FR2615327A1 - Photovoltaic device - Google Patents

Abstract

Photovoltaic device using a semiconductor based on hydrogenated amorphous silicon, formed by adding hydrogen to silicon, as photo-active layer 3b. The ratio of the number of silicon atoms being bound to hydrogen atoms to the total number of silicon atoms is less than or equal to 1 %, and the oscillating bond of silicon atoms is less than or equal to 1 x 10<17> cm<-3>. The advantages of the invention are that the cost may be reduced, by formation of a thinner layer and by obtaining a greater surface area of photo-active layer and that the photoelectric-conversion yield is improved, which also prevents photodeterioration.

Description

PBDTOVOLTAIC DEVICE
The present invention relates to a photovoltaic device that is used as a solar cell, photosensor or the like, and more particularly relates to a photovoltaic device using a hydrogenated amorphous silicon semiconductor in which hydrogen is added to silicon as a photoactive layer. .

A photovoltaic device is constituted in such a way that a front electrode, a power generating element and a rear electrode are deposited in this order on an insulating and light transmitting substrate, and, if a light enters the device of the photovoltaic system. side of the substrate, a photoactive layer of the energy generating element absorbs part of the light and generates a phototension, the generated phototension being extracted from the device.

A photovoltaic device is already known which uses a hydrogenated amorphous silicon semiconductor, in which hydrogen is added to the silicon as a photoactive layer. Hydrogenated amorphous silicon semiconductors have excellent thin film performance compared to monocrystalline silicon semiconductors, and therefore, a larger photoactive layer forming area can be obtained for the same mass, which provides effective cost reduction. Thus, the use of hydrogenated amorphous silicon semiconductors in large-scale solar cells is growing.

However, photovoltaic devices using hydrogenated amorphous silicon semiconductors have drawbacks, in that the photoelectric conversion efficiency is low compared to that of photovoltaic devices using monocrystalline silicon semiconductors, and in so doing. that photo-deterioration (the efficiency of photoelectric conversion is reduced by irradiation with intense light) may occur (Japanese Patent Publication No. sho-54274/1984).

The present invention aims to avoid the aforementioned drawbacks.

Consequently, a first object of the present invention is to provide a photovoltaic device capable of preventing photodeterioration, by increasing the absorption coefficient and therefore reducing the thickness of the layer.

A second object of the present invention is to provide a photovoltaic device capable of improving the efficiency of the photoelectric conversion, by reducing the optical band gap of the photoactive layer and therefore increasing the coefficient d. absorption of light in the long wavelength region.

According to the present invention, a hydrogenated amorphous silicon semiconductor is used as the photoactive layer in which the number of silicon atoms bonding to the hydrogen atoms is less than or equal to 1% of the total number of atoms of silicon, and in that the pendant or oscillating link is less than or equal to 1 x 1017cl ~ 3.

The above objects and advantages of the invention will appear better in the light of the detailed description below, with reference to the accompanying drawings in which
Figure 1 is a schematic section illustrating a basic structure of a photovoltaic device according to the present invention
Figure 2 is a graph illustrating the relationship between the proportion of silicon atoms bonding to hydrogen atoms and the photoelectric conversion efficiency
Figure 3 is a graph illustrating the relationship between the oscillating bond of silicon atoms and the photoelectric conversion efficiency, and
FIG. 4 is a schematic section illustrating a basic structure of another embodiment of a photovoltaic device according to the present invention.

An embodiment in accordance with the present invention will now be described.

FIG. 1 represents a basic structure of a photovoltaic device according to the invention and the example illustrated in FIG. 1 is that of the case of a single cell. In FIG. 1, the reference 1 designates a substrate which is insulating and which transmits light; this substrate is made of transparent glass. On the substrate 1. a front electrode 2 "TCC". for example in SnO2, In203, or "ITO" which is a mixture of these s-ubs # ar, these, or a film or thin layer of metal such as Au, Ai, Ni O'J r, or a grid of metal in Al, Ni, Cr or the like, an energy generator element 3 comprising a semi-conductive junction of the purple PIN type described later, and a rear electrode 4 in Al, are deposited in this order.

