1. Introduction

Semiconducting conjugated polymers have attracted significant attention because of their potential applications in optoelectronic devices. Since the discovery of the metallic properties of doped polyacetylene, remarkable progress has been made in synthesizing conjugated polymers [1]. The discovery of electroluminescence in conjugated poly(p-phenylene-vinylene) (PPV) [2] and its soluble derivative Poly[2-methoxy-5-(2’-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) [3] have opened up many optical and electrical applications. PPV-based materials have proven to be very attractive, and applications such as lasers [4,5], light emitting diodes (LEDs), [6] photovoltaic cells [7], photodetectors [8], and field effect transistors [9] have been developed. Due to the enormous potential for applications, the understanding of the photophysics properties of MEH-PPV is essential for the maturity of these devices.

2. Sample preparation and optical characterization

Gilch-type MEH-PPV was purchased from Aldrich with average molar number M_n=70,000-100,000 g/mol. MEH-PPV solutions were prepared from toluene at three different concentrations (0.38, 2, and 5 g/dm³). To enhance the dissolution process, the solutions were placed in an ultrasonic bath for 6 hours and kept in a dark environment at room temperature until ready for characterization. The thin films were prepared by spin-coating the solution onto microscope slide substrates from the various concentrations at a spin speed of 1200 rpm for 30 seconds. The average thickness of the samples spun from the 5 g/dm³ concentration were about 100 nm as measured by a Tencor model P10 surface profilometer.

The photoluminescence (PL) and amplified spontaneous emission (ASE) were measured using a Q-switched frequency-doubled, Quanta Ray (Spectra Physics) Nd:YAG laser operating at 532 nm, 8 ns pulse length with a repetition rate of 10 Hz, and with a pulse energy of 200 mJ as the excitation source. An HR2000 Ocean Optics fiber-coupled CCD spectrometer with a resolution of 0.1 nm was used to record the spectra. The experimental setup is shown in Fig 1.

 

Fig. 1. The experimental setup for the PL and ASE measurements 3. Results and discussion

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3. Results and discussion

Figure 2 shows the PL spectra of MEH-PPV in solution and in thin film obtained from the 5 g/dm3 concentration. The PL spectra have been normalized to facilitate comparison. The PL profiles show two emission peaks. The first one is located at 561 nm and 594 nm in the solution and film respectively, and it is associated with the 0-0 first vibronic transition [10]. The second peak intensity was found to have a small dependence on solution concentration and appeared at 600 nm and 637 nm respectively, and is due mainly to the 0-1 second vibronic transition of the polymer and to a smaller extent to the formation of aggregated species [11]. Notice that the PL emission in film is shifted by about 35 nm compared to the solution, which is consistent with the “gas-to-crystal” effect. This effect is a consequence of the stronger interaction of the MEH-PPV dipoles with its surroundings in the highly polarizable environment of a conjugated polymer film compared to the solution form [12].

 

Fig. 2. Photoluminescence spectra of MEH-PPV in solution and film. The films were cast from the 5 g/dm³ solution. The excitation source was a Nd:YAG pulsed laser operating at 532 nm.

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To observe the spectra line-narrowing (SLN), the samples were pumped at energy densities beyond photoluminescence. Figure 3 shows the dependence of the emission on solution concentration. A decrease in the bandwidth and a red shift of the emission peak was observed at higher concentrations. The reduction in the SLN bandwidth at higher concentrations is an indication that as concentration increases, the proportion of aggregates and interchain species also increases compared to the single chain polymer. Therefore, the ability of the solution to produce SNL is also diminished.

The mechanism of spectral narrowing in MEH-PPV has been the subject of much debate [13]. Results which show line-narrowing can only happen when the index of refraction of the polymer is higher than the substrate and when the film is thick enough to support waveguiding suggest that spectral narrowing results from amplified spontaneous emission [14]. Sheridan et al [15] demonstrated the tuneability of amplified spontaneous emission over a 40 nm range by controlling the waveguide thickness. This might suggest that the waveguide geometry is the primary factor that determines the SLN characteristics. However this mechanism is incapable of explaining the SLN in solution, as shown in Fig. 3. Other mechanisms such as superfluorescence [16] biexcitons [17] and condensed excitons [18] have been proposed to explain the SLN.

