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Doped polymer semiconductors with ultrahigh and ultralow work functions for ohmic contacts

Nature volume 539, pages 536–540 (2016)Cite this article

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Abstract

To make high-performance semiconductor devices, a good ohmic contact between the electrode and the semiconductor layer is required to inject the maximum current density across the contact. Achieving ohmic contacts requires electrodes with high and low work functions to inject holes and electrons respectively, where the work function is the minimum energy required to remove an electron from the Fermi level of the electrode to the vacuum level. However, it is challenging to produce electrically conducting films with sufficiently high or low work functions, especially for solution-processed semiconductor devices. Hole-doped polymer organic semiconductors are available in a limited work-function range1,2, but hole-doped materials with ultrahigh work functions and, especially, electron-doped materials with low to ultralow work functions are not yet available. The key challenges are stabilizing the thin films against de-doping and suppressing dopant migration3,4. Here we report a general strategy to overcome these limitations and achieve solution-processed doped films over a wide range of work functions (3.0–5.8 electronvolts), by charge-doping of conjugated polyelectrolytes5,6,7 and then internal ion-exchange to give self-compensated heavily doped polymers. Mobile carriers on the polymer backbone in these materials are compensated by covalently bonded counter-ions. Although our self-compensated doped polymers superficially resemble self-doped polymers8,9, they are generated by separate charge-carrier doping and compensation steps, which enables the use of strong dopants to access extreme work functions. We demonstrate solution-processed ohmic contacts for high-performance organic light-emitting diodes, solar cells, photodiodes and transistors, including ohmic injection of both carrier types into polyfluorene—the benchmark wide-bandgap blue-light-emitting polymer organic semiconductor. We also show that metal electrodes can be transformed into highly efficient hole- and electron-injection contacts via the self-assembly of these doped polyelectrolytes. This consequently allows ambipolar field-effect transistors to be transformed into high-performance p- and n-channel transistors. Our strategy provides a method for producing ohmic contacts not only for organic semiconductors, but potentially for other advanced semiconductors as well, including perovskites, quantum dots, nanotubes and two-dimensional materials.

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Figure 1: Stabilization of the doping profile by counter-ion immobilization in self-compensated doped polymer organic semiconductors.
Figure 2: Ohmic charge-injection and charge-collection characteristics.
Figure 3: Blue-light-emitting diode with ultrahigh- and ultralow-work-function charge injection layers.
Figure 4: Facile electrode differentiation by self-aligned assembly of ultrahigh- or ultralow-work-function interlayers.

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Acknowledgements

We thank C. Hu, H. Guo, D. Belaineh, X.-Y. Hou, M.-H. Teo, K. Yeo, S.-N. Tan, Z.-W. Tan, J. Chen, K. Lim, Y.-S. Soh, P. Tanay and S.-C. Lee for contributions to the experimental work. We thank I. Grizzi, R. Wilson and the CDT/Sumitomo Chemical Co. team for materials and support. We particularly thank R. H. Friend for inspiration and insights. This research is partially supported by the National Research Foundation, Prime Minister’s Office, Singapore under its Competitive Research Programme (CRP Award No. NRF-CRP 11-2012-03: R-144-000-339-281, R-143-000-608-281), and the Ministry of Education, Singapore (R-144-000-324-112). Solar Energy Research Institute of Singapore (SERIS) is sponsored by the National University of Singapore and Singapore’s National Research Foundation through the Singapore Economic Development Board.

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Author notes

  1. Cindy G. Tang, Mervin C. Y. Ang and Kim-Kian Choo: These authors contributed equally to this work.

Authors and Affiliations

  1. Department of Physics, National University of Singapore, Lower Kent Ridge Road, S117550, Singapore

    Cindy G. Tang, Jun-Kai Tan, Rui-Qi Png, Lay-Lay Chua & Peter K. H. Ho

  2. Solar Energy Research Institute of Singapore (SERIS), National University of Singapore, 7 Engineering Drive 1, S117574, Singapore

    Cindy G. Tang, Mervin C. Y. Ang, Jun-Kai Tan, Rui-Qi Png, Lay-Lay Chua & Peter K. H. Ho

  3. Department of Chemistry, National University of Singapore, Lower Kent Ridge Road, S117552, Singapore

    Mervin C. Y. Ang, Kim-Kian Choo, Venu Keerthi, Mazlan Nur Syafiqah & Lay-Lay Chua

  4. Cambridge Display Technology Ltd, Building 2020, Cambourne Business Park, Cambridge, CB3 6DW, UK

    Thomas Kugler & Jeremy H. Burroughes

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Contributions

R.-Q.P. led the heterostructure and interface work, L.-L.C. led the materials chemistry work, and P.K.H.H. led the device physics work. C.G.T., K.-K.C. and M.C.Y.A. fabricated and characterized devices. K.-K.C., M.C.Y.A., V.K. and M.N.S. synthesized materials. J.-K.T. simulated device behaviour. J.H.B. and T.K. contributed directions. All authors discussed the experiments and results. R.-Q.P., L.-L.C. and P.K.H.H. wrote the manuscript.

