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https://doi.org/10.29013/ESR-21-11.12-40-49
Rakhmonov Tokhirbek Imomalievich, Doktorant of PhD of Fergana Polytechnic Institute, Uzbekistan
E-mail: radiofizik2014@gmail.com Mamadieva Dilkhumor Tolibjonovna, Senior Lecturer of Fergana Polytechnic Institute, Uzbekistan
E-mail: zferfizika@mail.ru Yuldashev Nosirjon Khaydarovich, DSc, Professor of Fergana Polytechnic Institute, Uzbekistan
PHOTOELECTRIC PHENOMENA IN THIN POLYCRYSTALLINE CdTe, CdSe, CdS FILMS UNDER MECHANICAL DEFORMATION
Abstract. Some issues of improving the technology of obtaining thin-film (d — 1.0 ^m) elements with anomalous photovoltaic properties from cadmium chalcogenides by thermal evaporation in vacuum in separate portions on transparent dielectric substrates are considered. The experimental results of studying the current-voltage, lux-ampere, lux-voltage and deformation characteristics of CdTe, CdSe, CdS polycrystalline films are analyzed. It is shown that the obtained samples have linear I - V, L - A, temperature (T — 120-320 K) and deformation characteristics (e — -3 • 10 -s3 3 • 10 3 arb. units). Therefore, they can serve as promising photocells for the producing of film photodetec-tors sensitive to mechanical deformation in the visible and near-IR regions of the radiation spectrum.
Keywords: thin polycrystalline films, photovoltaic properties, method of discrete evaporation in vacuum, substrate temperature, relative deformation, strain-sensitivity coefficient, current-voltage, lux-amp, lux-volt and spectral characteristics.
1. Introduction crystals. Until now, there is no consensus regarding
Currently, there are quite a few works devoted to the physical nature of formation and the mechanism
a comprehensive study of the effect of generation of of this effect [8-13]. Undoubtedly, the APV effect
an abnormally large photo-voltage (APV, anomalous is associated with the presence of a large number of
photo-voltaic effect (APVE), Uapv ~ 10 2 - 10 4 V/cm) micro-photosensitive elements corresponding to the
in various semiconductor films with intrinsic and granular crystal structure of thin films. Nevertheless,
impurity absorption of light (see, for example, [1- the exact mechanisms of the appearance of photo-
3]). As shown by numerous experiments [4-7], the EMF in each of the micro-photocells, as well as the
APVE is observed exclusively in obliquely depos- methods or methods of their summation, are not yet
ited polycrystalline thin (d — 1 ^m) films grown on completely clear. In any case, according to some au-
dielectric substrates using a special technology in thors (e.g., [8; 9]), the physical mechanisms leading
vacuum (with a residual vapor pressure of P ~ 10-1 - to the appearance ofthe APV effect in semiconductor
- 10-2Pa)atmoderate temperatures(T~250-350K).). films, supposedly with a periodic p-n-p - ... structure
This effect is practically not observed in relatively thick (which causes fair objections from many experts), ap-
(d 3 > lD, where lDis the Debye screening length) semi- parently, are associated with incomplete compensa-
conducting conductive and amorphous films or single tion of photo-voltage in p-n- and n-p- junctions, which
is caused by a special technology of oblique deposition offilms on a dielectric substrate. Incomplete compensation of the photo-voltage in the p-n-p- cell can arise, for example, due to asymmetric illumination, or differences in the dark saturation currents ofp-n- and n-p- junctions. Analysis of literature data shows that photovoltaic films have a complex internal structural structure, obviously, they consist ofvarious asymmetric micro-potential barriers of different nature [1013]. Therefore, it is reasonable to associate the nature and mechanism of the APVE with the technological nature of formation and the photovoltaic property of micro-potential barriers at the boundaries of crystal grains responsible for the generation ofAPV.
The aim of this work is to obtain new information on the mechanism of APV generation through a comprehensive study of the effect of technological conditions for obtaining polycrystalline photovoltaic films on their electro physical, photoelectric, spectral and tens electric properties. Below we will consider the fundamental issues of improving the technology for obtaining thin films with the APV property from CdTe, CdSe, Cd S. The results of an experimental study of the volt-ampere, lux-ampere, lux-volt and deformation characteristics of the films produced are presented. To identify the type of barriers responsible for the APV effect, a new method is proposed here, in which photosensitive films are subjected to elastic mechanical deformations under illumination. At the same time, an analysis of the experimental results showed that internal micro-heterojunctions at grain boundaries with different phase compositions cause a relatively high tens sensitivity of the fabricated photovoltaic films.
