MeO-TPD); e)2,7-bis(9-carbazolyl)-9,9-spirobifluorene (S-2CBP); f)Mole ratio (MR) that compares the amount of dopant molecules to the number of host molecules.

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resulted in a higher CGE than ReO3 or MoO3, even with lower ∆ECT when doped in NPB, demonstrating the importance of the dispersion characteristics for CGE. Based on these observations, Lee et al. proposed that the CGE (ηCG) in doped OSs can be expressed by the product of the dispersion efficiency (ηdsp) of the dopant and charge transfer efficiency (ηCT) of a dopant molecule from or to a host molecule as expressed by Equation (2).[2]

ηCG = ηdsp × ηCT

Figure 4. Hole densities at room temperature as a function of doping concentration for ReO3 doped m-MTDATA, 2-TNATA, NPB, and TAPC, determined by Schottky-Mott analysis of the C−V characteristics of the MIS structure. The dashed lines are linear fits.[70]

(2)

Mityashin et al. added the concept of the dissociation of the CT complexes to the CGE, because a large portion of the CT complexes remains bound without dissociation into free carriers caused by the strong Coulomb binding energy due to the low dielectric permittivity of OSs.[72] With consideration of the dissociation efficiency (ηdis), which is defined as the ratio of the free carrier density and the density of CT complex, ηCG of a dopant can be expressed as follows: (3) ηCG = ηdsp × ηCT × ηdis

increased from 2 mol% to 15 mol% and were linearly proportional to the doping concentration (Figure 4).[70] However, the hole densities were different for different hosts. ReO3 doped 2-TNATA and m-MTDATA showed the highest The total carrier density (ρ) in doped OSs can be carrier densities followed by NPB and TAPC. A higher ∆ECT expressed as resulted in a higher CGE in the sequence of m-MTDATA ≈ ρ = ρ int + ρext 2-TNATA > NPB > TAPC. The same tendency was found (4) = ρ int + N dopant ηCG in the different combinations of host and dopant molecules.[23,24] It is interesting to note that the CGEs are low = ρ int + N dopant (ηdsp × ηCT × ηdis ) in the range of ≈0.3−4% in the experiments. The same group reported that the formation of nanowhere ρint is the intrinsic carrier density and ρext is the clusters accounts for the low CGE in TMO doped systems extrinsic carrier density, which is defined as the product as manifested in the image taken by transmission electron microscopy as displayed in Figure 5.[25] The MoO3 and ReO3 doped (2 mol%) NPB films clearly showed dark spots originating from dopant nanoclusters. Inhomogenous dispersion of the dopant will certainly reduce the number of contacts between the dopant and host molecules to reduce the CGE. Formation of filamentous nanostructures of MoO3 in the CBP matrix was also observed by another group using electron spectroscopic imaging.[71] In contrast to TMOs, organic dopants or organo−metallic complexes based dopants have better chances for homogenous dispersion and hence better CGEs. A recent study on Mo(tfd)3 (EA = 5.59 eV) showed no indication of the formaFigure 5. Bright-field TEM images of a) undoped NPB, b) ReO3- and c) MoO3-doped NPB tion of nanoclusters as visualized by (25 mol%). d,e) Diffraction patterns of (a) and (b), respectively. f) High-resolution TEM TEM images (Figure 6).[26] Because of image of (b). The NPB film doped with TMO shows dark spots in the image resulting the homogenous dispersion, Mo(tfd)3 from the TMO nanoclusters. Reproduced with permission.[25] Copyright 2011, Elsevier B.V.



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Figure 6. Bright-field TEM images of a) undoped NPB, b) ReO3doped NPB, c) Mo(tfd)3-doped NPB, and d) F4-TCNQ-doped NPB (2 mol%). The insets show the corresponding diffraction patterns. Reproduced with permission.[26] Copyright 2011, American Institute of Physics.

of the density of dopant molecules and ηCG, while Ndopant is the density of dopant molecules. 3.2. Models for Charge Transfer Processes Since charge carriers are generated by a charge transfer process between the host and the dopant molecules followed by the dissociation of the CT complexes into free carriers,[1,72] understanding of the doping process in OSs requires the understanding of the charge transfer process above all. Two models have been proposed to explain the host−dopant ground-state interaction. One is the integer charge transfer (ICT) model and the other one is the hybrid charge transfer complex (CTC) model. The difference between the models is whether or not a host molecule and a dopant molecule form a hybrid molecular orbital in the charge transfer state. The process is schematically expressed in Figure 7. 3.2.1. Integer Charge Transfer (ICT) Model The ICT model assumes that an integer charge is transferred between a host molecule and an appropriate dopant molecule to form a polaron and an ionized dopant. The polaron and the ionized dopant are bound by Coulomb interaction which can be overcome by thermal energy or an electric field.

