What Is the Energy of Blue ãžâ»400 Nm Photons

Solar Spectra

Nanostructured Surfaces

X. Chen , H. Zhu , in Comprehensive Nanoscience and Technology, 2011

3.01.5 Gold/Semiconductor Photocatalysts

In the whole energy of the incoming solar spectrum, ultraviolet radiation accounts for less than 4%, while the visible light (wavelength >400   nm) constitutes around 43% of solar energy [85]. Hence, one of the great challenges for catalysis study is to devise new catalysts that possess high activity when illuminated by visible light. It will allow us to use sunlight, the abundant and clean energy source with low cost, to drive chemical reactions.

As is well known, semiconductors can generate electron–hole pairs when light irradiation energy is enough to overcome the band gap. Then the degradation of the organic compounds proceeds [86]. However, semiconductors as effective photocatalysts have a big drawback in that they fail to utilize visible light, due to the bandgap. As the band gap of TiO₂ semiconductor is about 3.2 eV, electron–hole pairs and degradation of organic compounds can only occur in the UV-illuminated process, in which the wavelength is shorter than that of visible light.

One of the methods to extend the visible-light activity of semiconductor photocatalysts is the surface modification with gold [87–90]. Once gold particles contact with the semiconductor surface, the gold Fermi level shifts close to the Fermi level of the semiconductor. Then the electrons generated from the semiconductor under light irradiation are transferred to gold nanoparticles resulting in effective charge separation. Moreover, oxygen can trap the electrons from gold nanoparticles readily and enhance the photocatalytic activity.

The mechanism is supported by Sonawane's report [87]. By studying thin films of Au/TiO₂ prepared by a simple sol–gel dip coating method, Sonawane showed that the photocatalytic activity of phenol decomposition by Au/TiO₂ photocatalyst was improved by 2–2.3 times that of undoped TiO₂. Similar experimental results on the photocatalytic activity of Au/TiO₂ thin films were reported for the reaction of methylene blue degradation [88]. The effect of the doped gold to increase in photoactivity may be attributed to the improvement of the charge separation process through electron migration from TiO₂ conduction band to gold surface. The transfer of photoelectrons results in the decrease of recombination of electrons and holes; consequently, the Au/TiO₂ samples prepared by washing treatment showed higher photocatalytic activity for methyl orange photodegradation than that prepared by rotary evaporation [89]. Such a mechanism is also supported by the study of Wu et al. on the mechanism of methanol reforming on Au/TiO₂ photocatalyst [90]. Four basic steps are involved in the reforming reaction: (1) photogeneration of excited electrons in the semiconductor conduction band; (2) the electrons transfer to gold particles and reduce the protons to produce hydrogen; (3) the holes oxidize H₂O and CH₃OH, and its reaction intermediate products adsorbed on TiO₂; and (4) the final intermediate HCOOH is oxidized to CO₂.

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ULTRAVIOLET RADIATION

K. Stamnes , in Encyclopedia of Atmospheric Sciences, 2003

Spectrum of Electromagnetic Radiation for the Sun

An overview of the various parts of the solar spectrum is provided in Table 1. The spectral variable is the wavelength λ=c/ν, where c is the speed of light and ν is the frequency (s⁻¹ or Hz). In the UV and visible spectral range, λ is expressed in nanometers (1   nm=10⁻⁹  m). The irradiance in each spectral range is listed as well as the known percentage solar variability, defined as the maximum minus minimum divided by the minimum.

Table 1. Subregions of the spectrum

Sub region	Irradiance (W   m−2)	Solar variability	Comments
Far UV (100<λ< 200   nm)	<1	7–80%	Dissociates O2. Discrete electronic excitation of atomic resonance lines.
Middle UV or UV-C (200<λ<280   nm)	6.4	1–2%	Dissociates O3 in intense Hartley bands. Potentially lethal to biosphere.
UV-B (280<λ<320   nm)	21.1	<1%	Some radiation reaches surface, depending on O3 optical depth. Damaging to biosphere. Responsible for skin erythema.
UV-A (320<λ<400   nm)	85.7	<1%	Reaches surface. Benign to humans. Scattered by clouds, aerosols, and molecules.
Visible or PAR (400<λ<700   nm)	532	≤0.1%	Absorbed by ocean, land. Scattered by clouds, aerosols, and molecules. Primary energy source for biosphere and climate system.
Near IR (0.7<λ<3.5   μm)	722	—	Absorbed by O2, H2O, CO2 in discrete vibrational bands.

PAR: photosynthetically active radiation.

Adapted with permission from Thomas GE and Stamnes K (1999). Radiative Transfer in the Atmosphere and Ocean. Cambridge: Cambridge University Press.

In Figure 1 we show the UV, visible and near-infrared part of the spectral solar irradiance (wavelengths shorter than 1000   nm) measured on board an earth-orbiting satellite, above the atmosphere. Spectra of ideal blackbodies at several temperatures are also shown in Figure 1. Requiring that the total energy emitted is the same as a blackbody, one finds that the Sun's effective temperature is 5778   K. If the radiating layers of the Sun had a uniform temperature at all depths, its spectrum would match one of the theoretical blackbody curves exactly. Therefore, the deviations are the result of emission from a non-isothermal solar atmosphere. Some of the more important aspects of the UV/visible spectrum are: (1) Most of the emission arises within the photosphere where the Sun's visible optical depth reaches unity. The finer structure is due to Fraunhofer absorption by gases in the cooler (higher) portions of the photosphere. (2) For 125nm<λ<380, the effective radiating temperature falls to values as low as 4500   K, due to increased numbers of overlapping absorbing lines. At still shorter wavelengths, some of the emission originates in the hotter chromosphere which overlies the photosphere, and the effective temperature increases. (3) The UV irradiance is noticeably dependent upon the solar cycle, being more intense at high solar activity than at low solar activity.

Figure 1. Extraterrestrial solar irradiance, measured by a spectrometer on board an Earth-orbiting satellite. The UV spectrum (119&lt;λ&lt;420nm was measured by the SOLSTICE instrument on the UARS satellite (modified from a diagram provided by GJ Rottmann, private communication, 1995).The vertical lines divide the various spectral subranges defined in Table 1. The smooth curves are calculated blackbody spectra for a number of emission temperatures.

