ApplPhysLett_90_091106_2007 (3 pages)

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APPLIED PHYSICS LETTERS 90 091106 2007 Destruction of amplified spontaneous emission via chemical doping at low work function metal conjugated polymer interfaces Bertrand Tremolet de Villers and Benjamin J Schwartza Department of Chemistry and Biochemistry University of California Los Angeles California 90095 1569 Received 4 January 2007 accepted 25 January 2007 published online 28 February 2007 The authors investigate how the use of different metal electrodes affects the ability of poly 2 methoxy 5 2 ethyl hexyloxy 1 4 phenylene vinylene MEH PPV films to undergo amplified spontaneous emission ASE High work function metals such as Ag or Au have little effect on the ASE threshold but low work function metals such as Ca or Al completely shut off ASE ASE is restored when a thin spacer layer such as a few nanometers of polystyrene or oxidized Ca is introduced between the MEH PPV film and the Ca or Al electrode This suggests that low work function metals chemically dope the polymer creating polarons that destroy ASE not only by lowering the gain through emission quenching but primarily by increasing the loss via optical absorption Thus the exponential sensitivity of ASE to optical losses provides a spectroscopic probe of conjugated polymer metal interfaces 2007 American Institute of Physics DOI 10 1063 1 2710188 It has been over a decade since amplified spontaneous emission1 ASE and optically pumped lasing2 3 were observed in films of semiconducting polymers yet in the intervening years no one has produced an electrically pumped polymer based diode laser Although there are several possible explanations as to why electrically pumped lasing has been so difficult to achieve here we focus on the nature of the metal electrodes used in fabricating conjugated polymerbased optoelectronic devices In particular we show that the use of low work function metal electrodes which are critical to high current injection in conjugated polymer based diodes leads to chemical doping of the polymer and that the resulting polarons introduce enough optical loss to destroy the ability of the polymer film to undergo ASE or lasing Thus because of its exponential sensitivity to optical loss ASE can be used as an indicator of chemical interactions at polymer metal interfaces To investigate how different electrode metals affect the ability of conjugated polymer films to undergo lasing and ASE we prepared samples of poly 2 methoxy 5 2 ethyl hexyloxy 1 4 phenylene vinylene MEH PPV one of the most well studied semiconducting polymers We synthesized MEH PPV by following method b of Neef and Ferraris 4 except that we halved the concentrations of potassium t butoxide and monomer and we performed the recrystallization from methylene chloride instead of tetrahydrofuran In an inert atmosphere we spun 1 w v solutions of MEH PPV in chlorobenzene onto precleaned glass substrates at 2000 rpm to produce films with thicknesses ranging from 150 to 200 nm optical densities of 1 5 1 9 at the 500 nm absorption maximum the films were baked at 50 C for 1 h to drive off any excess solvent We then prepared samples in two basic configurations glass MEH PPV M or glass MEH PPV spacer M where M Ag Al Ca or Au and spacer is a thin layer of either polystyrene or oxidized Ca as discussed further below The metals were thermally evaporated under a vacuum of 10 8 bar at evaporation rates of 0 1 1 5 nm s to give a total metal layer a Electronic mail schwartz chem ucla edu thickness of 100 nm The ability of our samples to undergo ASE was investigated by exciting them from their nonmetallated side with 100 fs pulses of 490 nm light whose energy was controlled between 10 and 800 nJ the details of our laser setup are described elsewhere 5 6 Emission was collected 50 55 from the sample normal and analyzed using a fiber optic spectrometer Figure 1 shows the emission collected from a 150 nm thick film of MEH PPV with no top layer as a function of the 490 nm excitation energy It is clear that once the excitation energy becomes large enough the emission spectrum changes from the characteristically broad fluorescence spectrum associated with the continuous wave excitation of MEH PPV to a much narrower emission dominated by a single peak near 620 nm This turn on of line narrowing above a well defined threshold is the classic signature of ASE 1 3 To characterize the ASE threshold we integrated the intensity of the ASE peak in each spectrum after subtracting the below threshold broad fluorescence whose shape is independent of excitation energy The ASE threshold of 22 nJ pulse was then determined as the intersection of least squares fit lines to the low and high energy portions of the integrated spectral data as shown in the inset of Fig 1 The ASE thresholds determined this way for all of our samples are reported in Table I Table I shows that samples with Ag and Au top layers exhibit ASE thresholds that are approximately twice as large as that of the MEH PPV film without any metal This modest ASE threshold increase could result either from the fact that adding a metallic top layer modifies the waveguide that confines the emitted light in the gain region or from the fact that some of the polymer film s emission is likely quenched by image dipoles in the metal 7 resulting in a slight lowering of the gain Even though these metals slightly raise the ASE threshold however it is clear that the presence of Ag and Au does not destroy the gain behavior of the MEH PPV films consistent with other studies in which these metals have been used to create feedback structures for optically pumped conjugated polymer based lasers 2 3 8 0003 6951 2007 90 9 091106 3 23 00 90 091106 1 2007 American Institute of Physics Downloaded 03 Mar 2007 to 169 232 128 68 Redistribution subject to AIP license or copyright see http apl aip org apl copyright jsp 091106 2 Appl Phys Lett 90 091106 2007 B Tremolet de Villers and B J Schwartz FIG 1 Normalized emission spectra of a glass MEH PPV sample showing the signature line narrowing of ASE as the optical excitation energy is increased Inset Integrated intensity of the ASE peak centered at 620 nm after subtraction of the below threshold emission spectrum The solid lines are least squares fits to the low and high energy portions of the data The value of the ASE threshold is determined from the intersection of the lines to be 22 nJ pulse In contrast to Au and Ag when either of the common cathode metals Ca or Al is evaporated on top of a MEH PPV


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