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Figure 7. Two different models for charge transfer process. a) Integer charge transfer (ICT) model and b) hybrid charge transfer complex (CTC) model.

The ICT model is supported by ultraviolet-visible-near infrared (UV-VIS-NIR) and Fourier transform infrared (FTIR) absorption spectra as shown in Figure 8.[23] Doping of the metal complex dopants (CuI, MoO3, ReO3) in 2-TNATA generated a peak at the wavelength of 467 nm and a broad peak at the wavelength of 1226 nm in the UV-VIS-NIR absorption spectra, which were assigned to the CT peaks formed upon doping (Figure 8a). The IR spectrum of the doped film showed the appearance of new peaks at 1167 and 1567 cm−1 at the expense of the emission at 1108 and 1497 cm−1 (Figure 8b). The new peaks were assigned to the absorption of the 2-TNATA cation based on density functional theory (DFT), and no change of peak positions of the spectra was observed for different dopants, which was taken as evidence of the integer charge transfer from the host molecule to the TMO dopant. Similar results were reported for different systems including F4-TCNQ doped 4,4′,4″-tris(N-3-methylphenyl-N-phenylamino)triphenylamine (m-MTDATA) and F4-TCNQ doped zinc-phthalocyanine (ZnPC).[1] A clearer assignment of the radical absorption to the newly appearing IR absorption peaks was reported by Glaser et al. using a CBP film doped with MoO3 in combination with DFT calculations as shown in Figure 9.[73] When MoO3 was doped into a CBP matrix, additional absorption bands appeared at 1571 and 1594 cm−1. These new absorption peaks that appeared upon doping correspond to the absorption of CBP+ at 1604 and 1617 cm−1 as confirmed by DFT calculation, indicating that these spectral changes due to doping can be explained by





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extract that ≈50−70% of the dopant molecules were ionized by charge transfer from P3HT to F4-TCNQ. However, the mobile charge density in the doped films was about 5%, indicating that only a small portion of the ionized dopants contributed to mobile charges. The low dissociation probability was explained by the large binding energy of the charge transfer complex due to the low permittivity of OSs. The ICT model was also applied to explain the doping effect observed even when the HOMO level of the host is lower than the LUMO level (or conduction band edge) of dopants by taking trap states into consideration.[79] 3.2.2. Hybrid Charge Transfer Complex (CTC) Model

Figure 8. a) UV-VIS-NIR absorbance spectra of doped (25 mol%) 2-TNATA films with ReO3 (dashed-dotted curve), MoO3 (dashed curve), CuI (dotted curve), and undoped 2-TNATA film (solid curve). Inset: the molecular structure of 2-TNATA. (b) FT-IR spectra of doped 2-TNATA films with 25 mol% ReO3 (dashed-dotted curve), MoO3 (dashed curve), CuI (dotted curve), and undoped 2-TNATA film (solid curve). Reproduced with permission.[23] Copyright 2009, American Institute of Physics.

the formation of CBP cations. These cations are formed by the integer charge transfer between CBP molecules and MoO3. Matsushima et al. also clearly identified cation peaks in the UV-VIS-NIR absorption spectrum shown in Figure 10 which were obtained from a NPB film deposited on TMOs (MoO2, MoO3, WO3) layers.[74] By comparing the absorption spectra of the solution containing electrochemically formed radicals without any dopant molecules, they found that the broader new absorption peak at a wavelength of ≈1400 nm originates from NPB+ while the new peak at a wavelength of ≈500 nm is from NPB2+. The degree of charge transfer between host molecules and dopant molecules was determined from the absorption spectra of a F4-TCNQ doped poly(3-hexylthiophene) (P3HT) solution and films by Pingel et al.[55] Using the known absorption spectra of neutral and fully ionized F4-TCNQ and P3HT molecules,[75–78] they were able to