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IONOSPHERE

M.C. Kelley , in Encyclopedia of Atmospheric Sciences, 2003

Sources and Fundamental Features of the Ionosphere

The ionosphere is formed primarily when the most energetic component of the solar spectrum – the X-rays and extreme ultraviolet (EUV) light – impact the illuminated side of the Earth. These high-energy photons strike the daytime side of the Earth, ionizing the upper atmosphere and losing energy in the process. As the beam penetrates the atmosphere, the ionizing beam becomes weaker and weaker, leaving behind a layer of ionization. Part of the energy goes into heat as well as into ionizing the air, resulting in the temperature also rising to values much higher than in any part of the dense atmosphere below. Life on Earth is thus protected by its upper atmosphere from these dangerous photons, just as the ozone layer absorbs the lower-energy, but still harmful, ultraviolet component of the Sun's spectrum.

We compare and contrast the atmosphere and ionosphere in Figure 1. The most important atmospheric parameter is temperature, which is plotted versus height in (A). The key ionospheric parameter is the number of electrons (which equals the number of positive ions) per cubic centimeter. This is plotted in (B) for typical nighttime and daytime conditions.

Figure 1. Typical profiles of neutral atmospheric temperature (A) and ionospheric plasma density (B) with the various layers designated. (Reprinted with permission from Kelley MC (1989). Copyright 1989 by Academic Press.)

As anticipated above, the atmospheric temperature rises from its lowest value near the mesopause (near 200   K) to well over 1000   K in the thermosphere in the same height range where the daytime ionosphere is produced. A glance at (Figure 1B) shows that the ionosphere does not entirely disappear at night, even though the sunlight is no longer present to create new ionization. This is one of the key characteristics of the Earth's ionosphere and explains, for example, how Marconi was first able to send wireless signals across the Atlantic Ocean at night. To understand why some of the ionosphere remains through the night, we must consider the ion chemistry of the region.

At high altitudes (>300   km), production (P) and loss (L) of ionospheric plasma are both small. The balance between diffusion and gravitation results in the so-called hydrostatic equilibrium in which the plasma pressure (p) is of the form of eqn [1]).

[1] $p=p_0{\text{e}}^{-h/H_{\text{p}}}$

In eqn [1], where e is the base of the natural logarithms; h is height above some reference; p ₀ is the pressure at the reference altitude; and H _p is the plasma scale height, given by eqn [2].

[2] $H_{\text{p}}=\frac{k_{\text{B}}T}{\left(M/2\right)g}=\frac{2k_{\text{B}}T}{Mg}$

In eqn [2], M is the average ion mass and g is the gravitational acceleration. According to eqn [1], the pressure falls by a factor of about 2.7 for each altitude increase of H _p. M is quite close to the average mass of the neutral atmospheric particles surrounding the plasma. The factor of 2 comes from the fact that the average plasma mass is half the ion mass, since the electron mass is so tiny. The neutral atmosphere behaves like eqn [1] except that the neutral scale height H _n is half as large. One conclusion from the above is that, because the electrons are so light, the ionosphere extends higher into space than the neutral atmosphere surrounding it. For reference, H _n is about 50   km and H _p is about 100   km in the middle ionosphere.

At these altitudes the composition of the atmosphere is no longer similar to the surface composition (which is 79% N₂, 20% O₂ + minor constituents). The atmosphere is no longer mixed, and lighter atoms can reach higher altitudes. Also, O₂ is photodissociated into free oxygen atoms. Figure 2) shows the composition in terms of various atoms, molecules, and ions versus height for the mid-latitude ionosphere/thermosphere. We see that oxygen becomes dominant at 200   km and hydrogen above 700   km. Similarly, the ionosphere is primarily made up of H⁺ (with some He⁺) at very high altitude, O⁺ in the height range near the peak density, and a mixture of O₂ ⁺, N₂ ⁺, and NO⁺ in the lower thermosphere. Hydrogen is so light that it can escape the Earth's gravity and form the Earth's geocorona, a halo of hydrogen analogous to the Sun's glowing corona seen during an eclipse. By chance, hydrogen and oxygen have almost identical ionization potentials, so charge exchange is a very easy process, as shown in eqn [II].

Figure 2. International Quiet Solar Year (IQSY) daytime atmospheric composition. (Reprinted with permission of the MIT Press from Johnson CY (1969). Ion and neutral composition of the ionosphere. Ann IQSY 5. Cambridge, MA: MIT Press. Copyright 1969 by MIT.)

[II] $\text{H}+{\text{O}}^{+}\rightleftharpoons \text{O}+{\text{H}}^{+}$

Thus, if O⁺ is surrounded by H gas, after a while an oxygen ion will give up its charge to form a hydrogen ion (H⁺). This explains why O⁺ ions formed at low altitudes during daytime become H⁺ ions at very high altitudes.

Gravity and pressure are not the only forces with which the ionosphere must deal. The Earth's dipole magnetic field lines force the hydrogen ions to travel along closed trajectories between the hemispheres, since following the magnetic lines is easy but moving across them is not. The particle motion is helical, the particles moving in circles around the magnetic field lines while freely moving parallel or antiparallel to the direction of the field lines. The result of this motion is that the entire region, in a toroidal shape (shown in Figure 3), becomes filled with a hydrogen plasma during the daytime (whose source is sunlight ionization of oxygen coupled with charge exchange). During the night, this region – called the plasmasphere – starts to unload downward into the ionosphere by the reverse process, tending to maintain the oxygen plasma in the ionosphere during the night with the whole process starting over the next day. The reason the plasmasphere abruptly ends at about 4 earth radii (60° magnetic latitude) is very interesting and is discussed below.

Figure 3. A toroidal region of high plasma density exists around the Earth on average within the region shown. These magnetic flux tubes are filled with plasma of ionospheric origin during the day and discharge only slowly at night.

Refilling from above is not the entire reason that the ionosphere lasts all night, however. It turns out that a charged atom cannot easily recombine with an electron, since in a reaction such as ([III]) it is very difficult simultaneously to conserve both energy and momentum, and the reaction rate is very small.

[III] ${\text{O}}^{+}+{\text{e}}^{-}\to \text{O}$

However, in reactions [IVa] and [IVb] there are two end products, and this difficulty does not arise.

