Research in the use of organic polymers as the active semiconductors in light-emitting diodes has advanced rapidly, and prototype devices now meet realistic specifications for applications. These achievements have provided insight into many aspects of the background science, from design and synthesis of materials, through materials fabrication issues, to the semiconductor physics of these polymers.

Electroluminescence in conjugated polymers

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

In recent years, organic solar cells utilizing π-conjugated polymers have attracted widespread interest in both the academic and, increasingly, the commercial communities. These polymers are promising in terms of their electronic properties, low cost, versatility of functionalization, thin film flexibility, and ease of processing. These factors indicate that organic solar cells, although currently producing relatively low power conversion efficiencies (∼5-7%),1–3 compared to inorganic solar cells, have the potential to compete effectively with alternative solar cell technologies. However, in order for this to be feasible, the efficiencies of organic solar cells need further improvement. This is the focus of extensive studies worldwide. The backbone of a π-conjugated polymer is comprised of a linear series of overlapping pz orbitals that have formed via sp2 hybridization, thereby creating a conjugated chain of delocalized electron density. It is the interaction of these π electrons that dictates the electronic characteristics of the polymer. The energy levels become closely spaced as the delocalization length increases, resulting in a ‘band’ structure somewhat similar to that observed in inorganic solid-state semiconductors. In contrast to the latter, however, the primary photoexcitations in conjugated polymers are bound electron-hole pairs (excitons) rather than free charge carriers; this is largely due to their low dielectric constant and the presence of significant electron-lattice interactions and electron correlation effects.4 In the absence of a mechanism to dissociate the excitons into free charge carriers, the exciton will undergo radiative and nonradiative decay, with a typical exciton lifetime in the range from 100 ps to 1 ns. Achieving efficient charge photogeneration has long been recognized as a vital challenge for molecular-based solar cells. For example, the first organic solar cells were simple single-layer devices based on the pristine polymer and two electrodes of different work function. These devices, based on a Schottky diode structure, resulted in poor photocurrent efficiency.5–7 Relatively efficient photocurrent generation in an organic device was first reported by Tang in 1986,8 employing a vacuum-deposited CuPc/ perylene derivative donor/acceptor bilayer device. The differing electron affinities (and/or ionization potentials) between these two materials created an energy offset at their interface, thereby driving exciton dissociation. However, the efficiency of such bilayer devices is limited by the requirement of exciton diffusion to the donor/acceptor interface, typically requiring film thicknesses less than the optical absorption depth. Organic materials usually exhibit exciton diffusion lengths of ∼10 nm and optical absorption depths of 100 nm, although we note significant progress is now being made with organic materials with exciton diffusion lengths comparable to or exceeding their optical absorption depth.9–12 The observation of ultrafast photoinduced electron transfer13,14 from a conjugated polymer to C60 and the * To whom correspondence should be addressed. E-mail: j.durrant@ imperial.ac.uk. Chem. Rev. 2010, 110, 6736–6767 6736

Charge Photogeneration in Organic Solar Cells

The electron-hole pair created via photon absorption in organic photoconversion systems must overcome the Coulomb attraction to achieve long-range charge separation. We show that this process is facilitated through the formation of excited, delocalized band states. In our experiments on organic photovoltaic cells, these states were accessed for a short time (

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The Role of Driving Energy and Delocalized States for Charge Separation in Organic Semiconductors

Organic (opto)electronic materials have received considerable attention due to their applications in thin-film-transistors, light-emitting diodes, solar cells, sensors, photorefractive devices, and many others. The technological promises include low cost of these materials and the possibility of their room-temperature deposition from solution on large-area and/or flexible substrates. The article reviews the current understanding of the physical mechanisms that determine the (opto)electronic properties of high-performance organic materials. The focus of the review is on photoinduced processes and on electronic properties important for optoelectronic applications relying on charge carrier photogeneration. Additionally, it highlights the capabilities of various experimental techniques for characterization of these materials, summarizes top-of-the-line device performance, and outlines recent trends in the further development of the field. The properties of materials based both on small molecules and on conjug...

Organic Optoelectronic Materials: Mechanisms and Applications

We have studied electronic excited states in films of poly(p-phenylenevinylene) using picosecond transient and cw photomodulation, photoluminescence, and their excitation spectra, as well as electroabsorption spectroscopy. We have determined all the important energy levels of singlet excitons with odd and even parity, the onset of the continuum band, the two-electron (biexciton) states, and the two relevant triplet states, and show that good agreement exists with models involving electron correlation.