Corresponding authors

Correspondence to Rui-Qi Png, Lay-Lay Chua or Peter K. H. Ho.

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The authors declare no competing financial interests.

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Reviewer Information Nature thanks A. Facchetti, D. Seferos and the other anonymous reviewer(s) for their contribution to the peer review of this work.

Extended data figures and tables

Extended Data Figure 1 Chemical structures of materials used.

The value of x denotes the doping level, in carriers per repeat unit (r.u.), with x = 0 denoting an undoped polymer and x = 1 a fully doped polymer.

Extended Data Figure 2 Schematic of the preparation of self-compensated heavily doped polymer organic semiconductors.

Top, solution-state doping method. The undoped polymer dissolved in a suitable solvent is doped in solution by the desired dopant, and purified by repeated precipitation with a non-solvent while keeping the unwanted ions in solution. This process yields the fully self-compensated doped polymer as a solid, which can be dissolved in the desired solvent. Bottom, film-state doping method. The undoped polymer film is deposited on the desired substrate, doped by contact with a dopant solution, and purified by repeated contact with a non-solvent that dissolves the unwanted ions. This directly yields the fully self-compensated doped polymer film.

Extended Data Figure 3 Doping-profile migration in the presence of mobile counter-ions.

a, b, Schematic (a) and experimental data (b) for transfer of holes and counter-ions from a hole-doped organic semiconductor layer with mobile counter-anions (OSC-1) to an adjacent organic semiconductor layer (OSC-2) during solution processing. OSC-1 is P0 (IP = 5.9 eV), hole-doped to x ≈ 0.8, with SbF6− as counter-ion. OSC-2 is OC1C10-PPV (IP = 5.0 eV), deposited by spin-casting on p-doped P0. c, d, Schematic (c) and experimental data (d) for transfer of holes and counter-ions from an incompletely self-compensated hole-doped organic semiconductor (OSC-1) to an adjacent organic semiconductor layer (OSC-2) during solution processing. OSC-1 is incompletely self-compensated hole-doped P1 (x ≈ 0.8) containing 50% residual Na+ and SbF6−. OSC-2 as in a and b. Nearly complete transfer of holes and counter-anions from the doped underlayer to the OC1C10-PPV overlayer occurred during spin-casting, as evidenced by emergence of the polaron band of OC1C10-PPV (600–800 nm) and bleaching of the polaron band of P0 and P1 (>900 nm). This doping transfer is driven by thermodynamics from a high-IP layer to a low-IP layer. Ef, Fermi energy; VL, vacuum level.

Extended Data Figure 4 Characterization of selected self-compensated, heavily doped polymers.

a–d, Triarylamine–fluorene copolymer P1. a, Optical spectra of doped P1 in solution (top), and of a P1 film before and after doping (bottom). For solution-state doping, 0.12 mM r.u. P1 in propylene carbonate was doped with NOSbF6 and self-compensated, yielding a final doping level of 0.5 holes per r.u. The optical spectrum is collected in a cell with a 2.0-mm path length. For film-state doping, a 100-nm-thick P1 film was doped with NOSbF6 and self-compensated, yielding a final doping level of 0.9 holes per r.u. b, c, UPS (b) and XPS spectra (c; N1s and F1s) of undoped and doped P1 (x ≈ 0.5) films. d, FTIR evidence for full self-compensation in a P1 film. Spectra were acquired before doping (red), after film-state doping with 6 mM NOSbF6 in acetonitrile (green) and after self-compensation by excess ion removal (Na+ and SbF6−) via spin-rinse with acetonitrile (blue). Infrared active vibration modes (IRAV; 1,100–1,600 cm−1) indicate that the film is doped. SbF6− (anti-symmetric stretching frequency νas = 660 cm–1) is present before, but not after, self-compensation (residual, <1%). e–g, Naphthalenetetracarboxydiimide–thiophene copolymer N1. e, Optical spectra of doped N1 in solution (top), and of an N1 film before and after doping (bottom). For, solution-state doping, 0.22 mM r.u. N1 in dimethylsulfoxide was doped with sodium 9,10-diphenylanthracenide and self-compensated, yielding a final doping level of 1.0 electrons per r.u. The optical spectrum is collected in a cell with a 2.0-mm path length. For film-state doping, a 25-nm-thick N1 film was doped with sodium 9,10-diphenylanthracenide and self-compensated, yielding a final doping level of 1.0 electrons per r.u. f, g, UPS (f) and XPS spectra (g; N1s and I3d5/2) of undoped and doped N1 (x ≈ 1.0) films. h–k, Fluorene copolymers N2 and N3. h, i, Optical spectra of 40-nm-thick N2 (h) and N3 (i) films, before and after film-state doping with sodium naphthalenide solution and spin-rinsing with tetrahydrofuran. Final doping levels, 0.9 electrons per r.u. (N2; h) and 0.4 electrons per r.u. (N3; i). j, k, UPS spectra of undoped and n-doped N2 (j) and N3 (k) films. All binding energies are referenced to vacuum level. The Fermi energy EF is marked for doped films; the ionization potential IP is marked for undoped films.