[IVa] ${\text{O}}_2^{+}+{\text{e}}^{-}\to \text{O}+\text{O}$

[IVb] ${\text{NO}}^{+}+\text{e}\to \text{N}+\text{O}$

Reactions [IVa,b] are called dissociative recombination and are very fast. This explains why the molecular ions (seen in Figure 2) at low altitudes disappear at night, leaving the O⁺ plasma above as the distinct nighttime layer (shown in Figure 1). In fact, reaction [III] is so slow that O⁺ is actually lost through a two-step process such as charge exchange [Va] followed by [Vb].

[Va] ${\text{O}}^{+}+{\text{O}}_2\to {\text{O}}_2^{+}+\text{O}$

[Vb] ${\text{O}}_2^{+}+{\text{e}}^{-}\to \text{O}+\text{O}$

An alternative route is ion–atom interchange followed by [VIb]).

[VIa] ${\text{O}}^{+}+{\text{N}}_2\to {\text{NO}}^{+}+\text{N}$

[VIb] ${\text{NO}}^{+}+\text{e}\to \text{N}+\text{O}$

Both [Va] and [VIb] leave oxygen in an excited state, which emits both red (630   nm) and green (557.9   nm) light that is visible from the ground to sensitive cameras. Such emissions are called airglow and provide a tool for visualizing the ionosphere, as shown in the next section.

To summarize thus far: the ionosphere is created during the daytime by X-rays and EUV from the Sun, which are absorbed while heating and ionizing the outer layer of the atmosphere. This heats the gas to temperatures over 1000   K, explaining why it is called the thermosphere. The plasma, which is primarily O⁺ above 200   km, diffuses upward against gravity, reaching high enough that charge exchange with the geocorona converts the ionosphere to a H⁺ plasma, which can escape gravity. The plasma is constrained by the dipole magnetic field to a toroidal configuration, filled during the day and emptied at night. Molecular ions dominate in the lower thermosphere, but they disappear quickly after sunset, leaving a slowly decaying O⁺ layer.

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Solar Detoxification and Disinfection

S. Malato-Rodríguez , in Encyclopedia of Energy, 2004

1 Solar Photochemistry

A specific characteristic of solar photochemical processes is that the photons from the solar spectrum must be absorbed by some component(s) of the reacting system, transferring their energy to the chemical system. Solar photons can be directly absorbed by reactants (direct photochemistry) and/or by a catalyst or sensitizer (photocatalytic processes). Solar detoxification and disinfection are included within photocatalysis.

Photochemical processes use the intrinsic energy of photons (i.e., wavelength) from the solar irradiation to provoke specific reactions. The energy of a photon is a function of its wavelength, according to Eq. (1):

(1) $E=\frac{hc}{\lambda },$

where h is Planck's constant (6.626×10⁻³⁴ Js), c is the speed of light, and λ is the wavelength. Approximately 4 or 5% of the sunlight at the earth's surface is in the near-ultraviolet (UV) region (wavelengths from 300 to 400   nm), approximately 45% is in the visible range (400–760   nm), and the rest is in the near-infrared (IR) and IR regions. Solar photons from the visible and near-UV spectrum are energetic enough to be used in photochemical processes. Infrared photons are normally useless in photochemistry and can be considered as waste heat in chemical processes.

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Radiation (SOLAR)

Qiang Fu , in Encyclopedia of Atmospheric Sciences, 2003

Solar Spectrum and Solar Constant

The distribution of solar radiation as a function of the wavelength is called the solar spectrum, which consists of a continuous emission with some superimposed line structures. The Sun's total radiation output is approximately equivalent to that of a blackbody at 5776   K. The solar radiation in the visible and infrared spectrum fits closely with the blackbody emission at this temperature. However, the ultraviolet (UV) region (<0.4   μm) of solar radiation deviates greatly from the visible and infrared regions in terms of the equivalent blackbody temperature of the Sun. In the interval 0.1–0.4   μm, the equivalent blackbody temperature of the sun is generally less than 5776   K with a minimum of about 4500   K at about 0.16   μm. The deviations seen in the solar spectrum are a result of emission from the nonisothermal solar atmosphere.

The solar constant is the amount of solar radiation received outside the Earth's atmosphere on a surface normal to the incident radiation per unit time and per unit area at the Earth's mean distance from the Sun. The solar constant is an important value for the studies of global energy balance and climate. Reliable measurements of solar constant can be made only from space and a more than 20-year record has been obtained based on overlapping satellite observations. The analysis of satellite data suggests a solar constant of 1366   W   m⁻² with a measurement uncertainty of ±3   W   m⁻². Of the radiant energy emitted from the Sun, approximately 50% lies in the infrared region (>0.7   μm), about 40% in the visible region (0.4–0.7   μm), and about 10% in the UV region (<0.4   μm).

The solar constant is not in fact perfectly constant, but varies in relation to the solar activities. Beyond the very slow evolution of the Sun, a well-known solar activity is the sunspots, which are relatively dark regions on the surface of the Sun. The periodic change in the number of sunspots is referred to as the sunspot cycle, and takes about 11 years, the so-called 11-year cycle. The cycle of sunspot maxima having the same magnetic polarity is referred to as the 22-year cycle. The Sun also rotates on its axis once in about 27 days. Satellite observations suggest that the solar cycle variation of the solar constant is on the order of about 0.1%, which might be too small to directly cause more than barely detectable changes in the tropospheric climate. However, some indirect evidence indicates that the changes in solar constant related to sunspot activity may have been significantly larger over the last several centuries. Furthermore, solar variability is much larger (in relative terms) in the UV region, and induces considerable changes in the chemical composition, temperature, and circulation of the stratosphere, as well as in the higher reaches of the upper atmosphere.

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SOOT

P. Chylek , ... R. Pinnick , in Encyclopedia of Atmospheric Sciences, 2003

Soot and Direct Radiative Effect of Aerosols

Soot incorporated within an aerosol particle will increase the particle's absorption in the visible part of solar spectrum and thus it will decrease the particle's single scattering albedo. The direct top of the atmosphere radiative forcing, ΔF, of an optically thin aerosol layer is given by

[2] $\Delta F=-\frac{S_0}{4}{T}_{\text{atm}}^2(1-N)\left[{(1-a)}^{2}2\beta {\tau }_{\text{sc}}-4a{\tau }_{\text{abs}}\right]$

where S ₀ is the solar constant, N the fraction of sky covered by clouds, T _atm the transmittance of the atmosphere above the aerosol layer, a the surface albedo, β the fraction of the scattered radiation that is scattered into the upper hemisphere, and τ _sc and τ _abs the scattering and the absorption optical thickness of an aerosol layer.

