rus
Developed technology for producing photovoltaic components on the basis of hydrogenised amorphous and nanocrystalic silicon-hydrogen compound thin-films leads to an 11.2 percent increase of solar energy to electricity conversion.
We have studied various film parameters of the amorphous and nanocrystalline alloy of silicon–carbon (a-nc-Si1–xCx:H (x = 0–1)) doped with phosphorus (PH3) and boron (B2H6). We have studied also the properties of the films obtained on various substrates of quartz, glass and silicon coated with Fe, Al, Pd, Ni, Ti, Ag. The morphology of the obtained nanotubes which length is 1–4 microns has been studied by means of translucent electron microscopy (TEM). Also, the structural properties of the films were studied by means of infrared spectroscopy and X-ray diffraction. We found out that changing of the synthesis parameters (inner diameter of the electrode nozzle, the flow rate of methane and substrate temperature) in the wide range; allow to obtain such carbon microstructures as diamond particles, carbon nanowires, different types of carbon nanotubes and cone-shaped micro particles of Silicon. There were created the tandem-type solar cells with area of S=1,2 cm2, with structures /OIO p+-a-SiC:H/i-a-Si:H/n+-nc-Si:H/p+-nc-Si:H/i-nc-Si:H/n+-nc-Si:H/Ag/Al and having efficiency coefficient of 11.2%.
Introduction
The films of hydrogenated amorphous and nanocrystalline alloy of silicon-carbon (a-nc-Si1–хСх:Н (x=0–1)) as compared with the films of amorphous silicon and germanium have larger band gap and better optoelectronic properties in the visible spectrum, and also are more stable thermodynamically and radiation-resistant [1–2]. These properties allow using them in the number of nano-and microelectronics areas, as well as for creation of solar cells [3–5].
Experiments show that having changed the process parameters and conditions, the amorphous films are deposited at different structural phases. Furthermore, depending on the deposition conditions, both the film structures themselves as well as their optoelectronic properties strongly depend on the deposition rate, substrate temperature, substrate type and geometry of the metal coatings.
In this research there have been also studied some parameters of thin films of amorphous and nanocrystalline alloy of silicon–carbon a-nc-(Si1-xCx:H, (x=0–1)). There has been also studied the process of this film doping with phosphorus (PH3) and boron (B2H6). The morphology of the received nanotubes, the length of which depending on the deposition conditions, is 1–4 microns has been studied by means of translucent electron microscopy method (TEM). Structural properties of the films were analyzed by infrared spectroscopy and X-ray diffraction (XRD) method. It was noted that depending on the conditions of film formation, there occurs such change of parameters, which is characteristic for nanocrystalline thin films.
Experimental part and results
In this research we studied the films of amorphous and nanocrystalline alloy of silicon–carbon a-nc-(Si1-xCx:H (x=0–1)) doped with phosphorus (PH3) and boron (B2H6) on various substrates of quartz, glass and silicon coated with Fe, Al, Pd, Ni, Ti, Ag.
Since Al and Ag have small diffusion barriers and poor wetting of surface with single-walled carbon nanotubes (SWCNTs), they tend to aggregation and formation of large clusters. On the other hand, the binding energy between Fe and SWCNT is large, but because of the large corrosion energy and poor wetting Fe can form isolated clusters. It should be noted that the SWCNTs are obtained using gas mixture of CH4 and Ar. Microplasma HF jet is generated under atmospheric pressure by means of single-tube electrode and is directed onto Si substrate coated with Fe film.
Having changed the synthesis parameters within a wide range (the inner diameter of the electrode nozzle, the flow rate of methane and substrate temperature), there were obtained such carbon microstructures as diamond particles, carbon nanowires, carbon nanotubes, cone-shaped Si microparticles.
Also it has been studied the influence of growth conditions, the flow rate of methane and the type of substrate on the distribution of structures and properties of two-stage carbon nanotubes (TCN). At flow rate of 600 sm3/min there are formed TCN mainly with semiconducting properties. At higher flow rate (700 cm3/min) there is formed a mixture of single- and double-walled nanotubes, most part of which are semi conductive. At lower flow rates
(300–500 cm3/min) metallic multiwall carbon nanotubes are preferably formed. The length of the obtained nanotubes is 1–4 μm.
