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Multivalent anions as universal latent electron donors

Nature volume 573, pages 519–525 (2019)Cite this article

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Abstract

Electrodes with low work functions are required to efficiently inject electrons into semiconductor devices. However, when the work function drops below about 4 electronvolts, the electrode suffers oxidation in air, which prevents its fabrication in ambient conditions. Here we show that multivalent anions such as oxalate, carbonate and sulfite can act as powerful latent electron donors when dispersed as small ion clusters in a matrix, while retaining their ability to be processed in solution in ambient conditions. The anions in these clusters can even n-dope the semiconductor core of π-conjugated polyelectrolytes that have low electron affinities, through a ground-state doping mechanism that is further amplified by a hole-sensitized or photosensitized mechanism in the device. A theoretical analysis of donor levels of these anions reveals that they are favourably upshifted from ionic lattices by a decrease in the Coulomb stabilization of small ion clusters, and by irreversibility effects. We attain an ultralow effective work function of 2.4 electronvolts with the polyfluorene core. We realize high-performance, solution-processed, white-light-emitting diodes and organic solar cells using polymer electron injection layers with these universal anion donors, demonstrating a general approach to chemically designed and ambient-processed Ohmic electron contacts for semiconductor devices.

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Fig. 1: Observation of Ohmic electron injection into the benchmark low-EA semiconductor PFOP.
Fig. 2: Diode Vbi and subgap absorption by d.c. bias-dependent electroabsorption spectroscopy.
Fig. 3: Computed anion donor energies plotted on the vacuum scale.
Fig. 4: Evidence for electron doping by Ox2− induced by anion dehydration and via hole sensitization of a semiconductor core.
Fig. 5: High-performance white LEDs and organic solar cells with multivalent-anion-doped polymer electron contacts.

Data availability

The data supporting the findings of this study are available within this Article and its Supplementary Information and Figure files.

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Acknowledgements

We thank D. W. Y. Teo, Y. Wang, M. C. Y. Ang and A. Y. H. Ang for contributions to the experimental work, and Sumitomo Chemical for support. This research is partially funded 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).

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Author notes

  1. These authors contributed equally: Cindy G. Tang, Mazlan Nur Syafiqah

Authors and Affiliations

  1. Department of Physics, National University of Singapore, Singapore, Singapore

    Cindy G. Tang, Chao Zhao, Rui-Qi Png, Lay-Lay Chua & Peter K. H. Ho

  2. Department of Chemistry, National University of Singapore, Singapore, Singapore

    Mazlan Nur Syafiqah, Qi-Mian Koh, Jamal Zaini, Qiu-Jing Seah & Lay-Lay Chua

  3. Cambridge Display Technology Limited, Godmanchester, UK

    Michael J. Cass, Martin J. Humphries, Ilaria Grizzi & Jeremy H. Burroughes

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Contributions

R.-Q.P. led the device heterostructure and interface work. L.-L.C. led the materials chemistry and development work. P.K.H.H. led the theory work and performed the solid-state corrections. M.N.S, Q.-M.K., J.Z. and Q.-J.S. synthesized and characterized the materials. C.G.T. performed the DFT calculations. Fabrication and characterization of the diagnostic devices was performed by C.G.T., M.N.S. and Q.-M.K.; by C.Z. for the organic solar cells; and by M.J.H. and M.J.C. for the white LEDs. J.H.B., I.G. and M.J.H. provided industry insights. All authors discussed the experiments and results. R.-Q.P., L.-L.C. and P.K.H.H. wrote the manuscript.

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Correspondence to Rui-Qi Png or Lay-Lay Chua.

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Extended data figures and tables

Extended Data Fig. 1 Chemical structure and ion exchange characterization.

a, Chemical structures of the materials used in this work. b, Transmission infrared spectroscopy of: top, N2(X) film before (red spectrum) and after (green) TfO− exchange with Ox2− by the contact ion-exchange protocol; and bottom, N6(Ox) film from the one-pot protocol (blue). Key vibrational modes of TfO− (magenta dashed lines) and Ox2− (green dashed lines) are marked. c, X-ray photoemission spectroscopy of the N3(X) film before (red spectrum) and after (blue) TfO− exchange with Ox2−. The N:S:C ratio for N3(TfO) is 1.95:2.0:54.0 (from measurement) and 2.0:2.0:54.0 (from theory); for N3(Ox) it is 2.0:0.03:56.0 (from measurement) and 2.0:0.0:56.0 (from theory). The large upshift in binding energy upon ion exchange is due to shift of Fermi level as a consequence of electron doping in the XPS chamber.