Extended Data Figure 5 Electrode potentials of H2O and O2 redox couples in pH 7 water, plotted on the vacuum scale.

Conditions: neutral H2O; pressure, 1 bar (O2, H2); concentration, 1 M (H2O2, O2•−, HO•, HO2•). The reference voltage (0.00 V) on the standard hydrogen electrode (SHE) scale corresponds to 4.46 eV on the vacuum scale. Electrode potentials of O2/O2•− were computed from ref. 31; other electrode potentials were computed from ref. 32. p-doped films with ultrahigh work functions (UHWF) of greater than about 5.3 eV and n-doped films with low work functions (LWF) of less than about 5.3 eV can oxidize neutral H2O to O2, and reduce O2 to neutral H2O, respectively, and so cannot remain indefinitely stable in the presence of these species. If the oxygen reduction reaction proceeds through one-electron steps, then the relevant first one-electron couple (O2/O2•−) lies in the range 4.15–3.65 eV, depending on the dielectric medium, which opens up a stability window for films with ultralow work functions (ULWF). The stability of the films with ultralow work functions is ultimately limited by the electrode potential at the minimum practical O2 concentration during film preparation (p.p.m. range). To overcome this limitation, precursor n-dopants can be incorporated into the self-compensated platform (see main text). Conversely, the relevant first one-electron couple for water oxidation ((H+ + HO•)/H2O) lies at 6.75 eV, which opens up a stability window for films with ultrahigh work functions. DMSO, dimethyl sulfoxide; BMIM BF4, 1-butyl-3-methylimidazolium tetrafluoroborate.

Extended Data Figure 6 Ambient stability of ultrahigh- and ultralow-work-function films.

a, b, Self-compensated p-doped P4. a, Time-dependent optical spectra of a 12-nm-thick film in ambient conditions (295 K, 65% relative humidity (RH)). The initial doping level was x ≈ 0.8. b, To compare the stability imparted by different counter-anions, the doping level is shown as a function of exposure time for P4 (with tethered trifluoromethylsulfonylimide counter-anion; blue) and P3 (with tethered sulfonate counter-anion; red). The films were hole-doped with 1 mM NOSbF6 in acetonitrile and self-compensated by spin-rinse with acetonitrile in a N2 glove box, and then exposed to ambient conditions and measured after specified intervals. The doping level was evaluated from the integrated polaron band intensity (460–540 nm). Time to 50% doping level: P4, 50 h; P3, 15 min from initial doping level of about 0.8 holes per r.u. c, d, Self-compensated n-doped N1. c, Time-dependent optical spectra of a 7.5-nm-thick self-compensated film in ambient conditions. The initial doping level was x ≈ 1.0. d, To compare the stability of self-compensated and non-self-compensated films, the doping level is shown as a function of exposure time for N1 (with tethered trimethylammonium counter-ion; blue) and N0 with unbonded Na+ (red) or Cc+ (cobaltocenium; green) counter-ions. The films were electron doped with 15 mM sodium 9,10-diphenylanthracenide in diglyme and self-compensated by spin-rinse with acetonitrile. The doping level was evaluated from the integrated band intensity (540–700 nm) after calibration. Time to 50% doping level: N1, 2 h; N0 counter-balanced by Cc+, 20 min; N0 counter-balanced by Na+, 4 min from initial doping level of about 1.0 electrons per r.u. Effective ionic radius rion: Na+, 1.0 Å; Cc+, 3. Å; −(CH2)3NMe3+, 3.2 Å. De-doping recovers the original π–π* spectrum.

