Photophysics of poly„2, 3-diphenyl-5-hexyl-p-phenylene vinylene… Arthur Dogariu* and A. J. Heeger Institute for Polymers and Organic Solids, University of California at Santa Barbara, Santa Barbara, California 93106 Hailiang Wang UNIAX Corporation, 6780 Cortona Road, Santa Barbara, California 93117 Received 7 December 1999 Time resolved photoinduced absorption and stimulated emission measurements, with temporal resolution of approximately 100 fs, demonstrate that the emissive properties of the blue-green emitting conjugated polymer, poly2, 3-diphenyl-5-hexyl-p-phenylene vinyleneDP6-PPV, are fully consistent with a simple ‘‘four-level’’ model. The stimulated emission spectrum is the same as the photoluminescence spectrum, and the stimulated emission time decay is the same as the photoluminescence time decay. INTRODUCTION Conjugated polymers are promising as high-gain materi- als for solid-state lasers. The -* transition is allowed and the absorption depth is small a few tens of nm. Since the cross sections for absorption and stimulated emission are the same to first order, semiconducting polymers are intrinsi- cally high optical gain materials. In conjugated polymers, the emission is redshifted with respect to the absorption; the Stoke’s shift arises from a combination of structural relax- ation in the excited state and disorder. As a result of the Stoke’s shift, residual self-absorption is relatively weak. The existence of structural relaxation in the excited state bond relaxation after photoexcitation implies that semiconducting polymers can be modeled as ‘‘four-level’’ systems. Thus, population inversion should be achievable at relatively low excitation levels. Finally, concentration quenching, which is typically a major problem in laser dyes, is not of great im- portance in semiconducting polymers; neat films with photo- luminescence PL quantum efficiencies in excess of 60– 70% have been demonstrated. The absence of excitation quenching arises directly from the delocalized nature of the photoexcitations; the weak Davidov splitting is comparable to spectral shifts from local disorder. As a result, interchro- mophore interactions do not lead to quenching of the PL. In the classic ‘‘four-level’’ model of a laser dye, the pho- toluminescence spectrum is identical with the stimulated emission spectrum the ‘‘gain’’ spectrum. In conjugated polymers, however, excited state absorption losses often overcome the gain over a significant fraction of the emission spectrum. Moreover, the stimulated emission SE lifetime is typically significantly less than the PL decay time. As a re- sult, the potential advantages of conjugated polymers as ma- terials for optical gain have not been fully realized. Although time resolved pump-probe experiments offer the possibility of exploring the lifetime of the excited state, the results are often not internally consistent. Ideally, when the pump energy is coincident with the excited state energy, the SE spectrum will be identical to the PL spectrum. However, if there are higher energy excited states, PA to those higher energy excited states can compete with SE and change the gain spectrum. It is generally accepted that both the SE in the visible and the photo-induced absorption PA in the near infrared, arise from the same delocalized *-excited state. However, the SE and PA lifetimes are almost never the same. While the lifetime of the PA is usually on the order of hundreds of picoseconds, 1–5 the SE lifetimes range from a few picoseconds 1,2 to hundreds of picoseconds equal to 3 or somewhat smaller than the PA. 4–6 There is disagreement in the literature even at the most basic level. A good example is MEH-PPV, where initial measurements implied that stimulated emission was not possible, 1,2 while later results showed the opposite. 4,7,8 More recently, it has become apparent that the detailed processing, specific solvents used, 9 the presence or absence of oxygen, 10 etc, all affect the chain packing and microstruc- ture, and ultimately the optical properties of the conjugated polymers. To address these issues, we report results of time resolved photoinduced absorption and stimulated emission measure- ments, with temporal resolution of approximately 100 fs, which demonstrate that the emissive properties of the blue- green emitting conjugated polymer, poly2, 3-diphenyl-5- hexyl-p-phenylene vinylene (DP6- PPV ), are fully consis- tent with a simple ‘‘four-level’’ model. The stimulated emission and photoluminescence spectra are the same, and the stimulated emission and photoluminescence time decay are the same. EXPERIMENTAL DETAILS The synthesis of DP6 was recently reported by Hsieh et al. 11 The samples used in our experiments were 200 nm thick films spin cast from solutions of DP6-PPV in xylene. The samples were deposited on sapphire substrates to pre- vent wave guiding and hence, amplified spontaneous emission. 12 All experiments were performed with the samples under dynamic vacuum. The pump power was kept sufficiently small that bimolecular interactions had little or no effect on the decay of photogenerated excited state; i.e., when pumping at photon energies above the -* energy gap, the excited state population density was kept below or about 10 18 cm -3 . When two-photon excitation was used for PHYSICAL REVIEW B 15 JUNE 2000-I VOLUME 61, NUMBER 23 PRB 61 0163-1829/2000/6123/161834/$15.00 16 183 ©2000 The American Physical Society