The energy-generating element 3 has a three-layer structure, comprising a layer 3a of amorphous P-type Sic: H (film composition: Si 90 mol%, C10 mol% film thickness: 15 nm), a Amorphous Si: H layer 3b of type
I (film thickness: 200 nm), and an N-type amorphous Si: H layer 3c (film thickness: 30 rem), in that order from the side of the front electrode 2. According to the present invention, for type I amorphous Si: H layer 3b (photoactive layer), the ratio of silicon atoms bonding to hydrogen atoms, relative to all silicon atoms, is 1% or less, and the oscillating bond of silicon atoms is 1 x 1017 cm ~ 3 or less.

A method of manufacturing a photovoltaic device having such a structure is now described. The front electrode 2 is formed, for example by evaporation of SnO2 on the substrate 1. Then, the amorphous P-type layer 3a of SiC: H is formed on the front electrode 2, by chemical vapor deposition (DVC) in plasma using a radiofrequency corona discharge, the amorphous layer 3b of type I of Si: H is formed on the amorphous layer 3a of type P of SiC: H by chemical vapor deposition of excited radical or photo-deposition chemical vapor phase, and the N-type amorphous Si: H layer 3c is formed on the Si: H type I amorphous layer 3b by plasma chemical vapor deposition using radiofrequency corona discharge. Table I below shows an example of reaction conditions for the formation of each of these three layers. Finally, the rear electrode 4 is formed by evaporation of Al on the amorphous N-type layer 3c of Si: H. In this table a-P-, a-I- and a-N- denote respectively amorphous structures of types P, I and N.

TABLE 1

<tb><SEP> 3a <SEP> 3b <SEP><SEP> 3c
<tb><SEP> aP-SiC: H <SEP> aI-Si: H <SEP> aN-Si: H
<tb> Method <SEP> of <SEP> unload <SEP> of ef- <SEP> DVC <SEP> radical <SEP> i <SEP><SEP> unload
<tb> reaction <SEP> fluve <SEP> excited <SEP> effluvium
<tb> Power <SEP><SEP> Radiofrequency <SEP> Microwaves <SEP> Radiofrequency
<tb> applied <SEP> (13.56 <SEP> MHz) <SEP> (2.45 <SEP> GHz) <SEP><SEP> (13.6 <SEP> MHz) <SEP>
<tb><SEP> (W) <SEP><SEP> 30 <SEP> 100 <SEP> 30
<tb> Temperature <SEP> of the
<tb> substrate <SEP> (C) <SEP> 200 <SEP> 300 <SEP> 200
<tb> Pressure <SEP> (torr) <SEP> 0.3 <SEP> (a) <SEP> 0.001 <SEP> (b) <SEP> 0.3 <SEP> (a)
<tb> input <SEP> of <SEP> gases <SEP> CH4 / SiH4 = l <SEP> SiR2F2 / H2 = O, l <SEP><SEP> PH3 / SiH4 = O, l <SEP>
<tb> of <SEP> reaction <SEP> B2H4 / SiH4 = 0.01 <SEP>
<tb> Notes
<tb> (a) <SEP> 0.3 <SEP> torr <SEP> corresponds approximately <SEP><SEP> to <SEP> 40 <SEP> Pa,
<tb> (b) <SEP> 0.001 <SEP> torr <SEP> corresponds approximately <SEP><SEP><SEP> to <SEP> 0.13 <SEP> Pa.
<tb>

We now refer to the ratio of silicon atoms bonding to hydrogen atoms and to the numerical oscillation value of silicon atoms. Fig. 2 is a graph illustrating the relationship between the rate of silicon atoms bonding to hydrogen atoms and the photoelectric conversion efficiency. The abscissa represents the rate (%) of silicon atoms bonding to hydrogen atoms, and the ordinate represents the photoelectric conversion efficiency (%), respectively. In addition, the graph illustrates a relationship between these two quantities in the case where the oscillating link is 5 x 1016 cm ~ 3 and the irradiation condition is 100 mW / cm2 for the value AM = 1.

AM denotes the length of the path that sunlight directly follows to enter the atmosphere, and. Is defined by the relation
AM = b / bo sec Z where,
bo is the standard pressure,
b is the pressure during the measurement,
Z is the zenith distance from the sun, and
sec Z is the secant of angle Z.