2. Technology
2.1. Method of obtainingfilm structures
The analysis of the known technological methods for obtaining film photovoltaic cells showed [1-7, 13, 14] that the authors obtained polycrystalline films, mainly by the method of open vacuum deposition with an oblique incidence of a molecular beam on pre-heated dielectric substrates. At the same time,
some features of evaporation and condensation of materials are not sufficiently taken into account. As a result, the "optimal" conditions for obtaining film elements, established by various authors, differ significantly from each other. Based on the analysis of literature data on the method of obtaining photovoltaic films and taking into account the peculiarities of the processes of evaporation and condensation, we have improved the known methods [1, 13; 14] and developed a method ofvacuum co-deposition of the evaporated starting material in certain portions, i.e. method of discrete thermal evaporation in vacuum.
To obtain the films, we used fine powders of CdTe, CdSe, CdS with the "Extra pure" stamp, as well as mechanical mixtures of individual components of the above binary compounds. Naturally, the structure and properties of film photovoltaic cells are significantly influenced by the composition of the vapor phase, which in turn depends on the composition of the initial sample, the characteristics and conditions of its evaporation. Therefore, taking into account these technological factors, we have obtained film elements from the following material components: 1) Samples of CdTe, CdSe and CdS mechanical mixtures of stoichiometric composition; 2) Defined portions of the evaporated material; 3) Mechanical mixtures of CdTe and CdS powders with an excess of Cd up to 20 wt.%; 4) Mechanical mixtures of CdSe powders with an excess of Cd up to 15 wt.%.
2.2. Influence of evaporation conditions
To study the influence of the specific features and conditions of evaporation of weighed portions of CdTe, CdSe, CdS and an excess of cadmium on the photovoltaic properties of film elements, layers were obtained from various portions of the evaporated material. A controlled amount of the sample material evaporated from the weighed portions of cadmium telluride and excess cadmium. The photovoltage value was measured in the obtained films. Studies have shown that with an increase in the first portion from 5% to 30%, the photovoltage generated under normal conditions by film elements under
illumination grows to 2500 V per 1 cm of the length of the film obtained from cadmium telluride and 1200 V from cadmium sulfide, and then (above 30%) are falling slowly. With an increase in the amount of non-evaporated part (with an increase in the third portion) from 0 to 10%, the maximum APV value increases, and then (above 10%) begins to fall. The highest APV value was obtained under the following conditions: I portion 30%, II portion 60%, III portion 10%. The lowest photovoltage was generated by film elements obtained from all the starting material. The highest photovoltage ~ 5 kV was generated by CdSe films obtained from portions I (25% of the starting material). The smallest optimal masses of the starting material have been determined. They were: CdTe-40 mg, CdSe and CdS-50 mg. This effect of a portion of the starting material on the value of the generated photovoltage is due to the fact that the molecular composition of the sample changes during evaporation and this causes a change in the phase composition and structure of the resulting layers.
2.3. Influence of the composition of the starting material
To study the effect of the composition of the starting material on the electrophysical and photoelectric properties of film elements, photovoltaic layers were obtained from mixtures with different contents of individual components. The results showed that the APN value almost linearly depends on the relative mass of the excess of components over stoi-chiometry up to ~ 10% for cadmium telluride and ~ 5% for cadmium selenide. With a further increase in the excess of components, the value of the generated photovoltage decreases. A change in the composition of the evaporated material and the temperature of its evaporation significantly affects the conditions for condensation of the vapor phase and causes a change in the composition and structure of film elements. From the results obtained, it can be concluded that with a deviation from stoichiometry by 10 wt.% for telluride and 5 wt.% for cadmium selenide, apparently favorable conditions are created for the forma-
tion of photovoltaic active elements in films in the form of micro-heterojunctions of the p-n, p-p, n-n type, for example, Te -CdTe , Se -CdSe , S -CdS .
/ ' -L'p pp nn n
These structures can form at crystal grain boundaries or different heterophases.
