Centimetre-scale electron diffusion in photoactive organic heterostructures

Abstract

The unique properties of organic semiconductors, such as flexibility and lightness, are increasingly important for information displays, lighting and energy generation. But organics suffer from both static and dynamic disorder, and this can lead to variable-range carrier hopping1,2, which results in notoriously poor electrical properties, with low electron and hole mobilities and correspondingly short charge-diffusion lengths of less than a micrometre3,4. Here we demonstrate a photoactive (light-responsive) organic heterostructure comprising a thin fullerene channel sandwiched between an electron-blocking layer and a blended donor:C70 fullerene heterojunction that generates charges by dissociating excitons. Centimetre-scale diffusion of electrons is observed in the fullerene channel, and this can be fitted with a simple electron diffusion model. Our experiments enable the direct measurement of charge diffusivity in organic semiconductors, which is as high as 0.83 ± 0.07 square centimetres per second in a C60 channel at room temperature. The high diffusivity of the fullerene combined with the extraordinarily long charge-recombination time yields diffusion lengths of more than 3.5 centimetres, orders of magnitude larger than expected for an organic system.

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Figure 1: Device structure, experimental set-up and distance-dependent photocurrent.
Figure 2: Energetics of materials used in devices.
Figure 3: Temperature dependence of channel currents.

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Acknowledgements

This work was supported by the United States Department of Energy SunShot Program under awards DE-EE0006708 and DE-EE0005310, and the Air Force Office of Scientific Research under award FA9550-14-1-0245. We thank M. Ware for discussions regarding numerical simulations.

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Authors

Contributions

Q.B. fabricated all samples, performed all measurements, and assisted with data fitting and analysis. C.C. assisted with current measurements, formulated theory, and performed simulations and data analysis. X.C. analysed mismatch in organic photovoltaic devices between over- and under-filled device illumination measurements. A.P. assisted with ultraviolet photoelectron spectral measurement of film energy levels. Y.Q. calculated lateral electric field strength in the channel. S.R.F. supervised the project and analysed data. Q.B., C.C. and S.R.F. wrote the manuscript together.

Corresponding author

Correspondence to Stephen R. Forrest.

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The authors declare no competing financial interests.

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Reviewer Information Nature thanks N. Banerji and the other anonymous reviewer(s) for their contribution to the peer review of this work.

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Extended data figures and tables

Extended Data Figure 1 Impact of channel disruption on channel currents.

Room-temperature transient currents were measured on a DTDCTB-(10 nm C60)-mixed device at L = 3 mm before and after a series of razor blade cuts was made to the organic layers, as shown in the inset. The peak height of the current pulse was greatly reduced and the peak arrival time was delayed for devices with a ‘partial cut’ that was transverse to, and spanned the width of, the ITO anode between the illumination position and the silver cathode, compared with the pristine, uncut device. Charge-diffusion simulations were performed for both geometries, where the only difference was a blocking boundary condition at the position of the partial cut. We find that charge diffusion around the cut accounts for the differences between the cut and uncut device transients, as demonstrated by the agreement between fits (lines) and the data. The partial cut was also extended such that there was no continuous organic path between the illumination position and the cathode (a ‘full cut’ device). This eliminated the response except for a residual current at time t < 200 ms arising from scattered light absorbed in the organic layers between the cathode and cut. This effect was observed in all devices exhibiting channel currents.

Extended Data Figure 2 Electron diffusion in an electrical injection device.

a, A device was fabricated to characterize charge diffusion in an electron-only electrically injected channel, with the structure shown here. Charges were injected into the C60 channel by applying a 50 V pulse between the injecting contact and the silicon substrate for 5 s. b, The transient current collected at the buried contact. A steady-state current of 0.56 μA is observed approximately 3 s after the start of the pulse, with an exponential decay time of about 400 ms. A simulation of the turn-off transient using the same optically measured parameters for D and k in Table 1 for the DTDCPB-(10 nm C60)-neat device is also shown in b (solid line). The small deviations of the fits to the electrical data are probably due to slow de-trapping of charges in the BPhen and SiO2 that are injected during the 50 V pulse. Inset, photograph of the device with 1 cm scale bar.

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Burlingame, Q., Coburn, C., Che, X. et al. Centimetre-scale electron diffusion in photoactive organic heterostructures. Nature 554, 77–80 (2018). https://doi.org/10.1038/nature25148

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