N-Type Molecular Doping in Organic Semiconductors: Formation and

Apr 13, 2016 - Department of Creative Convergence Engineering, Hanbat National University, Daejeon 305-719, Korea. ∥ Molecular-Level Interface ...
0 downloads 0 Views 1MB Size
Download PDF
Article pubs.acs.org/JPCC

N‑Type Molecular Doping in Organic Semiconductors: Formation and Dissociation Efficiencies of a Charge Transfer Complex Jae-Min Kim,† Seung-Jun Yoo,† Chang-Ki Moon,† Bomi Sim,† Jae-Hyun Lee,*,§ Heeseon Lim,∥ Jeong Won Kim,⊥ and Jang-Joo Kim*,†,‡ †

Department of Materials Science and Engineering, Seoul National University, Seoul 151-742, Korea Research Institute of Advanced Materials (RIAM), Seoul National University, Seoul 151-744, Korea § Department of Creative Convergence Engineering, Hanbat National University, Daejeon 305-719, Korea ∥ Molecular-Level Interface Research Center, Department of Chemistry, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, Korea ⊥ Korea Research Institute of Standards and Science, Daejeon 305-340, Korea ‡

S Supporting Information *

ABSTRACT: Electrical doping is an important method in organic electronics to enhance device efficiency by controlling the Fermi level, increasing the conductivity, and reducing the injection barrier from the electrode. To understand the charge generation process of the dopant in doped organic semiconductors, it is important to analyze the charge transfer complex (CTC) formation and dissociation into a free charge carrier. In this paper, we correlate the charge generation efficiency with the CTC formation and dissociation efficiency of an n-dopant in organic semiconductors. The CTC formation efficiency of Rb2CO3 linearly decreases from 82.8% to 47.0% as the doping concentration increases from 2.5 to 20 mol %. The CTC formation efficiency and its linear decrease with the doping concentration are analytically correlated with the concentration-dependent size and number of dopant agglomerates by introducing the degree of reduced CTC formation. Lastly, the behavior of the dissociation efficiency is discussed on the basis of the picture of the statistical semiconductor theory and the frontier orbital hybridization model.

1. INTRODUCTION Electrical doping is widely used to enhance the conductivity of organic semiconductors by increasing the charge carrier density. However, doping characteristics of organic semiconductors are different from those of inorganic semiconductors because of the intrinsic properties, which include the Gaussian density of states, charge transfer process of molecular doping, low dielectric constant, and so on.1,2 Two different models have been proposed to describe the electrical doping in organic semiconductors: the integer charge transfer model and the orbital hybridization model.3−5 In the integer charge transfer model, doping generates free charge carriers through integer charge transfer between the host and dopant molecules. The ionized dopants and free carriers are generated simultaneously, and since they have an opposite polarity, the ionized dopants act as Coulomb traps to the free carriers.6 The low charge generation efficiency in organic semiconductors (OSs) compared to inorganic semiconductors can be attributed to a strong Coulomb binding energy, because common organic semiconductors have a low dielectric permittivity of about 3.7 In contrast to the integer charge transfer model, the orbital hybridization model states that the doping effect is caused by © 2016 American Chemical Society

the formation of hybrid charge transfer complexes upon partial charge transfer between the host and dopant molecules.4,5 In this model, the doping property originates from the hybrid charge transfer complex, which has a narrow band gap. As a result of the difference between the Fermi energy level and the unoccupied states of hybrid charge transfer complexes, only a fraction of the hybrid charge transfer complexes can be ionized at room temperature, resulting in a low charge generation efficiency. It is also reported that dopant aggregation8,9 and a low dissociation probability related to the disorder at the transport level10 are other factors that cause low charge generation efficiency. Recently, the modified Fermi energy level of n- or p-doped organic semiconductors was statistically investigated on the basis of the combination of classical semiconductor physics and energy distribution theory.11,12 It was argued that the process of molecular doping in organic semiconductors, be it through integer charge transfer or orbital hybridization, must be regarded as a modification of the density Received: February 3, 2016 Revised: April 12, 2016 Published: April 13, 2016 9475

DOI: 10.1021/acs.jpcc.6b01175 J. Phys. Chem. C 2016, 120, 9475−9481

Article

The Journal of Physical Chemistry C of states in the organic semiconductor upon dopant admixture, and the Fermi−Dirac occupation at a certain temperature determines the doping efficiency.13 No matter which model we can apply to the doping process in organic semiconductors, charge generation needs to be analyzed in terms of two processes, namely, charge transfer complex (CTC) (or ion pair) formation between the host and dopant molecules (an ionized dopant−host pair for the integer charge transfer model and a hybrid charge transfer complex for the orbital hybridization model, hereafter represented by CTC for both pairs), and dissociation of free carriers from the CTCs. Therefore, the charge generation efficiency can be expressed by ηCG =

