Photoelectric Effect in Photocells

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  Field Electron Emission and Electron States (Concepts and Applications)

  • (Author)
  • 2014(Publication Date)
  • Learning Press

    (Publisher)

  Effect on wave–particle question The photoelectric effect helped propel the then-emerging concept of the dualistic nature of light, that light simultaneously possesses the characteristics of both waves and particles, each being manifested according to the circumstances. The effect was im-possible to understand in terms of the classical wave description of light, as the energy of the emitted electrons did not depend on the intensity of the incident radiation. Classical theory predicted that the electrons would 'gather up' energy over a period of time, and then be emitted. Uses and effects Photodiodes and phototransistors Solar cells (used in solar power) and light-sensitive diodes use a variant of the photoelectric effect, but not ejecting electrons out of the material. In semiconductors, light of even relatively low energy, such as visible photons, can kick electrons out of the valence band and into the higher-energy conduction band, where they can be harnessed, creating electric current at a voltage related to the bandgap energy. ________________________ WORLD TECHNOLOGIES ________________________ Photomultipliers These are extremely light-sensitive vacuum tubes with a photocathode coated onto part (an end or side) of the inside of the envelope. The photocathode contains combinations of materials such as caesium, rubidium and antimony specially selected to provide a low work function, so when illuminated even by very low levels of light, the photocathode readily releases electrons. By means of a series of electrodes (dynodes) at ever-higher potentials, these electrons are accelerated and substantially increased in number through secondary emission to provide a readily detectable output current. Photomultipliers are still commonly used wherever low levels of light must be detected. Image sensors Video camera tubes in the early days of television used the photoelectric effect; newer variants used photoconductive rather than photoemissive materials.

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  Electronic Properties of Crystalline Solids

  An Introduction to Fundamentals

  • Richard Bube(Author)
  • 2012(Publication Date)
  • Academic Press

    (Publisher)

  Electrical processes in the material m a y also be influenced by carriers injected electrically from these contacts. Photoelectronic effects c o m m o n l y involve a variety of imperfection levels. In these circumstances the formal complete mathematical solution of the problem becomes intractable. By defining a steady-state Fermi level a n d a 449 450 12 Photoelectronic Effects corollary demarcation level, to define whether or not the occupancy of a level is determined by thermal or kinetic processes, it is possible to describe a n u m b e r of critical processes in a semiquantitative and conceptually help-ful way. The increase in conductivity of a material under illumination, k n o w n as photoconductivity, has been explained t h r o u g h the use of a n u m b e r of different models. It is clear today that sensitive p h o t o c o n d u c t o r s can exist in both homogeneous-material form, a n d in heterogeneous-material form involving junctions or potential barriers. T h e length of time t h a t a carrier excited by light remains free is deter-mined in part by the probability that it will recombine with a carrier of the opposite type. In this recombination process, the excess energy of the carriers must be dissipated as p h o n o n s , p h o t o n s or excited carriers. In some cases the probability of recombination can be calculated by a consideration of the recombination processes. Photovoltaic effects, in which light generates either a short-circuit current or an open-circuit voltage in the presence of a potential barrier, a n d p h o t o -magnetoelectric effects, in which light generates similar current or voltage in the presence of a magnetic field in a h o m o g e n e o u s material, are related p h e n o m e n a . 12.1 General Concepts W h e n optical absorption by a semiconductor or insulator produces additional free carriers, the electrical conductivity of the material is in-creased in the p h e n o m e n o n of photoconductivity.

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  Advanced Concepts for Solar Cells

  • P. Pérez Rodríguez(Author)
  • 2019(Publication Date)
  • Arcler Press

    (Publisher)

  hv Eq. 2.1 where h is Planck’s constant (6.626 × 10 −34 m 2 kg/s) and v is the frequency of the incident light. The main steps involved in the PV effect are as follows: Generation of charge carriers when photons are absorbed in one or both of the materials that form the junction. This process is governed both by the optical and material properties of the two materials used. Separation of the photo-generated charge carriers in the junction. If this separation does not happen, the two charge carriers would recombine, and the energy would be released back, either as light or heat. Therefore, to be able to use the energy, the junction created by the two materials must be able to separate the charge carriers. This ability depends mainly on the electrical properties of the semiconductor materials used in the junction, as will be explained in this section. Collection of the photo-generated charge carries at the junction terminals. To be able to perform work with the energy of the potential energy generated, the charge carriers need to be collected in the opposite terminals of the device, so that electrical energy is produced. For that purpose, metal electrodes are added to the junction. It is important to understand each of these processes more in detail to be able to see their limitations. This chapter would explain the main physical principles and limits governing solar cells, based on the PV effect. The Working Principles of Solar Cells 13 2.2. OPTICAL CONSIDERATIONS The first consideration when designing a solar cell is the optical properties of that cell. A solar cell must be able to absorb the photons coming from the solar radiation as efficiently as possible in order to create as many charge carriers as possible. Therefore, this section focuses on understanding the physical principles behind that absorption and how it can be optimised for the solar radiation.