It was found out that in obtained from gas mixtures SiH4+H2PH3 or SiH4+H2+B2H6 on quartz or Si substrate films in case of PH3 concentration increase, the average grain size (d) and the proportion of crystal grain of volume (Vc) is decreased. When doped with boron, in case of B2H6 concentration increase, the d value is changed, and Vc value is decreased.
In nanocrystalline SiC films with thickness of 0,5–1 mm those obtained from plasma 80%H2+20%Ar on quartz substrate, in case of substrate temperature increase from 200 to 600°C, an increase of SiC nanocrystals density has been observed, which average size was ~12–24 nm [2]. These results were also verified by method of infrared absorption spectra. Based on these results, it can be stated that the monohydride Si-H and dihydride Si-H2, act integrally as space barrier in the volumes of films and they change the growth of nanocrystals (Fig. 1) [6].
Let’s use the representation of Bragg–Wolf in order to analyze the pattern formed on Debye powder diagram. To obtain a certain order of reflection of some series of densities the crystal should be oriented in such a way that these planes could form with the incident beam the θ angle, which satisfies the equation:
2d sin θ = nλ. (1)
Knowing the striker angle and the wavelength it’s possible to determine the diameter of nanocrystals from the equation (1). Using formula (1) for each line, it’s possible to determine the ratio of the inter-complex distance of the reflective series of grids to the reflection order:
. (2)
Value for all lines is the final result obtained directly from the diagram.
Amorphous triple components alloys а-Si1-хСх:Н were obtained from gas mixtures of SiH4, CH3, H2. Hydrogen was added in the following proportions:
for n-type of films,
for p-type of films.
а-Si1-хСх:Н and nc-Si1-хСх:Н films are obtained in case of gas mixture deposition [SiH4+СH4]. It is assumed that in the films, the relative content of carbon and silicon should correspond to the ratio: . Doped layer was manufactured as follows: and when x=0–1. It should be noted that the obtained films also differ by morphology and structure.
There was performed X-ray diffraction analysis of the films, as well as there were measured infrared absorption spectra by means of spectrometer IKS-29. Using the half-width analysis of X-ray lines (by diffraction peak of reflection from the planes <111>, <220> and <311>) there was calculated the average crystallite size (δ), which was 12 nm for films with area of 95 nm2 and films doped with phosphorus by means of high frequency discharge capacity Wrf = 250 W and the substrate temperature Ta = 600˚C.
Distance of X-rays from planes <111> of crystalline silicon, angular peak positions 2θ, their height Jp and half-width Δ(2θ), for doped and undoped films are different (Fig. 2). The figure shows the dependence of Jp, 2θ, and Δ(2θ) for maximum reflection of X-rays with the planes <220> for undoped nc–Si1-xCx:H films. Together with the annealing temperature increase in the range of 300–500°C, Jp value increases monotonically, while the half-width Δ(2θ), which determines the size of the nanocrystals, up to the annealing temperature Ta = 500°C, remains constant. This means that together with the annealing temperature increase in the specified range, the number of nanocrystals in the film increases, and the average size remains constant.
Together with further increase of the annealing temperature T > 500°C, Jp rate increases sharply with simultaneous decrease of Δ(2θ), indicating the increase of nanocrystals sizes in the film. As shown above, the obtained results for nc–Si1-xCx: H films of which the hydrogen was completely withdrawn, within this temperature range, were also tested by means of infrared absorption spectra method. After temperature increase, after heat treatment at 700°C there takes place the hydrogen effusion, and its concentration in the film decreases (Fig. 1c).
However, in case of higher flow rate about 700 cm3/min, while increasing of the long-lived radicals concentration, the adatom mobility on the surface of the film increases, but the content of dihydrides (SiH2) and hydrogen decreases, in comparison with the films obtained at the same temperature in a high frequency system. The reduction of dihydrides content, as shown in figure [7], leads to the decrease of heterogeneity of the films microstructure, the quantity of nonradioactive recombination centers, the density of localized states in the mobility gap. In case of reactive magnetron sputtering the minimum content of dihydrides is 20% in relation to monohydrates (γ = SiH2/SiH = 0,2) [ 7].