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Optical probes of excited states in poly(p-phenylenevinylene).

We show that one-electron band theory fails to describe optical absorption in PPV and the absorption spectrum can be described only within a Coulomb-correlated model. The lowest optical state is an exciton, whose binding energy is estimated theoretically to be 0.90\ifmmode\pm\else\textpm\fi{}0.15 eV. Measurements of the photoconductivity quantum efficiency in poly[2-methoxy,5-(${2}^{\mathcal{'}}$-ethyl-hexyloxy)-1,4 phenylenevinylene] reveal a binding energy in good agreement with the theoretical value.

Excitons in poly(para-phenylenevinylene)

Photoexcited states in transpolyacetylene [t-(CH${)}_{\mathit{x}}$], and in alkoxy derivatives of poly(paraphenylenevinylene) (PPV) nondegenerate ground-state polymers, have been studied by the technique of absorption detected magnetic resonance. In addition to the stable charged photoexcitations, which are charged solitons in t-(CH${)}_{\mathit{x}}$ and bipolarons in PPV, we clearly identify neutral photoexcitations with nonzero spins. In t-(CH${)}_{\mathit{x}}$ these are spin-1/2 neutral solitons ${\mathit{S}}^{0}$(S\ifmmode\bar\else\textasciimacron\fi{} $^{0}$) with an optical transition at 1.35 eV. The neutral photoexcitations in PPV form triplet excitons which cannot dissociate into ${\mathit{S}}^{0}$S\ifmmode\bar\else\textasciimacron\fi{} $^{0}$ pairs due to the confinement caused by the nondegenerate backbone structure.

Studies of photoexcited states in polyacetylene and poly(paraphenylenevinylene) by absorption detected magnetic resonance: The case of neutral photoexcitations.

Photoexcitations in poly[2-methoxy-5-(${2}^{\ensuremath{'}}$-ethyl-hexyloxy)-p-phenylene vinylene] (MEH-PPV) and composites of MEH-PPV and ${\mathrm{C}}_{60}$ (MEH-PPV/${\mathrm{C}}_{60}$) were studied by photoinduced absorption and absorption-detected magnetic-resonance spectroscopies. We report direct evidence that the prominent triplet photoexcitations in pristine MEH-PPV are effectively quenched in MEH-PPV/${\mathrm{C}}_{60}$. In contrast, the dominant photoexcitations in MEH-PPV/${\mathrm{C}}_{60}$ are spin-1/2 excitations: polarons on the polymer chains; and polarons on the ${\mathrm{C}}_{60}$ (${\mathrm{C}}_{60}^{\mathrm{\ensuremath{-}}}$). \textcopyright{} 1996 The American Physical Society.

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Absorption-detected magnetic-resonance studies of photoexcitations in conjugated-polymer/C60 composites.

We have studied polaron pair photoexcitations in a variety of {pi}-conjugated polymer films by photoinduced absorption (PA) and optically detected magnetic resonance (ODMR). Both luminescent and nonluminescent polymers have been studied. An exchange interaction between polarons broadens their magnetic resonance spectrum, permitting the unambiguous identification of the polaron pair species by ODMR. Both isotropic and anisotropic exchange interactions have been observed. An isotropic exchange interaction broadens the spin-(1)/(2) resonance associated with polarons, whereas an anisotropic exchange interaction gives rise to both full-field and half-field powder patterns. We have successfully modeled the observed spin-(1)/(2) ODMR spectra and estimate the strength of the isotropic exchange interaction I. An anisotropic exchange interaction has been observed in C{sub 60}-doped 2,5-dioctyloxy poly(p-phenylene vinylene) and cis-rich polyacetylene. Both polarons and polaron pairs are characterized by two absorption bands. However, the high-energy PA band of polaron pairs is blueshifted with respect to that of isolated polarons and the low-energy PA band of polaron pairs is relatively weak. We have also observed neutral soliton pair excitations in polymers with degenerate ground states. {copyright} {ital 1997} {ital The American Physical Society}

X. Wei

Papers

Optical probes of excited states in poly(p-phenylenevinylene).

Excitons in poly(para-phenylenevinylene)

Studies of photoexcited states in polyacetylene and poly(paraphenylenevinylene) by absorption detected magnetic resonance: The case of neutral photoexcitations.

Absorption-detected magnetic-resonance studies of photoexcitations in conjugated-polymer/C60 composites.

Spin and spectral signatures of polaron pairs in π-conjugated polymers