NCTC Nfree‐carrier = ηCTCηdisso Ndopant NCTC

(1)

where ηCG is the charge generation efficiency and Ndopant, NCTC, and Nfree‑carrier are the number densities of the dopant molecules, CTCs, and free charge carriers, respectively. ηCTC and ηdisso are the formation and dissociation efficiencies of the CTC, respectively. The behaviors of these two efficiencies should be investigated separately to establish a quantitative description of the doping characteristics. While a change of the electrical properties upon doping has been reported in many studies,6,9,10,14−25 there have been few results relating the charge generation process itself to the CTC formation and dissociation up to now.3,8,11,26 Recently, ηCTC of a p-doped organic semiconductor was measured to be in the range of 13− 77% as determined by UV−vis or FT-IR spectroscopy.3,26 In the case of ηdisso, no quantitative analyses on the dependence of ηdisso on the doping concentration were reported to the best of our knowledge because it is hard to consider the dopant aggregation or the charge trapping effect. Furthermore, there are few results for n-doped organic semiconductors related to the charge generation process available to facilitate a comprehensive understanding of the doping phenomena due to their instability in the presence of air.27−32 In this study, we comprehensively investigate the charge generation process of n-doped organic semiconductors using Rb2CO3-doped 2,2,2-(1,3,5-benzinetriyl)-tris(1-phenyl-1H-benzimidazole) (TPBi). We measure ηCTC at various doping concentrations from the change in the UV−vis−NIR absorption spectra of the host molecules upon doping. ηCTC linearly decreases from 82.8% to 47.0% as the doping concentration (C) increases from 2.5 to 20 mol %. The equation of the CTC formation efficiency including the constant of reduced CTC formation is first introduced to quantitatively characterize the aggregation property of the dopant. ηdisso calculated with ηCTC and the carrier density decreases from 3.4% to 1.6%. The change of the dissociation efficiency with the doping concentration is interpreted in view of the classical semiconductor model and the orbital hybridization model.

Figure 1. (a) UV−vis−NIR absorption spectra of a Rb2CO3-doped TPBi thin film and (b) calibrated absorption spectra without reflection. The number of TPBi molecules in all samples is fixed as that of the 50 nm TPBi pristine thin film.

TPBi (304 nm) is reduced and those of CTC (479 and 600 nm) are increased. The change in the absorption spectra indicates CTC formation between the TPBi and Rb2CO3 molecules. Since absorbance is linearly dependent on the concentration of molecules according to the Beer−Lambert law, we can calculate the reduced number of neutral TPBi molecules, i.e., the number of CTCs, from the peak intensity of neutral TPBi at 304 nm. To remove the reflected light loss from the surface of the analyzed film, we calibrated the absorption spectra of all samples by subtracting the calculated reflection using the transfer matrix method with the refractive indices (n, k) of the TPBi film measured by variable-angle spectroscopic ellipsometry. Figure 1b represents the UV−vis−NIR absorption spectra after the calibration. The CTC formation efficiency (ηCTC), i.e., the ratio of the number density of CTCs (NCTC) to dopant molecules (Ndopant), can then be derived according to the following equation: ηCTC =

NCTC 100 − C NCTC = Ndopant C Nhost

(2)

where C and Nhost are the doping concentration (mol %) and the number density of the host molecules, respectively. NCTC/ Nhost can be obtained by the relative ratio of the reduced peak intensity of neutral TPBi. The CTC formation efficiency obtained from this method is displayed in Figure 2. The CTC formation efficiency linearly decreases from 82.8% to 47.0% as the doping concentration increases from 2.5 to 20 mol %. We consider that a decrease in ηCTC originates from the aggregation of dopant molecules, assuming all encounters between the host and dopant molecules lead to the formation of CTCs.9,10 To analyze the aggregation property quantitatively, the degree of

2. RESULTS AND DISCUSSION 2.1. CTC Formation Efficiency. Rb2CO3-doped TPBi films are thermally evaporated onto a glass substrate and encapsulated in a glovebox immediately after deposition. The number of TPBi molecules in all samples is monitored by the quartz crystal monitor and is fixed as that of the 50 nm thick TPBi pristine film. Figure 1a shows the UV−vis−NIR absorption spectra of Rb2CO3-doped TPBi thin films. As the doping concentration increases, the peak intensity of intrinsic 9476

