polymer interfaces

ELSEVIER

Synthetic

Metals

101 (1999)

105-106

Excited-State Energy- and Charge-Transfer

at Polymer/Polymer Interfaces

J.J.M. Halls’, J. Cornil’, D.A. dos Santos’, D.-H. Hwang3% A.B. Holmes3 J.L. Bredas’ and R.H. Friend’ ‘Ca\wdish ‘Lr~borurory fiw Chemisfry ‘dlel~‘ille luboratoq,%r

Luborcuoty Madingley Roud, Cambridge CB3 OHE (UK) o~Nowl Materials, Ukwsity ofMons-Hri~riairr. B-7000 Mm (Belgium) polymer Syrhesis, Pernbmke Street, Curnbridge CB2 3RA (UK)

Abstract When an exciton diffuses to the interface between two conjugated polymers, either energy- or charge-transfer may take place. We present an experimental investigation of these processes in various binary polymer systems. These results are consistent with calculations of the relative energies of the intrachain and interchain excited states on pairs of different PPV-related chains, In particular, we show experimentally and theoretically that charge transfer occurs at the interface between MEH-PPV and a cyano-substituted PPV derivative, whereas energy transfer occurs at the interface of PPV with the same cyano-substituted polymer. Keg\r,ords:

1.

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Photocorzdlrctilli~;

Poly(pher~ylerze

Introduction

Polymer/polymer interfaces play an important role in devices which use polymers in multi-layer or composite It is the behaviour of excitons when they configurations. encounter such an interface which make these systems function efficiently as LEDs or solar cells and photodiodes. When an exciton in a conjugated polymer reaches the interface with a second polymer it may move to the second material, where it may decay radiatively. Alternatively, it may dissociate by transfer of a charge to the second material, leaving behind an opposite charge. The separation of photogenerated carriers has been shown to be efficient at the interface between certain materials with differing ionisation energies and electron affinities. Recently, conjugated polymers have been combined with electron acceptors such as Chtr to make efficient photovoltaic cells.[l] Efficient photodiodes have also been fabricated from blends of MEH-PPV with the cyano-substituted PPV derivative, CN-PPV.[2, 31 Interfaces between organic materials have also been employed to drive energy-transfer. Efficient polymer LEDs have been made by combining CN-PPV with PPV in a double layer structure.[4] Electrons and holes recombine across the interface and excitons decay radiatively from the orange CN-PPV.

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and deril,atirses;

Polymer/polymer

inteffirces.

Absorption spectra were obtained with a Perkin-Elmer Lambda-9 spectrophotometer. PL efticiency measurements were made using the integrating sphere technique.[S] Samples were excited with an argon ion laser and emission was detected with a CCDarray coupled to a spectrograph (Oriel instaspec IV). 3.

Results

Charge

Transfer

it1 the CN-PPVMEH-PPV

sysrem

MEH-PPV and CN-PPV, the structures of which are depicted in figure 1, have similar bandgaps (2.1 eV). The electron withdrawing cyano-groups attached to the backbone of CN-PPV increase its ionisation potential and electron affinity by approximately 0.5 eV relative to MEH-PPV. Polymer blend films were fabricated by spin coating from a solution containing both materials. Electron microscopy revealed the formation of an interpenetrating network of the two polymer phases within the plane of the film. on a lengthscale of order 1Onm.

iiC,H,l

2.

Experimental

A

methods

Evidence for charge- and energy-transfer was obtained from photovoltaic and photoluminescence (PL) measurements Photovoltaic cells were fabricated by spin-coating a polymer layer onto ITO-coated glass substrates, which acts as the hole collecting electrode. The top contacts (Al or Mg) were deposited over the polymer by evaporation. Cells were fabricated in a nitrogen-filled glovebox and characterised under vacuum. Cells were illuminated using a xenon lamp coupled to a monochromator. The short-circuit photocurrent was measured with a Keithley 617 electrometer. Samples for optical measurements were made by spincoating ‘pure’ or blended polymer onto quartz substrates.

Fig. 1. (A) DMOS-PPV,

1270-3

C

(B) MEH-PPV

and (C) CN-PPV

Figure 2 (open circles) shows the absolute PL efficiencies of blends of different composition. Values of 10% and 32% were obtained for MEH-PPV and CN-PPV respectively, whereas the PL in the blends is efficiently quenched to a level of 2% to 5% for composites containing 10% to 60% of CN-PPV. This significant though incomplete quenching is consistent with the scale of the phase separation and the exciton diffusion range. Figure 2 (filled circles) plots the quantum yield of polymer blend photocells of different composition. The blend photodiodes have efficiencies in the range 1.5% to 4%, whereas the MEH-PPV and CN-PPV devices are much less efficient

0379~6779/99/$ - see front matter 0 1999 Elsevier Science S.A. All rights reserved. PII: SO379-6779(98jO

B

J.J.M. Halls

106

et al. I Synthetic

(0.05% and 0.004% respectively). This is further evidence for the dissociation of excitons in the blend, driven by charge transfer at the distributed MEH-PPV/CN-PPV interfaces.