In case of different discharge densities the relative content of carbon in the films is greater than the relative content of carbon in the gas mixture. This means that the reactive content of CH4 in comparison with SiH4, is more effective. However, in case of capacity increase of high-frequency discharges, these values are equalized. The concentration of carbon and hydrogen in a–Si1-xCx:H films depends on the deposition conditions, even in the case of constant content of SiH4, CH4, H2 in the initial gas mixture.
Fig. 3 shows the change of deposition rate depending on the hydrogen pressure. As can be seen from the figure, the dependence vanishes at 5∙10-4 Torr, and deposition rate increases with the increase of gas pressure. Deposition rate of 0,5 Å/s at pressure of about 10-3 Torr , was compared with data received by means of high-frequency deposition method.
Fig. 4 shows the dependence of high-frequency discharge capacity in the areas of high pressures. There was also varied widely the gas penetration in the areas of high pressure, since the value of the initial gas pressure was constant (5∙10-4 Torr). The gas pressure is regulated by a valve. With increasing of high-frequency discharge capacity, the gas pressure is reduced and becomes constant in the area of 100-250 W. This phenomenon is not observed in argon gas discharge, and is similar to the result of dissolution and reaction of gas mixture due to the high-frequency discharge.
Fig. 5 shows the relation between high-frequency discharge capacity and the deposition rate. When high-frequency discharge capacity is below 100 W, the deposition rate increases and the penetration of gas does not depend on high-frequency capacity. If high-frequency capacity is above 100 W, the deposition rate increases, but it increases sharply at high values of gas penetration. This means that the part of gas is activated at higher values of high-frequency capacity, and only activated part enters into reaction. As a result of this research, we can conclude that the deposition is controlled by high-frequency capacity at constant temperatures and constant cathode potential.
Fig. 6 shows the temperature dependence of the activation energy of electrical conductivity (ΔE) for a–Si1-xCx:H films. These data suggest that in the films obtained in high-frequency discharge, the activation energy is greater than in the case of low-frequency discharge. In the first case, the dependence of the activation energy on the band gap Ea = f(Eg opt) is approximated by the equation: Ea = 0,5Eg opt. As it can be seen from the data, the temperature range of electrical conductivity has not hopping but activation character. In this case, the concentration of paramagnetic centers has a low value and depending on the hydrogen pressure (PH2=0–4Torr) ranges within 1018–1016 cm-3eV-1. At rather low temperatures (T≤80 K), one could expect the hopping mechanism of conductivity as it is characteristic for all amorphous materials, including a-Si1-xCx:H films [8].
Figures 7, 8 show the infrared absorption spectra of amorphous a-Si1-xCx:H films obtained by magnetron sputtering method in high frequency system. As seen in the infrared spectrum area there were observed three major absorption areas: broad peak of the area at 760 cm–1, which bears the resemblance to a shoulder by shape; peaks at 1000 cm-1, respectively refer to the stretch Si-C vibrations and sweep vibrations of CH4 group attached to silicon atom and stretching mode of C–H bond is in 2800–3000 cm-1 area [6, 9]. Infra red absorption spectra were calculated using the formula:
(3)
where R1, R2, R3 – are the reflection coefficients, in case of interaction of air-film-substrate and substrate-air, respectively. For highly absorbing areas R1=R2=R3=R, which are defined by band gap E0. To determine E0 the Tauts model [8–10] is used.
The elements were exposed to light under the stream of photons N = 1017–1818 m-2c-1 in short-circuit mode. Carrier collection efficiency (EC) Y(λ) at different wavelengths was determined as the ratio of the number of incident photons to the number of free carriers connected by the external circuit [11, 12].
Y(λ) = Jp(λ)/eN(λ),
where Jp(λ) – photocurrent density which has 10 mA/cm2 value, N(λ) – the stream of incident photons, e – the charge of free carriers.