DOI: 10.1021/acs.jpcc.6b01175 J. Phys. Chem. C 2016, 120, 9475−9481

Article

The Journal of Physical Chemistry C

efficiency with the doping concentration indicates that the degree of reduced CTC formation is linearly proportional to the doping concentration. δCTC of Rb2CO3 increases from 0.17 to 0.53. This means that at a high doping concentration half of the CTCs are formed due to aggregation. With the equation of the CTC formation efficiency, the characteristics of dopant aggregation can be described. In the extreme case where δCTC is constant and η0 is 100%, aggregation of dopants will not occur. If δCTC is constant and η0 is lower than 100%, only the number of aggregated dopant clusters will increase with a fixed average size as the doping concentration is increased. The linear decrease of the CTC formation efficiency with increasing doping concentration of Rb2CO3 and η0 lower than 100% in our results indicate that both the number and the size of the aggregates increase simultaneously. In this case, if each cluster is able to transfer one electron, the average number of dopant molecules per cluster would increase from 1.21 at 2.5 mol % to 2.13 at 20 mol %. The number density of CTCs increases from 1.54 × 1019 to 1.42 × 1020 cm−3. Introduction of the equation will facilitate quantitative comparison of the aggregation property of various dopants. Future work will include a detailed investigation of the aggregation property for various dopants using the behavior of the CTC formation efficiency. This analysis of the CTC formation efficiency is based on in situ Xray photoemission spectroscopy (XPS) results that show 1.34 effective dopants are produced per Rb2CO3 molecule during thermal deposition, as described below. 2.2. Decomposition of Rb2CO3 Based on in Situ XPS Measurements. The decomposition of alkali-metal compounds during thermal evaporation was reported using in situ XPS.33−35 Accordingly, the thermal decomposition of Rb2CO3 is investigated by in situ XPS. Figure 3 shows the XPS core level spectra of (a) Rb 3d, (b) O 1s, and (c) C 1s for a Rb2CO3 film deposited on a Au substrate. The Au substrate is cleaned by Ar+ sputtering before deposition. In the region of the C 1s core level, the spectrum fitted with Voigt functions is composed of three distinct bonding features that have binding energy peaks at 286.1, 287.3, and 290.3 eV, respectively. These carbon peaks do not result from contamination of the deposition chamber because carbon peaks are not observed when intrinsic Rb is deposited. The Rb2CO3 molecule has only one bonding feature corresponding to a carbon of carbonate, CO32−, and represents the binding energy of ∼290 eV.36 Thus, the three binding energy peaks suggest that evaporated Rb2CO3 would have at

Figure 2. CTC formation efficiency of a Rb2CO3-doped TPBi thin film with various dopant densities. The dashed line is the linear fit of the data. The CTC formation efficiency is the ratio of the number density of CTCs to dopant molecules. The linear decrease of the CTC formation efficiency means that both the number and the size of the aggregates increase simultaneously.

reduced CTC formation, δCTC, is introduced and defined by eq 3. From mathematical derivation of the definition, doping concentration-dependent ηCTC and δCTC can be expressed by δCTC =

0 NCTC − NCTC 0 NCTC

= f (Ndopant)

(0 ≤ δCTC ≤ 1) (3)

and ηCTC = η0 − η0δCTC

(Ndopant > 0)

(4)

where η0 and are the values of ηCTC and NCTC in the situation where all dopants are surrounded by host molecules. The degree of reduced CTC formation is defined as the ratio between the reduced number of CTCs due to aggregation and the number of CTCs at extreme dilution. By the definition, the degree of reduced CTC formation is dependent on the dopant concentration and has a value between 0 and 1. If the aggregation property of the dopant is strong, the degree of reduced CTC formation will have a high value. From the experimental result, a linear decrease of the CTC formation N0CTC

Figure 3. In situ XPS core level spectra of (a) Rb 3d, (b) O 1s, and (c) C 1s for an evaporated Rb2CO3 film on a Au substrate. The decomposed peaks represent the results of peak fitting with Voigt functions. 9477

DOI: 10.1021/acs.jpcc.6b01175 J. Phys. Chem. C 2016, 120, 9475−9481

Article

The Journal of Physical Chemistry C least three kinds of carbon bonding, including the carbonate bond. Considering these characteristics, we can speculate on the thermal deposition process of Rb2CO3 as follows. First, Rb2CO3 decomposes into Rb, O2, and CO2 when the temperature is raised: Rb2 CO3 → 2Rb + 0.5O2 + CO2

Nfree‐carrier =

2 qε0εr

∂C p−2

(7)

∂V

where q, ε0, εr, Cp, and V are the elementary charge, vacuum permittivity, dielectric permittivity, capacitance, and dc voltage, respectively. The Mott−Schottky plot and loss spectrum of the MIS devices are displayed in Figure S1 in the Supporting Information. In Figure 4, the carrier density of Rb2CO3-doped