The negative value of radiative forcing implies cooling of the system, while a positive value implies heating. For nonabsorbing aerosol τ _abs = 0, and eqn [2] implies always a cooling effect. When soot is present within an aerosol, aerosol absorption increases and the direct aerosol effect will be either cooling or heating, depending on the relative magnitudes of the terms inside the bracket on the right-hand side of eqn [2]. For an optically thin aerosol layer, ω = τ _sc/(τ _sc + τ _abs). The critical single scattering albedo, ω _sc, which determines whether an aerosol will heat or cool the system, is derived from eqn [2] in the form

[3] ${\omega }_{\text{cr}}=\frac{2a}{\beta {(1-a)}^2+2a}$

For given surface albedo, a, and backscattering fraction, β, an aerosol with single scattering albedo ω > ω _cr will cool the system, while aerosols with ω < ω _cr will cause heating.

Thus the sign of a direct top-of-the-atmosphere aerosol forcing depends – in addition to the fraction of radiation scattered into the upward hemisphere and the albedo of an underlying surface – on the amount of soot within an aerosol particle (which determines the single scattering albedo ω). Most aerosols will cause cooling over the ocean and heating over fresh snow. Thus, the soot heating effect will be especially significant over clouds, ice, and snow.

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TOTAL1 (BROADBAND) RADIATION UNDER CLOUDLESS SKIES

Muhammad Iqbal , in An Introduction to Solar Radiation, 1983

7.1 Introduction

For most engineering problems, determining the amount of radiation over a certain bandwidth or over the complete solar spectrum is required. Radiation over a certain bandwidth can be evaluated only by adding up the spectral values. On the other hand the radiation over the entire solar spectrum can be computed either by integrating over the complete solar spectrum or by what may be called a broadband approach. Integration of the monochromatic values can be carried out with the help of the material presented in Chapter 6. However, such a procedure is very time consuming. Consequently, simple broadband equations have been developed to account for the attenuation by each of the atmospheric constituents.

Under the broadband approach, we discuss the parameterization method and the ASHRAE algorithm. For each method, formulations to compute direct, diffuse, and global radiation are presented. We present first the equations treating spectral integration of irradiance.

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GROUND ALBEDO

Muhammad Iqbal , in An Introduction to Solar Radiation, 1983

9.4 The Overall Albedo

In this section, the basic features of albedo values for a number of ground covers integrated over the solar spectrum and solar height are briefly discussed.

The soil surface albedo has strong dependence on moisture content and tillage conditions: moisture decreases the albedo. The plant albedo can have large variations depending on the season and the phase of growth. The albedo of ice has not been well studied; however, it is known that the albedo of an ice layer over water can be substantially low and that of ice under snow can be very high. The albedo of snow and ice can also fluctuate a great deal depending on the freshness of the snow, the water under the ice, and the amount of dirt on the surface. Figure 9.3.2 may also be used to obtain the albedo of snow for clear-sky conditions; for completely cloudy skies and isotropic diffuse radiation, these values should be increased by 10−15%.

For solar height greater than 10°, the albedo of a smooth water surface under beam radiation may be obtained from Fig. 9.3.2. A water surface under diffuse radiation has an albedo of 0.08−0.11, the lower value being for clear-sky diffuse radiation and the higher value for diffuse radiation from cloudy skies. These albedo values need to be slightly modified to take into account the depth of the water basin and the fact that a large water surface is not horizontal because of the earth's curvature.

Albedo is generally measured by two back-to-back pyranometers mounted in a horizontal position a few meters above the underlying surface. The pyranometer facing the sky measures the global incident solar radiation and the one facing the earth measures the reflected energy (albedo is the ratio of the reflected to the incident energy). Average values of such measurements taken under various cloud conditions and solar heights are reported in the literature. Such measurements characterize the albedos of small underlying surfaces.

However, engineering and architectural applications require measurement of the weighted-average albedo of a large land mass that a receiving surface sees. This land mass may itself consist of patches of various surfaces, and for each patch the albedo for diffuse radiation and the variation of albedo with solar height must be determined. Although such detailed information for an individual situation may not be available, Hunn and Calafell [3], using a photographic method, have presented the albedo for some typical winter landscapes in the United States. However, it seems that recourse has still to be made to a subjective selection of monthly weighted-average albedo, representative of a large area composed of patches of different surfaces.

Table 9.4.1 lists the albedo of natural ground covers. Most of these data are based on Russian authors who have done extensive studies of this subject.