Extended Data Fig. 2 Atomic force microscopy of the film surfaces before and after ion exchange.

a, N3(TfO) (10-nm thick) film cast from 2,2,3,3,4,4,5,5-octafluoro-1-pentanol onto a fused silica substrate cleaned with O2 plasma. b, N3(Ox) film, from a after contact ion-exchange with 80 mM Na2Ox in 4:1 v/v H2O:MeOH mixture. The nodular surface topography, characteristic of π-conjugated polyelectrolyte films deposited from moderately good solvents, is substantially unaffected by ion exchange. Root-mean-square roughness (Rrms) is 4.7 nm and 3.1 nm for N3(TfO) and N3(Ox), respectively. The image size is 3 × 3 μm and the full z scale (shown at right) is 40 nm.

Extended Data Fig. 3 Invariance of J–V characteristics with N3(Ox) thickness.

The device structure is ITO/30 nm PEDT:PSSH/100 nm PFOP/N3(Ox)/Ag. Data from two representative diodes (solid and open symbols) are shown for each EIL thickness. The characteristics overlap, indicating a negligible voltage drop across the EIL even at a high current density of 1,000 mA cm−2. This is in contrast to the nominally undoped ‘transport’ interlayers reported in the literature, in which low conductivity limits the usable film thickness and useful current density to less than 5 nm and 100 mA cm−2, respectively.

Extended Data Fig. 4 Observation of electron and hole SCLC in PFOP diodes.

a–c, Electron-dominated devices are ITO/20 nm PEDT:PSSCsH/d nm PFOP/EIL, where the EIL is 10 nm N3(SO3)/Ag (a); 3.5 nm LiF/10 nm Ca/Al (b); and 30 nm Ba/Ag (c). d, The hole-dominated devices are ITO/20 nm mTFF-C2F5SIS/d nm PFOP/Al, where mTFF-C2F5SIS is a self-compensated, hole-doped poly(fluorene-alt-phenylene-(m-trifluoromethylphenylimino)-phenylene) with a work function of 5.75 eV. The thickness d for each PFOP layer is indicated by the colour of the symbols, and data from two representative diodes (solid and open symbols) are shown for each d. Data for the electron-dominated devices are plotted for a second sweep, high-to-low direction, at −5 V s−1. Data for the hole-dominated devices are plotted for a first sweep, high-to-low direction, at −5 V s−1. Electron SCLC behaviour is observed in a and b. Hole SCLC behaviour is observed in d, where the apparent hole mobility increases from 7 × 10−5 to 3 × 10−4 cm2 V−1 s−1 as the thickness of PFOP increases from 50 nm to 100 nm.

Extended Data Fig. 5 Diode Vbi and subgap absorption by d.c. bias-dependent electroabsorption spectroscopy in other diodes.

a–c, In-phase electroabsorption spectra (left panels) and energy level diagram (right panels) for ITO/PEDT:PSSH/PFOP/10 nm N3(SO3)/Ag (a); ITO/PEDT:PSSH/TFB/Ca/Al (b); and ITO/PEDT:PSSH/OC1C10-PPV/10 nm N3(Ox)/Ag (c). The key for a–c is given below the plots. Vdc is applied to the ITO. The Vbi of a diode identical to c but with Ca/Al in place of N3(Ox)/Ag is 1.95 V, which gives the effective work function of OC1C10-PPV/Ca to be 3.0 eV. All semiconductor layers were 100-nm thick; feature and parameter explanations are as given in Fig. 2.

Extended Data Fig. 6 Hess cycle for computation of the solid-state adiabatic electron detachment Gibbs free energy ∆GD,s.

X•(n−1)−(Mm+)pn(Xn−)pm−1‡(g) denotes the ion cluster frozen in the initial configuration, whereas the electron-detached anion has been relaxed; X•(n−1)‒(Mm+)pn(Xn−)pm−1(g) denotes the final fully relaxed ion cluster.

Extended Data Fig. 7 Dehydration of Ox2− in high vacuum (base pressure, 1 × 10−6 mbar).

In situ transmission Fourier-transform infrared spectra, showing loss of hydrogen-bonded H2O (stretching ν, 3,050–3,600 cm−1; bending δ, 1,640 cm−1) accompanied by red shift of both the symmetric stretching νs and anti-symmetric stretching νa COO modes of Ox2− (marked by green dashed lines). The initial hydration level in nominally dry nitrogen (pH2O, 1–10 p.p.m.) is about 4 H2O per Ox2−. Electron doping does not occur until the hydration level decreases below 0.5 H2O per Ox2−. Dehydration occurs slowly in thick films. The thickness of the N3(Ox) film was 1.1 µm. HV, high vacuum.