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  High Resolution Imaging

  Detectors and Applications

  • Swapan K. Saha(Author)
  • 2015(Publication Date)
  • Jenny Stanford Publishing

    (Publisher)

  A photoelectric light detection occurs when a photon interacts with a sensor to create an electrical signal. The interaction of light of appropriate frequency with materials results in the absorption of photons and the creation of electron–hole pairs (see Section 2.2.4.1). The photon absorption is accomplished by a change in the energy state of an electron in the sensor material. If the energy is sufficient to release the electrons from the substance, the charge carriers under the influence of an external electric field drift to the electrodes of opposite polarity produce an electric current, called photocurrent. For the light with frequencies below a certain cut-off value, ν c , the photoemission does not occur. For ν > ν c , where ν c is the cut-off frequency of the photon required to produce photoelectric emission and is a constant for the material, a major fraction of the excess energy [ = h ( ν − ν c )] appears as kinetic energy of the emitted electron. The non-measurable time lag between the incidence of the radiation and the ejection of the electron follows from the corpuscular nature of the radiation. When an oscillator emits a photon, it drops from energy, nh ν to a level ( n − 1) h ν . The energy of the photon is expended in liberating the electron from the metal and imparting a velocity to it. The maximum kinetic energy, E k , of the ejected electrons is linearly related to the energy of the absorbed photons, E ( = h ν ), and the work function, φ 0 , of the PE surface (see Table 5.1) E k = h ν − φ 0 = h ν − h ν c = h ( ν − ν c ), (2.1) 82 High Resolution Imaging: Detectors and Applications with E k ( = 1 / 2 mv 2 ) as the kinetic energy of the ejected photoelec-tron from the metal surface, m the mass of the electron, v the velocity of the electron, and ν c the cut-off frequency, which is a characteristic of the metal. The work function of a metal is defined as the minimum energy, measured in electron volts, needed to release an electron from atomic binding.

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  Physics, Volume 2

  • David Halliday, Robert Resnick, Kenneth S. Krane(Authors)
  • 2019(Publication Date)
  • Wiley

    (Publisher)

  Figure 45-5, eV 0  K max . V 0   V  45-3 The Photoelectric Effect 1019 Vacuum Quartz window Incident light Electrons E V C i i Sliding contact – + A FIGURE 45-3. An apparatus for studying the photoelectric effect. The arrows show the direction of the photoelectric current in the external circuit, which is opposite to the motion of the (neg- atively charged) electrons. The effective potential difference be- tween the emitter E and the collector C, which is read by the volt- meter V, is the algebraic sum of the potential difference associated with the batteries and that associated with the contact potential difference between the emitter and the collector. Because these components are made of different materials they form a “battery” in their own right. FIGURE 45-4. A plot (not to scale) of data taken with the ap- paratus of Fig. 45-3. The intensity of the incident light is twice as great for curve b as for curve a. The emitter and the wavelength of the incident light are the same for both runs. Note that the stop- ping potential is the same for each run. Current i Potential difference ∆V b a V 0 0 – + in which we plot the stopping potential against the fre- quency (rather than the wavelength) of the incident light, shows the result for an emitter made of sodium. The plot is a straight line with an intercept f 0 on the frequency axis, suggesting a second fact about the photoelectric effect: Fact 2. The frequency of the light falling on a given emitter must be greater than a certain value f 0 . Other- wise the photoelectric effect will not occur. This cutoff frequency f 0 depends only on the material of which the emitter is made and is totally independent of the intensity of the incident light. A third fact about the photoelectric effect has been firmly established by separate experiments: Fact 3. Photoelectrons are emitted without delay once the incident light reaches the surface of the emitter.

