Photovoltaic Phenomena Research Articles

Polymeric photovoltaic materials

Recent developments in conjugated polymer-based photovoltaic elements have been reviewed. The photophysics of such photoactive devices is based on the photoinduced electron transfer from donor-type semiconducting conjugated polymers onto acceptor-type conjugated polymers or acceptor molecules such as Buckminsterfullerene, C 60. Photoinduced electron transfer in solid composite films of fullerenes embedded into conjugated polymers is reversible, ultrafast (within 300 fs) with a quantum efficiency approaching unity, and metastable. Similar to the first step in natural photosynthesis, this photoinduced electron transfer leads to a number of potentially interesting applications, which include sensitization of the photoconductivity and photovoltaic phenomena. Furthermore, using the conjugated polymer donors in polymer blends with another conjugated polymer acceptor, similar photovoltaic elements have been realized. Examples of photovoltaic architectures are discussed with their potential in terrestrial solar energy conversion.

Effect of fluorine on metal-insulator-silicon solar cell performance with an low-temperature deposited SiO 2 layer

Metal-insulator-silicon solar cells with an insulator layer fabricated by the liquid phase deposition method, denoted as LPD-MIS solar cells, were studied. Under the illumination, an output efficiency up to 7.2% was obtained. A photovoltaic phenomenon occurred even for an oxide thickness ranging from 50 to 150 Å, which was much thicker than the limit of 30 Å predicted by MIS theory. It was suggested that these interesting results may be due to barrier height enhancement and diode quality factor lowering caused by a fluorine content incorporated in the SiO 2 layer during the LPD-MIS solar cell fabrication process. The existence of fluorine content and its effect on LPD-MIS solar cell performances were investigated through AES, FTIR, and C–V characterizations. The hopping conduction mechanism was proposed to explain the effect of the oxide traps, which were believed to be due to the fluorine incorporated in the LPD-SiO 2 film. The results provided a guideline for the fabrication of LPD-SiO 2 related devices and showed that an extreme thin insulating layer in LPD-MIS solar cell was not a necessity.

Investigation of the photovoltage in por-Si/p-Si structures by the pulsed-photovoltage method

Photovoltaic phenomena in the structures por-Si/p-Si were investigated by the pulsed photovoltage method in the time interval 100 ns-10 ms using irradiation with nanosecond laser pulses with photon energies 1.4, 2.0, 2.8, and 3.7 eV. The data obtained show that besides the barrier photovoltage, due to the separation of nonequilibrium charge carriers in the space-charge region of p-Si at the por-Si/p-Si interface, there also exists an efficient mechanism of photovoltage formation due to charging of the surface of the por-Si nanostructure. This mechanism is explained as “optical doping” of a semiconductor and develops in a manner peculiar to semiconductor nanostructures.

Anomalous electrical resistivity and photovoltaic phenomenon in the fast-mixed conductor lithium silicide Li12Si

The anomalous electrical resistivity and photovoltaic phenomenon in the fast-mixed conductor lithium silicide (${\mathrm{Li}}_{12}$${\mathrm{Si}}_{7}$), which is classified as a diamagnetic semiconductor, are presented for the first time. An abrupt change in the electrical resistivity near 230 K is associated with the ordering of vacancies on the Li sublattice. The conductive behavior of ${\mathrm{Li}}_{12}$${\mathrm{Si}}_{7}$ is characterized as a degenerate semiconductor resulting from the off-stoichiometric composition. A photovoltage on the order of ${10}^{\mathrm{\ensuremath{-}}6}$ V is produced instantly by illumination at 300 K. The phenomenon is described in terms of the Dember effect.

Photovoltaic effect on fast mixed conductor LiAl.

The photovoltaic phenomena in the fast mixed conductor LiAl (\ensuremath{\beta} phase) having a semimetallic band structure are presented for the first time. Photovoltage on the order of ${10}^{\mathrm{\ensuremath{-}}6}$ V is produced with a relaxation time of \ensuremath{\sim}90 sec by illumination at 290 K. The phenomena make a reversible process by turning on and off the light. The photovoltaic effect is described in terms of a photoenhanced Li internal diffusion.