The hybrid CTC model assumes that a host molecule and a dopant molecule form a hybrid orbital in the CT state. For p-doping, the supramolecular hybrid orbitals are formed between the HOMO level of the host molecule and the LUMO level of the dopant molecule, and unoccupied hybrid states are located in the fundamental gap of the OSs. Holes are created due to the electron de-occupation of the OS HOMO and electron occupation of the CTC LUMO as a consequence of the Fermi-Dirac statistics. As the empty hybrid states are still several tenths of eV above the HOMO level of OSs, only a fraction of the hybridized CTCs are ionized at room temperature, resulting in a low doping efficiency. Figure 7b illustrates the hybrid CTC model of molecular doping which relies on the formation of supramolecular hybrid orbitals. The hybrid CTC model was proposed by Salzmann et al. based on ultraviolet photoelectron spectroscopy (UPS) data and DFT calculations using pentacene as host and F4-TCNQ as dopant.[80] Firstly, they reported that the IE of heavily doped pentacene with F4-TCNQ (1:1 ratio) increased to 5.75 eV which is much higher than the ionization energy (IE) of flat-lying pentacene (5.45 eV).[81] Secondly, no photoemission features were observed near EF even though the polaronic state is expected to be created near EF in heavily doped systems according to the ICT model. The existing ICT model seems to be unable to explain all the phenomena described above. Instead, they assumed that supramolecular hybrid orbitals are formed between the HOMO level of pentacene and the LUMO level of F4-TCNQ based on DFT calculations, and the formation of these supramolecular hybrid orbitals was experimentally validated by UV-VIS spectroscopy as shown in Figure 11. The considerable splitting of the bonding and anti-bonding hybrid orbital was calculated when the orbital overlap was maximized. Such a splitting was also observed in other systems.[13,82–85] The experimentally observed new absorption peaks from p-doped pentacene at 1.28 and 1.42 eV matched well with the energy levels calculated using DFT. The increase of the new absorption peak was inconsistent with the decrease of polaron den-


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because the inter-gap between the pentacene HOMO and hybrid CTC LUMO is still about ≈0.6−0.7 eV, only a fraction of hybrid CTCs can be ionized at a certain temperature, resulting in a low doping efficiency. The splitting energy gap of the hybrid orbital upon interaction of host and dopant molecules is a critical parameter influencing the doping efficiency in the hybrid CTC model. The magnitude of the splitting energy gap of the hybrid orbital must be minimized by reducing the intermolecular resonance integral β. Preventing the dopant molecular orbitals from overlapping with those of the OSs by steric shielding therefore emerges as a promising strategy for the chemical design of improved dopants.[86] It is difficult at this stage to say which model is more appropriate to describe the doping process or which model is applicable in which systems. Further quantitative and systematic analyses of charge generation characteristics are required to understand the doping process.

4. Density-of-States (DOS) of Doped Organic Semiconductors The density of states of disordered OSs is often described by a Gaussian distribution:[87] Figure 9. Top: DFT-calculated IR spectra of CBP and CBP+1. Bottom: Measured IR spectra of CBP, CBP:MoO3 with varying doping concentrations, and MoO3, as indicated in the middle. All the doped layers contain the same amount of deposited CBP molecules, corresponding to a pseudo thickness of 50 nm CBP. The thickness of the MoO3 layer (22 nm) corresponds to the pseudo thickness of MoO3 in the doped film with 54 mol% doping concentration. Vertical dotted lines indicate mode assignments according to assignment of selected calculated and measured absorption bands of CBP and CBP+1 to dominating atomic displacements. The vertical line at 991 cm−1 indicates the stretching vibration of the terminal oxygen of MoO3. Reproduced with permission.[73] Copyright 2012, Elsevier B.V.

sity measured by continuous wave electron paramagnetic resonance, indicating that the new absorption peak was not generated by fully ionized pentacene. The free holes were created due to electron de-occupation of the pentacene HOMO and electron occupation of the hybrid CTC LUMO described by the Fermi-Dirac statistics. Since the inter-gap between the bonding and anti-bonding of the hybrid orbital (≈1.07−1.43 eV) is much smaller than that of pristine pentacene (2.54 eV), the fraction of ionized hybrid CTCs is higher than that of pristine pentacene. However,

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(

g( E ) = 2πσ 2

)

−1 /2

(

exp − E 2 /2σ 2

)