2.4. Influence of temperature and substrate material
It is known [13-15] that the properties of film elements obtained on preheated substrates depend on the substrate temperature. To study the effect of the substrate temperature on the properties of film elements, films were obtained from CdTe, CdSe, and CdS, as well as from mixtures of their components on glass substrates that were pre-cooled, not heated, and heated to 303-373 K. The results of photovoltage measurements at room temperature were shown that the APV generated by the film elements obtained on previously unheated or cooled substrates were of the same order of magnitude, and with an increase in the substrate temperature, the photovoltage decreases. According to the polarity of the photovoltage, all the film elements were of the same type. Apparently, the condensation of a vapor flow onto cooled and unheated substrates creates more favorable conditions for the formation of a finely dispersed film with a large number of micro-heterojunctions. Obviously, with an increase in the substrate temperature, the composition and structure (part of the material re-evaporates) of the condensate changes, the sizes of crystallites increase, and the number of micro-het-erojunctions decreases. It was found that the highest photovoltage is generated by CdTe, CdSe film elements obtained on previously unheated or cooled substrates. Films grown on quartz, mica dielectric substrates generated an APN of no more than 103 V/cm at room temperature.
3. Research methodology
In order to reveal the physical nature of anomalous photoelectric effects in polycrystalline layers of CdTe, CdSe, and CdS, we studied the influence of external mechanical stresses on their electrical and photoelectric properties. The investigated film
elements were obtained on a plexiglass substrate by the method of open vacuum deposition of certain portions of the evaporated material. In this case, the influence of the preparation conditions and the geometric dimensions of the films on the possibility of the formation of various structural defects were taken into account. To study the effect of mechanical stresses on the properties of photovoltaic cells, a conventional deformation device was assembled from a steel cantilever beam, with the help of which the film samples glued to it were stepwise or smoothly subjected to deformation of unilateral tension or compression by bending the substrate. The magnitude of the relative deformation e was calculated using the well-known expression: s = 3abAx / £3, where a - is the distance from the neutral axis of the cantilever beam to the film, b- is the distance from the point of application of the force to the middle of the film sample, Ax- is the deflection of the free end of the plate at the point of application of the force, £ -is the length of the plate between the reference point and the point of application of the force. The deformation values varied in the range from e = + 2.10 3 arb. units, up to e = - 2.10 3 arb. units, which made it possible to repeatedly deform the same film without destroying it. Below, we will consider the effect of mechanical deformation on the electrophysical, photoelectric, and spectral properties of the prepared CdTe, CdSe, and CdS films.
4. Results and discussion of experiments
4.1. Current-voltage characteristics at deformation
It is known [1-3, 13, 14] that the current-voltage characteristics of pho tovoltaic films made of cadmium chalcogenides are linear in the dark and at illumination values within L = 0 - 3.5.10 5 lux up to an electric field strength of ~10 3 V/cm and in the temperature range T = 150-300 K. This can be explained by the fact that the number of micro-heterojunctions reaches N ~ 10 5 per 1 cm and at electric fields ~ 10 3 V/cm on each of them a very low voltage U. ~
~ 0.01 Vdrops, at which they naturally have a linear I - V characteristic.
The electrical properties of CdTe, CdSe film elements with a thickness of d ~ 1 ^m and an area of 5 x x 20 mm 2, obtained according to optimal technological conditions, were studied; their I - V characteristics were recorded in the dark and at an illumination of L = 5.10 4 Ix in the range of electric field strengths from -3 kV/cm to + 3 kV/cm. It turned out that the I - V characteristics of these films are also linear in the dark and under illumination, but their dark resistance is one order of magnitude higher than the resistances of similar films obtained by known methods [1; 2; 13; 14], and is R ^ 10 12 - 10 13 Om. This is apparently due to the fact that a larger number of active micro-photocells (micro-heterojunctions) are formed in photovoltaic films prepared by the method of fractional evaporation, and the number of inactive (shunt layers) decreases, and a strong compensation of donor and acceptor impurities in crystal grains.
Figure 1, a shows the I - V characteristics of the film elements CdTe, CdS in the dark in the absence and superposition of external elastic mechanical deformation. It can be seen from the characteristics that the linear character of the I - V characteristic does not change upon deformation. Under unilateral compression, the conductivity of the film noticeably increases, and under tension it decreases. This is due to a change in the parameters of potential barriers at the boundaries of heterophases under the action of deformation. Obviously, during compression, the height and width of the barrier decreases, and during stretching, it increases. Tensosensitivity coefficient K of film samples at con stant voltage U = const was calculated by the formula K = AI/IQe, where AI = I - I is the absolute change in the current strength, I and I0 are the current through the sample in the presence and absence of deformation, e is the relative deformation. Estimates have shown that the value of K reaches from 60 to 100 arb. units for the investigated films.