ChemInform Abstract: Photoresponsive Organic Molecules

In the context of the importance of photoregulation of functions of organic substances, recent developement of photochromic molecules is described, focusing attention on the following topics.a) New photochromic compounds appearring in recent literatures. b) Molecular and environment design for getting the kinetic stability of photochrome isomers. c) What kind of property of materials, into which a photochromic molecule is incorporated, can be regulated by the photoisomerization of the chromophore ? d) Hitherto unknown phenomenon accompanying photochromism.Successful example for b) is heteroaromatic homologues of fulgide system which reveales a remarkable kinetic stability of the colored form and is immune to fatigue. A variety of properties of polymers, which are photochemically regulated, are related closely to the fundamental character of the photochromism of chromophoric unit in the polymer. Photo-voltaic phenomenon of the thin film of triphenylformazan is a unique two photon process consisting of photoisomerization of the red-form to the yellow-form and the successive photo-ionization of the latter.

光応答性有機分子

In the context of the importance of photoregulation of functions of organic substances, recent developement of photochromic molecules is described, focusing attention on the following topics.a) New photochromic compounds appearring in recent literatures. b) Molecular and environment design for getting the kinetic stability of photochrome isomers. c) What kind of property of materials, into which a photochromic molecule is incorporated, can be regulated by the photoisomerization of the chromophore ? d) Hitherto unknown phenomenon accompanying photochromism.Successful example for b) is heteroaromatic homologues of fulgide system which reveales a remarkable kinetic stability of the colored form and is immune to fatigue. A variety of properties of polymers, which are photochemically regulated, are related closely to the fundamental character of the photochromism of chromophoric unit in the polymer. Photo-voltaic phenomenon of the thin film of triphenylformazan is a unique two photon process consisting of photoisomerization of the red-form to the yellow-form and the successive photo-ionization of the latter.

Optical Effects on the Static and Dynamic Characteristics of a GaAs MESFET (Short Paper)

In this paper, we describe the effect of light on the S-parameters of a GaAs MESFET. The photon energy is greater than the gap bandwidth of the semiconductor. The photoconductive and photovoltaic dc phenomena in the channel and in the depletion layer are theoretically analyzed with a unidimensional model to describe the light effect on the dc transconductance g/sub m/. The comparison between the dc transconductance, without and under illumination, and the theoretical model shows a very close agreement.

Photovoltaic Effects in Liquid Crystal Cells Containing Organic Dyes

Abstract Dye sensitization on photovoltaic phenomena under visible light stimulation are described in smectic, nematic, and isotropic phases of p-cyano-p'-n-octylbiphenyl cells with a symmetrical electrode arrangement. Methyl red is used as a sensitizer. The dependence of short-circuit photocurrents on excitation light intensity, light pulse width, temperature, and the cell thickness are measured. The photocurrent consists of three components, that is, the small negative peak with a fast response, the second positive peak following after the first peak, and the third negative peak with a slow response. The second and the third components of the photocurrents cannot be observed in the solid crystalline state. Both of them decrease with an increase in the cell thickness, and their response time becomes shorter as the temperature rises. These two components are induced by the diffusion processes of the carriers produced by the light stimulation.

Hot photo-carrier and hot electron effects in p- n junctions

When a p- n junction of semiconductor (with bandgap energy E g ) is illuminated by light beam (with photon energy hv) in the condition of E g > hv, such as in Ge p- n junction illuminated by CO 2 laser beam, an electromotive force (emf) was induced between the terminals of the p- n junction, which indicated the opposite polarity to the ordinary photovoltaic effect like a solar cell. Such an anomalous photovoltaic phenomenon was explained by an optically excited hot carrier effect, through the following experiment with electrical excitation. Using a rod of n- or p-type Ge with a p- n junction at the surface of its center and an ohmic contact at each terminal of the rod, the same kind of phenomena was observed when electric field is applied along the length of the rod. The perpendicularly induced voltage or current had the same polarity instead of the reverse change of the applied electric field, and increases with increasing the applied field strength. The perpendicularly induced emf was caused by warm or hot carriers crossing the potential barrier of the p- n junction, which is very sensitive to the departure from thermally equilibrium velocity distribution of carriers.

