Destruction of amplified spontaneous emission via chemical dopingat low-work-function metal/conjugated polymer interfacesBertrand Tremolet de Villers and Benjamin J. Schwartza兲Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095-1569共Received 4 January 2007; accepted 25 January 2007; published online 28 February 2007兲The authors investigate how the use of different metal electrodes affects the ability ofpoly关2-methoxy-5-共2⬘-ethyl-hexyloxy兲-1,4-phenylene vinylene兴共MEH-PPV兲 films to undergoamplified spontaneous emission 共ASE兲. High-work-function metals such as Ag or Au havelittle effect on the ASE threshold, but low-work-function metals such as Ca or Al completelyshut off ASE. ASE is restored when a thin spacer layer, such as a few nanometers of polystyrene oroxidized Ca, is introduced between the MEH-PPV film and the Ca or Al electrode. This suggeststhat low-work-function metals chemically dope the polymer, creating polarons that destroy ASE notonly by lowering the gain through emission quenching but primarily by increasing the loss viaoptical absorption. Thus, the exponential sensitivity of ASE to optical losses provides aspectroscopic probe of conjugated polymer/metal interfaces. © 2007 American Institute of Physics.关DOI: 10.1063/1.2710188兴It has been over a decade since amplified spontaneousemission1共ASE兲 and optically pumped lasing2,3were ob-served in films of semiconducting polymers, yet in the inter-vening years, no one has produced an electrically pumpedpolymer-based diode laser. Although there are several pos-sible explanations as to why electrically pumped lasing hasbeen so difficult to achieve, here we focus on the nature ofthe metal electrodes used in fabricating conjugated polymer-based optoelectronic devices. In particular, we show that theuse of low-work-function metal electrodes, which are criticalto high current injection in conjugated polymer-based diodes,leads to chemical doping of the polymer and that the result-ing polarons introduce enough optical loss to destroy theability of the polymer film to undergo ASE or lasing. Thus,because of its exponential sensitivity to optical loss, ASE canbe used as an indicator of chemical interactions at polymer/metal interfaces.To investigate how different electrode metals affectthe ability of conjugated polymer films to undergo lasingand ASE, we prepared samples of poly关2-methoxy-5-共2⬘-ethyl-hexyloxy兲-1,4-phenylene vinylene兴共MEH-PPV兲,one of the most well-studied semiconducting polymers. Wesynthesized MEH-PPV by following method b of Neef andFerraris,4except that we halved the concentrations of potas-sium t-butoxide and monomer and we performed the recrys-tallization from methylene chloride instead of tetrahydrofu-ran. In an inert atmosphere, we spun 1 % w/v solutions ofMEH-PPV in chlorobenzene onto precleaned glass substratesat ⬃2000 rpm to produce films with thicknesses rangingfrom 150 to 200 nm 共optical densities of 1.5–1.9 at the500 nm absorption maximum兲; the films were baked at⬃50 ° C for ⬃1 h to drive off any excess solvent. We thenprepared 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 polysty-rene or oxidized Ca, as discussed further below. The metalswere thermally evaporated under a vacuum of 10−8bar atevaporation rates of 0.1–1.5 nm/s to give a total metal layerthickness of ⬃100 nm. The ability of our samples to undergoASE was investigated by exciting them from their nonmet-allated side with ⬃100 fs pulses of 490 nm light whose en-ergy was controlled between ⬃10 and 800 nJ; the details ofour laser setup are described elsewhere.5,6Emission was col-lected ⬃50° –55° from the sample normal and analyzed us-ing a fiber optic spectrometer.Figure 1 shows the emission collected from a⬃150-nm-thick film of MEH-PPV with no top layer as afunction of the 490 nm excitation energy. It is clear that oncethe excitation energy becomes large enough, the emissionspectrum changes from the characteristically broad fluores-cence spectrum associated with the continuous-wave excita-tion of MEH-PPV to a much narrower emission dominatedby a single peak near 620 nm. This turn on of line narrowingabove a well-defined threshold is the classic signature ofASE.1,3To characterize the ASE threshold, we integrated theintensity of the ASE peak in each spectrum after subtractingthe below-threshold broad fluorescence, whose shape is in-dependent of excitation energy. The ASE threshold of⬃22 nJ/pulse was then determined as the intersection ofleast-squares fit lines to the low- and high-energy portions ofthe integrated spectral data, as shown in the inset of Fig. 1.The ASE thresholds determined this way for all of oursamples are reported in Table I.Table I shows that samples with Ag and Au top layersexhibit ASE thresholds that are approximately twice as largeas that of the MEH-PPV film without any metal. This modestASE threshold increase could result either from the fact thatadding a metallic top layer modifies the waveguide that con-fines the emitted light in the gain region or from the fact thatsome of the polymer film’s emission is likely quenched byimage dipoles in the metal,7resulting in a slight lowering ofthe gain. Even though these metals slightly raise the ASEthreshold, however, it is clear that the presence of Ag and Audoes not destroy the gain behavior of the MEH-PPV films,consistent with other studies in which these metals have beenused to create feedback structures for optically pumped con-jugated polymer-based lasers.2,3,8a兲Electronic mail: [email protected] PHYSICS LETTERS 90, 091106 共2007兲0003-6951/2007/90共9兲/091106/3/$23.00 © 2007 American Institute of Physics90, 091106-1Downloaded 03 Mar 2007 to 169.232.128.68. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jspIn contrast to Au and Ag, when either of the commoncathode metals Ca or Al is evaporated on top of a MEH-PPVfilm, Table I shows that ASE is completely shut off: the ASEthreshold rises above the optical damage limit of the poly-mer. To investigate whether the reflectivity of Ca or Alplayed a role in altering the ASE threshold, we preparedsamples in which a ⬃30-nm-thick spacer layer of polysty-rene was spin cast between the MEH-PPV film and the Al orCa top layer. We found that introduction