Meials

IOI

105-106

(1999)

confirming CN-PPV.

that excitons are transferred to the lower

Blend - - - - - DMOS.pp”

-. 0.2

.._._ ..._CN.pp” 3.0 Energy / eV Weigh!

transfer

4.0

g: z

0.0

fraction of CN-PPV

Fig. 2. The open circles show the absolute PL efficiencies of films of the MEH-PPV:CN-PPV composite (488 nm excitation). The filled circles show the short-circuit photocurrent quantum = yields of polymer blend photodiodes (500 nm excitation).

Energy

3.5

bandgao

in the PPVKN-PPV

sptem

Double-layer PPV/CN-PPV photovoltaic devices were fabricated and characterised. These cells were less efficient than single layer indicating that efficient exciton dissociation driven by charge-transfer does not occur at the interface. Indeed, the extra layer appears to suppress the photocurrent. This is in contrast with work using, for example, the electron acceptor Chll in a double-layer structure with PPV. Photovoltairquantum yields of up to 9% were obtained in devices of this type, some ten times higher than in single-layer PPV cells.[l] This, along with the high efficiency of PPV/CN-PPV LEDs, indicates that the probability of exciton dissociation at the PPV/CN-PPV interface is low, and points rather to efficient energy transfer to the CNPPV. PPV can not be blended with CN-PPV as its precursor does not share a common solvent with CN-PPV. In order to compare the MEH-PPV/CN-PPV more closely with the PPV/CN-PPV system we used the polymer DMOS-PPV as a substitute for PPV (see figure 1). DMOS-PPV is soluble in chloroform and is electronically similar to PPV.[6] The absorption spectra of spin-cast films of DMOS-PPV (dashed line), CN-PPV (dotted line) and a 2:l (by weight) blend (solid line) are shown in figure 3. The absorption spectrum of the blend is a superposition of two homopolymer spectra. The absolute quantum efficiencies of the DMOS-PPV and CN-PPV samples were 63% and 33% respectively. The blend had a PL yield of 37%. Thus luminescence is not efficiently quenched in the blend. PL spectra of these three films (excited at 457 nm) are shown in Figure 3. The PL spectrum of the blend is identical to that of the CN-PPV sample, with no trace of any PL from the DMOS-PPV. These results imply that excitons in the DMOS-PPV are transferred to the lower band-gap CN-PPV where they decay radiatively. The high efficiency of this energy transfer indicate that phase separation in the blend exists on a smaller scale than the exciton diffusion range. Photovoltaic cells made using the composite were found to have quantum yields smaller by a factor of 5000 than their MEHThis points to the PPVICN-PPV composite counterparts. absence of charge transfer in the DMOS-PPVICN-PPV system,

Fig. 3. Absorption and PL spectra of thin spin-cast films of DMOS-PPV (dashed lines), CN-PPV (dotted lines) and a 2:l DMOS-PPV:CN-PPV blend by weight of (solid line). 4.

Predictions

based on Molecular

Modelling

Cornil er al have shown that these observations are consistent with calculations of the relative energies of the HOMOs and LUMOs of the polymers ~investigated here.[7] Their findings are reported in more detail elsewhere in these proceedings. Using a calculated exciton binding energy of 0.75 eV, the interchain (charge-transfer, CT) state of the CNPPVIMEH-PPV pair lies 0.31 eV above the lowest intrachain state. In the case of the CN-PPV/PPV system, the interchain state is 0.58 eV above the lowest intrachain state. Thus in both cases the dissociated state is higher in energy and one would expect energy transfer. In conjugated polymers, polarisation effects are expected to lower the exciton binding energy, making the CT state lower in energy. A polarisation energy in the range of 0.3-0.6 eV is sufficient to make the CT state the lowest in the MEH-PPV/CN-PPV system (indicating charge-transfer), whilst leaving the CT state in the PPV/CN-PPV system higher in energy than the intrachain state in the CN-PPV (indicating energy-transfer). This level of polarisation energy reduces the binding energy from 0.75 eV to 0.15-0.45 eV, consistent with 1 independent evaluations ol‘the exciton binding energy.@] 5. References [1] J.J.M. Halls, K. Pichler, R.H. Friend, S.C. Moratti, A.B. Holmes, Appl. Phys. Lett. 68 (1996) 3120. [2] J.J.M. Halls, CA. Walsh,-N.C. Greenham, E.A. Marseglia, R.H. Friend, SC. Moratti, A.B. Holmes, Nature 376 (1995) 498. [3] G. Yu, J. Gao, J.C. Hummclen, F. Wudl. A.J. Heeger, Science 270 (1995) 1789. [4] N.C. Greenham, SC. Moratti, D.D.C.Bradley, R.H. Friend, A.B. Holmes, Nature 365 (1993) 628. [5] J.C.d. Mello, H.F. Wittmann, R.H. Friend, Adv. Mater. 9 (1997) 230. [6] D.H. Hwang, C.B. Yoon, K.J. Moon, H.K. Shim, Mol. Cryst. Liq. Cryst. 280 (1996) 175. [7] J. Cornil, D.A.dos Santos, R. Siibey, J.L. Bredas, Synth. Met. to be published f 1998). [S] J.L. Bredas, J. Cornil, A.J. Heeger, Adv. Mater. 8 (1996) 447.