(5)

where E is the measured energy relative to the center of the DOS and σ is the width of the Gaussian distribution, which is the energetic (diagonal) disorder parameter. Equation (5) assumes that the energies of adjacent sites are uncorrelated. The Gaussian disorder model (GDM) was modified by including the charge carrier-dipole correlation of adjacent states (correlated disorder model) to explain the Poole-Frenkel effect across a wide range of electric fields.[88,89] An exponential DOS frequently used to explain the gate bias- and temperature-dependent carrier mobility in field-effect-transistors (FETs)[90] has been unified with the GDM by showing that the exponential DOS is a good approximation of the tail states of the Gaussian-type DOS in the energy range where the Fermi level is varied by the carrier density.[91–93]





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In contrast to intrinsic OSs, doping in OSs generates CT states, mobile carriers, and ionized dopants at the same time. The ionized dopants are expected to trap the charge carriers by Coulomb attraction, thereby generating trap states. The Coulomb traps in OSs are deeper than those in inorganic semiconductors due to the low dielectric permittivity of only about 3 in OSs so that the ionized dopant will have a significant effect on the DOS of OSs. The modification of the DOS in doped OSs has been considered recently. Arkhipov et al. theoretically predicted that the presence of Coulomb traps caused by ionized dopants in OSs broadens the DOS in doped organic layers.[56] In this theoretical model, the effective DOS distribution of doped OSs is simplified by considering the localized site nearest to a dopant ion as a Coulomb trap and the electrostatic energy ∆ counted from the top of the potential barrier, which is formed by the Coulomb and external fields: ∆= Figure 10. a) UV-VIS-NIR absorption spectra of stacked films of NPB and metal oxide (2 nm NPB on 10 nm metal oxide). b) UV-VISNIR absorption spectra of 10−4 M NPB solution before and after applying Epa for 1 h. Inset: CV of 1 mM NPB solution. Reproduced with permission.[74] Copyright 2013, Elsevier B.V.

e3 F e2 − πε 0 ε r 4πε 0 ε r a

(6)

In Equation (6), a is the distance between a dopant ion and the nearest intrinsic localized site. This is equal to the intermolecular distance which is typically ≈0.6−1.0 nm. With this assumption, the effective DOS of doped OSs can be described by a linear combination of the original DOS

Figure 11. a) Schematic of frontier-orbital hybridization and orbital isosurface plots for a pentacene(PEN)-F4-TCNQ complex. Circled a, b, and c indicate optical transitions assigned in (c); experimental energy values (exp.) and those for the empty antibonding hybrid orbital estimated via the optical gap assuming an exciton binding energy equal to that of PEN as well as DFT values (cal.) are given in parentheses. b) Top: Individual PEN HOMO and F4-TCNQ LUMO orbitals; bottom: representative local-minimum mutual orientation. c) Experimental UV-VIS absorption spectra. Transitions assigned to pristine PEN (circled a), the PEN-F4-TCNQ complex (circled b) in films of equal PEN content, and pristine F4-TCNQ (circled c) for a drop-cast reference film. d) Corresponding cwEPR spectra normalized to the number of molecules in the respective films. Reproduced with permission.[80] Copyright 2012, American Physical Society.



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Figure 12. The effect of doping on the DOS distribution in a disordered organic semiconductor. The Coulomb interaction between ionized dopants and charge carriers creates additional deep traps and broadens the deep tail of the DOS. Reproduced with permission.[56] Copyright 2005, American Physical Society.

distribution and the Coulomb traps of the ionized dopants which itself can be described by: gd ( E ) =

Ni − N d Nd gi ( E ) + gi ( E + ∆ ) Ni Ni

(7)

where Ni is the total density of intrinsic OS molecules, Nd is the density of ionized dopants, gi(E) is the Gaussiantype DOS of intrinsic OSs, and gd(E) is the effective DOS of doped OSs. Nd is equal to the extrinsic carrier density, since the neutrality of the organic matrix has to be maintained. Also, it is assumed that the DOS induced by Coulomb traps is of Gaussian shape with the same width as the intrinsic Gaussian-type DOS. The calculated broadening of the Gaussian distribution upon doping is shown in Figure 12. The broadening of the DOS in doped OSs was experimentally observed by Hulea et al.[94] from electrochemically doped poly(2-methoxy-5-(3′-7′-dimethyloctyloxy)-p-phenylenevinylene) (OC1C10-PPV) using an electrochemically gated transistor (EGT). The experimentally obtained HOMO DOSs are displayed in Figure 13. The HOMO DOS of the doped PPV layers was almost the same regardless of the type of counter ions and fitted well to a Gaussian distribution (area A in Figure 13) with an energetic width of 0.19 eV. However, on linear-log scale (Figure 13b), the HOMO DOS of the doped PPV showed a small amount of an additional energy distribution at the lower energy region which was fitted well by a Gaussian distribution (area B in Figure 13b) with an energetic width of 0.11 eV. This broadening of the Gaussian-type DOS upon doping was also experimentally observed in F4-TCNQ doped NPB[96] and P3HT[55] systems using Kelvin probe potential measurements. While the HOMO DOS distribution of pristine NPB not only shows a Gaussian-type distribution with an energetic width of 0.1 eV but also a small