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  Let There Be Light: The Story Of Light From Atoms To Galaxies (2nd Edition)

  The Story of Light from Atoms to Galaxies

  • Alex Montwill, Ann Breslin(Authors)
  • 2013(Publication Date)
  • ICP

    (Publisher)

  It varies from metal to metal, and determines the minimum photon energy required for the photoelectric effect to take place at that metallic surface. Photoelectric work functions of some elements. Element Al Cs Cu Hg K Na Pb W (eV)* 4.28 2.14 4.65 4.49 2.30 2.75 4.25 The most energetic photons of visible light are at the violet end of the spectrum. It can be seen by comparing the data in the tables above and below that such photons have enough energy to liberate electrons from caesium, potassium and sodium, but not from aluminium, copper, mercury or lead. Energies of some visible photons. Wavelength (nm) 400 550 700 Photon energy, hf (eV) 3.11 2.26 1.61 Practical applications Quite apart from its theoretical importance, the photoelectric effect has numerous practical applications. It makes it possible to convert a light signal into an electric current. Television cameras, burglar alarms, barcode readers and light sensors of every descrip-tion are based on the photoelectric effect. Perhaps the most remarkable practical example is the photo-multiplier , which amplifies the electrical signal to such an extent that it is possible to detect the arrival of a single photon. The use 392 Let There Be Light 2nd Edition of this instrument in some fascinating fundamental experiments with individual photons will be discussed in Chapter 14. In 1907, the Russian scientist Boris Rosing (1869–1933) realized that light from a cathode ray tube could be thrown onto a screen and made into a pic-ture. One of his students, Vladimir Kosma Zworykin (1889–1982), was fascinated by this notion and applied for a pat-ent for what was in effect the idea for the first television cam-era. The patent application was lodged in 1923, although Zworykin had no working model at the time. Zworykin went to work for Westinghouse Electric and Manufacturing Company in America and began to develop a practical version of the camera tube called the iconoscope.

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  Light

  • R. W. Ditchburn(Author)
  • 2013(Publication Date)
  • Dover Publications

    (Publisher)

  Planck’s original theory was concerned only with the interaction of radiation and matter. He hoped that it would involve only minor modifications of the classical electromagnetic theory of light and the classical electron theory of matter. This expectation was not fulfilled and it became necessary to modify the theory of light to include the fact that energy of the radiation field which represents a beam of light can change only by an integral number of quanta. It is also necessary to assume that an atom can change its energy only by discrete amounts, and hence that atoms can exist only in certain states separated by finite differences of energy. Nothing in the classical laws of electromagnetism, which are based on experiments with static fields or alternating fields of low frequency, would lead us to expect this effect.

  The quantum theory thus becomes both a theory of radiation and a theory of matter instead of being merely a special hypothesis concerning their interaction. The theory is a connected whole and we cannot deal with a section of the theory concerning radiation without referring extensively to atomic structure. In this chapter we discuss the experimental basis of the quantum theory, giving special prominence to experiments on light and on electromagnetic radiation of shorter wavelength. The order is chosen for convenience of exposition and does not follow the historical order.

  17.2. The Photo-electric Effect.

  It is found that electrons are ejected from the surfaces of metals by light and by radiation of shorter wavelength (X-rays and y-rays). If the radiation is able to penetrate the substance, electrons in the interior may be removed from their equilibrium positions. In this paragraph we are concerned with the emission of electrons from surfaces, and we shall call this the photo-electric effect, although, strictly, it should be called the surface photo-electric effect. The number and velocities of electrons emitted have been measured for different metals and for different wavelengths. Fig. 17.1 shows in a diagrammatic way a simple apparatus. Fig. 17.2 shows a more elaborate experiment in which the surface of the metal is freshly cut in vacuo immediately before the measurements are made. The results of the experiments in which monochromatic light is incident normally upon the surface of a metal may be summarized as follows:

  Fig. 17.1.—Photo-electric effect. Diagram of simplified apparatus

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  Energy Autonomous Micro and Nano Systems

  • Marc Belleville, Cyril Condemine, Marc Belleville, Cyril Condemine(Authors)
  • 2012(Publication Date)
  • Wiley-ISTE

    (Publisher)

  4.5 ]

  4.3. Photovoltaic cell: physical principle and model

  4.3.1. Physical principle

  A photovoltaic cell is a component that generates electric energy when exposed to light. Photovoltaic cells are based on semiconductor materials, mostly: – bulk monocrystalline (c-Si) or multicrystalline (mc-Si) silicon;

  – thin films of amorphous or polycrystalline materials: amorphous silicon (a-Si), microcrystalline silicon (μc-Si), amorphous silicongermanium alloy (a-SiGe), cadmium telluride (CdTe), Cu(In,Ga)Se2 (CIGS) ternary chalcopyrite compounds;

  – thin films of monocrystalline III-V compounds, prepared through epitaxy; and – thin films of organic materials.

  The operating principle of a photovoltaic cell is outlined in Figure 4.4 .