Extended Data Figure 7 Apparent work function evaluated by in situ built-in potential measurements.

a, Stark-effect electro-absorption spectra of hole-only diodes with p-doped PEDT:PSSH (left), P2 (middle) and P4 (right) as hole contacts, PFOP as the organic semiconductor and Al as the electron-blocking contact. Normalized modulated reflection spectra (ΔR/R) are shown for different d.c. biases Vdc applied to the hole contact, where ΔR is the modulated root-mean-square reflectance in-phase with the forward-bias half-cycle and R is the d.c. reflectance33. Negative ΔR/R therefore indicates induced absorption in-phase with the forward-bias half-cycle. The dotted line gives the null background. Temperature, 30 K; modulation frequency, 535 Hz; root-mean-square voltage Vrms, 0.5 V. b, Apparent energy-level alignment diagrams deduced from the built-in potential Vbi, assuming rigid alignment at the electron contact. Energies are plotted relative to the vacuum level (VL) of the organic semiconductor. The Stark band at 2.87 eV near the π–π* absorption edge of PFOP shows polarity inversion at 1.70 V for PEDT:PSSH, at 2.35 V for P2 and at 2.4 V for P4. Estimated uncertainty, ±0.03 V. Consequently, the apparent work functions are: PEDT:PSSH, 3.4 + 1.7 = 5.1 eV; P2, 3.4 + 2.35 = 5.75 eV; P4, 3.4 + 2.4 = 5.8 eV. In reality, the effective work function at the electron contact decreases in response to an increase in work function at the hole contact, driving it into the pinned regime. This then gives rise to pinning at about 5.6 eV. The apparent work functions therefore confirm that the work functions of P2 and P4 are both larger than 5.6 eV, with the work function of P4 greater than that of P2.

Extended Data Figure 8 Characteristics of solar cells with an ultralow-work-function polymer as the bottom electron-collection contact.

Current density versus voltage for an 80-nm-thick P3HT:PCBM photoactive layer (weight ratio of 1: 0.8) under 100 mW cm−2 illumination (spectral-mismatch corrected) with an ultralow-work-function polymer as the bottom electron-collection contact and PEDT:PSSH as the top hole-collection contact. 20-nm-thick n-doped N1 was deposited as the ECL via spin-coating from DMSO solution onto the ITO layer. The photoactive layer (PAL) was deposited from chlorobenzene. 30-nm-thick PEDT:PSSH was deposited as the HCL via spin-coating from aqueous solution onto the top of the PAL and capped with evaporated Ag. A control cell was fabricated in the same batch by omitting the N1 layer, so that ITO provided the electron-collection contact. The control cell (red) shows poor behaviour with an S-shaped J–V characteristic, whereas the test cell (blue) gives good performance similar to standard cells with evaporated Ca as the electron-collection contact. Open-circuit voltage Voc is given in V; short-circuit current Jsc in mA cm−2, and power conversion efficiency PCE in %. FF, fill factor. Both cells have been corrected for shunt leakage.

Extended Data Figure 9 Facile electrode differentiation by self-aligned assembly of ultrahigh- and ultralow-work-function interlayers on Ag.

a, Schematic of the FET structure. b, c, Transfer characteristics on a linear (b) and semi-logarithmic (c) scales. A monolayer of p-doped P2 or n-doped N1 was self-assembled onto the Ag electrode array from dilute solution. Channel length, 100 μm; channel width, 3 mm; Vd was stepped from 0 V (red) or to −20 V (blue) in −5 V steps. The control FET shows the expected ambipolar behaviour34, whereas the modified electrodes show gate–source Vgs threshold shifts, similar to that observed on Au electrodes.

Extended Data Figure 10 Integration of precursor n-dopant methodology into self-compensated doped polymers.

a, b, N4 (a) and N0 (b) with (blue) and without (red) DMBI-H. Pristine polymer films (15-nm-thick N4 and 20-nm-thick N0) and composite films with DMBI-H (1:2 mol/mol r.u.) were spin-cast and baked at 120 °C on a hotplate, and then measured by transmission optical spectroscopy. DMBI-H is (4-(1,3-dimethyl-2,3-dihydro-1H-benzoimidazoyl-2-yl)phenyl)dimethylamine, a precursor n-dopant35. c, Authentic n-doping of N4 films, as a reference. A 30-nm-thick N4 film was spin-cast and then electron-doped to 1.0 electron per r.u. by contact with cobaltocene (Cc; 2 mM in diglyme). N0 has an identical π-conjugated organic semiconductor core to N4, but lacks tethered counter-ions. Whereas N4 exhibits strong doping, N0 exhibits only weak doping under identical conditions, possibly owing to the lower stability of its n-doped state.

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Tang, C., Ang, M., Choo, KK. et al. Doped polymer semiconductors with ultrahigh and ultralow work functions for ohmic contacts. Nature 539, 536–540 (2016). https://doi.org/10.1038/nature20133

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