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Figure 13. E dependence of the experimentally determined DOS (g(E)), E with respect to the vacuum level. The horizontal dashed line marks the HOMO position found from cyclic voltammetry. a) E vs g(E) using PF6− and ClO−4 as anions. A Gaussian function with σ = 0.19 eV (area A) fits the data well. b) E vs g(E) for PF6− on a linear-log scale. Below g = 0.1 states/(eVmonomer) deviations from the Gaussian fit are visible. The filled squares are the PPV FET-data by Tanase et al.,[91] which are well described by an exponential function. At the lowest values of g(E) an additional structure appears, which in a limited energy range would allow a description with a Gaussian with a width of 0.11 eV (area B) as used for PPV LEDs by Martens et al.[95] and Tanase et al.[91] Reproduced with permission.[94] Copyright 2004, American Physical Society.

portion of exponentially distributed tail states, F4-TCNQ doped NPB shows not only the HOMO DOS of pristine NPB but also an additional Gaussian-type distribution. The additional Gaussian-type distribution is fitted well by Equation (7), indicating that ionized F4-TCNQ molecules in the NPB matrix act as Coulomb traps.[96] Considering the Coulomb binding energy between carriers and ionized dopants, other experiments also showed good fitting between experimental and theoretical data on the variation of the work function of F4-TCNQ doped P3HT films with varying the thickness on Cu and Al electrodes.[55]

5. Charge Mobility of Doped Organic Semiconductors In spite of decades of research on electrical doping and its utilization in organic electronics, the effect of doping on the charge mobility is still under discussion, because different findings have been reported for different systems and experimental methods. For example, the hole mobility increased with increasing doping concentration in F4-TCNQ doped ZnPc, poly(2-methoxy-5(2′-ethylhexyloxy)-p-(phenylene vinylene) (MEH-PPV), poly[2,6(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4b0]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole) (PCPDTBT), and m-MTDATA, as well as MoO3 doped NPB and 2-ethylbenzenesulfonic acid (EBSA) doped P3HT.[12,52,97–100]





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Table 2. Summary of the doping-concentration-dependent charge mobility.

Methods

Doping ratio

Mobility [cm2 V−1 s−1]

FET

≈0.1−2%c)

≈2 × 10−4−2 × 10−3[12]

Drift Currentb)

≈1:600 (Dopant:Host)

d)52

MoO3/NPB

SCLC

≈0−10 wt%

≈1.6 × 10−3−4.6 × 10−2[99]

EBSA/P3HT

FET

≈0−1 wt%

≈2.0 × 10−4−3.0 × 10−2[100]

F4-TCNQ/PCPDTBT

FET

≈0−0.3 wt%

≈1.03 × 10−5−4.57 × 10−5[97]

Drift Currentb)

≈0.4−2 wt%

d)[98]

F4-TCNQ/P3HT

Admittance Spectroscopy

≈0−1:10000 (Dopant:Host)

≈1.7 × 10−4−7 × 10−5[102]

ReO3/2-TNATA

Ohmic Current

≈2−15 mol%

≈1.8 × 10−6−5 × 10−7[101]

Doped systemsa) F4-TCNQ/ZnPC F4-TCNQ/MEH-PPV

F4-TCNQ/m-MTDATA

left: dopant materials, right: host materials; b)the full range of J−V characteristics at room temperature are described by the numerical solution of drift current with consideration of electric field- and carrier density-dependent mobility.; c)Molar dopant concentration; d) increased mobility upon doping. a)