(5)

These gas components could be the reason for the increasing base pressure in the vacuum chamber during thermal evaporation. Thereafter, O2 and CO2 gases in the vacuum chamber react with Rb again in the process of deposition, and Rb2CO3 and Rb2CO3−x are produced by the following reaction: 2aRb + 0.5ΔaO2 + ΔaCO2 → baRb2 CO3 + (1 − b)aRb2 CO3 − x

(6)

a is introduced for the purpose of calculating the amount of reacted Rb, and b is the relative ratio between Rb2CO3 and Rb2CO3−x. Δa is the nonperfect stoichiometry of the reaction. Therefore, the peak at 290 eV originates from Rb2CO3,36 and other peaks can be attributed to Rb2CO3−x. The relative area ratio of the Rb 3d peak with respect to the C 1s peak of carbonate (290 eV) must be kept in a ratio of 2:1 when a Rb2CO3 molecule is deposited without thermal decomposition during the deposition. In our measurements, however, the ratio is shown to be 3.03:1. To determine the effective dopant density, we quantitatively calculate the amounts of Rb, Rb2CO3, and Rb2CO3−x from the relative area ratio between each Rb 3d and C 1s peak of 290 eV and the others. On the basis of this analysis, 0.68, 0.22, and 0.44 Rb, Rb2CO3, and Rb2CO3−x molecules are produced per Rb2CO3 molecule, respectively. Since the oxygen states in Rb2CO3 and Rb2CO3−x in the O 1s spectra are not clearly resolved within instrumental resolution (∼1 eV), O 1s spectra are not considered for quantitative analysis. Assuming that each molecule (i.e., Rb, Rb2CO3, and Rb2CO3−x) can take part in the doping process, we use Ndopant as the effective dopant density as with 1.34 effective dopants per Rb2CO3 molecule. The CTC formation efficiency is obtained from the effective dopant density. 2.3. Charge Generation Efficiency and Dissociation Efficiency. The CTC formation efficiency of Rb2CO3-doped TPBi is higher than the charge generation efficiency for all doping concentrations. In their recent paper, Pingel et al. analyzed the UV−vis absorption spectra of fully ionized P3HT and F4-TCNQ molecules to determine the fraction of ionized molecules.3 From these results and our experimental data, it is shown that the formation of CTCs from dopant and host molecules is not the limiting process of molecular doping in organic semiconductors when the energy level difference between the HOMO (for p-type doping) or LUMO (for ntype doping) of a host and the Fermi level of the dopant is sufficient. To determine the dissociation efficiency, we fabricated metal−insulator−semiconductor (MIS) devices to measure the charge carrier density.24,25,37 The device structure is indium tin oxide (ITO) (150 nm)/LiF (150 nm)/Rb2CO3doped TPBi (200 nm)/Rb2CO3 (1 nm)/Al (100 nm). The intrinsic Rb2CO3 layer is used for efficient electron injection to equalize the electron injection barrier in all devices. The ac frequency of impedance spectroscopy is 200 Hz as determined from the loss spectrum (G/ω), and the amplitude is 50 mV. We obtain the charge carrier density using the Mott−Schottky equation

Figure 4. Charge carrier density of a Rb2CO3-doped TPBi layer from Mott−Schottky analysis. The device structure is ITO (150 nm)/LiF (150 nm)/Rb2CO3-doped TPBi (200 nm)/Rb2CO3 (1 nm)/Al (100 nm). The dashed line is the linear fit of the data (Nfree‑carrier = sNdopant + n0).

TPBi linearly increases from 8.9 × 1017 to 2.2 × 1018 cm−3 with increasing doping concentration. Linearity between the carrier density and the doping concentration is reported in many doped systems.24,25,37,38 Accordingly, the carrier density can be expressed by the following equation: Nfree‐carrier = sNdopant + n0

(8)

where s and n0 are the slope of the change in carrier density and the intercept, respectively. At this point, the equation describes the change of the carrier density in the range of a few mole percent. Therefore, there is a lack of correspondence between n0 and the intrinsic carrier density. The values of s and n0 are 4.47 × 10−3 and 8.74 × 1017 cm−3. The charge generation efficiency and dissociation efficiency can finally be expressed from the linearity of the CTC formation efficiency and the carrier density with increasing doping concentration as follows: ηCG =

ηdisso =

⎞ Nfree‐carrier ⎛ n0 = ⎜⎜ + s⎟⎟ Ndopant ⎝ Ndopant ⎠

sNdopant + n0 Nfree‐carrier = −δCTCNdopant + η0Ndopant NCTC

(9)

(10)