 

Fig. 3. Spectral line narrowing of MEH-PPV at different concentrations in solution. The excitation source was a Nd:YAG pulsed laser operating at 532 nm.

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In thin films, we argue that the polymer chains that are closer to the glass substrate behave differently than those that are further away. This is a consequence of the strong interaction of the polymer’s dipoles on its surrounding. The spin coating process may have a different impact on the polymer chains closer to the glass interface compared to the chains further away from the interface. This may cause a variation of the interchain coupling [12] and the polymer morphology at different positions in the film. The influence of the polymer morphology on the emission spectra has been extensively studied in MEH-PPV [11]. The red shifted PL and SLN emission at lower temperatures [19] suggest that the polymer morphology is strongly dependent on the physical conditions of the polymer chains in films.

In order to study the SLN photophysics of the polymer of both the inner and outer layers of the polymer, the samples were characterized immediately after they were spun, so that the properties under study were mostly of the outer polymer chains. The outer polymers were then carefully removed by increasing the laser power, which caused the evaporation of the outer layers (about 45-60 nm). Similar spectra were obtained by removing the outer layers of material with a cotton swab dipped in toluene, but the laser ablation technique was preferred since the contact of the polymer with the toluene can contribute to the formation of aggregate species. Since both procedures provided the same results, the possibility that the laser ablation itself can induce changes in the polymer photophysics that could shift the emission wavelengths was ruled out. Figure 4 shows the spectra corresponding to both the inner and outer layers of the polymer film. In an earlier publication [20] we explained that the two emission bands corresponding to the inner and outer polymer chains in the films were due to the 0-0 and 0-1 band transitions respectively. This explanation relied on the correspondence of the spectral position between the two emission bands of the PL profile (the emission wavelengths are roughly 10 nm red shifted compared to the 0-0 and 0-1 PL transitions). However, notice that both spectra in Fig. 4 show an emission peak around 570 nm and 600 nm respectively, which are at shorter wavelengths compared to the SLN emission. These emission bands are most likely the result of the 0-0 band transition. As a consequence, the SLN in both cases is observed in the 0-1 band transition (just as in solution). The red shifted emission coming from the outer polymer (which was shifted around 40 nm compared to the emission of the inner chains) is attributable to the “gas-to-crystal” effect. In other words, the polarization environment of the outer layer of the polymer is quite different than the inner layer that is closer to the glass substrate. It is interesting to point out that when the samples are excited for the first time, one would expect that emissions from both the inner and outer polymer chains should be observed. Instead only the emission of the outer polymer was recorded. Since the emission from the inner polymer chains appears at shorter wavelengths compared to the outer polymer chains, the emission from the inner polymer chains is most likely absorbed by the outer polymer chains. Figure 5 illustrates that it is also possible to observe line-narrowing from both transitions at the same time by removing just enough material from the sample surface (30-35 nm).

 

Fig. 4. Spectral line narrowing of MEH-PPV in film. Two emission bands corresponding to the inner and outer layers of the polymer are observed. The excitation source was a Nd:YAG pulsed laser operating at 532 nm.

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The mechanisms described in Ref. [15] are important for explaining the shape of the ASE spectra. However if waveguide confinement were the only mechanism for the SLN, that will imply a total emission bandwidth of around 60 nm (590 - 650 nm). To the best of our knowledge, such large ASE bandwidths in MEH-PPV have not been reported. The emission from solution shows, where there is no waveguide confinement, a bandwidth of only 20 nm (at a concentration of 5 g/dm³). The waveguiding model cannot explain the two ASE bands of Fig. 5. Therefore these experiments indicate that the crystal to gas effect may be important in explaining the line-narrowing properties of MEH-PPV in films.

 

Fig. 5. Spectral line narrowing of MEH-PPV film cast from the toluene solution. Laser ablation removed just enough material from the surface of the samples so ASE emission from both inner and outer polymer chains takes place.

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4. Conclusions

In summary we have shown that the “gas-to-crystal” effect in MEH-PPV may play an important role in the spectral line-narrowing emission. These results suggest that in thin films, two line-narrowing emission bands are observed as a consequence of the interaction of the polymer chains in different polarizable environments. These results could have important implications for devices based on MEH-PPV, especially for lasing applications. Since the SLN radiation of the inner most polymer chains can be absorbed by the outer polymer chains, this can significantly reduce the effective gain in a laser structure.