The effects of substitute side chain were investigated using donor-acceptor (D-A) conjugated copolymers consisting of a cyclopentadithiophene (CPDT) derivative and dithienyl-benzothiadiazole (DTBT). The intrinsic properties of the copolymers were significantly altered by perturbations of the intramolecular charge transfer (ICT). The absorption of PCPDT-ttOTBTOT (P2), which assumed a tail-tail configuration, tended to blue-shift relative to the absorption of PCPDT-TBTT (P1). The absorption of PCPDT-hhOTBTOT (P3), which assumed a head-head configuration, was blue-shifted relative to that of P2. The electrical transport properties of field-effect transistors (FETs) were sensitive to the side chain position. The field-effect mobility in P2 (μ2=1.8×10–3 cm2/V·s) was slightly lower that in P1 (μ1=4.9×10–3 cm2/V·s). The mobility of P3, however, was very low (μ3=3.8×10–6 cm2/V·s). Photoexcitation spectroscopy showed that the charge generation efficiency (shown in transient absorption spectra) and polaron pair mobility in P1 and P2 were higher than in P3, yielding P1 and P2 device performances that were better than the performance of devices based on P3.

We have studied the carrier transport in regio-regular polythiophene field effect transistors (FETs) by four-probe measurements of the steady-state channel conductance from room temperature to 4.2 K. At high gate voltage (constant total carrier density, n = 5×1012 cm−2) and at low temperatures, we demonstrate that the gate voltage and source-drain voltage combine to induce the insulator-to-metal transition. In the insulating regime, the carrier transport is well described by phonon assisted hopping in a disordered Fermi Glass (with Coulomb interactions between the hopping charge carrier and the charge left behind). At the highest gate voltages and at sufficiently high source-drain voltages, the data imply a zero-temperature transition from disordered insulator to metal.

Luminescence quenching experiments on oriented and drop-cast poly(phenylene vinylene), (PPV), films identify the excitons in these polymer films as Wannier-Mott excitons. The data are interpreted in terms of a distribution of exciton binding energies (Eb). We find that the width of this distribution is well correlated with the degree of structural disorder. The relatively small mean EbEb, approximately 60 meV, results in field-induced exciton disociation that limits the quantum efficiency obtained from polymer-based light emitting diodes (PLEDs) which typically are operated at applied fields of 106 V/cm or greater where the field-induced exciton disociation is significant. We present data and analysis that support these conclusions, as well as a discussion on the implications of our findings on polymer-based display technology.

Prior to receiving the Nobel Prize in chemistry in 2000 for my work in polymers, polymer science had been recognized three times. The first Nobel Prize in chemistry for polymer science was awarded in 1953 to Hermann Staudinger, for his pioneering work in the 1920s. At that time, the concept of macromolecules was new, and his ideas were controversial. However, the data prevailed, and he was awarded the Prize “for his discoveries in the field of macromolecular chemistry.” The next major event in polymer science was the discovery and invention of nylon by Wallace Carothers at the Dupont Company in 1935. Although Carothers died as a young man, his discoveries created an industry. I have little doubt that his work was deserving of a Nobel Prize and probably would have been awarded. The next related Prize went to Karl Ziegler and Giulio Natta in 1963 for their work on polymer synthesis in the 1950s. The Ziegler–Natta catalysts made possible the large-scale production of polymers such as polypropylene. They were awarded the Nobel Prize in chemistry “for their discoveries in the field of chemistry and technology of high polymers.” In 1974, the Prize for chemistry went to Paul J. Flory, who was a giant in this field. He was awarded the Nobel “for his fundamental achievements, both theoretical and experimental, in the physical chemistry of macromolecules.”

Ultrafast photoinduced absorption by infrared-active vibrational modes (IRAV) is used to detect charged photo-excitations (polarons) in solid films of conjugated luminescent polymers. Experiments, carried out in zero applied electric field, show that polarons are generated within 100 fs with quantum efficiencies of approximately 10%. The ultrafast photoinduced IRAV Absorption, the weak pump-wavelength dependence, and the linear dependence of charge density on pump intensity indicate that both charged polarons and neutral excitons are independently generated even at the earliest times. Measurements of the excitation profile of the transient and steady-state photoconductivity of poly(phenylene vinylene) and its soluble derivatives over a wide spectral range up to hν = 6.2 eV indicate an apparent increase in the “photoconductivity” at hν > 3-4 eV that arises from external currents generated by electron photoemission (PE). After quenching the PE by addition of CO2+SF6 (90%:10%) into the sample chamber, the bulk photoconductivity is nearly independent of photon energy in all polymers studied, in a good agreement with the IRAV spectra. The single threshold for photoconductivity is spectrally close to the onset of π-π* absorption, behavior that is inconsistent with a large exciton binding energy.

The exciton binding energy (Eb) and the band gap energy (Eg) of poly(phenylene vinylene), PPV, have been determined by photoconductivity excitation profile spectroscopy as a function of light polarization, applied electric field, and temperature. The spectral signature of the exciton is a narrow peak (100 meV full width at half maximum) that emerges just below the band edge upon increasing the external field, the temperature or the defect density. The exciton peak is observed only for light polarized parallel to the chain axis. The exciton binding energy is obtained from the energy of the exciton peak with respect to the band edge and, independently from analysis of the field dependence of the exciton dissociation. It is Eb ≈ 60 meV.

Conjugated polymers and polymer blends were developed for photosensing applications. Large size photosensors fabricated in ITO/polymer/metal configuration show high photosensitivity, 0.1-0.4 A/Watt in visible and near UV, as good as that made with inorganic semiconductors. The processability of the polymeric materials allows these photosensors be made in large size, onto substrates in desire shape, or even in flexible form, hybridized or integrated with optical devices or electronic devices. Large area, full-color, digital image sensing is demonstrated using photodiode arrays made from semiconducting polymers. The photodiode arrays, fabricated by casting the semiconducting polymer from solution at room temperature, have high photosensitivity, low dark current and large dynamic range. Photo-detection with desired spectral response or multi-band selection are also demonstrated.