The film thickness d at the known refraction coefficient is defined under the conditions of interference phenomena. Silicon substrates are used to avoid the complicated calculations and interference phenomena. When using the silicon substrate the absorption coefficient outside the area of the fundamental absorption edge is determined by the formula using films [6, 10]
for transmission coefficient (T):
(4)
where, T0 is transmission coefficient of silicon substrate, T = T0 = 0.54; nsubstrate = nfilm = 3,42 (α = 0).
The equation (4) is valid with an accuracy of ±10% at αd ≥ 0,1. The reflection coefficient R1, R2, R3 is theoretically determined by the ratio for different types of substrate:
(5)
n and n1 (=1,5) respectively show the absorption of the film and the substrate. Here , this relation is valid for weakly absorbing light areas (Fig. 9).
In the most of amorphous materials, including a-Si:H films and its alloys, at photon energies below E0, in all studied samples the absorption varies exponentially with energy and is described by formula:
(6)
where const = 4π/nc; n – refraction index (determined by the position of the interference peaks in the transmission and reflection spectra, c-light speed, E1 – order energy E0. β does not depend on the temperature at 300 K and is defined by formula β ≈ 0,8/kT. The optical absorption coefficients α were determined by the following formulas from equations (3):
или (7)
Equations (7) are the working formulas for determining of the absorption coefficient of the films on various substrates and at different wavelengths.
These results show that nanocrystalline phase in amorphous grid is 70% of the total film in as-deposited undoped films. For nc–Si1-xCx:H films doped with phosphorus (PH3) the total volume of the crystallites in the film is 50%, when doped with boron it is 30%. Similar results are also observed for the planes <220> and <311> of silicon crystalline grids.
Creation of tandem type solar cells
The amorphous a-Si:H films and their alloys are often used for manufacturing of electronic devices [1]. Such alloys are characterized by two phases: amorphous and nanocrystalline, the most interesting of which are located on the border of crystallinity, which are considered to be the most stable for the creation of electronic devices.
Tandem cell with structure of glass /ОИО/р+-а-SiC:H/i-a-Si:H/n+-nc-Si:H/p+-nc-Si:H/i-nc-Si:H/n+-nc-Si:H/Ag/Al are obtained in the following way: a-SiC:H p-type layer, which acts as a window, is doped with boron [B2H6/(SiH4 + CH4) = 0,1%] and with thickness of 300 Å and is deposited on the transparent conductive film of indium tin oxides (ITO) which was sprayed before onto the glass substrate. Then, the undoped i-layer a-SiC:H, with thickness d = 5000 Å was deposited with the following applying of nc-Si n-type layer on it, which was doped with phosphorus (RN3/SiH4 = 0.5%) with thickness of 400 Å. Accordingly, the subsequent p+-nc-Si:H, i-nc-Si:H, n+-nc-Si:H layer was deposited by the above mentioned method. The contact of Ag/Al alloys (Fig. 10) was deposited the last one. In case of use of too thin "window" for outcoupling, the voltage value of the open circuit (V∞) increases and in case of too large thickness of the "window" the current density of short circuit (Jsc) increases. Therefore the optimum thickness of the "window" was selected. These are the biggest V∞ and Jsc values which are obtained when the thickness of the "window" is 300 Å. This, in its turn, describes the efficiency coefficient of the cells. Optical absorption coefficient (α) for i-layer in the visible area of the spectrum was about 8·104 cm-1 and was described by the ratio:
, (9)
where the coefficient B = 530 eV-1cm-1 was determined by extrapolation of linear dependence of the photon energy hν; E0 = 1,85 eV – band gap.
For photovoltaic effect, the cells were exposed to light source with an intensity of about 100 mW/cm2 within the range of wavelength about 300–900 nm. The cell area was 1.2 cm2 and had the following characteristics: V∞ = 0,882V, Jsc = 18,0 mA/cm2, load factor ξ = 0,709, efficiency coefficient η = 11,2% (Fig. 10).
For formation of tandem type solar cells on a-Si:H and nc-Si:H films there is used the 20-minutes exhaust cycle before the deposition, i-layer and p-layer cover thoroughly (in the form of B-P complexes) the most of P-atoms on the walls of chamber that leads to high quality of cells on a-Si:H, nc-Si:H films with a slight performance degradation. Unlike the most of the manufacturing technology, this method does not require the purification of the chamber after deposition of each p-type layer.