The charge generation efficiency and dissociation efficiency are depicted in Figure 5a. In the low doping concentration regime (Ndopant < 2 × 1020 cm−3), the charge generation efficiency and dissociation efficiency decreased substantially with the doping concentration. The dissociation efficiency saturates to 1.6% in the high doping concentration regime (Ndopant > 2 × 1020 cm−3). This concentration-dependent 9478

DOI: 10.1021/acs.jpcc.6b01175 J. Phys. Chem. C 2016, 120, 9475−9481

Article

The Journal of Physical Chemistry C

of the dopant. A similar discussion can be applied to the orbital hybridization model. According to the model, frontier orbital hybridization between the host and dopant results in formation of a hybrid charge transfer complex whose HOMO level is located below the LUMO level of the host. Since the Fermi level increases with increasing doping concentration, the ionization probability of the CTC to the LUMO of the host will be reduced again upon increasing the doping concentration.

(11)

3. CONCLUSION We investigate the charge generation efficiency in terms of the CTC formation efficiency and dissociation efficiency of a Rb2CO3-doped TPBi thin film. Analysis of the peak intensities in the TPBi UV−vis−NIR spectra can lead to quantitative derivation of the CTC formation efficiency, which linearly decreases as the doping concentration increases. For the first time the quantitative method for characterizing the aggregation property is introduced. Given that the slope of the curve of the CTC formation efficiency against the doping concentration is negative in our experimental results, it is shown that both the number and size of the dopant aggregates increase simultaneously. Moreover, the degree of reduced CTC formation as a function of the doping concentration is introduced. The degree of reduced CTC formation will provide an intuitive perspective of the effectiveness of dopants at various doping concentrations. The low charge generation efficiency (2.8%) compared to the high CTC formation efficiency (82.8%) indicates that the dissociation of bound charge from the charge transfer complex is the most impeding process in the electrical doping of organic semiconductors. The doping concentration-dependent behavior of the dissociation efficiency is first analyzed experimentally. The dissociation efficiency decreases from 3.4% at 3.15 × 1019 cm−3 to 1.6% at 3.07 × 1020 cm−3. The reduction of the dissociation efficiency with increasing doping concentration can be explained by the reduced ionization probability originating from the shift of the Fermi level located above the donor level away from the donor level (CTC HOMO level in the orbital hybridization model) due to the increased electron density with increasing doping concentration. This study contributes to further understanding the charge generation process of doping in organic semiconductors. In the future, doping technology based on an established principle and theory will make precise parameter control and an efficient doping process feasible for organic electronics such as organic light-emitting diodes, organic photovoltaic devices, and organic thin-film transistors.

EF, ED, kB, and T are the number density of the ionized dopant, the Fermi level, the donor level, the Boltzmann constant, and the temperature, respectively. In this equation, Ndopant can be substituted by NCTC since NCTC is the actual number density of the dopant considering dopant aggregation. Using the equation, the doping concentration-dependent energy difference between the Fermi level and the donor level can be obtained from the dissociation efficiency. The change of the position of the Fermi level relative to the donor level is displayed in Figure 5b. The Fermi level shifts from 86.5 meV at 3.15 × 1019 cm−3 to 106.1 meV at 3.07 × 1020 cm−3 above the donor level as the doping concentration increases. The shift of the Fermi level away from the donor level toward the LUMO level with increasing dopant density can be understood from the increased electron density with increasing doping concentration leading to a lower activation probability

4. EXPERIMENTAL SECTION 4.1. UV−Vis−NIR Absorption Spectroscopy Analysis. We investigate a doped organic semiconductor consisting of 1,3,5-tris(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl (TPBi) (Nichem Co. Ltd.) as the host molecule and Rb2 CO3 (Sigma-Aldrich Co. LLC) as the dopant molecule. All doped thin films are fabricated by coevaporating TPBi and Rb2CO3 molecules at room temperature. Samples are encapsulated with a moisture absorber in the dry nitrogen glovebox prior to the measurement. The absorption spectra are measured using the Cary 5000 UV−vis−NIR spectrophotometer and calibrated. 4.2. X-ray Photoelectron Spectroscopy (XPS) Measurement. XPS measurements are performed using a hemispherical electron energy analyzer. Since the analysis chamber is directly connected to the evaporation chamber, a thermally

Figure 5. (a) Formation and dissociation efficiencies of a CTC. Data denoted by symbols are obtained from the experiment. The dashed line is the fitted line of the CTC formation efficiency, and the solid lines represent the calculated dissociation efficiency and charge generation efficiency using the constant derived from the experimental data. (b) Energetic position of the Fermi level relative to the donor level based on the Fermi−Dirac distribution of the single donor level.