On the other hand, other studies reported that the hole mobility is reduced by increasing the doping concentration, e.g., in ReO3 doped 2-TNATA[101] and F4-TCNQ doped P3HT.[102] These different characteristics of doping induced charge mobility are summarized in Table 2.[12,52,97–102] Increasing mobilities with doping concentration were mostly reported from the analysis of J−V characteristics of FETs or single carrier devices in the space-charge-limited current (SCLC) region. An example of the dopingconcentration-dependent FET mobilities extracted from the transfer curves of F4-TCNQ doped ZnPC is shown in Figure 14.[12] Regardless of the growth behavior of ZnPC, the FET mobilitiesincreased with increasing doping concentration. An increased hole carrier mobility upon doping was also reported by SCLC measurements on F4TCNQ doped MEH-PPV films.[52] The current density in the low voltage region increased by ≈1−3 orders of magnitude upon doping, indicating that the hole mobility increased due to the doping. The full range of the J−V characteristics at room temperature was described by the numerical solution of drift current with consideration of a carrier density dependent mobility as shown in Figure 15. The increased mobility upon doping was explained by filling of the tail states of the Gaussian DOS by the generated charges,[91,92,103–106] while this model was developed to explain the significant difference between the SCLC and FET mobilities as shown in Figure 16. The mobilities were plotted against the voltage dependent average charge densities in the SCLC measurements and the gate bias dependent carrier densities at the interface of the OS/insulator interface of the FET channel. The carrier density dependent mobility was nicely fitted with a model considering the trap filling of the tail states of the Gaussian DOS by injected or accumulated charges (solid lines in Figure 16). One problem of the FET and SCLC measurements is that the total carrier density in the OSs during the


measurements is dominated by the injected carriers from the electrodes, not by the extrinsic carrier density via doping. Since the mobilities of amorphous OSs strongly depend on the charge carrier density,[91,92] it is necessary to devise a method where the carrier density generated by doping is much higher than that introduced during the measurement to clearly establish the relationship between the carrier mobility and doping in organic semiconductors. Furthermore, the measured charge mobility with SCLC is critically dependent on the contact properties between electrode and OSs.[61,107] If the charge mobility of OSs is extracted from the quadratic behavior of J−V characteristics assuming SCLC when the contact is not Ohmic, then the experimental results may easily be misinterpreted because the SCLC assumed no contact resistance. One example of the contact issues is clearly manifested in Figure 17 displaying the current densityelectric field (J−F) characteristics of hole-only devices based on NPB with different hole injection layers (HILs) of 1,4,5,8,9,11-hexaazatripheylene hexacarbonitrile (HATCN), MoO3, and ReO3 (ITO/HIL (1 nm)/NPB/Ag).[61] Different HILs resulted in orders of magnitude different current densities and the difference must be related to the injection characteristics rather than the transport in the NPB layer. Analysis using the zero-field mobility (μ0) and Poole-Frenkel coefficient (β) of NPB measured using the time-of-flight (TOF) methods showed that only the ReO3 interlayer forms a perfect Ohmic contact with an injection efficiency of almost 100%, where the injection efficiency was defined as the ratio of the measured current density of a hole or electron-only device to the SCLC of the device.[108] The MoO3 injection layer which is commonly believed to form an Ohmic contact yields a hole injection efficiency lower than 10%. This low injection efficiency at the MoO3/NPB interface was also reported for thick MoO3 films (≈5−20 nm).[107,109,110] If one extracts the mobility value using the MoO3 layer from the J−F curves in the


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Figure 15. Current density versus voltage characteristics of MEH-PPV hole-only devices for various dopant:host molar ratios. (Square: undoped MEH-PPV, Circle: doping ratio of 1:860 (dopant:host), Triangle: doping ratio of 1:750 (dopant:host), Inverted triangle:doping ratio of 1:600 (dopant:host)) Also included (solid lines) are the numerically calculated current densities taking into account a model that describes the chargecarrier density dependent hole mobility. Reproduced with permission.[52]

Figure 14. a) Transfer characteristics of a thin film transistor (type I) using a 30 nm polycrystalline ZnPc layer as active semiconductor (doping ratio 0.7%). b) FET mobility and Seebeck-mobility vs molar doping ratio: the FET mobility increases with increasing dopant concentration and the Seebeck-mobility seems to be independent of the doping concentration (within the experiment error). Reproduced with permission.[12] Copyright 2001, American Physical Society.