Conclusion
The results of this study show perspectives for formation of films of amorphous and nanocrystalline silicon-carbon deposited by means of reactive magnetron sputtering method. It was shown how the distinctive features of the films structure affect the efficiency of crystallization. It was also determined that the process parameters (substrate temperature, deposition rate of the films, high frequency discharge capacity) affect the physical properties of amorphous and nanocrystalline silicon – carbon. The research results of this study show that a–Si1-xCx:H and nc–Si films are the perspective materials for creation of tandem type solar cells.
Introduction
The films of hydrogenated amorphous and nanocrystalline alloy of silicon-carbon (a-nc-Si1–хСх:Н (x=0–1)) as compared with the films of amorphous silicon and germanium have larger band gap and better optoelectronic properties in the visible spectrum, and also are more stable thermodynamically and radiation-resistant [1–2]. These properties allow using them in the number of nano-and microelectronics areas, as well as for creation of solar cells [3–5].
Experiments show that having changed the process parameters and conditions, the amorphous films are deposited at different structural phases. Furthermore, depending on the deposition conditions, both the film structures themselves as well as their optoelectronic properties strongly depend on the deposition rate, substrate temperature, substrate type and geometry of the metal coatings.
In this research there have been also studied some parameters of thin films of amorphous and nanocrystalline alloy of silicon–carbon a-nc-(Si1-xCx:H, (x=0–1)). There has been also studied the process of this film doping with phosphorus (PH3) and boron (B2H6). The morphology of the received nanotubes, the length of which depending on the deposition conditions, is 1–4 microns has been studied by means of translucent electron microscopy method (TEM). Structural properties of the films were analyzed by infrared spectroscopy and X-ray diffraction (XRD) method. It was noted that depending on the conditions of film formation, there occurs such change of parameters, which is characteristic for nanocrystalline thin films.
Experimental part and results
In this research we studied the films of amorphous and nanocrystalline alloy of silicon–carbon a-nc-(Si1-xCx:H (x=0–1)) doped with phosphorus (PH3) and boron (B2H6) on various substrates of quartz, glass and silicon coated with Fe, Al, Pd, Ni, Ti, Ag.
Since Al and Ag have small diffusion barriers and poor wetting of surface with single-walled carbon nanotubes (SWCNTs), they tend to aggregation and formation of large clusters. On the other hand, the binding energy between Fe and SWCNT is large, but because of the large corrosion energy and poor wetting Fe can form isolated clusters. It should be noted that the SWCNTs are obtained using gas mixture of CH4 and Ar. Microplasma HF jet is generated under atmospheric pressure by means of single-tube electrode and is directed onto Si substrate coated with Fe film.
Having changed the synthesis parameters within a wide range (the inner diameter of the electrode nozzle, the flow rate of methane and substrate temperature), there were obtained such carbon microstructures as diamond particles, carbon nanowires, carbon nanotubes, cone-shaped Si microparticles.
Also it has been studied the influence of growth conditions, the flow rate of methane and the type of substrate on the distribution of structures and properties of two-stage carbon nanotubes (TCN). At flow rate of 600 sm3/min there are formed TCN mainly with semiconducting properties. At higher flow rate (700 cm3/min) there is formed a mixture of single- and double-walled nanotubes, most part of which are semi conductive. At lower flow rates
(300–500 cm3/min) metallic multiwall carbon nanotubes are preferably formed. The length of the obtained nanotubes is 1–4 μm.
It was found out that in obtained from gas mixtures SiH4+H2PH3 or SiH4+H2+B2H6 on quartz or Si substrate films in case of PH3 concentration increase, the average grain size (d) and the proportion of crystal grain of volume (Vc) is decreased. When doped with boron, in case of B2H6 concentration increase, the d value is changed, and Vc value is decreased.