Hot Carrier-Voltaic Effect in p-n Junction and Its Application to Electron Emitters and Light Detectors

When a p-n junction is illuminated by intense light beam in a condition of band gap energy larger than photon energy, electromotive force is induced across the p-n junction with the polarity opposite to ordinary photo-voltaic effect such as a solar cell. Such an anomalous photovoltaic phenomenon has been explained as optically excited hot carrier-voltaic effect by studying the induced voltage across the p-n junction due to hot carriers excited in electric field parallel to the junction plane. As useful applications of the hot carrier-voltaic effect, high efficiency electron emitter and high sensitivity light detector are proposed.

Interband photoconduction edge in zinc sulphide thin films

Photovoltaic phenomena similar to phototransition effects are induced in ZnS sputtered films by light of energy beneath the band-gap (not more than 0.22 eV). It is advanced that hot electron may escape from the photoionized coulombic centre and moves to some level within shallow traps distribution. Thickness dependent phenomena confirm the existence of a critical thickness in which the density of coulombic centres is higher.

Reversal of photovoltaic phenomena in sputtered zinc sulphide

Reversal of photovoltaic phenomena in sputtered zinc sulphide

Photovoltaic Effect in Photoconductors

On the basis of the assumption that the induced minority and majority carrier densities are large compared with the ``dark'' densities, a theory of the photovoltaic effect in photoconductors is presented. The theory is initially set up in general terms and then applied to specific cases: (i) abrupt homojunctions and heterojunctions between two photoconductors, (ii) abrupt homojunctions and heterojunctions between a semiconductor and a photoconductor. The existence of interfacial recombination is included in the analysis and the self-consistency of the theory is examined. The photovoltage is evaluated in terms of the photovoltaic current and the change in Fermi energy across the interface and, in addition, an expression for the short-circuit current is obtained. The results are used to discuss the suitability of some of the physical mechanisms which have been proposed to account for the photovoltaic phenomenon in photoconductors.

The thermodynamics of non-barrier layer photo-voltaic phenomena

By using quasi-Fermi levels introduced by Shockley, which have the meaning of partial chemical potentials of an assembly of electrons and an assembly of holes, and an analogy with galvanic cells, an equation was derived for the photo-voltage of a non-barrier layer semi-conductor photo-cell on the basis of thermodynamic laws. For the case that classical statistics are valid the basic equation leads to the result derived by kinetic considerations. Attention is thus drawn to the application of thermodynamics to photo-voltaic phenomena in semi-conductors.

The theory of a bulk photo-voltaic phenomenon in semiconductors

The theory of a bulk photo-voltaic phenomenon in semiconductors

The theory of a bulk photo-voltaic phenomenon in semi-conductors

It is shown theoretically that for a photo-voltaic phenomenon in semi-conductors to exist neither the presence of localised potential barriers with rectifying properties at the contact of a semi-conductor with a metal (Mott's theory 1939) or at a p-n junction (Sosnowski, 1947) nor the presence of non-rectifying contacts (Dember's phenomenon, Lashkarev's theory 1948) is necessary. The case is considered that the illuminated part of the semi-conductor is sufficiently distant from both contacts so that these will not assert themselves; further, it is assumed that there are no localised potential barriers in the semi-conductor. Under these conditions a photo-voltage is produced in a semi-conductor if its conductivity varies throughout its length in such a way that in the dark it is different at the beginning and end of the illuminated part. For this type of “non-barrier and non-contact” photo-voltage we propose the name “bulk” photo-voltage. The underlying law of this phenomenon, the general procedure for calculating the photo-voltage, an approximation for weak illumination and the dependence on the intensity of illumination are derived and its physical aspects are discussed.