SCLC region, significantly lower mobility values will be obtained due to the low hole injection efficiency. Doping in OSs will alter not only the mobility of the layer, but also the contact property, making the analysis of mobility very difficult due to the mixing of effects. Recently Yoo et al. resolved these issues by investigating the carrier mobility in the Ohmic current region, where the carrier density generated by doping is much higher than the injected carrier density.[101] Therefore, the carrier density in the doped organic layer of each sample must be constant during the measurement and independent of the electric field. They also used a hole-only device with a simple Mp++pp++M (metal/heavily p-doped organic layer/ p-doped organic layer/heavily p-doped organic layer/ metal) structure, where ReO3 doped 2-TNATA was used

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as the amorphous p-doped organic layer (p) and indiumtin-oxide (ITO) and Al were used as the electrodes. The Ohmic contacts between the electrodes and the p-doped organic layer were achieved by inserting thin, heavily doped organic layers (p++, 50 mol%) for both contacts. Linear J−F relationships for various doping concentrations (≈2−15 mol%) allowed the unambiguous determination of the conductivity. The conductivity of the doped film increased with the doping concentration and was in the range of ≈4−9 × 10−8 S cm−1. This conductivity is four orders of magnitude greater than that of pristine 2-TNATA

Figure 16. Temperature-dependent mobility μh(p, T) vs hole density for OC1C10-PPV. The solid lines are the calculated μh(p, T). Reproduced with permission.[92] Copyright 2004, American Physical Society.





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Figure 17. J−F characteristics of hole-only devices (based on NPB) with various high electron affinity (EA) materials (HAT-CN, MoO3 and ReO3) as the interlayer or without an interlayer on logarithmic axes. The current density designates the absolute magnitude. The open symbols represent measured values and the solid line represents calculated field-dependent SCLCs. Reproduced with permission.[61] Copyright 2014, Nature Publishing Group.

(≈10−12 S cm−1). The hole carrier densities in the p-doped layers were measured separately by Schottky-Mott analysis of the C−V characteristics, using the MIS structure. The hole mobility of the p-doped films was analyzed from the measured conductivity and the carrier density using the Drude model. J = qµh (T , p ( x ) , F ( x )) p ( x ) F ( x )

(8)

Since the density of carriers injected from the electrodes is lower than the carrier density generated by doping in the Ohmic current region, the carrier density and the electric field are independent of x (the distance from the anode). Therefore, we can neglect the effect of electric field on the mobility, and the drift current density and conductivity can be expressed as J = qµh (T , p ) pF ,

σ = qpµh (T , p )

(9) (10)

The hole mobility at room temperature was plotted as a function of the doping concentration in Figure 18. The results showed that the hole mobility in the doped system decreased as the doping concentration increases. The hole mobility was in the order ≈10−6−10−7 cm2 V−1 s−1, one to two orders of magnitude lower than that of pristine films (≈10−5 cm2 V−1 s−1 deduced from TOF measurements.[111] However, these results were obtained at a relatively high doping range between ≈2−15 mol%. In order to generalize the trends of reduced mobility by doping, the charge mobility should be analyzed at a low doping range. This reduction of the mobility upon doping was also reported previously from F4-TCNQ doped P3HT at low-to-medium


Figure 18. Hole mobilty of ReO3-doped 2-TNATA films at room temperature. The hole mobility was analyzed from the meausred conductivity and the carrier density within the Drude model. Reproduced with permission.[101] Copyright 2013, AIP Publishing LLC.

doping levels.[102] Admittance spectroscopy was used to obtain the conductivity and carrier density using a MIS structure of ITO/PSQ (insulator)/F4-TCNQ doped P3HT/ MoO3/Al. The obtained hole density, bulk conductivity, and hole carrier mobility are displayed at Figure 19. The hole mobility was steadily reduced from 1 × 10−4 cm2 V−1 s−1 to 5 × 10−5 cm2 V−1 s−1 (at a doping ratio of 3 × 10−5). The decreased hole mobility upon doping can be interpreted as extension of the lower tail of the intrinsic Gaussian DOS of the organic host matrix by Coulomb traps. Arkhipov et al. simulated the mobility considering the broadening of the Gaussian DOS upon doping.[56] Figure 20 displays the simulated doping-concentrationdependent carrier mobility in doped OSs having different energetic disorder parameters. The simulation predicted that the mobility will be reduced upon doping due to the trap states except for energetically very disordered organic systems. The decreased mobilities upon doping in ReO3 doped 2-TNATA and F4-TCNQ doped P3HT are consistent with the theoretical prediction because 2-TNATA and P3HT have low energetic disorder parameters of 69 meV[112] and 78 meV,[55] respectively. Reduction of mobility by doping was also reported in an n-doped crystalline organic semiconductor (3,6-bis(dimethylamino)acridine doped C60) in early times, which was interpreted by scattering by impurities.[113]

