James H. Sharp

Phthalocyanine organic solar cells: Indium/x‐metal free phthalocyanine Schottky barriers

A thin organic film of polycrystalline particles of x‐metal free phthalocyanine (x‐H2Pc) dispersed in a polymer binder, when sandwiched between tin oxide (NESA) and indium electrodes, is shown to exhibit a strong photovoltaic effect. The photovoltaic and rectification properties of In/x‐H2Pc/NESA sandwich cells are reported. From the photovoltaic action spectra, the active region responsible for electric power generation was found to be confined to the metal/semiconductor interface. A Schottky barrier width of 300 A was determined, which allows the capture of 30% of solar irradiance. An electron trap density of 3×1017/cm3 and a Schottky barrier built‐in potential of 0.63 V are estimated from C‐V measurements. At low voltage, the dark current in the forward direction varies exponentially with voltage: from this dependence values of 2×10−9 A/cm2 and 1.3–2.6 for the saturation current J0 and diode quality factor n are determined. At higher voltage, a super quadratic dependence of forward current on voltage i...

Photovoltaic properties of metal‐free phthalocyanines. I. Al/H2Pc Schottky barrier solar cells

The photovoltaic properties of Schottky barier solar cells, made by dispersing particles of the x form of mental‐free phthalocyanine in a binder polymer and sandwiching between NESA (SnO2/Sb) and aluminum electrodes, have been studied. A power conversion efficiency of over 6% for transmitted light at low power densities (0.06 W/m2) has been obtained for monochromatic irradiation at 670 nm. At peak solar power density (1400 W/m2) the extrapolated power conversion efficiency (η) for transmitted 670 nm irradiation decreases to 0.01%. The decrease in η with intensity was attributed to a space charge limitation due to nonlinear resistance. The devices exhibit Voc’s as high as 1.1 V, but are still limited by a field dependent quantum efficiency. Analysis of the action spectra of these devices revealed the formation of a thin photoactive depletion region (∼400 A) at the semiconductor/metal interface. These devices are capable of capturing 30% of the solar spectrum within the photoactive region. The effect of pig...

Pulsed photoconductivity action spectra of β‐metalfree phthalocyanine thin films

Using pulsed photoconductivity techniques, transient action spectra have been recorded on vacuum sublimed β‐metalfree phthalocyanine thin films in the wavelength range 360♯740 nm. Quantum efficiencies for carrier generation and carrier ranges were deduced assuming (a) range limited transport and (b) wavelength and field independent quantum efficiency. Carrier generation is shown to be a bulk phenomenon and the lowest excited singlet state is the common intermediate in the carrier generation process. Two sets of measurements have been performed, 5 and 12 weeks after sample preparation. The quantum efficiency changed from 2.5×10−4 to 10−3, probably due to an increase in the amount of absorbed oxygen.

ESR of α‐Copper Phthalocyanine. II. Powder Line Shape and MO Calculations

The ESR spectrum of polycrystalline α‐Cu phthalocyanine, reported by us previously, has been analyzed in detail by powder line shape and MO calculations. The superhyperfine (shf) tensors are shown to be essentially isotropic and hence the system can be described by an axial spin Hamiltonian. The low‐field spectrum consists of four well‐defined peaks from which the spin‐Hamiltonian parameters g‖ and A can be readily determined. The high‐field spectrum is a superposition of four sets of nine‐component lines over‐lapping to result in 15 almost equally spaced peaks. This fact suggests that the hyperfine (hf) constant B is about twice the shf constant aN. This relation has been used to estimate the values of g⊥, B, and aN. Only a slight adjustment of parameters was needed to synthesize a derivative line shape which fitted well to the experimental spectra. For one of the hf components (M = 32), the resonance field is not a monotonic function of θ, the angle between molecular axis and the magnetic field. The eff...

Electronic transitions of vanadyl phthalocyanine in solution and in the solid state

Abstract The electronic absorption and luminescence of vanadyl phthalocyanine (Pc) in dilute solution. and solids at 300 K were measured and analyzed qualitatively with the Ake and Gouterman(AG) mode [1] and the exciton in molecular crystal model [2]. Good agreement between the group theoretical predictions and the observed spectra was found. The presence of trace aggregates in VOPc dilute solutions was clearly illustrated by their characteristic O-O fluoreseences. The dimer fluoresces on the high energy side of the monomer, whereas the signal from the heavier aggregates appears on the low energy side. The presence of an unpaired electron in the monomer results in a much smaller fluorescence yield as compared to the dimeric form in which the odd electrons are paired. The absorption and fluorescence vibronic progressions suggest that the monomer is quite stable upon optical excitation to the first excited singlet. Our spectra show that there are at least two crystal forms in the phase I solid in agreement with Griffiths et al.`s calorimetric observation [3]. One of the two forms is dimeric as proposed by Griffiths et al. and confirmed by us. The extremely low radiative yield of the dimeric form is attributed to ultrafast exciton migration and to the intrinsic low yield of the molecules. In the phase II crystal, two prominent electronic absorptions were predicted and assigned. The low luminescence yield of this phase is ascribed to the rapid 2 Q ⇝ 2 T 1 relaxation which also leads to the greatly red shifted luminescence (i.e. 2 T 1 → 2 T) relative to the absorption, 2 T → 2 Q. Molecules in this phase are treated as monomeric rather than dimeric as in one of the phase I forms. Thus, the characteristics of the unpaired electron of the single molecule are preserved in the phase II crystal.

Electron Spin Resonance of the Organic Semiconductor, α‐Copper Phthalocyanine

Various concentrations of polycrystalline powders of α‐copper phthalocyanine diluted in nonparamagnetic α‐metal‐free phthalocyanine matrices have been studied by 8‐mm ESR at room temperature. The superhyperfine (SHF) spin Hamiltonian used in analyzing earlier results of ESR investigations on square‐bonded copper complexes has been reconsidered and a new SHF term is derived. The concentration‐dependent study has revealed enough separate features to enable observations to be interpreted by essentially considering the contributions of only two molecular orientations to the over‐all derivative line shape. Although single crystals of many organo‐metallic complexes, including α‐copper phthalocyanine, are not available, this study indicates that the relevant parameters of the spin Hamiltonian can in some cases be readily deduced from polycrystalline spectra.

Singlet–triplet fusion in metal‐free phthalocyanine

Fluorescence quenching has been observed in 10−4 to 10−7M solutions of metal‐free phthalocyanine in 1‐chloronaphthalene at 77°K. This quenching is attributed to singlet–triplet fusion. A triplet lifetime of 200 μsec was measured at 77°K. A dipole–dipole interaction formulation for singlet excitation migration was adapted to the singlet–triplet fusion process. A critical distance of about 200 A was computed for the system under study. Unlike in singlet excitation migration, this critical distance depends on the experimental illumination conditions. If singlet–triplet fusion plays a role in photogeneration of charge carriers in metal‐free phthalocyanine thin films, a photocurrent linear in light intensity, in the absence of recombination and trapping effects, can be understood.

Electric-field-induced modulation of fluorescence from rhodamine 610 dye in a thin plastic film

Abstract The influence of strong external electric fields on the fluorescence intensity from rhodamine 610 dye molecules dispersed in a polycarbonate matrix is reported. Differences in dipole moments and electric polarizabilities between the ground and first excited states were obtained from an analysis of the wavelength dependence of the field-modulated fluorescence. A small field-induced decrease in the fluorescence quantum efficiency was also observed.