In general, b = bo and AM = 1 corresponds to the case where the sun shines at the zenith in relation to the observer. Figure 3 also illustrates the relationship between the oscillating bond of silicon atoms and the photoelectric conversion efficiency, the abscissa representing the oscillating bond value (cl ~ 3) and the ordinate representing the photoelectric conversion efficiency. (%), respectively. In addition, the graph illustrates a relationship between these two quantities in the case where the rate of silicon atoms bonding to hydrogen atoms is 0.5% and the irradiation state is 100 mW / cm2. (when AM = 1).

As seen in Fig. 2, when the rate of silicon atoms bonding to hydrogen atoms exceeds 1%, the photoelectric conversion efficiency decreases sharply. Therefore, it is preferable that the rate of silicon atoms bonding to hydrogen atoms is less than or equal to 1%. On the other hand, as can be seen in Fig. 3, when the oscillating bond of silicon atoms exceeds 1 x 1017 cl -3, the photoelectric conversion efficiency decreases sharply. Therefore, the digital value of oscillating link of silicon atoms is preferably less than or equal to ix 1017 cl ~ 3. As a result, a photovoltaic device using a hydrogenated amorphous silicon semiconductor as a photoactive layer, in which the rate of silicon atoms binding to hydrogen atoms is 1% or less and the oscillating bond of silicon atoms is less than or equal to 1 x 1017 cm-3, has high photoelectric conversion efficiency.

Table 2 below shows the layer characteristics of the type I amorphous layer of Si: H according to the present invention and of the usual type I amorphous layer of Si: H. Table 3 below shows the photoelectric conversion characteristics of a photovoltaic device according to the present invention and of a photovoltaic device, comprising the usual type I Si: H amorphous layer illustrated in Table 2. In addition , the numerical values recorded in table 3 indicate the characteristics in the case where the irradiation condition is when AM = 1 of 100 mW / cm2 for the two photovoltaic devices.

TABLE 2

<tb><SEP> Example <SEP> of <SEP> the <SEP> Example
<tb><SEP> presents <SEP> invention <SEP> usual
<tb> Rate <SEP> of atoms <SEP> of <SEP> silicon <SEP><SEP> se
<tb> binding <SEP> to <SEP> atoms <SEP> of hydrogen
<tb><SEP> (Z) <SEP><SEP> 0.5 <SEP> 20
<tb> Oscillating <SEP><SEP> value <SEP> link
<tb> of <SEP> atoms <SEP> of <SEP> silicon <SEP> (cm-3) <SEP> 5x1016 <SEP> 1x1016 <SEP>
<tb> Interval <SEP> of <SEP> tape <SEP> optical <SEP> (eV) <SEP> 1.60 <SEP> (a) <SEP> 1.70 <SEP> (b)
<tb> Absorption coefficient <SEP><SEP> at
<tb> 700 <SEP> nm <SEP> (cm-1) <SEP> 3x104 <SEP> 7x103
<tb> Photo-conductivity <SEP> orph <SEP>
<tb><SEP>(# -1 # -I) <SEP><SEP> 2x10-5 <SEP><SEP> lx10-5 <SEP>
<tb> Dark conductivity <SEP><SEP> 6r <SEP><SEP> d <SEP> 5x10-11 <SEP> 1x10-11 <SEP>
<tb><SEP> (D <SEP> lem ~ l) <SEP>
<tb> Notes
<tb> (a) <SEP> or approximately <SEP><SEP> 2.56 <SEP> x <SEP> 10-19 <SEP><SEP> J
<tb> (b) <SEP> or approximately <SEP><SEP> 2.72 <SEP> x-10-19 <SEP><SEP> J
<tb>
TABLE 3