6. Conclusions Electrical doping effects in disordered OSs have been described focusing on the charge generation and carrier mobility. The doping induced CGE is generally low in OSs even though high CGEs above a few tens of percent were reported in recent years. The low CGE has been explained


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Figure 19. a) Hole density of undoped and doped P3HT layers. The line models a linear increase of the hole density, starting from a background of 1.2 × 1021 m−3. b) Low-field bulk conductivity and c) hole mobility. The solid lines are calculated according to the model of Arkhipov et al.,[56] using the parameters given in the inset. Reproduced with permission.[102] Copyright 2012, American Institute of Physics.

by the integer charge transfer model and the hybrid charge transfer model. Both increasing and decreasing mobilities have been reported upon doping. Increasing mobilities with doping concentration were mostly reported from the analysis of J−V characteristics of FETs or single carrier devices in the SCLC region and were interpreted by filling of the tail states of the Gaussian DOS by generated charges. One problem of FET and SCLC measurements is that the total carrier density in OSs during the measurements is dominated by the injected carriers from the electrodes, not by the extrinsic carrier density via doping. Furthermore, the measured charge mobility with SCLC is dependent on the contact properties between electrode and OSs. These issues have been resolved. Reduced carrier mobilities were reported by investigating the carrier mobility in the Ohmic current region, where the carrier density generated by doping is much higher than the injected carrier density and also by admittance spectroscopy using a MIS structure. The reduced mobility was interpreted by the charge

trapping effect of the ionized dopants formed by the charge transfer process between hosts and dopants. Apparently, there are models and experimental results which seem to be inconsistent with each other. In order to fully understand the doping processes in OSs, further quantitative and systematic analyses of charge generation and transport characteristics must be accomplished in the field of Oss in the future. Acknowledgements: This work was supported by the IRTG program (2014001836) of National Research Foundation of Korea (NRF) and by the Mid-career Researcher Program (2014R1A2A1A01002030) through an NRF (National Research Foundation) grant funded by the MSIP (Ministry of Science, ICT and Future Planning). The graphical abstract was converted to color on June 02, 2015. None of the information represented by the figure was changed. Received: January 15, 2015; Revised: February 27, 2015; Published online: April 9, 2015; DOI: 10.1002/marc.201500026 Keywords: doping; charge generation efficiency; mobilities; coulomb traps; organic semiconductors

Figure 20. Dependence of the carrier mobility upon the concentration of dopants in materials with different variations of the intrinsic DOS distribution. Reproduced with permission.[56] Copyright 2005, American Physical Society.

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Influence of ionizing dopants on charge transport in organic semiconductors.

Molecular dynamics and charge transport in organic semiconductors: a classical approach to modeling electron transfer.

Charge carrier coherence and Hall effect in organic semiconductors.

Energy position of the transport path in disordered organic semiconductors.

QM approach to model energy disorder in amorphous organic semiconductors.

Charge carrier mobility in thin films of organic semiconductors by the gated van der Pauw method.

Madelung and Hubbard interactions in polaron band model of doped organic semiconductors.

Nanoscale Imaging of Charge Carrier and Exciton Trapping at Structural Defects in Organic Semiconductors.

Organic semiconductors: Healing contact.

Bulk charge carrier transport in push-pull type organic semiconductor.

Charge transport in nanoscale vertical organic semiconductor pillar devices.

Theoretical investigations on charge-transfer properties of pentacenequinone derivatives as n-type organic semiconductors.

Prediction of Charge Mobility in Amorphous Organic Materials through the Application of Hopping Theory.

Charge transport mechanisms and memory effects in amorphous TaNx thin films.

Polymorphism in new thienothiophene-thiazolothiazole organic semiconductors.

Polymorphism in new thienothiophene-thiazolothiazole organic semiconductors.

Trap-limited exciton diffusion in organic semiconductors.

Amorphous metal-organic frameworks.

Heavily Doped, Charge-Balanced Fluorescent Organic Light-Emitting Diodes from Direct Charge Trapping of Dopants in Emission Layer.

Enhancement of charge transport properties of small molecule semiconductors by controlling fluorine substitution and effects on photovoltaic properties of organic solar cells and perovskite solar cells.

Interface Structure of MoO3 on Organic Semiconductors.

Nature of charge transport and p-electron ferromagnetism in nitrogen-doped ZrO2: an ab initio perspective.

Noble-metal-free plasmonic photocatalyst: hydrogen doped semiconductors.

Probing charge transfer between molecular semiconductors and graphene.