Table 9.4.1. Albedo of Natural Ground Covers

Item	Albedo ρ	Reference
I. Crops
Alfalfa	0.02−0.05	5
Beets (sugar)	0.18	1
	0.25	6
Cotton	0.20−0.22	1
Grass
Dry	0.15−0.25	8
dry, wizened in sun	0.19	8
dry, no sun	0.19−0.22	8
dry, high	0.31−0.33	8
Green	0.26	1
high, fresh	0.26	8
wet, no sun	0.14−0.26	8
wet, sun	0.33−0.37	8
Heather	0.10	1
Lettuce	0.22	1
Lucerne	0.23−0.32	1
	0.22−0.24	6
Maize:
15−20 cm, 40−50% cover	0.16	10
40−50 cm, 70−75% cover	0.18	10
140−200 cm, green cobs, 80% cover	0.20	10
200−250 cm, fully ripe	0.23	10
Rice	0.12	1
Rye
Winter	0.21	1
Green	0.18	10
mass fluorescence	0.16	10
end of fluorescence	0.15	10
beginning of fading of leaves	0.13	10
faded leaves, < 50% cover	0.11	10
Wheat
Summer	0.10−0.25	1
milky light green	0.13	10
yellow ripeness	0.17	10
full ripeness	0.21	10
II. Other agricultural and waste (nonarable) lands
Soils
chestnut soil, gray red:
dry, leveled	0.20	10
moist, leveled	0.12	10
dry, ploughed	0.15	10
moist, ploughed	0.07	10
clay soils:
blue, dry	0.23	1, 10
blue, moist	0.16	1, 10
gray sandy soils:
level, dry	0.25	10
level, moist	0.18	10
ploughed, dry	0.20	10
ploughed, moist	0.11	10
black earth, dark gray:
level, dry	0.13	10
level, moist	0.08	10
ploughed, dry	0.08	10
ploughed, moist	0.04	10
unspecified soil:
dry, ploughed	0.20−0.25	8
Sand—Deserts
fine, light sand	0.37	1
gray sand	0.21	1
death valley	0.25	8
Mojave Desert	0.24−0.28	8
quartz (white) sand	0.35−0.40	1
river sand	0.43	1
wet sand	0.09	8
yellow sand	0.35	1
white sand, New Mexico	0.60	11
valleys, plains and slopes	0.27	11
Forests (see also snow)
exfoliating in dry season	0.24	1
exfoliating in wet season	0.18	1
green forest	0.03−0.06	8, 12
	0.04−0.10	8
bare ground—some trees	0.07	8, 12
coniferous forest	0.12	11
coniferous and deciduous:
Jun θZ = 39−55°	0.14−0.17	8
Aug θZ = 30−41°	0.12−0.16	8
Sep θZ = 26−38°	0.19−0.10	8
Steppes	0.40−0.52	8
tops of fir	0.10	1
tops of pine	0.14	1
tops of oak	0.18	1
Tundra	0.11−0.23	8
marsh	0.10−0.18	8
III. Snow and ice
Snow
forests
new, fallen	0.82	10
wet, fine grained	0.65	10
wet, medium gained	0.56	10
wet, large grained	0.47	10
against background of mixed landscape	0.34	10
separate spots	0.31	10
stable snow cover	0.65	1
unstable in spring	0.25	1
unstable in fall	0.30	1
fields
new fallen	0.82	10
wet, fine grained	0.73	10
wet, medium grained	0.64	10
wet, large grained	0.55	10
new fallen
dry, bright, white, clean	0.72−0.98	10
wet, bright, white	0.80−0.85	10
compacted
dry, clean	0.66−0.80	8
wet, gray, white	0.61−0.75	8
melting (soaked with water)	0.35	8
stable cover:
above lat = 60°	0.80	1
below lat = 60°	0.70	1
Ice (general ice forms)
coastal ice, no snow	0.4−0.5	10
melting pack ice, no snow	0.49−0.67	10
frozen puddles	0.42−0.50	10

It is also useful to note an important element in average albedo data of a natural ground cover located at two different latitudes. Since the two locations will have different average solar altitudes, the albedo values will be different. Further, the albedo of a surface at two different locations of the same latitude but under different sunshine conditions may not be identical owing to unequal ratios of beam and diffuse radiation. It is useful to keep in mind these factors while applying the tabulated data.

Table 9.4.2 contains albedos of building materials. These data are obtained from Gubareff et al. [4], who carried out an extensive survey of the radiative properties of different kinds of engineering and building materials.

Table 9.4.2. Albedo of Building Materials ^a

Item	Albedo ρ	Item	Albedo ρ
Bricks		Limestone
clay, cream, glazed	0.64	anston	0.40
lime clay, French	0.54	bath	0.47
Red	0.32	Portland	0.64
Stafford blue	0.11	White marble	0.56
white glazed	0.74	Reddish granite	0.45
Tiles		Slate
clay, purple (dark)	0.18	blue-gray	0.13
Red	0.33	gray dark	0.10
concrete, uncolored	0.35	purple	0.14
concrete, black	0.09	Wood	0.22
concrete, brown	0.15	Aluminum	0.85
Asphalt		Iron
New	0.09	new galvanized	0.35
pavement	0.15	galvanized, very dirty	0.08
pavement, weathered	0.18	Steel	0.80
Roofing		Copper	0.74
bituminous felt	0.12	Paint
sheet, green	0.14	aluminum	0.46
sheet, black matte surface	0.13	oil paint, cream, light	0.70
Asbestos cement		oil paint, green, light	0.50
Aged	0.25
Red	0.31
white	0.39

a

From Gubareff et al. [4].

Regional albedos of some U.S. and Canadian sites are listed in Tables 9.4.3 and 9.4.4, respectively.

Table 9.4.3. Monthly Average Ground Albedo for Some U.S. Locations ^a

	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Albuquerque, NM	0.478	0.432	0.394	0.303	0.280	0.280	0.280	0.280	0.280	0.280	0.409	0.478
Apalachicola, FL	0.140	0.140	0.140	0.140	0.140	0.140	0.140	0.140	0.140	0.140	0.140	0.140
Bismark, ND	0.660	0.660	0.660	0.468	0.289	0.180	0.180	0.180	0.212	0.404	0.628	0.660
Brownsville, TX	0.280	0.280	0.280	0.280	0.280	0.280	0.280	0.280	0.280	0.280	0.280	0.280
Boston, MA	0.365	0.331	0.287	0.160	0.140	0.140	0.140	0.140	0.140	0.140	0.192	0.322
Cape Hatteras, NC	0.180	0.180	0.180	0.180	0.180	0.180	0.180	0.180	0.180	0.180	0.180	0.180
Caribou, ME	0.660	0.660	0.660	0.539	0.244	0.140	0.140	0.140	0.192	0.400	0.573	0.660
Charleston, SC	0.140	0.140	0.140	0.140	0.140	0.140	0.140	0.140	0.140	0.140	0.140	0.140
Columbia, MO	0.608	0.539	0.435	0.192	0.140	0.140	0.140	0.140	0.140	0.157	0.400	0.556
Dodge City, KS	0.644	0.564	0.500	0.276	0.180	0.180	0.180	0.180	0.180	0.212	0.468	0.628
El Paso, TX	0.280	0.280	0.280	0.280	0.280	0.280	0.280	0.280	0.280	0.280	0.280	0.280
Ely, NV	0.660	0.660	0.660	0.556	0.365	0.209	0.140	0.157	0.296	0.539	0.628	0.660
Ft. Worth, TX	0.215	0.206	0.190	0.180	0.180	0.180	0.180	0.180	0.180	0.180	0.186	0.206
Fresno, CA	0.140	0.140	0.140	0.140	0.140	0.140	0.140	0.140	0.140	0.140	0.140	0.140
Great Falls, MT	0.591	0.573	0.573	0.365	0.192	0.140	0.140	0.140	0.175	0.279	0.487	0.556
Lake Charles, LA	0.140	0.140	0.140	0.140	0.140	0.140	0.140	0.140	0.140	0.140	0.140	0.140
Madison, WI	0.660	0.608	0.573	0.296	0.140	0.140	0.140	0.140	0.140	0.209	0.469	0.643
Medford, OR	0.469	0.400	0.313	0.192	0.140	0.140	0.140	0.140	0.140	0.192	0.365	0.418
Miami, FL	0.140	0.140	0.140	0.140	0.140	0.140	0.140	0.140	0.140	0.140	0.140	0.140
Nashville, TN	0.310	0.270	0.227	0.149	0.140	0.140	0.140	0.140	0.140	0.149	0.227	0.310
New York, NY	0.339	0.309	0.244	0.149	0.140	0.140	0.140	0.140	0.140	0.140	0.175	0.296
N. Omaha, NE	0.660	0.591	0.521	0.244	0.140	0.140	0.140	0.140	0.140	0.192	0.487	0.643
Phoenix, AZ	0.280	0.280	0.280	0.280	0.280	0.280	0.280	0.280	0.280	0.280	0.280	0.280
Raleigh, NC	0.296	0.270	0.227	0.149	0.140	0.140	0.140	0.140	0.140	0.149	0.218	0.309
Santa Maria, CA	0.140	0.140	0.140	0.140	0.140	0.140	0.140	0.140	0.140	0.140	0.140	0.140
Seattle, WA	0.279	0.209	0.175	0.140	0.140	0.140	0.140	0.140	0.140	0.140	0.157	0.227
Washington, DC	0.313	0.287	0.227	0.149	0.140	0.140	0.140	0.140	0.140	0.140	0.192	0.300