<tb><SEP> Example <SEP> of <SEP> the <SEP> Example
<tb><SEP> presents <SEP> invention <SEP> usual
<tb> Current <SEP> of <SEP> circuit <SEP> Isc
<tb><SEP> (mA / cm2) <SEP> 17.2 <SEP> 14.5
<tb> Voltage <SEP> in <SEP> circuit <SEP> open
<tb><SEP> Voc <SEP> (V) <SEP> 0.84 <SEP> 0.85
<tb> Factor <SEP> of <SEP> filling <SEP> FF <SEP> 0.65 <SEP> 0.68
<tb> Yield <SEP> of <SEP> conversion
<tb> photo-electricf <SEP><SEP> O <SEP> (%) <SEP> 9.39 <SEP> 8.38
<tb> Yield <SEP> of <SEP> conversion
<tb> photoelectric <SEP> after
<tb> irradiation <SEP> at <SEP> 100 <SEP> mW / cm2
<tb> for <SEP> 100 <SEP> h <SEP> Q <SEP><SEP> (%) <SEP> 8.64 <SEP> 6.79
<tb> ~ <SEP><SEP> 0 <SEP><SEP> 0.92 <SEP> 0.81
<tb>
For the conventional device, the photoelectric conversion efficiency is 8.38 X, while it is 9.39 Z for the present invention, an increase of 1 Z or more.

Further, the reduction rate of the photoelectric conversion efficiency after irradiation with intense light radiation is 19% for the usual device while it is only 8% for the present invention, which shows that the photodeterioration is avoided.

In the photovoltaic device according to the present invention, as shown in Table 2, the optical band gap of the type I Si: H amorphous layer is reduced and the absorption coefficient of the long wavelength region is reduced. increased and therefore the long wavelength light, which is not actually used in the conventional device, can be used to produce a photo-voltage, so that the photoelectric conversion characteristics are improved as seen in Table 3 In addition, as shown in Table 2 above, the absorption coefficient at 700 nm is increased to 3 x 104 cm ~ 1 and, as a result, the film thickness which should be 400 to About 500nm can be reduced to 300nm or less, and preferably 200nm or less, thereby avoiding photodeterioration.

As detailed above, with the photovoltaic device according to the present invention, the photoelectric conversion efficiency can be improved and photodeterioration can be suppressed.

Also known is the photovoltaic device of so-called tandem structure (US Pat. No. 4,496,788) in which the energy generating elements are deposited in multiple layers, for example in two or three layers or more, and, in such cells. in tandem, the photoelectric conversion efficiency can be improved by adjusting the optical band gap in each power generating element.

A second embodiment in accordance with the present invention is described below, relating to the aforementioned tandem cells.

Figure 4 illustrates a basic structure of tandem stacks according to the present invention. In view from the incident light side, the front TCO electrode 12, a first power generation element 13 having a three-layer PIN-like structure 13a, 13b and 13c in that order from the front electrode 12, and a second power generating element having a PN-type semiconductor junction 23a and 23b in that order from the side of the front electrode 12, are deposited in this order on a rear electrode 14 made of Ti which has a high melting point and gives excellent ohmic performance with the N-type semiconductor layer 23b. Thus, each of the first and second power generation elements 13 and 23 individually operates substantially as a generation element of energy. Table 4 below illustrates an embodiment of layer configurations of the first and second energy generation elements 13 and 23. In this table, the references "a" and "> 4C" denote respectively amorphous and monocrystalline structures. .

Further, according to the present invention, in the type I amorphous layer (photoactive layer) 13b, the ratio of silicon atoms bonding to hydrogen atoms to all silicon atoms is 1% or less, and the oscillating bond of silicon atoms is 1 x 1017 cm 3 or less.

