J. Partee

Effects of film order on the optical properties of α-sexithiophene

Abstract The optical properties of ordered and disordered α-sexithiophene (α-6T) thin films are investigated by a variety of spectroscopies. The dominant low-lying even and odd parity excitons are identified by absorption and electroabsorption spectroscopies. Intermolecular interactions in ordered α-6T films results in Davydov splitting of the absorption band of the lowest odd parity exciton. Charged excitations in α-6T films were studied by photoinduced absorption and optically detected magnetic resonance. Both polarons and bipolarons are observed in disordered samples. As a consequence of higher carrier mobilities, only bipolarons were observed in ordered samples. Spin pairing of excitations broadens the observed ODMR spectra.

Photoluminescence and Auger spectroscopy of porous Si: Solvent, reactive ion etching, annealing, and substrate boron level effects

The photoluminescence (PL) at λ≤850 nm of boron‐doped porous Si films anodically etched, passively etched, annealed, and reactive ion etched (RIE) under systematically varied conditions is described and discussed. As previously observed, the PL yield η of films etched from 7 to 20 Ω cm wafers in HF/H2O/C2H5OH solutions rapidly degraded during 40 mW illumination in air at 488 nm. In addition, it was totally quenched by O2 annealing or RIE but not by H2 RIE. However, the yield of films etched in HF/H2O only decreased by less than 10% following similar illumination for 7 h, and O2 annealing or RIE reduced it by 0%–50% only. The instability under illumination, the effects of oxygen annealing and RIE, Auger line shape measurements, and surface charge buildup during Auger analysis are consistent with a significantly higher density of weak Si—Si bonds in films etched in ethanol‐containing solutions. These bonds are apparently broken by photoinduced oxidation, oxygen annealing, or O2 RIE, but the oxygen does not ...

Absorption and luminescence of pyridine-based polymers

We summarize the low energy photophysics of the pyridine-based polymers poly(p-pyridine)(PPy), poly(p-pyridyl vinylene) (PPyV) and copolymers made up of PPyV and poly(p-pheneylene vinylene) (PPyVPV). The absorption and luminescence properties are morphology dependent. The primary photoexcitations within these polymers are singlet excitons which may emit from individual chains following a random walk to lower energy segments, depending upon the excitation energy. Films display redshifted absorption and emission properties with a decrease in photoluminescence efficiency which can be attributed to aggregate formation in comparison to powder and solution forms. Photoinduced absorption (PA) studies show direct conversion of singlet to triplet excitons on the ps time scale. Polaron signatures and the transition between triplet exciton states are seen in powder forms using ms PA techniques. Film forms display only a polaron signature at millisecond times indicating that morphology plays a key role in the long-time photophysics for these systems. Photoluminescence detected magnetic resonance studies also have signatures due to both polarons and triplet excitons. The size of the triplet exciton is limited to a single ring suggesting that the triplet exciton may be trapped by extrinsic effects.

Solid State Effects in the Electronic Structure of Ladder-Type Poly(p-phenylene)s and Oligo(p-phenylene)s

Abstract In this paper, the spin-dependent photophysical properties of poly( para -phenylene)-type ladder-polymers (LPPP), a highly luminescent polymer with emission yields of ∼30% in film and ∼100% in solution, and of five- and seven-member ladder-type oligo( para -phenylenes) (LOPP5 and LOPP7, respectively) are determined via their photoluminescence (PL) emission, PL excitation, and PL-detected magnetic resonance (PLDMR) spectra. In the latter experiment, we investigated spin-sensitive properties, such as the formation of polaron pairs as a function of conjugation length and interchain interaction. The observation of a PLDMR quenching resonance upon increasing the temperature and/or the excitation wavelength is discussed in relation with the formation of polarons and polaron pairs from delocalized and localized states, respectively.