a

From SOLMET vol.2 [13]

Table 9.4.4. Monthly Average Ground Albedo for Some Canadian Locations ^a

Month	Vancouver	Edmonton	Saskatoon	Winnipeg	Toronto	Ottawa	Montreal
Jan	0.18	0.58	0.49	0.54	0.50	0.62	0.32
Feb	0.17	0.57	0.50	0.55	0.50	0.60	0.33
Mar	0.17	0.46	0.42	0.47	0.38	0.43	0.25
Apr	0.14	0.32	0.24	0.31	0.28	0.12	0.22
May	0.14	0.26	0.18	0.20	0.25	0.13	0.20
Jun	0.14	0.25	0.20	0.21	0.25	0.19	0.20
Jul	0.14	0.25	0.21	0.21	0.25	0.19	0.20
Aug	0.14	0.25	0.22	0.23	0.25	0.20	0.20
Sep	0.14	0.26	0.22	0.23	0.25	0.20	0.20
Oct	0.14	0.28	0.22	0.24	0.25	0.25	0.21
Nov	0.15	0.39	0.27	0.32	0.29	0.21	0.23
Dec	0.18	0.53	0.27	0.45	0.39	0.56	0.28

a

Courtesy of Dr. John Hay.

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Epitaxy for Energy Materials

Roberto Fornari , in Handbook of Crystal Growth: Thin Films and Epitaxy (Second Edition), 2015

1.3.2.2 III–V Cells on GaAs

The forced bandgap combinations of the lattice-matched alloys are not optimal for maximum photovoltaic conversion efficiency of the solar spectrum. There are models that allow calculation of the maximum theoretical efficiency of a series-connected triple-junction solar cell for a range of bandgap combinations assuming GaAs-like material parameters. Such simulations are very helpful for optimizing the solar cell design. By applying such semi-empirical models, it was suggested that efficiency may be maximized corresponding to bandgap combinations of 1.86, 1.34, and 0.93   eV and 1.75, 1.18, and 0.70   eV [13]. They could respectively be 5.2% and 4.9% more efficient than the usual lattice-matched GaInP/GaInAs/Ge (1.86, 1.39, and 0.67   eV) structure. Thus, in order to grow a more efficient monolithic solar cell structure with improved bandgap combination, lattice-mismatched alloys with low defect densities are required. Compositionally graded buffer layers may be used to accommodate the lattice mismatch via formation of misfit dislocations within the buffer while inhibiting the propagation of these dislocations into the active junction. The resulting structure, incorporating active layers of different lattice constants separated by the graded layer, is commonly referred to as an inverted metamorphic structure [13].

An efficiency of 40.7% at 240 suns has been obtained in a metamorphic triple-junction device using a Ge bottom junction and two coupled metamorphic junctions that are both 0.5% misfit from the substrate with the band gaps 1.80, 1.29, and 0.67   eV [19]. This design allows for an efficiency improvement with respect to lattice-matched devices; however, its bandgap combination is still far from optimized and some threading dislocations are introduced into the highest power-producing top junction. Further decreasing the top two-junction band gaps by this approach is very difficult because the performance is very sensitive to dislocations in the top GaInP junction. By inverting the direction of growth and then removing the substrate, a device with two high-quality lattice-matched top junctions, Ga_0.5In_0.5P and GaAs, and highly lattice-mismatched (1.9% misfit) Ga_0.73In_0.27As bottom junction with band gaps (1.84, 1.41, and 1.00   eV) can be grown on GaAs substrates, which further increases the efficiency by approximately 3% relative to the lattice-matched design on Ge. This inverted design on GaAs containing a single metamorphic junction has several advantages over the conventional triple-junction designs that use a Ge bottom junction, but it does not allow for the optimal bandgap combination. A further step in that direction was made by using an improved inverted triple-junction solar cell containing two independent metamorphic junctions (see Figure 1.5). The band gaps in this inverted design are 1.83, 1.34, and 0.89   eV, which more closely approach the theoretical global maximum. A performance of 40.8% efficiency at 326 suns was demonstrated for this design [13], but further development could potentially boost the efficiency by more than 4% beyond that of the lattice-matched design.

FIGURE 1.5. Ion beam image and composite 220 dark-field transmission electron microscopy of a focused ion beam (FIB) cross-section of an inverted triple-junction solar cell structure. (After Ref. [13] .) Note the two tunnel junctions (TJ) and the graded buffer layers that separate the individual homo-junctions.

