Singlet Intrachain Exciton Generation and Decay in Poly(p-phenylenevinylene)

Abstract

We present excitation spectra, picosecond decay time, and quantum efficiency measurements of the photoluminescence (PL) signal in poly(p-phenylenevinylene) (PPV). In pristine PPV we observe a constant efficiency with excitation wavelength for singlet intrachain exciton generation, which we model to be close to unity. In contrast, photo-oxidized PPV samples contain a distribution of quenching centers following the profile of the absorption depth, which causes the PL efficiency to decrease with increasing excitation energy, and the PL efficiency and decay time to be no longer simply related.[S0031-9007(96)00969-6] PACS numbers: 78.55. Kz, 71.35.Cc, 78.47. + p The photophysics of conjugated polymers has been of interest for some time, [1] particularly since the discovery of electroluminescence (EL) in the polymer poly(pphenylenevinylene) (PPV) in a light-emitting diode (LED) structure [2]. Since the initial observation of green electroluminescence from PPV, LED's with various colors of emission [3-7] and improved efficiencies [8] have been reported.Much research has centered on the arylene-vinylenebased polymers, of which PPV is the simplest and most studied. However, there is still controversy surrounding the nature of the primary photoexcitation in these materials. Debate has often centered on the binding energy of the photoexcited state which can arise from both Coulombic and lattice distortion contributions. If this is small then description within a band model may be appropriate [9][10][11]; in contrast, if the electron and hole are strongly bound, they form an intrachain exciton [12][13][14][15]. We consider vide infra that an exciton model is more appropriate. However, it has also been suggested that nonemissive spatially indirect excitons ("bound polaron pairs" [12]) are the primary photoexcited species in these materials [16][17][18][19]. Luminescence in PPV is due to radiative decay of the intrachain singlet exciton, and measurements of photoluminescence (PL), provide important information about the nature of the photoexcitations. There is a discrepancy apparent between high values of PL efficiency (up to 0.27) reported recently [20] and the estimates for the quantum yield of "polaron pairs" [16 -19]. A high quantum yield of a nonemissive species such as polaron pairs also invalidates the presumed relationship [20,21] between PL quantum efficiency and lifetime. Furthermore, conventional estimates of maximum LED efficiencies of 25% of the PL quantum yield [22] may be too low if photoexcitation forms polaron pairs with high efficiency, but electrical injection does not [17,19]. A resolution of these contradictory results is essential for a greater understanding of photophysical processes in conjugated polymers, and of great importance to the development of LED's based on these materials.We have addressed these issues through measurements of photoluminescence excitation spectra, PLE, PL quantum yield, and PL transient decay on pristine and photooxidized samples of PPV. We find that the pristine material sh...

Controlling Interchain Interactions in Conjugated Polymers: The Effects of Chain Morphology on Exciton−Exciton Annihilation and Aggregation in MEH−PPV Films

“…As mentioned in the Introduction, several groups have argued that photooxidation of conjugated polymer films can take place quite readily, even at relatively low excitation intensities. 39,40,49,74,75 The photophysical signatures of oxidative damage are a reduced PL quantum yield and the appearance of an excited-state absorption in the spectral region where SE is expected to occur. One suggestion for reducing photodamage is to excite the polymer film through the transparent substrate.…”

“…Many studies have reported a link between photooxidation and reduced PL quantum yield. [74][75][76] Several groups also have argued that irreversible photodamage can occur in conjugated polymer samples at relatively low (e10 18 cm -3 ) excitation intensities, 39,49 even in the vacuum of a cryostat. 40 The chief signature of photochemical damage is a rapid decay of SE and the appearance of PA in the emission region; the PA in the near-infrared is only slightly affected.…”

Efficiency of Exciton and Charge Carrier Photogeneration in a Semiconducting Polymer

“…Indeed, exciton-exciton annihilation [30], generating free charges, may explain the small real component of the slowly decaying conductivity in Fig. 2(b).…”

Photophysics of poly(p-phenylene vinylene) films

“…As the oxidation is not oxygen limited, the film is not uniformly oxidized. A carbonyl concentration profile is formed [7] following the laser intensity decay in accordance with the Beer-Lambert law. As a consequence of chain shortening process, an inhomogeneous profile of PPV segments of different sizes is continuously generated from the interface air/polymer into the film.…”

“…Models of photoluminescence (PL) decay dynamics [1,5,6] and photoluminescence excitation spectroscopy (PLE) [2,[7][8][9] reveal details of carrier dynamics, quenching mechanisms and oxidation processes. The quenching process was accounted by attributing an additional non radiative decay channel to excitons [1][2][3][4][5][6][7]. This effect has been ascribed to the creation of carbonyl groups.…”