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Exciton Regeneration at Polymeric Semiconductor Heterojunctions

Arne C. Morteani, Paiboon Sreearunothai, Laura M. Herz, Richard H. Friend, and Carlos Silva
Phys. Rev. Lett. 92, 247402 – Published 18 June 2004
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Abstract

Control of the band-edge offsets at heterojunctions between organic semiconductors allows efficient operation of either photovoltaic or light-emitting diodes. We investigate systems where the exciton is marginally stable against charge separation and show via E-field-dependent time-resolved photoluminescence spectroscopy that excitons that have undergone charge separation at a heterojunction can be efficiently regenerated. This is because the charge transfer produces a geminate electron-hole pair (separation 2.2–3.1 nm) which may collapse into an exciplex and then endothermically (EA=100–200  meV) back transfer towards the exciton.

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  • Received 16 October 2003

DOI:https://doi.org/10.1103/PhysRevLett.92.247402

©2004 American Physical Society

Authors & Affiliations

Arne C. Morteani, Paiboon Sreearunothai, Laura M. Herz*, Richard H. Friend, and Carlos Silva†

  • Cavendish Laboratory, University of Cambridge, Madingley Road, Cambridge CB3 0HE, United Kingdom

  • *Current address: Clarendon Laboratory, University of Oxford, Parks Road, Oxford OX1 3PU, United Kingdom.
  • †Corresponding author. Email: cs271@cam.ac.uk

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Vol. 92, Iss. 24 — 18 June 2004

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  • Figure 1
    Figure 1
    Potential energy diagram describing the energetics and kinetics at type II polymer heterojunctions. The energetic order of ∣A−D+⟩r=∞ and ∣A*D⟩r=∞ may be reversed for PFB:F8BT vs TFB:F8BT. The inset shows the band offsets at a type II heterojunction (see also 7).Reuse & Permissions
  • Figure 2
    Figure 2
    (a) Photoluminescence intensity (PL, solid circles) and reduction of photoluminescence intensity due to an applied reverse bias of 10 V (ΔPL, continuous line) for a PFB:F8BT blend device at 340 K. PL and −ΔPL are plotted in the same scale and reflect their relative intensities. (b) ΔPL spectra (at 10 V) from the same device as in (a) at different temperatures. (c) PL (solid circles) and ΔPL at a reverse bias of 15 V (continuous line) for a TFB:F8BT blend device at 340 K. (d) ΔPL spectra from the same device as in (c) at different temperatures. For comparison, the PL spectrum from an F8BT-only device (open circles) is plotted in both parts (a) and (c). The structures of PFB, F8BT, and TFB are also shown.Reuse & Permissions
  • Figure 3
    Figure 3
    (a) Photoluminescence decay measured using TCSPC (excitation: 407 nm, <4   nJ/cm2; detection: 640 nm) from a PFB:F8BT device at room temperature under 0 V (continuous line), 13 V (circles), and 30 V (triangles) applied reverse biases. (b) PLUC measurements (excitation: 405 nm, 42  nJ/cm2; detection: 550 nm) from a similar device at 0 V (continuous line), 5 V (squares), and 12.5 V (triangles) reverse bias. For comparison, data for a device with pure F8BT at 0 V (continuous line) and 12 V (circles) are also plotted.Reuse & Permissions
  • Figure 4
    Figure 4
    Relative electric field quenching of the PFB:F8BT and TFB:F8BT exciplex photoluminescence intensities (measured at 700 and 580 nm, respectively), in the same devices as in Fig. 2, versus electric field at 230 K (solid squares), 250 K (open and solid circles), 290 K (solid triangles), and 295 K (open diamonds). The solid lines through the data are Onsager simulations (parameters for PFB:F8BT: ϵ=3.5, rgp=3.0   nm at T=230   K and 3.1 nm at 250 and 290 K; for TFB:F8BT: ϵ=3.5, rgp=2.3   nm at T=250   K and 2.2 nm at 295 K).Reuse & Permissions
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