Dynamics of Long-lived Polarons in Poly(p-phenylene)-Type Ladder Polymers (LPPP)

Abstract The X-band spin 1/2 photoluminescence-detected magnetic resonance (PLDMR) of LPPP films and solutions featuring a high degree of intrachain order and high PL quantum yields (24% in films and 84% in solutions) is described. Excitation at 458 run yields a PL-enhancing polaron resonance at g~2.00 of width ΔH 1/2 ~8G and 4G in films and frozen solutions, respectively. The resonances are assigned to enhanced nonradiative recombination of long-lived polaron pairs, which are singlet exciton quenching centers. Their behavior is consistent with two distinct polaron pair lifetimes or a very broad distribution of lifetimes which are ~10 times longer in solutions than in films. The distinct lifetimes which are ~40 and ~670μs in films and ~370 and ~3300μs in solutions, are discussed in relation to longer-lived intrachain interconjugation segment pairs and shorter-lived interchain pairs. The spectral dependence of the resonance demonstrates that the emission from charge transfer excitons in aggregates of the films is much more spin-dependent than that from single polymer chains of the frozen solutions.

Dynamics of Polarons in Guest-Host-System Polymer Light Emitting Devices

The conjugated ladder-type poly(paraphenylene) is an attractive material for blue polymer light emitting devices (PLED). Blending the active layer with small amounts of a red emitting guest polymer, the emission shifts from blue to red with increasing guest concentration due to efficient excitation energy transfer. The results of photoluminescence detected magnetic resonance, electroluminescence detected magnetic resonance measurements and current detected magnetic resonance measurements on PLEDs based on 0.05 w%--2 w% red emitting poly(perylene-co-diethynylbenzene) (PPDB) in the active layer of the PLED are presented and discussed.

Excitation energy migration in highly emissive semiconducting polymer blends probed by photoluminescence detected magnetic resonance

Abstract The excitation energy migration (EEM) in methyl-substituted ladder-type poly(para-phenylene) (m-LPPP) doped with small amounts of the red emitter poly(perylene-co-diethynylbenzene) (PPDB) was studied by photoluminescence (PL) detected magnetic resonance (PLDMR). It is suggested that the EEM process proceeds via two steps: (1) migration within the host and (2) transfer from the host to the guest. The contributions of the emissions from m-LPPP and PPDB to the PL-enhancing polaron PLDMR at g=2, which are due to a reduction in the density of polarons acting as singlet exciton (SE) quenching centers, evolve differently with temperature. This provides clear evidence for SE migration in m-LPPP. The triplet exciton (TE) PLDMR at g=4 shows a distinct peak for each polymer, with the intensity of the PPDB feature being proportional to its concentration. However, the spectral dependence recorded at the peak of each resonance is the same. This rules out the triplet–triplet-annihilation-mechanism in these blends for simple energetic reasons. Instead we propose that the resonance at g=4 is due to a SE-quenching mechanism similar to that for the polaron resonance at g=2. At the field-for-resonance the number of TE’s decreases, and hence the PL increases.

Delayed Fluorescence (DF) and Photoluminescence (PL)-Detected Magnetic Resonance (PLDMR) Studies of Triplet-Triplet (T-T) Annihilation and Other Long-Lived Processes in π-Conjugated Polymers

Abstract Recent studies of frequency resolved DF due to T-T annihilation to singlet excitons (1S*) in π-conjugated polymers are reviewed, and the significance of this process is compared to that of other long-lived processses affecting the PL, in particular the nonradiative quenching of 1S* by polarons or triplet excitons. While the DF studies suggest that T-T annihilation typically accounts for up to ∼3% of the total PL, PLDMR and other studies suggest that singlet quenching by polarons and triplets may be a very significant process in the films and organic light emitting devices (OLEDs), especially at high excitation densities.

Optically detected magnetic resonance studies of tetrathienylene vinylene

We have investigated films and dilute glasses of tetrathienylene vinylene (4TV) by optically detected magnetic resonance. We calculate the g-values of polarons and zero field splitting parameters for triplet excitons. The strength of the ODMR signals, but not their spectral lineshapes, are sensitive to the sample concentration. Photoexcitation lifetimes were also estimated.

Frequency-resolved delayed fluorescence (DF) and photoluminescence detected magnetic resonance (PLDMR) studies of triplet and polaron dynamics in π-conjugated materials and devices

The DF and PLDMR of triplet exciton and polaron dynamics in (pi) -conjugated materials and devices is reviewed, and the significance of various processes involving these long-lived excitations is considered. These include the generation of singlet excitons by triplet-triplet annihilation, which leads to DF, and nonradiative quenching of such singlets by polarons or triplet excitons. The clear differences between the roles of these processes in (pi) -conjugated polymers vs small molecules and their implications for organic light emitting devices are discussed.