dissociation behavior can be described by activation from the donor level according to the classical semiconductor theory:39 Nfree‐carrier ≈ ND+ =

Ndopant 1 + exp[(E F − E D)/kBT ]

N+D,

9479

DOI: 10.1021/acs.jpcc.6b01175 J. Phys. Chem. C 2016, 120, 9475−9481

Article

The Journal of Physical Chemistry C deposited Rb2CO3 thin film is characterized without breaking the vacuum condition. The XPS core level scan is performed by Mg Kα (ℏω = 1253.6 eV) photon lines. Using an Alq3 thin film, which is a thermally stable compound, and Rb2CO3 powder, the different cross sections of the C 1s, O 1s, and Rb 3d core level peaks are considered for the quantitative analysis. For the Rb 3d core level spectra, the reference value of the spin−orbit splitting energy (1.49 eV) and intensity ratio for two spin−orbit split peaks (3:2) is considered as a fitting parameter. All spectra are fitted on the basis of a Shirley-background correction. 4.3. Fabrication of Metal−Insulator−Semiconductor (MIS) Devices and Impedance Spectroscopy. The structure for all devices is ITO (150 nm)/LiF (150 nm)/ Rb2CO3-doped TPBi (200 nm)/Rb2CO3 (1 nm)/Al (100 nm). Before evaporation, ITO is cleaned using acetone and isopropyl alcohol. Impedance analysis is carried out by a 1260 impedance/gain-phase analyzer and a 1287 electrochemical interface (Solartron). Considering the capacitance loss spectrum, the ac frequency is determined to be 200 Hz, and the amplitude is 50 mV.

■

Intermolecular Hybridization Governs Molecular Electrical Doping. Phys. Rev. Lett. 2012, 108, 035502. (6) Yoo, S.-J.; Lee, J.-H.; Lee, J.-H.; Kim, J.-J. Doping-concentrationdependent Hole Mobility in a ReO3 Doped Organic Semiconductor of 4,4′,4″-tris(N-(2-naphthyl)-N-phenyl-amino)-triphenylamine. Appl. Phys. Lett. 2013, 102, 183301. (7) Bernardo, B.; Cheyns, D.; Verreet, B.; Schaller, R. D.; Rand, B. P.; Giebink, N. C. Delocalization and Dielectric Screening of Charge Transfer States in Organic Photovoltaic Cells. Nat. Commun. 2014, 5, 3245−3251. (8) Lee, J.-H.; Kim, H.-M.; Kim, K.-B.; Kabe, R., Jr; Anzenbacher, P.; Kim, J.-J. Homogeneous Dispersion of Organic P-dopants in an Organic Semiconductor as an Origin of High Charge Generation Efficiency. Appl. Phys. Lett. 2011, 98, 173303. (9) Lee, J.-H.; Kim, H.-M; Kim, K.-B.; Kim, J.-J. Origin of Charge Generation Efficiency of Metal Oxide P-dopants in Organic Semiconductors. Org. Electron. 2011, 12, 950−954. (10) Mityashin, A.; Olivier, Y.; Van Regemorter, T.; Rolin, C.; Verlaak, S.; Martinelli, N. G.; Beljonne, D.; Cornil, J.; Genoe, J.; Heremans, P. Unraveling The Mechanism of Molecular Doping in Organic Semiconductors. Adv. Mater. 2012, 24, 1535−1539. (11) Tietze, M. L.; Pahner, P.; Schmidt, K.; Leo, K.; Lussem, B. Doped Organic Semiconductors: Trap-filling, Impurity Saturation, and Reserve Regimes. Adv. Funct. Mater. 2015, 25, 2701−2707. (12) Tietze, M. L.; Burtone, L.; Riede, M.; Lussem, B.; Leo, K. Fermi Level Shift and Doping Efficiency in P-doped Small Molecule Organic Semiconductors: A Photoelectron Spectroscopy and Theoretical Study. Phys. Rev. B: Condens. Matter Mater. Phys. 2012, 86, 035320. (13) Salzmann, I.; Heimel, G.; Oehzelt, M.; Winkler, S.; Koch, N. Molecular Electrical Doping of Organic Semiconductors: Fundamental Mechanisms and Emerging Dopant Design Rules. Acc. Chem. Res. 2016, 49, 370. (14) Chan, C. K.; Kim, E.-G.; Bredas, J.-L.; Kahn, A. Molecular nType Doping of 1,4,5,8-Naphthalene Tetracarboxylic Dianhydride by Pyronin B Studied Using Direct and Inverse Photoelectron Spectroscopies. Adv. Funct. Mater. 2006, 16, 831−837. (15) Qi, Y.; Sajoto, T.; Barlow, S.; Kim, E.-G.; Bredas, J.-L.; Marder, S. R.; Kahn, A. Use of a High Electron-Affinity Molybdenum Dithiolene Complex to p-Dope Hole-Transport Layers. J. Am. Chem. Soc. 2009, 131, 12530−12531. (16) Olthof, S.; Tress, W.; Meerheim, R.; Lussem, B.; Leo, K. Photoelectron Spectroscopy Study of Systematically Varied Doping Concentratioins in an Organic Semiconductor Layer Using a Molecular p-Dopant. J. Appl. Phys. 2009, 106, 103711. (17) Meyer, J.; Hamwi, S.; Schmale, S.; Winkler, T.; Johannes, H.-H.; Riedl, T.; Kowalsky, W. A Strategy Towards p-Type Doping of Organic Materials with HOMO Levels beyond 6 eV Using Tungsten Oxide. J. Mater. Chem. 2009, 19, 702−705. (18) Kroger, M.; Hamwi, S.; Meyer, J.; Riedl, T.; Kowalsky, W.; Kahn, A. P-type Doping of Organic Wide Band Gap Materials by Transition Metal Oxides: A Case-Study on Molybdenum Trioxide. Org. Electron. 2009, 10, 932−938. (19) Meerheim, R.; Olthof, S.; Hermenau, M.; Scholz, S.; Petrich, A.; Tessler, N.; Solomeshch, O.; Lussem, B.; Riede, M.; Leo, K. Investigation of C60F36 as Low-volatility p-Dopant in Organic Optoelectronic Devices. J. Appl. Phys. 2011, 109, 103102. (20) Olthof, S.; Mehraeen, S.; Mohapatra, S. K.; Barlow, S.; Coropceanu, V.; Bredas, J.-L.; Marder, S. R.; Kahn, A. Ultralow Doping in Organic Semiconductors: Evidence of Trap Filling. Phys. Rev. Lett. 2012, 109, 176601. (21) Menke, T.; Ray, D.; Kleemann, H.; Hein, M. P.; Leo, K.; Riede, M. Highly Efficient p-Dopants in Amorphous Hosts. Org. Electron. 2014, 15, 365−371. (22) Lou, Y.-H.; Xu, M.-F.; Zhang, L.; Wang, Z.-K.; Naka, S.; Okada, H.; Liao, L.-S. Origin of Enhanced Electrical and Conducting Properties in Pentacene Films Doped by Molybdenum Trioxide. Org. Electron. 2013, 14, 2698−2704.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b01175. Mott−Shottky plot of the MIS devices of a Rb2CO3doped TPBi thin film using impedance spectroscopy and capacitance loss spectrum (G/ω) of the MIS devices (PDF)