This structure was grown by atmospheric-pressure MOVPE on a (001) GaAs substrate miscut 2° toward (111)B. The top 1.83-eV Ga_0.51In_0.49P junction was grown first and lattice-matched to the GaAs substrate. The middle 1.34-eV Ga_0.96In_0.04As junction was grown next after gradually increasing the lattice constant by 0.3% with an (Al)GaInP step-grade buffer. Finally, the bottom 0.89-eV Ga_0.63In_0.37As junction (2.6% misfit) was grown after further increasing the lattice constant by inserting a GaInP step-grade. The thicknesses of the junctions were approximately 2.5–2.9 μm. As the band gaps were closed to optimum values, current-matching was achieved without the need to thin any junctions. The graded layers were discovered to be transparent in the spectral range required by the junctions below them and to produce a negligible strain within the active junctions. As clearly visible in Figure 1.5, each n-on-p homojunction was clad with passivating window and back-surface-field layers of an (Al)GaInP composition with higher bandgap than the junction but identical lattice constant. Tunnel junctions were grown between each junction before the graded layers. The tunnel junction between the top and middle junction was thus lattice-matched to the top junction and the substrate, but the tunnel junction between the middle and bottom junction was grown on the metamorphic middle junction. This thin metamorphic junction consisted partly of Ga_0.96In_0.04As doped with Se matching the metamorphic middle junction lattice constant on which it was grown, but also of Ga_0.70Al_0.30As doped with C, which was grown in tension on the metamorphic middle junction. The doping of Ga_0.96In_0.04As with C was avoided because In-containing alloys are difficult to dope with carbon.

Although this tunnel junction design is not ideal because of the strained GaAlAs layer, it provided low-resistance Ohmic-like conduction in the device up to a concentration of 826 suns. X-ray diffraction investigations showed the metamorphic junctions to be only 0.029% and 0.014% compressive strain. Plan-view, spectrally resolved cathodoluminescence images indicated approximately 1   ×   10⁵  cm⁻² and 2–3   ×   10⁶  cm⁻² threading dislocations in the middle and bottom junctions, respectively, whereas the threading dislocations in the top junction were below the detection limit (5   ×   10⁴  cm⁻²). Additional investigations by transmission electron microscopy (TEM; also shown in Figure 1.5) showed many dislocations within the two graded buffer layers, but almost none in the three active junction regions. The measured dislocation densities are believed to have just minor influence on recombination of photocarriers.

The monolithic device with metamorphic junctions is normally processed as shown in Figure 1.6. A gold back contact is first applied and the inverted structure is bonded to a silicon handle with epoxy. The original GaAs substrate is then removed, the front metal grids are applied, the devices are isolated, and an antireflective coating is deposited (see Ref.[27] for details).

FIGURE 1.6. Another type of inverted triple-junction structure. (After Ref. [27] .) Here, it is clear how the final structure is bonded top-down to another support (e.g., Si or glass) before the original GaAs substrate is etched away and electrodes are formed.

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Cu(InGa)Se2 Based Thin Film Solar Cells

Subba Ramaiah Kodigala , in Thin Films and Nanostructures, 2010

5.1.2 CuInSe₂

The CuInSe₂ (CIS) has band gap of 0.95–1.05 eV that is slightly lower to match with the solar spectrum to acquire most percentage of photons and its absorption coefficient is in the order of 10⁵ cm^{−   1} at fundamental absorption region [18]. The variation of transmission with photon energy (hν) and a plot of (αhν)² versus hν for the stoichiometric CuInSe₂ thin films grown onto corning 7059 glass substrates by vacuum evaporation are depicted in Figure 5.3A and B , respectively [19]. The absorption coefficient (α) can be extracted from the transmission spectrum employing above formulae. The sharp transmission spectrum of CuInSe₂ thin films prepared by spray pyrolysis technique is observed in high resistance samples (Figure 5.4A) relating to band gap of 1.02 eV, whereas the transmission signal is not in detectable range in low resistance samples due to high carrier absorption [20]. The spectrum of an amorphous 80 nm thick p-CuInSe₂ is depicted in Figure 5.4B, from which band gap is calculated to be 1.2 eV [21]. In fact, the band gap is higher in the amorphous CIS than that in the single crystal or polycrystalline due to difference in nature of band structure.

Figure 5.3. (A) Transmission spectrum of CuInSe₂ thin film and (B) (αhν)² versus hν.

Figure 5.4. (A) Transmission spectrum of CuInSe₂ thin films grown by spray pyrolysis technique and (B) Transmission spectrum of an amorphous p-type CuInSe₂ thin film.

The Cu-rich and nearly stoichiometric In-rich CuInSe₂ thin films exhibit band gaps of 0.99 and 1.02 eV, respectively, which are determined from transmission spectra of the layers, as given in Figure 5.5 [22]. Figure 5.6 shows transmission spectra of stoichiometric and Cu-rich CuInSe₂ thin films grown onto glass substrates. The poor trasmission is observed in the Cu-rich CuInSe₂ thin films due to free carrier absorption [23]. Unlike transmission spectrum alone, the more precise determination of band gap for CuInSe₂ thin films can be found out from both transmission and reflectance spectra using Equation (5.1). The band gaps of 1.02 and 0.94 eV are found for CuInSe₂ thin films deposited onto glass substrates by evaporation of three constituent elements of Cu/In/Se at two different temperatures of 450 and 350 °C, which had Cu:In:Se compositions of 21.9:25.8:52.3 and 24.6:24.9:50.5, respectively (Figure 5.7). On contrast, the latter shows slightly higher tail at below the fundamental absorption region due to excess Cu in the layers. Keeping Se composition of 47.6–48.7% with respect to metal composition of (Cu+ In), the band gap of CIS increases from 1.015, 1.031, 1.043, 1.086, 1.088 to 1.087 eV with decreasing Cu/In ratio from 0.57, 0.53, 0.49, 0.48, 0.44 to 0.43, respectively [24]. After annealing the sphalerite structure films under vacuum at 450 °C for 30 min, the band gap slightly increases and structure changes to chalcopyrite. The CIS films with Cu/In   ≥   1.0 show ρ  <   1.0 Ω-cm whereas the films with Cu/In   <   1.0 had ρ  >   10 Ω-cm [25].

Figure 5.5. Transmission spectra of Cu-rich and nearly stoichiometric In-rich CuInSe₂ thin films.

Figure 5.6. Transmission spectra of CuInSe₂ thin film: (A) stoichiometric (Cu:In:Se   =   25:25:50), and (B) Cu-rich (Cu:In:Se   =   42.32:15.1:42.58).

Figure 5.7. Transmission and reflectance spectra of CuInSe₂ thin films deposited at: (A) 450 °C, Cu:In:Se   =   21.9:25.8:52.3 (solid line) and (B) 350 °C, Cu:In:Se   =   24.6:24.9:50.5 (dashed line).

