A review was presented to demonstrate a historical description of the synthesis of light-emitting conjugated polymers for applications in electroluminescent devices. Electroluminescence (EL) was first reported in poly(para-phenylene vinylene) (PPV) in 1990 and researchers continued to make significant efforts to develop conjugated materials as the active units in light-emitting devices (LED) to be used in display applications. Conjugated oligomers were used as luminescent materials and as models for conjugated polymers in the review. Oligomers were used to demonstrate a structure and property relationship to determine a key polymer property or to demonstrate a technique that was to be applied to polymers. The review focused on demonstrating the way polymer structures were made and the way their properties were controlled by intelligent and rational and synthetic design.

Synthesis of Light-Emitting Conjugated Polymers for Applications in Electroluminescent Devices

Upconversion luminescent materials: advances and applications.

Alkylsabstituted polyfluorenes have emerged as a very attractive class of conju gated polymers, especially for display applications, owing to their pure blue and efficient electroluminescence coupled with a high charge-carrier mobility and good processability. The availability of specific and highly regroselective coupling reactions provides a rich variety of taifored polyfluorene-type paly mers and copolymers. The focus of this review, therefore, is on reliable structure-property relationships regarding (i) the interplay of substitution pattern, solid state morphology, and optical/electronic properties, and (ii) the key role of degradation-induced keto defect sites in fluorerse-type polymers, and strategies for obtaining defect-free polyfluorenes.

Semiconducting Polyfluorenes—Towards Reliable Structure–Property Relationships

Over the last five years, organic photovoltaic devices have emerged as a new competitor to silicon-based solar cells. In particular, the bulk heterojunction architecture (BHJ), in which the photoactive layer consists of a bicontinuous blend of an electron donor and an electron acceptor, has allowed power conversion efficiencies around 8%. We will present in this review the latest conjugated polymers used in such BHJ solar cells. We will mainly focus on electron-donating (p-type) polymers based on thiophenes, 1,3,2-benzodiathiazoles, pyrrolo[3,4-c]pyrrole-1,4-diones, benzo[1,2-b;3,4-b]dithiophenes, and few other materials with more exotic structures. This review should be helpful to evaluate which are the most promising materials and where this research field is going in the years to come.

Processable Low-Bandgap Polymers for Photovoltaic Applications†

Designing pi-conjugated polymers for organic electronics

The Effect of Keto Defect Sites on the Emission Properties of Polyfluorene-Type Materials

We present photoluminescence studies as a function of temperature from a series of conjugated polymers and a conjugated molecule with distinctly different backbone conformations. The organic materials investigated here are: planar methylated ladder type poly para-phenylene, semi-planar polyfluorene, and non-planar para hexaphenyl. In the longer-chain polymers the photoluminescence transition energies blue shift with increasing temperatures. The conjugated molecules, on the other hand, red shift their transition energies with increasing temperatures. Empirical models that explain the temperature dependence of the band gap energies in inorganic semiconductors can be extended to explain the temperature dependence of the transition energies in conjugated molecules.

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Temperature dependent photoluminescence of organic semiconductors with varying backbone conformation

We report on measurements of phosphorescence (Ph) and delayed fluorescence (DF) in poly(2,7-(9,9-bis(2-ethylhexyl)fluorene)) (PF2/6) present as dilute solid solution, as well as in bulk films. From combined experimental investigations of the time and intensity dependence of DF and Ph, as well as the temperature dependence of DF, we are able to show that DF in PF2/6, both in solution and film, is dominated by triplet-triplet annihilation (TTA). As a consequence of the intrachain diffusion and, hence, relaxation of triplets, the kinetics of the bimolecular reaction show a turnover from dispersive to nondispersive regime as borne out by the DF decay. This is in accord with theoretical predictions of TTA in disordered solids. From the transition to equilibrium kinetics, we are able to estimate the width of the density of states of triplets to 40 meV in good agreement with the width of the inhomogeneous broadenend S0←T1 transition of PF2/6.

Triplet-triplet annihilation in a poly(fluorene)-derivative

Spectra, kinetics, temperature, and excitation-dose dependencies of phosphorescence and delayed fluorescence intensities in thin films of the conjugated polymer poly(9,9-di(ethylhexyl)fluorene) [PF2/6] have been investigated via time-resolved spectroscopy. The results reflect a limited number of available energy sites (traps) for triplet-excited states in the long time region after laser excitation. Triplets captured in these traps decay mono-exponential with a time constant of several 100 ms. In the first few milliseconds after laser excitation, pronounced delayed fluorescence is observed due to triplet–triplet annihilation. The decay kinetics of the delayed fluorescence obeys a power law at all times in accordance with the framework in which triplets find each other after performing a random walk (hopping) in an energetic disordered media, which is represented by variations of the conjugation length in the polymer.

R. Guentner

Papers

The Effect of Keto Defect Sites on the Emission Properties of Polyfluorene-Type Materials

Temperature dependent photoluminescence of organic semiconductors with varying backbone conformation

Triplet-triplet annihilation in a poly(fluorene)-derivative

Trap influenced properties of the delayed luminescence in thin solid films of the conjugated polymer Poly (9,9-di(ethylhexyl)fluorene)

The role of keto defect sites for the emission properties of polyfluorene-type materials