■

AUTHOR INFORMATION

Corresponding Authors

*Phone: +82-42-821-1970. E-mail: [email protected]. *Phone: +82-2-880-7893. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the Midcareer Researcher Program through a National Research Foundation grant funded by the Ministry of Science, ICT and Future Planning (2014R1A2A1A01002030).

■ ■

ABBREVIATIONS HOMO, highest occupied molecular orbital; LUMO, lowest unoccupied molecular orbital REFERENCES

(1) Yoo, S.-J.; Kim, J.-J. Charge Transport in Electrically Doped Amorphous Organic Semiconductors. Macromol. Rapid Commun. 2015, 36, 984−1000. (2) Lussem, B.; Riede, M.; Leo, K. Doping of Organic Semiconductors. Phys. Status Solidi A 2013, 210, 9−43. (3) Pingel, P.; Neher, D. Comprehensive Picture of P-type Doping of P3HT with The Molecular Acceptor F4TCNQ. Phys. Rev. B: Condens. Matter Mater. Phys. 2013, 87, 115209. (4) Mendez, H.; Heimel, G.; Opitz, A.; Sauer, K.; Barkowski, P.; Oehzelt, M.; Soeda, J.; Okamoto, T.; Takeya, J.; Arlin, J.-B.; et al. Doping of Organic Semiconductors: Impact of Dopant Strength and Electronic Coupling. Angew. Chem., Int. Ed. 2013, 52, 7751−7755. (5) Salzmann, I.; Heimel, G.; Duhm, S.; Oehzelt, M.; Pingel, P.; George, B. M.; Schnegg, A.; Lips, K.; Blum, R.-P.; Vollmer, A.; et al. 9480