In nanocrystalline SiC films with thickness of 0,5–1 mm those obtained from plasma 80%H2+20%Ar on quartz substrate, in case of substrate temperature increase from 200 to 600°C, an increase of SiC nanocrystals density has been observed, which average size was ~12–24 nm [2]. These results were also verified by method of infrared absorption spectra. Based on these results, it can be stated that the monohydride Si-H and dihydride Si-H2, act integrally as space barrier in the volumes of films and they change the growth of nanocrystals (Fig. 1) [6].
Let’s use the representation of Bragg–Wolf in order to analyze the pattern formed on Debye powder diagram. To obtain a certain order of reflection of some series of densities the crystal should be oriented in such a way that these planes could form with the incident beam the θ angle, which satisfies the equation:
2d sin θ = nλ. (1)
Knowing the striker angle and the wavelength it’s possible to determine the diameter of nanocrystals from the equation (1). Using formula (1) for each line, it’s possible to determine the ratio of the inter-complex distance of the reflective series of grids to the reflection order:
. (2)
Value for all lines is the final result obtained directly from the diagram.
Amorphous triple components alloys а-Si1-хСх:Н were obtained from gas mixtures of SiH4, CH3, H2. Hydrogen was added in the following proportions:
for n-type of films,
for p-type of films.
а-Si1-хСх:Н and nc-Si1-хСх:Н films are obtained in case of gas mixture deposition [SiH4+СH4]. It is assumed that in the films, the relative content of carbon and silicon should correspond to the ratio: . Doped layer was manufactured as follows: and when x=0–1. It should be noted that the obtained films also differ by morphology and structure.
There was performed X-ray diffraction analysis of the films, as well as there were measured infrared absorption spectra by means of spectrometer IKS-29. Using the half-width analysis of X-ray lines (by diffraction peak of reflection from the planes <111>, <220> and <311>) there was calculated the average crystallite size (δ), which was 12 nm for films with area of 95 nm2 and films doped with phosphorus by means of high frequency discharge capacity Wrf = 250 W and the substrate temperature Ta = 600˚C.
Distance of X-rays from planes <111> of crystalline silicon, angular peak positions 2θ, their height Jp and half-width Δ(2θ), for doped and undoped films are different (Fig. 2). The figure shows the dependence of Jp, 2θ, and Δ(2θ) for maximum reflection of X-rays with the planes <220> for undoped nc–Si1-xCx:H films. Together with the annealing temperature increase in the range of 300–500°C, Jp value increases monotonically, while the half-width Δ(2θ), which determines the size of the nanocrystals, up to the annealing temperature Ta = 500°C, remains constant. This means that together with the annealing temperature increase in the specified range, the number of nanocrystals in the film increases, and the average size remains constant.
Together with further increase of the annealing temperature T > 500°C, Jp rate increases sharply with simultaneous decrease of Δ(2θ), indicating the increase of nanocrystals sizes in the film. As shown above, the obtained results for nc–Si1-xCx: H films of which the hydrogen was completely withdrawn, within this temperature range, were also tested by means of infrared absorption spectra method. After temperature increase, after heat treatment at 700°C there takes place the hydrogen effusion, and its concentration in the film decreases (Fig. 1c).
However, in case of higher flow rate about 700 cm3/min, while increasing of the long-lived radicals concentration, the adatom mobility on the surface of the film increases, but the content of dihydrides (SiH2) and hydrogen decreases, in comparison with the films obtained at the same temperature in a high frequency system. The reduction of dihydrides content, as shown in figure [7], leads to the decrease of heterogeneity of the films microstructure, the quantity of nonradioactive recombination centers, the density of localized states in the mobility gap. In case of reactive magnetron sputtering the minimum content of dihydrides is 20% in relation to monohydrates (γ = SiH2/SiH = 0,2) [ 7].
In case of different discharge densities the relative content of carbon in the films is greater than the relative content of carbon in the gas mixture. This means that the reactive content of CH4 in comparison with SiH4, is more effective. However, in case of capacity increase of high-frequency discharges, these values are equalized. The concentration of carbon and hydrogen in a–Si1-xCx:H films depends on the deposition conditions, even in the case of constant content of SiH4, CH4, H2 in the initial gas mixture.