  The cell consists of a stack of two semiconductor layers: an n -type layer (in which the main charge carriers are electrons, i.e. negative charger carriers) and a p -type layer (in which the main charge carriers are holes, i.e. positive charge carriers). At such a pn junction, there is a space charge layer, and thus an electric field. When the cell is illuminated, each photon absorbed by the semiconductor creates an electron-hole pair. The electric field at the pn junction separates photogenerated electrons and holes by taking electrons toward the n -zone and the holes toward the p -zone. Illuminating the cell thus generates a current and voltage. Generally speaking, photovoltaic cells are electrically interconnected to form modules. The current and voltage supplied by a module can be adjusted through the number of cells and the interconnection scheme (in parallel or series).

  Figure 4.4.

  Photovoltaic cell operational principle. The electric field E at the pn junction separates the photo-generated electrons and holes

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  Oxide Semiconductors for Solar Energy Conversion

  Titanium Dioxide

  • Janusz Nowotny(Author)
  • 2016(Publication Date)
  • CRC Press

    (Publisher)

  The same effect was reported by Kudo et al. [5]. 2.3 LIGHT SOURCE The key performance-related quantity of PECs is its ECE. From the viewpoint of practical application, it is essential to assess the ECE for sunlight. On the other hand, the most commonly reported ECE data has been determined for different artificial light sources. These exhibit a wide range of spectral distribution, which is very dif-ferent from that of sunlight. Consequently, the related ECE data may differ sub-stantially from that corresponding to sunlight. Accordingly, ECE data reported for artificial light sources can only be considered as indicative. A comparison of specific irradiance spectra for different light sources is shown in Figure 2.7 [6,7]. The percentages of energy spectra available for absorption above given energy thresholds for artificial light sources are at considerable variance with those of sunlight. The ECE determined using different light sources may therefore lead to ECE values that do not allow direct comparison with those obtained for terrestrial sunlight. 2.4 LIGHT-INDUCED EFFECTS IN SEMICONDUCTORS Semiconductors are able to absorb light when the energy of photons is equal to or greater than the band gap energy. Exposition of semiconducting photocatalysts to light leads to intrinsic ionization over the band gap and results in the formation of an electron-hole pair: h ν → (e ′) CB + (h • ) VB (2.6) where the subscripts CB and VB are related to the conduction band and the valence band, respectively. The lifetime of the electron-hole pairs formed as a result of light-induced excite-ment is limited to nanoseconds. These charge carriers have a tendency to recombine, leading to the formation of heat energy. The recombination-related energy losses Light-Induced Effects 133 may be reduced when the light-induced charge carriers are separated in an electric field. Such field is imposed by an internal electrical potential barrier that is repre-sented by bending of bands.

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  Photorefractive Materials for Dynamic Optical Recording

  Fundamentals, Characterization, and Technology

  • Jaime Frejlich(Author)
  • 2020(Publication Date)
  • Wiley

    (Publisher)

  This is also probably the case for Bi 12 TiO 20 doped with Ru [27], where 𝜎 d decreases more than three-fold from undoped to [Ru] = 10 19 cm − 3 Ru-doped samples. 2.4 Photovoltaic Effect Photovoltaic is a bulk effect that is experimentally put into evidence by the generation of an electric current under the action of light of adequate wavelength without any externally applied electric field on the sample. This effect is observed in some ferroelectric crystals and is supposed to be produced by the photoexcitation of electrons from asymmetric impurity potentials [40, 41]. This effect appears in poled uniform single crystals with noncentrosymme-try. It is different from the P-N junction observed in semiconductors or metal-semiconductor interfaces. The photovoltaic effect has interesting practical applications because it results in a higher space-charge modulation, and therefore leads to enhanced diffraction efficiency for the recorded holograms and may have potential applications (although not yet practical) for photoelectric conversion. At the origin of photovoltaic effect there seems to be a nonsymmetric distribution of donors and acceptors so that the electron photoexcited from a trap is closer to an acceptor in a cer-tain sense rather than in the opposite one. Therefore, electrons do move preferentially along the same sense, the so-called “photovoltaic C axis”, when excited by the light. The resulting photovoltaic current density is j phv ( z ) = 𝜅 phv I ( z ) 𝛼 (2.64) I ( z ) 𝛼 = − 𝜕 I ( z ) 𝜕 z with I ( z ) = I ( 0 ) e − 𝛼 z (2.65) with 𝜅 phv being called the photovoltaic Glass constant, which depends on the nature of the absorbing center and the illumination wavelength, as reported in Tables 2.1 and 2.2, with I ( z ) 𝛼 2.4 Photovoltaic Effect 41 Table 2.1 Photovoltaic transport coefficient 𝜅 phv for Fe-and Cu-doped LiNbO 3 .

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