Recombination of triplet excitons and polaron pairs in a derived paraphenylene vinylene pentamer

Abstract

The triplet photoexcited states of the pentamer 2,5-di-n-octyloxy-1,4-bis ͑4',4''-bisstyryl͒styrylbenzene in a polystyrene matrix have been investigated by optically detected magnetic resonance ͑ODMR͒ spectroscopy in spin coated films at different concentrations of the blend. As previously reported for related para-phenylene vinylene ͑PPV͒ polymers, a part of the singlet excitons-the primary photoexcitations-are dissociating by intermolecular charge transfer, which results in Coulomb bound pairs of charge carriers, the distant polaron pairs ͑PPs͒. At low temperature and weak photo-excitation, however, the triplet exciton ͑TE͒ ODMR spectrum is dominant relative to that of the PPs, which points to a more efficient formation of the long living TEs than in PPV polymers. From a detailed analysis of the TE spectrum a rhombic (Dϭ0, Eϭ0.037 cm Ϫ1 ) zero-field splitting interaction could be determined. Under optical excitation a much faster saturation of TE compared to PP spectra is found, in qualitative agreement with the previously proposed triplet-triplet annihilation mechanism for ODMR. A quantitative analysis shows a stronger than expected intensity dependence at higher excitation levels, which is attributed to an increasing probability for triplet-triplet annihilation as a result of the decreasing average distance between pairs of triplet excitons. Furthermore, the PP signal shows a much faster response than that of the TEs (ϭ0.22 and 1.2 ms, respectively, measured at 2.5 K͒, which also calls for a refinement of the description of the processes involved. TE and PP signals also show markedly different behaviors under a variation of the oligomer concentration in the polystyrene matrix, and of the measuring temperature. These results are discussed in light of the different formation and recombination mechanisms of excitons and polarons in conjugated organic materials.

Quantitative evaluation of the preferential orientation ofpara-phenylene vinylene pentamers in polystyrene films by optically detected magnetic resonance

“…The experimental ODMR setup, based on an X-band (-9.4 GHz) EPR spectrometer (Bruker ESP300E) with an Oxford liquid helium flow cryostat, has been described in detail previously [9]. All ODMR measurements were performed at 2.5 K. A special holder with optical fiber access for the excitation was designed to be able to mount the thin film samples horizontally and vertically in the Xband spectrometer.…”

“…The dependence of the three different ODMR signals as a function of the optical excitation intensity has been measured in a spincoated film for each oligomer (not shown) and gives the same results as in ref. 9, from which we can conclude that the TTA model [9,11] is indeed a good description for all of these systems. For the simulation of the TE ODMR spectra we use an EPR simulation program [10] which we adapted for simulating the measured ODMR intensities.…”

“…In the case of the oligomers under study, magnetic resonance transitions between the TE states are found to influence the PL intensity IpL and to give rise to a triplet ODMR spectrum. Previous studies of PPV-derived oligomers [9,11] have shown that triplettriplet annihilation (TTA) yields a good model to describe the obtained ODMR signals. In the TTA model the coupling between two neighboring TEs yields a pair with a combined energy that is sufficient to create an SE, and this process is influenced by microwave-induced transitions between the spin states, leading to a change of/PE" In this paper, we introduce ODMR as an altemative technique to characterize the preferential o¡ of conjugated molecules in polymer films, and we apply it to three different PPV pentamers, with different side chains attached to the backbone.…”

Discrimination between spin-dependent charge transport and spin-dependent recombination in π-conjugated polymers by correlated current and electroluminescence-detected magnetic resonance

“…In this case, device current changes will, of course, also induce recombination rate changes. An example of a spin-dependent transport mechanism which has previously been resolved directly in MEH-PPV devices is the quenching interaction between triplet excitons and charge carriers, the socalled triplet exciton-polaron mechanism [14,[43][44][45][46][47][48][49]. This process depends on triplet density and triplet lifetime, and therefore on temperature and exhibits clear signatures of s=1 triplet species in the half-field magnetic resonance.…”

Coherent Spin Manipulation in Molecular Semiconductors: Getting a Handle on Organic Spintronics

“…[38] Obviously, electron spin resonance provides direct insight into this question, and was used early on to argue in favor of the existence of polaron pairs of both singlet and triplet character. [39][40][41] The remainder of the present article will deal with this discussion. It is worth noting, however, that the direct visualization of singlet and triplet excitons in organic semiconductors by fluorescence and phosphorescence, respectively, can be used to prove the presence of both singlet and triplet polaron pairs.…”