TABLE 4

<tb> Composition <SEP><SEP> of <SEP> film <SEP> Interval <SEP> Thickness
<tb><SEP> tape <SEP> optical <SEP> of <SEP> film <SEP> (nm) <SEP>
<tb> 13a <SEP> a <SEP> type <SEP> P-SiC: H <SEP> ~ <SEP><SEP> 2,0 <SEP> N <SEP> 20
<tb><SEP> a <SEP> a <SEP> type <SEP> P-Si: fi <SEP><SEP> 1.7 <SEP><SEP> 15 <SEP>
<tb><SEP><SEP> C <SEP><SEP> type <SEP> P-Si: H <SEP> o <SEP><SEP> 2,0 <SEP>> 2 <SEP><SEP> 20
<tb> |
<tb><SEP> 13b <SEP><SEP> a-Si :: H
<tb><SEP> weakly <SEP> hydrogenated <SEP> ~ <SEP> 1.6 <SEP> ~ <SEP> 100 100
<tb><SEP> 13c <SEP> a <SEP> type <SEP> N-Si: H <SEP> ru <SEP><SEP> 1,7 <SEP> cJ <SEP><SEP> 7
<tb><SEP> C <SEP> type <SEP> N-Si: H <SEP> ~ <SEP><SEP> 2,0 <SEP> r% J <SEP><SEP> 7
<tb><SEP> 23a <SEP> a <SEP> type <SEP> P-SiC: H <SEP> ~ <SEP><SEP> 2,0 <SEP> ~ <SEP> 8
<tb><SEP> a <SEP> type <SEP> P-Si: H <SEP> ru <SEP><SEP> 1,7 <SEP> ~ <SEP><SEP> 8
<tb><SEP> s <SEP> C <SEP><SEP> type <SEP> P-Si :: H <SEP> rv <SEP> 2,0 <SEP>#<SEP><SEP> 8
<tb><SEP> type <SEP> P <SEP> poly-Si <SEP> rv <SEP> 1,2 <SEP> ~ 300
<tb><SEP> 23b <SEP> type <SEP> N <SEP> poly-Si <SEP> cu <SEP><SEP> 1,2 <SEP> ~ <SEP> 10000
<tb><SEP> Notes
<tb><SEP> (a) <SEP> 1 <SEP> eV <SEP> corresponds approximately <SEP> to <SEP> to <SEP> 1.602 <SEP> x <SEP> 10-19 <SEP><SEP> J .
<tb>

Further, Table 4 gives an example of a PIN type and PN type configuration in that order from the front electrode side 12 but it goes without saying that, conversely, one can use a type configuration. NIP and NP type in that order from the side of the front electrode 12 and, in this case, the rear electrode 14 is preferably in Pt.

Table 5 shows the photoelectric conversion characteristics of the photovoltaic device having the tandem structure according to the present invention. Further, an embodiment (a) shown in Table 5 has a P-type amorphous layer configuration of SiC: H (13a), weakly hydrogenated type I amorphous SiC: H (13b), N-type microcrystalline. SiC: H (13c) P-type poly-Si (23a) and N-type poly-Si (23b) in that order from the front electrode side 12, and one embodiment (b) has a configuration N-type amorphous layers of
SiC: H (13a), amorphous type I weakly hydrogenated from SiC: H (13b), microcrystalline type P from SiC: H (13c), poly-Si type
N (23a) and P-type poly-Si (23b) in that order from the side of electrode 12. In addition, the numerical values in Table 5 give the characteristics in case the irradiation state is when AM = l of 100 mW / cm2 for the two photovoltaic devices.

TABLE 5

<tb><SEP>
<tb><SEP> Realization <SEP> Realization
<tb><SEP> (a) <SEP> (b)
<tb> Current <SEP> of <SEP> short circuit
<tb><SEP> 1sc <SEP><SEP> (mA / cm2) <SEP><SEP>.<SEP><SEP> 12.2 <SEP> 14.5
<tb> Voltage <SEP> to <SEP> circuit <SEP> open
<tb><SEP> Voc <SEP> (V) <SEP> 1.5 <SEP> 1.5
<tb> Factor <SEP> of <SEP> filling <SEP> FF <SEP> 0.68 <SEP> 0.70
<tb> Yield <SEP> of <SEP> conversion
<tb> photoelectric <SEP> tao <SEP> (%) <SEP><SEP> 12.4 <SEP> 15.2
<tb> Yield <SEP> of <SEP> conversion
<tb> photoelectric <SEP> after <SEP> irra
<tb> diation <SEP> to <SEP> 100 <SEP> mW / cm2 <SEP> during
<tb> ioo <SEP> h <SEP> n (%) <SEP> 11.4 <SEP> 14.0
<tb><SEP> 0.92 <SEP> 0.92
<tb>
As shown in Table 5, the photovoltaic device consisting of the tandem cells according to the present invention exhibits improved photoelectric conversion characteristics.

Further, in the tandem battery device described above, the second power generation element uses polysilicon as the main component. Without limiting the device described above, it is possible to employ a second energy generation element using amorphous silicon as the main constituent, or else a second energy generation element using monocrystalline silicon as the main constituent.