Fig. 3 shows the change of deposition rate depending on the hydrogen pressure. As can be seen from the figure, the dependence vanishes at 5∙10-4 Torr, and deposition rate increases with the increase of gas pressure. Deposition rate of 0,5 Å/s at pressure of about 10-3 Torr , was compared with data received by means of high-frequency deposition method.
Fig. 4 shows the dependence of high-frequency discharge capacity in the areas of high pressures. There was also varied widely the gas penetration in the areas of high pressure, since the value of the initial gas pressure was constant (5∙10-4 Torr). The gas pressure is regulated by a valve. With increasing of high-frequency discharge capacity, the gas pressure is reduced and becomes constant in the area of 100-250 W. This phenomenon is not observed in argon gas discharge, and is similar to the result of dissolution and reaction of gas mixture due to the high-frequency discharge.
Fig. 5 shows the relation between high-frequency discharge capacity and the deposition rate. When high-frequency discharge capacity is below 100 W, the deposition rate increases and the penetration of gas does not depend on high-frequency capacity. If high-frequency capacity is above 100 W, the deposition rate increases, but it increases sharply at high values of gas penetration. This means that the part of gas is activated at higher values of high-frequency capacity, and only activated part enters into reaction. As a result of this research, we can conclude that the deposition is controlled by high-frequency capacity at constant temperatures and constant cathode potential.
Fig. 6 shows the temperature dependence of the activation energy of electrical conductivity (ΔE) for a–Si1-xCx:H films. These data suggest that in the films obtained in high-frequency discharge, the activation energy is greater than in the case of low-frequency discharge. In the first case, the dependence of the activation energy on the band gap Ea = f(Eg opt) is approximated by the equation: Ea = 0,5Eg opt. As it can be seen from the data, the temperature range of electrical conductivity has not hopping but activation character. In this case, the concentration of paramagnetic centers has a low value and depending on the hydrogen pressure (PH2=0–4Torr) ranges within 1018–1016 cm-3eV-1. At rather low temperatures (T≤80 K), one could expect the hopping mechanism of conductivity as it is characteristic for all amorphous materials, including a-Si1-xCx:H films [8].
Figures 7, 8 show the infrared absorption spectra of amorphous a-Si1-xCx:H films obtained by magnetron sputtering method in high frequency system. As seen in the infrared spectrum area there were observed three major absorption areas: broad peak of the area at 760 cm–1, which bears the resemblance to a shoulder by shape; peaks at 1000 cm-1, respectively refer to the stretch Si-C vibrations and sweep vibrations of CH4 group attached to silicon atom and stretching mode of C–H bond is in 2800–3000 cm-1 area [6, 9]. Infra red absorption spectra were calculated using the formula:
(3)
where R1, R2, R3 – are the reflection coefficients, in case of interaction of air-film-substrate and substrate-air, respectively. For highly absorbing areas R1=R2=R3=R, which are defined by band gap E0. To determine E0 the Tauts model [8–10] is used.
The elements were exposed to light under the stream of photons N = 1017–1818 m-2c-1 in short-circuit mode. Carrier collection efficiency (EC) Y(λ) at different wavelengths was determined as the ratio of the number of incident photons to the number of free carriers connected by the external circuit [11, 12].
Y(λ) = Jp(λ)/eN(λ),
where Jp(λ) – photocurrent density which has 10 mA/cm2 value, N(λ) – the stream of incident photons, e – the charge of free carriers.
The film thickness d at the known refraction coefficient is defined under the conditions of interference phenomena. Silicon substrates are used to avoid the complicated calculations and interference phenomena. When using the silicon substrate the absorption coefficient outside the area of the fundamental absorption edge is determined by the formula using films [6, 10]
for transmission coefficient (T):
(4)
where, T0 is transmission coefficient of silicon substrate, T = T0 = 0.54; nsubstrate = nfilm = 3,42 (α = 0).
The equation (4) is valid with an accuracy of ±10% at αd ≥ 0,1. The reflection coefficient R1, R2, R3 is theoretically determined by the ratio for different types of substrate:
(5)
n and n1 (=1,5) respectively show the absorption of the film and the substrate. Here , this relation is valid for weakly absorbing light areas (Fig. 9).