As far as band gap concerned, the Cu-rich CuInSe₂ thin films show lower band gap than that of Cu-poor thin films. The Cu 3d levels dictate the valence band in the band structure (Figure 5.1) that means an increase in valence band position takes place to upwards if the Cu is doped in the films but not in the conduction band. Therefore, the optical band gap between valence band and conduction band decreases with increasing Cu or decreasing In content in the compound. The phase change occurs with increasing In in the system such as ordered vacancy compounds (OVC) those have higher band gaps. Bougnot et al., [26] observed that the band gap of spray deposited CuInSe₂ varies from 0.9 to 1.03 eV with changing Se atomic ratio with respect to unity of Cu/In in the starting chemical spray solution from 2.5 to 2.8. In another occasion, the band gap increases from 0.97 to 1.12 eV with decreasing Cu/In ratio from 1.1 to 0.8 by keeping Se to Cu/In ratio at 2.5 in the chemical solution. The band gaps of 0.9 and 1.05 eV are reported for the spray deposited CuInSe₂ thin films in the literature [27,28]. The band gap of p-CuInSe₂ layers deposited from 5% excess Se bulk CuInSe₂ decreases from 1.08 to 1.025 eV after annealing layers under vacuum at 300° C due to loss of Se [29]. The CuInSe₂ thin films synthesized by two-stage selenization with composition of Cu:In:Se   =   24.8:24.9:50.3 reveal band gaps of ~   1.019, 1.044, and 1.050 eV at RT, 78, and 4.2 K, respectively and its absorption coefficient is 1.5   ×   10⁵  cm^{−   1} at 78 K [30]. The band gap or band edge emission obtained from photoacoustic spectra for CuInSe₂ with Cu/In ratio of 1.79 grown onto GaAs (001) follows the Varshini formula,

(5.5) $E_g=E_g(0)-\alpha T^2⁄(\beta +T),$

where E _g(0) is band gap at 0 K, α and β are 3.6   ×   10^{−   4} eV/K and 350 K fitting parameters, respectively as shown in Figure 5.8 [31].

Figure 5.8. Variation of band gap or band edge of CuInSe₂ versus temperature.

After rapid thermal processing (RTP), the films experience slightly Se loss. All the Cu-rich, stoichiometric, and Cu-poor Cu–In–Se precursor layers rapid thermal processed at 700 °C with ramp up rate of 150 °C/s for 30 s show band gap of 1.0 eV and absorption coefficient of 5   ×   10⁴  cm^{−   1} at fundamental absorption region. The Cu-rich CuInSe₂ films contain secondary optical transition due to Cu_2−x Se phase, in comparison stoichiometric films show sharp optical absorption transition at fundamental absorption region [32]. In order to confirm the presence of Cu_2−δSe phase in the CIS, the CuInSe₂ and Cu_2−δSe layers are separately deposited onto transparent glass substrates by thermal evaporation at substrate temperatures of 350–500 and 400 °C, respectively. The Cu_2−δSe is silverish blue in color and exhibits absorption coefficient of 3   ×   10⁴ cm^{−   1}. The CuInSe₂ with composition of Cu:In:Se   =   22.2:25.4:52.4 shows sharp absorption in the fundamental absorption region. In the Cu-rich CuInSe₂ layers, the dominant subband gap absorption is observed due to secondary phase of Cu_2−δSe. The absorption nature of CuInSe₂ coincides with that of Cu_2−δSe by less magnitude. After NaCN treating the Cu-rich CuInSe₂ samples, the Cu:In:Se composition of 32.1:12.5:46.3 comes down to 23.6:26.8:49.6. Difference in compositions of 13.2:0:6.6 between etched and as-grown are close to that of Cu_2−δSe. In the optical absorption spectra, the subband gap disappears and sharp absorption occurs in the NaCN etched sample. The ρ value also increases from 10⁻³–10⁰ to 10⁰–10³ Ω-cm supporting elimination off Cu_2−δSe in the sample [33]. The transmission of CuInSe₂ thin films decreases at below the band gap with increasing Cu/In ratio from 0.94, 1.05, 1.3 to 2.15 in the layers due to free carrier absorption of secondary phase Cu_2−x Se, whereby the absorption and emission rates are under equilibrium condition depending on temperature and frequency, according to Kirchhoff's law [34].

The CuInSe₂ thin films grown by electrodeposition at potential of –1.0 V and annealed at 350, 450, and 550 °C exhibit band gaps of 0.92, 0.96, and 1.0 eV, respectively, while the films deposited at different potentials of −   0.8, −   0.9, and –1.0 V, followed by annealing at 550 °C for 2 h show band gaps of 0.978, 0.98, and 1.0 eV, respectively due to difference in composition [35]. The band gap of CuInSe₂ deposited onto ITO at –2.2 V versus SCE is 1.1 or 1.2 eV [36,37], whilst the CuInSe₂ thin films deposited on Au coated plastic substrates at potential of –1.5 V versus SCE and pH 1.65, followed by annealing under N₂ at 150 °C for 1 h show band gap of 1.18 eV and composition of Cu:In:Se   =   25.57:25.1:49.42 [38]. The CuInSe₂ thin films deposited at lower potential –0.7 eV shows band gap of 0.85 eV. After annealing, followed by etching the CIS films in KCN exhibit band gaps of 1.01 and 1.10 eV, respectively [39]. The band gap of spray deposited In-rich CuInSe₂ thin films is 1.22 eV for 0.54   <   In/(Cu   +   In)   <   0.67 but increases from 1.22 to 1.36 eV with increasing In/(Cu   +   In) ratio from 0.67 to 0.78 eV evidencing formation of likely CuIn₃Se₅ (OVC) [40]. The transmission spectra of CuInSe₂ and (CuIn₃Se₅  +   CuInSe₂) thin films are given in Figure 5.9. The optical band gaps of CuInSe₂ and CuIn₃Se₅ (OVC) are observed to be 1.01 and 1.24 eV from spectra, respectively [41]. The band gap increases from 1.05 to 1.32 eV with increasing x in the (Cu₂Se)(In₂Se₃)_1+x (OVC) compound that is, CuInSe₂ (1.05 eV) for x  =   0, CuIn₂Se_3.5 (1.215 eV) for x  =   1, CuIn₃Se₅ (1.18 eV) x  =   2, and CuIn₅Se₈ (1.32 eV) x  =   4.

Figure 5.9. Transmission spectra of CuInSe₂ and (CuIn₃Se₅  +   CuInSe₂) thin films.

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