I -1.7101", A
a)
Figure 1a. I - V characteristics of CdTe (1)
and CdS (2, left scale) under compression deformations: s = 0 (o), - 2.0 • 10 -3 (x - compression), 2.0 -10 -3 (A - stretching) arb. units
The light I - V characteristics were recorded under the action of mechanical deformation. When illuminated with light in the spectral region of intrinsic absorption of film elements, the slope of the I - V characteristic changes, i.e. the films exhibit appreciable photoconductivity. Figure 1 b shows the I - V characteristic of deformed cadmium telluride film elements under illumination. It can be seen from the obtained characteristics that the value of the photocurrent increases during compression, and decreases during stretching. It is interesting to note that at a certain critical value of Ucr, the resistance of the photovoltaic film does not depend on deformation, which is difficult to explain. 1b, we determine the value U = 280 V for CdTe films.
cr
A typical deformation characteristic f(e) = = A UAPV/UAPV of the photovoltage ofCdTe, CdS, and CdSe films is shown in (Fig. 2), from which it can be seen that during compression the value of UAPV decreases, and during tension it grows linearly and symmetrically along e. It can also be seen from the figure
b)
Figure1 b. I - V characteristics of CdTe under illumination L = 2.10 4 • Ix: o - in the absence of deformation, A - at • e = - 2.0 -10 -3 (compression), x - at • e = 2.0 -10 -3 (stretching) arb.units
that the tensosensitivity for U is the highest for CdS films: K ~ 102.
Figure 2. Deformation characteristics of CdTe (a), CdS (b) and CdSe (c) according to UAPV at room temperature at an incandescent lamp illumination L= 2.104 • Ix
Changes in the photocurrent and photovoltage upon deformation are obviously associated with a
change in the parameters of the micro-potential barriers of the intracrystalline structures of the films, created primarily by micro-heterojunctions at the interfaces of crystalline grain boundaries.
It is known that upon evaporation of CdTe, CdS and CdSe, the molecules of these materials partially decompose due to thermal dissociation. As a result, the composition of the obtained films can differ significantly from the degree of stoichiometry of the starting material. Violation of stoichiometry affects the phase composition, structure-sensitive properties, the nature and concentration of various defects and, consequently, the type of conductivity of the films, the maximum value of UAPV and its polarity.
We studied the effect of the composition of the sample on the tensoelectric properties of CdTe, CdS, and CdSe photovoltaic films. The results of the study showed that when the stoichiometry of the mixture is violated towards an excess of cadmium up to ACd = =10 wt.%, The value of the tensosensitivity coefficient K of films from CdTe and CdS increases, and with an increase in the excess of cadmium above 10 wt.% - decreases slowly. It turned out that such a
character of the K dependence for CdSe films occurs at a value of DCd = 5 wt%. Apparently, in CdTe, CdS with an excess of Cd up to 10 wt.% and for CdSe with an excess of Cd up to 5 wt.%, favorable conditions are created for the formation of photo-tenso-active defects in the film, and with an increase in Cd above the indicated values, layers of shunting-conducting phases can form in photovoltaic films.
4.2. Lux-volt and lux-ampere characteristics
The most important characteristic of the APVE is the dependence of the photovoltage UAPV on the intensity of the exciting light. The study of L-V characteristic during deformation allows one to obtain the necessary information about the nature and mechanism of this effect. For this purpose, we investigated the effect of deformation on the L-V characteristic of photovoltaic films from CdTe and Cd S. The obtained characteristics are shown in (Fig. 3 a). It can be seen from the figure that the L-V characteristics of telluride and cadmium sulfide qualitatively coincide and consist of two characteristic sections within the range of the excitation light illumination variation from 0 to 5.10 4 • Ix.
a)
Figure 3 a. Dependence of photovoltage on light intensity for CdTe (1) and CdS (2) at a value of relative deformation: e = 0 (*), 2.10-3 (A-stretching), -2.10-3 (x-compression) arb. units
b)
Figure 3 b. Lux-ampere characteristic of CdTe (1-right scale) and CdS (2) at:- s = 0(o), 2.10 -3 (x-compression), 2.10 -3 (A-stretching) arb.units
First, in the interval L = (0-104) • Ix, a rapid increase in the photovoltage UAPV is observed, and a further increase in the light intensity to a value of 5.104 • Ix leads to an increase in the photovoltage by only 20%, and then the process of generation of UAPV films reaches saturation.The figure also shows that the L-V characteristics of CdTe and CdS during deformation does not qualitatively change, but only shifts along the ordinate: under compression downward, and under tension - upward, which is unconditionally related to the corresponding changes in the height of the micro-potential barriers.