In addition, although the case of a configuration comprising two energy generation elements has been described, it is understood that a configuration comprising three or more energy generation elements can be adopted without departing from the scope of the present invention. This can be implemented in various forms without departing from the scope of its essential characteristics and the embodiments as described are not given in a limiting manner but by way of illustration.

According to one embodiment of the invention, the photoactive layer 3b is of type I. According to another embodiment, the type of conductivity of the energy generating element is P,
I, and N in this order from said front electrode 2. As a variant, the type of conductivity of the energy generating element is N, I and P in this order from said front electrode 2.

According to another embodiment, a photovoltaic device is also recommended, comprising a front electrode 12, a first energy-generating element 13, the main component of which is amorphous silicon, a second energy-generating element 23, and a rear electrode 14 , deposited in this order, characterized in that a photoactive layer of said first energy generating element is made of a hydrogenated amorphous silicon semiconductor in which the ratio of the number of silicon atoms bonding to atoms d hydrogen in the total number of silicon atoms is less than or equal to 1%, and the oscillating bond of silicon atoms is less than or equal to 1 x 1017 cm-3.

In this last embodiment, the second energy generating element 23 is an energy generating element, the main constituent of which is polysilicon.

In such a configuration the types of conductivity of said first and second energy generating elements are respectively P, I, N and P, N in that order, from said front electrode, or are N, I, P and N, P in that order, from said front electrode.

The essential constituent of the second energy generating element comprises as essential constituent amorphous silicon or monocrystalline silicon.

Preferably, the photoactive layer 13b of the first energy generating element is of a hydrogenated amorphous silicon semiconductor whose absorption coefficient at 700 nm is about 3 x 104 cm-i or more.

Advantageously, the thickness of said photoactive layer is 30 nm or less.

Claims (12)

1. Photovoltaic device, comprising a front electrode (2B, an energy generating element (3) whose main component is amorphous silicon, and a rear electrode (4), deposited in this order, said device being characterized in that a photoc-a ti ve section (3b) of said energy generating element is of a hydrogenated amorphous silicon semiconductor in which the ratio of the number of silicon atoms bonding to hydrogen atoms to the total number of silicon atoms is less than or equal to 1%, and wherein the oscillating bond of silicon atoms is less than or equal to 1 x 1017 cl ~ 3. 2. Photovoltaic device according to claim 1, characterized in that the type of conductivity of said photo-aotlveest layer of type I. 3. Photovoltaic device according to claim 2, characterized in that the type of conductivity of said energy generating element is P, I and N in this order from said front electrode (2). 4. Photovoltaic device according to claim 2, characterized in that the type of conductivity of said energy generating device is N, I, P in this order from said front electrode (2). 5. Photovoltaic device, comprising a front electrode (12), a first energy-generating element (13) whose main component is amorphous silicon, a second energy-generating element 23, and a rear electrode (14), deposited in this order, characterized in that a photoactive layer of said first energy generating element is made of a hydrogenated amorphous silicon semiconductor in which the ratio of the number of silicon atoms bonding to hydrogen atoms the total number of silicon atoms is less than or equal to 1%, and the oscillating bond of silicon atoms is less than or equal to 1017 çn # 3 6. Photovoltaic device according to claim 5, characterized in that the second energy generating element (23) is an energy generating element whose main constituent is polysilicon. 7. Photovoltaic device # according to claim lj, characterized in that the types of conductivity of said first and second energy-generating elements are respectively P, I, N and P, N in that order, from said front electrode. 8. Photovoltaic device according to claim 6, characterized in that the types of conductivity of said first and second energy-generating elements are respectively N, I, P, and N, P in that order from said front electrode. 9. Photovoltaic device according to claim 5, characterized in that said second energy generating element is an energy generating element whose main constituent is amorphous silicon. 10. Photovoltaic device according to claim 5, characterized in that said second energy generating element is an energy generating element whose main constituent is monocrystalline silicon. 11. Photovoltaic device comprising a front electrode, an energy generating element whose main constituent is amorphous silicon, and a rear electrode, characterized in that said energy-generating element comprises a photoactive layer contributing to the generation. energy, said photoactive layer of a hydrogenated amorphous silicon semiconductor whose absorption coefficient at 700 nm is about 3 x 104 cml or more. 12. Photovoltaic device according to claim; characterized in that the thickness of said photo-active layer is 30nm or less.