In the most of amorphous materials, including a-Si:H films and its alloys, at photon energies below E0, in all studied samples the absorption varies exponentially with energy and is described by formula:
(6)
where const = 4π/nc; n – refraction index (determined by the position of the interference peaks in the transmission and reflection spectra, c-light speed, E1 – order energy E0. β does not depend on the temperature at 300 K and is defined by formula β ≈ 0,8/kT. The optical absorption coefficients α were determined by the following formulas from equations (3):
или (7)
Equations (7) are the working formulas for determining of the absorption coefficient of the films on various substrates and at different wavelengths.
These results show that nanocrystalline phase in amorphous grid is 70% of the total film in as-deposited undoped films. For nc–Si1-xCx:H films doped with phosphorus (PH3) the total volume of the crystallites in the film is 50%, when doped with boron it is 30%. Similar results are also observed for the planes <220> and <311> of silicon crystalline grids.
Creation of tandem type solar cells
The amorphous a-Si:H films and their alloys are often used for manufacturing of electronic devices [1]. Such alloys are characterized by two phases: amorphous and nanocrystalline, the most interesting of which are located on the border of crystallinity, which are considered to be the most stable for the creation of electronic devices.
Tandem cell with structure of glass /ОИО/р+-а-SiC:H/i-a-Si:H/n+-nc-Si:H/p+-nc-Si:H/i-nc-Si:H/n+-nc-Si:H/Ag/Al are obtained in the following way: a-SiC:H p-type layer, which acts as a window, is doped with boron [B2H6/(SiH4 + CH4) = 0,1%] and with thickness of 300 Å and is deposited on the transparent conductive film of indium tin oxides (ITO) which was sprayed before onto the glass substrate. Then, the undoped i-layer a-SiC:H, with thickness d = 5000 Å was deposited with the following applying of nc-Si n-type layer on it, which was doped with phosphorus (RN3/SiH4 = 0.5%) with thickness of 400 Å. Accordingly, the subsequent p+-nc-Si:H, i-nc-Si:H, n+-nc-Si:H layer was deposited by the above mentioned method. The contact of Ag/Al alloys (Fig. 10) was deposited the last one. In case of use of too thin "window" for outcoupling, the voltage value of the open circuit (V∞) increases and in case of too large thickness of the "window" the current density of short circuit (Jsc) increases. Therefore the optimum thickness of the "window" was selected. These are the biggest V∞ and Jsc values which are obtained when the thickness of the "window" is 300 Å. This, in its turn, describes the efficiency coefficient of the cells. Optical absorption coefficient (α) for i-layer in the visible area of the spectrum was about 8·104 cm-1 and was described by the ratio:
, (9)
where the coefficient B = 530 eV-1cm-1 was determined by extrapolation of linear dependence of the photon energy hν; E0 = 1,85 eV – band gap.
For photovoltaic effect, the cells were exposed to light source with an intensity of about 100 mW/cm2 within the range of wavelength about 300–900 nm. The cell area was 1.2 cm2 and had the following characteristics: V∞ = 0,882V, Jsc = 18,0 mA/cm2, load factor ξ = 0,709, efficiency coefficient η = 11,2% (Fig. 10).
For formation of tandem type solar cells on a-Si:H and nc-Si:H films there is used the 20-minutes exhaust cycle before the deposition, i-layer and p-layer cover thoroughly (in the form of B-P complexes) the most of P-atoms on the walls of chamber that leads to high quality of cells on a-Si:H, nc-Si:H films with a slight performance degradation. Unlike the most of the manufacturing technology, this method does not require the purification of the chamber after deposition of each p-type layer.
Conclusion
The results of this study show perspectives for formation of films of amorphous and nanocrystalline silicon-carbon deposited by means of reactive magnetron sputtering method. It was shown how the distinctive features of the films structure affect the efficiency of crystallization. It was also determined that the process parameters (substrate temperature, deposition rate of the films, high frequency discharge capacity) affect the physical properties of amorphous and nanocrystalline silicon – carbon. The research results of this study show that a–Si1-xCx:H and nc–Si films are the perspective materials for creation of tandem type solar cells.
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