Extended Data Fig. 8 Evidence for photosensitized electron doping of N3 by Ox2−.

a, In situ transmission optical spectroscopy showing growth of the P2 polaron band in N3(Ox) film with exposure to 365-nm radiation. This wavelength is absorbed by the semiconductor core, but not by Ox2−. The upper plot is a 5 × magnification of the same data up to 2.7 eV that more clearly show the evolution of the P2 band with time (indicatedby arrow). The final saturation doping level is about 0.05 electrons per repeat unit. b, In situ transmission infrared spectroscopy, showing growth of the P1 polaron band and infrared active vibration (IRAV) bands with exposure to 365-nm UV light . The dose required to n-dope this thick film is considerably larger than for the thin film owing to the inner-filter effect. One dose unit is 80 mJ cm−2. The vacuum base pressure is 1 × 10−6 mbar.

Extended Data Fig. 9 Comparative ground-state and photosensitized electron doping of N2 by anions.

a, In situ transmission optical spectroscopy of N2(X) films in dry nitrogen (N2; pH2O, 1 p.p.m.), after high-vacuum pumping (HV; base pressure of 1 × 10−6 mbar for 2 h) and after exhaustive in situ irradiation at 365-nm wavelength (UV; 1.2 J cm−2). The plots in the X=TfO panel have been vertically demagnified by a factor of 4.  b, Difference spectra, showing the red shift of the π‒π∗ band upon drying, and the emergence of the 2.0-eV P2 polaron band with breaching of the 2.7-eV π‒π∗ band upon doping. The plots in the X=Ox panel have been vertically demagnified by a factor of 2; those in the X=TfO panel by a factor of 4. The spectra were obtained at a temperature of 295 K.

Extended Data Fig. 10 Evidence for electron doping from UV photoemission spectroscopy.

The vacuum work function is computed from the difference in photoelectron kinetic energies at the Fermi level (KFL) and at the low-energy cutoff (KLECO), that is, the work function is given by KLECO + hν − KFL, where hν is the photon energy (21.21 eV). The work functions for N3(X) are: Ox, 2.95 eV; CO3, 3.75 eV; SO3, 3.85 eV; AcO, 4.1 eV; SO4, 4.1 eV; and TfO, 4.85 eV. Relative to TfO, the work function shifts for Ox, CO3 and SO3 are −1.9 eV, −1.1 eV and −1.0 eV, respectively. The relative binding-energy shifts (measured from the Fermi level for C 1s and other core levels) are even larger: +2.0 eV, +1.55 eV and +1.4 eV, respectively. This indicates strong electron doping of the N3(Ox) film and weaker doping in the other films.The results also suggest the presence of a partially counteracting dipole at the surface of the films (negative on the outside). In the weakly-doped regime, the work function varies very strongly with the doping level34.

Extended Data Fig. 11 Quenching of Ox2− donor strength by added Cs2Ox salt in N5(Ox).

Plot of normalized saturation doping level of N5(Ox):Cs2Ox film after exhaustive photosensitized doping, against m/n of the added Cs2Ox. The doping level was determined from the integrated P2 polaron absorption band intensity at 2.05 eV, together with loss of the π–π∗ band intensity, and normalized to the saturation doping level attained without the added salt, which is 0.01 electrons per repeat unit. The added Cs2Ox increases the size of the ion clusters. The blue line is a guide to the eye.

Extended Data Fig. 12 Organic solar cell characteristics for N2(X) electron collection layers.

Plot of cell parameters against solar irradiation time. From top to bottom, the data show the open-circuit voltage (Voc), fill factor (FF), short-circuit current density (Jsc) and power conversion efficiency (PCE). The first data point at 0.5 min corresponds to the first measurement, given also in Fig. 5b. After a first continual exposure period of 10 h, the device was stored in the dark in nitrogen for 75 h, then re-measured in a second light-soak period to probe recoverability. The standard error is smaller than the symbol size. The irradiance is 100 mW cm−2 with an AM1.5 spectrum. The solar cells with multivalent anions are high-performance, robust and well behaved; those with mono-anions are lower performance, but improves slowly with light soak, albeit with partial reversibility in the dark. The light-soaking-induced improvement in FF lags the improvement in Voc and Jsc, probably as a consequence of the charge-extraction contact resistance that still remains at an insufficiently doped collection contact. The key for the coloured Xn− data points in all panels is given in the first panel of the fill factor row.

Supplementary information

Supplementary Tables

Supplementary tables of electron detachment energies for anions and their hydrated complexes in gas phase and solid state. (i) Table 1: Experimental and theoretical results for the gas phase from literature, (ii) Table 2: Theoretical results for the gas phase computed in the present work, (iii) Table 3: Corrections for the solid state with ion clusters embedded in an organic matrix, (iv) Table 4: Corrections for the `kinetic’ donor level due to irreversibility.

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Tang, C.G., Syafiqah, M.N., Koh, QM. et al. Multivalent anions as universal latent electron donors. Nature 573, 519–525 (2019). https://doi.org/10.1038/s41586-019-1575-7

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