We also studied the lux-ampere characteristics of CdTe and CdS films at room temperature in the range of illumination values from 0 to 5.104 Ix with and without deformation. The corresponding L-A characteristics are shown in (Fig. 3 b). It can be seen that in the range of illumination values from 0 to 5.104 • Ix, the characteristic is linear both in the presence and in the absence of deformation. Under the action of deformation, the qualitative appearance of the L-A characteristic does not change, but only shifts along the ordinate downward under tension, and upward under compression, i.e. the photocurrent increases during compression, and decreases during stretching. We also associate these changes with a change in the parameters of asymmetric micro-potential barriers [2; 3; 7; 10; 13] upon deformation ofphotovolta-ic films of cadmium chalcogenides, which leads to a significant photo- and tensosensitivity of these films.
4.3. Spectral characteristics
It is known [1-4, 13; 14] that the photovoltage UAPV and the corresponding short-circuit current in conventional CdTe photovoltaic films of stoichiometric composition or doped, significantly depends on the wavelength of the incident light. Studies of the UAPV (l) spectra ofCdTe: Ag films in the 563-1240 nm region have shown that a smooth spectrum is observed with a maximum near the intrinsic absorption edge, and depending on the wavelength of the exciting light, not only the value, but also the polarity of the photovoltage can change [4, 13]. The influence
of mechanical stresses on the spectral characteristics of photovoltaic films from cadmium chalcogenides has been little studied.
arhmkt.
L
30-
Ol-1-1-1-1-1-1— -
4iS 510 J4S 5S2 634 677 A. Jim
Figure 4. Effect of mechanical deformation on the spectral characteristics of CdTe. s =0; s= 2,0.10 -3 (stretching) and s = -2,0.10 -3 (compression) arb. units for lines with dots •, ■ and respectively
The spectral characteristics of U (1) were recorded for CdTe, CdSe films obtained by the method of partial evaporation with and without mechanical deformation. Typical curves U (\) for these films are qualitatively the same; therefore, (Fig. 4) shows fragments of the spectra in the short-wavelength region Eg only for the CdTe film. It can be seen from the figure that a characteristic spectral line M of photovoltage is observed, located approximately in the range (510-580) nm with a maximum at a wavelength of 1M = 548 nm (hwM = 2.26 eV), which was not previously detected in CdTe films and CdTe: Ag obtained by other authors using known technologies. Apparently, the appearance of the M line on the short-wavelength wings of the U (X) spectrum is associated with the formation of photovoltaic films with strongly developed asymmetric micropotential barriers with a slightly different structure by the method of partial evaporation in vacuum. We believe that the M line is certainly due to the generation of photovoltage by a micro-heterojunction structure of
the CdTe-CdO-CdTe type. Note that the band gap widths of CdTe and CdO at room temperature are 1.46 eV and 2.37 eV, and the latter is comparable to the value hw. = 2.26 eV.
Under the action of mechanical stress, the qualitative shape, the frequency of the maximum AM, and the half-width ofthe spectral distribution ofthe photovoltage curve for photovoltaic films of CdTe, CdSe do not change significantly, only shifts downward upon compression and upward upon stretching (Fig. 4). The polarity of the photovoltage in the investigated short-wavelength region of the spectrum does not change in any sample, both with and without deformation. It was found that the spectral maximum M of the photovoltage of the samples obtained by sputtering cadmium telluride with an excess of cadmium, with an increase in ACd < 10%, shifts to the shorter wavelength side of the spectrum, and at ACd > 10%, with an increase in the Cd concentration, it broadens and disappears in the same way as the AFVE.