DOI: 10.1021/acs.jpcc.6b01175 J. Phys. Chem. C 2016, 120, 9475−9481

Article

The Journal of Physical Chemistry C (23) Higgins, A.; Mohapatra, S. K.; Barlow, S.; Marder, S. R.; Kahn, A. Dopant Controlled Trap-filling and Conductivity Enhancement in an Electron-transport Polymer. Appl. Phys. Lett. 2015, 106, 163301. (24) Lee, J.-H.; Leem, D.-S.; Kim, J.-J. Effect of Host Organic Semiconductors on Electrical Doping. Org. Electron. 2010, 11, 486− 489. (25) Lee, J.-H.; Leem, D.-S.; Kim, H.-J.; Kim, J.-J. Effectiveness of Pdopants in an Organic Hole Transporting Material. Appl. Phys. Lett. 2009, 94, 123306. (26) Glaser, T.; Beck, S.; Lunkenheimer, B.; Donhauser, D.; Kohn, A.; Kroger, M.; Pucci, A. Infrared Study of the MoO3 Doping Efficiency in 4,4 0-bis(N-carbazolyl)-1,1 0-biphenyl (CBP). Org. Electron. 2013, 14, 575−583. (27) Naab, B. D.; Zhang, S.; Vandewal, K.; Salleo, A.; Barlow, S.; Marder, S. R.; Bao, Z. Effective Solution- and Vacuum-processed Ndoping by Dimers of Benzimidazoline Radicals. Adv. Mater. 2014, 26, 4268−4272. (28) Duong, D. T.; Phan, H.; Hanifi, D.; Jo, P. S.; Nguyen, T.-Q.; Salleo, A. Direct Observation of Doping Sites in Temperaturecontrolled, P-doped P3HT Thin Films by Conducting Atomic Force Microscopy. Adv. Mater. 2014, 26, 6069−6073. (29) Zhao, J.; Cai, Y.; Yang, J.-P.; Wei, H.-X.; Deng, Y.-H.; Li, Y.-Q.; Lee, S.-T.; Tang, J.-X. The Role of Cesium Carbonate on The Electron Injection and Transport Enhancement in Organic Layer by Admittance Spectroscopy. Appl. Phys. Lett. 2012, 101, 193303. (30) Choudhury, K. R.; Yoon, J.-H.; So, F. LiF as An N-dopant in tris(8-hydroxyquinoline) aluminum Thin Films. Adv. Mater. 2008, 20, 1456−1461. (31) Huang, J.; Xu, Z.; Yang, Y. Low-work-function Surface Formed by Solution-processed and Thermally Deposited Nanoscale Layers of Cesium Carbonate. Adv. Funct. Mater. 2007, 17, 1966−1973. (32) Yuan, Y.; Grozea, D.; Han, S.; Lu, Z. H. Interaction between Organic Semiconductors and LiF Dopant. Appl. Phys. Lett. 2004, 85, 4959−4961. (33) Li, Y.; Zhang, D.-Q.; Duan, L.; Zhang, R.; Wang, L.-D.; Qiu, Y. Elucidation of The Electron Injection Mechanism of Evaporated Cesium Carbonate Cathode Interlayer for Organic Light-Emitting Diodes. Appl. Phys. Lett. 2007, 90, 012119. (34) Liu, Q.; Duan, L.; Li, Y.; Qiao, J.; Yu, Z.; Zhang, D.; Wang, L.; Dong, G.; Qiu, Y. Study on The Electron Injection Mechanism of Thermally Decomposable Cs2CO3. Jpn. J. Appl. Phys. 2009, 48, 102302. (35) Lee, J.; Lee, H.; Jeon, P.; Jeong, K.; Kim, T. G.; Kim, J. W.; Yi, Y. Direct Evidence of N-type Doping in Organic Light-Emitting Devices: N Free Cs Doping from CsN3. Appl. Phys. Lett. 2012, 100, 203301. (36) Moudler, J. F.; Stickle, W. F; Sobol, P. E.; Bomben, K. D. Handbook of X-Ray Photoemission Spectroscopy; Perkin-Elmer Corp., Eden Prairie, MN, 1992. (37) Lehnhardt, M. S.; Hamwi, S.; Hoping, M.; Reinker, J.; Riedl, T.; Kowalsky, W. Charge Carrier Densities in Chemically Doped Organic Semiconductors Verified by Two Independent Techniques. Appl. Phys. Lett. 2010, 96, 193301. (38) Pingel, P.; Schwarzl, R.; Neher, D. Effect of Molecular P-doping on Hole Density and Mobility in poly(3- hexylthiophene). Appl. Phys. Lett. 2012, 100, 143303. (39) Sze, S. M.; Ng, K. K. Physics of Semiconductor Devices, 3rd ed.; John Wiley and Sons: Hoboken, NJ, 2007.

9481

DOI: 10.1021/acs.jpcc.6b01175 J. Phys. Chem. C 2016, 120, 9475−9481