4.4. Temperature dependences of photovoltage and photocurrent upon deformation
When studying the photovoltaic effect, important characteristics are the dependences of the photocurrent and photovoltage on temperature. In the deformed and undeformed CdTe, CdSe
films, the dependences of the photovoltage and short-circuit photocurrent on temperature were measured. Figure 5 a shows the temperature dependence of the photovoltage UAPV in the presence and absence of deformation. It can be seen from the figure that with decreasing temperature, the generated photovoltage for CdTe films without deformation first increases almost linearly from 2 • 10 3 V at T = 100 °C to 7 • 10 3 V at T = -50 °C, then sharply increases to a maximum (~ 15 fcV) at T = -160 ° C, and then decreases (a). For CdSe (Fig. 5 b), there is first a smooth sublinear growth from 3 • 10 3 V at T = 100 °C to 6 • 10 3 V at T = =40 °C, saturation to T = -40 °C occurs, then a repeated superlinear growth to 7.5 • 10 3 B at T = = -100 °C, and then a smooth fall. Such features of the temperature dependence of UAPV (T) indicate that the generation of photovoltage is significantly influenced by various mechanisms and scattering regions. Thus, for a CdTe film at T 3> -50 °C, carrier scattering in the quasineutral region of crystalline grains prevails, while at T < -50 °C the influences of near-surface regions of space charges and recharge of surface states dominate [2, 13]. Note that CdSe films have three characteristic temperature ranges: T 3 > 40 °C, T = (40 - -40) °C, T < -40 °C.
a)
b)
Figure 5. Temperature dependences of the photovoltage of CdTe (a) and CdSe (b) films under mechani-cal deformation: s = 0(o), 2.10 -3 (A-stretching), and 2.10 -3 (x-compression), arb. units L = 2.0-10 4 • Ix
During deformation, the form of the temperature characteristic UAPV(T) of both films hardly changes qualitatively, but only shifts along the ordinate upward under tension, and under compression - downward, and symmetrically with respect to e. Note that photovoltaic films become more sensitive to mechanical deformation with decreasing temperature. This is explained by an increase in the resistance of the films at low temperatures due to a decrease in the effective concentrations and mobilities ofcharge carriers, caused by a change in the height and width of micro-potential barriers, as well as in the charge states of defects.
Figure 6. Temperature dependences of the short-circuit photocurrent on a semi logarithmic scale: for cadmium telluride (1) and cadmium sulfide (2) at s = 0 (o), stretching (A), compression (x). L = 2.10 4 • lx
Figure 6 shows the temperature dependence of the short-circuit photocurrent I in the presence and absence of deformation for CdTe, CdS photovoltaic films. From the obtained characteristics I (T), it can be seen that the strength of the photocurrent generated by the anomalous photo-emf of these films during a short circuit depends very little on temperature in the range of ~ 120-300 K, which means that the mechanism of current passage in the samples under study is practically of a tunnel nature [1, 13; 14]. Under the
action of mechanical deformation, the character of the dependence of the photocurrent on temperature does not change. When the film is compressed, the photocurrent increases, and when the film is stretched, it decreases. Apparently, this is also associated with a change in the parameters of micro-potential barriers at the boundaries of crystal grains upon deformation.
5. Conclusion. Based on the above results of studying the photo-tensoelectric properties of CdTe, CdSe, and CdS photovoltaic films prepared by batch evaporation in a vacuum, the following conclusions can be drawn:
1. Photovoltaic properties of thin (d ~ 1 y,m) polycrystalline films of CdTe, CdS and CdSe grown on an organic glass substrate with a temperature T = 250-350 K in vacuum with a residual gas pressure of P ~ 10-1-10-2 Pa by batch Evaporation from the starting material in 40-50 mg exhibits a number of features that differ from similar films investigated by other authors.
2. The photovoltaic films of CdTe, CdS and CdSe obtained by the method proposed in this work have linear volt-ampere, lux-ampere, deformation characteristics, which makes it possible to manufacture new film photosensitive devices on their basis.
3. Analysis of the spectral characteristics and temperature dependences of the photovoltage and short-circuit photocurrent upon deformation showed that embedded chains of micro-photocells can be responsible for the high-voltage effect in the studied films of CdTe, CdSe, and CdS, the potential barriers in which are obviously of the nature of micro-heterojunction structures of the type CdTe-CdO-CdTe.
4. Among thin polycrystalline films of CdTe, CdSe and CdS, with the APV property, the most promising are CdTe films for the manufacture of photodetectors sensitive to deformation in the near infrared region of the radiation spectrum.
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