Photovoltaic Device

1. Field of the Invention

The present invention relates to a photovoltaic device used in solar generation of electric power, optical detection, and the like.

2. Description of Related Art

A photovoltaic device with an amorphous semiconductor as a photo-active layer is already known. It basically comprises a transparent substrate made of glass or the like, on which a lightreceiving-side electrode made from transparent conductive oxides (hereinafter denoted as TCO for short), such as ITO or SnO.sub.2, a semiconductor film having a semiconductor junction, such as pn and pin type, and a back electrode in ohmic contact with said semiconductor film, layered in this order.

Also, a photovoltaic device comprising a metallic substrate made of stainless steel or the like on which a semiconductor film with a semiconductor junction and a light-receiving-side electrode made from TCO layered in this order is known.

Almost all of the existing photovoltaic devices, in order to minimize the absorption of light in a p-type or n-type conductivity type impurity doped layer disposed on the light receiving side and obtain a higher open circuit voltage (V.sub.oc), employ hydrogenated amorphous silicon carbide (hereinafter denoted as a-SiC:H for short) having a wide band gap as this impurity doped layer. In the latest investigations, in order to obtain a still higher V.sub.oc, a film formed of hydrogenated microcrystalline silicon carbide (hereinafter described .mu.c-SiC:H for short), for example, has been tried in place of the a-SiC:H film as an impurity doped layer disposed on the light receiving side or the opposite side (H. Sasaki et al. : Proc. of Int'l PVSEC-2 pp 467-470 (Aug. 1986)).

When forming a microcrystalline impurity doped semiconductor film by a glow discharge capable of forming an amorphous semiconductor, the impurity doped layer is not significantly microcrystalline during the first stage of the film-formation, and

remains amorphous. Since the film-thickness is limited, the impurity doped layer of which only a slight thin part is microcrystallized, exhibits no effect due to microcrystallization. If it is intended to microcrystallize the impurity layer in a sufficient thickness for achieving the desired effects of microcrystallization, the thickness of the impurity layer must be extremely thick, e.g. to an extent of 400 .ANG. (H. Itozaki et al. : Proc. of Int'l. PVSEC-1 pp 707-710 (Nov. 1984)). As a result, since light is now absorbed in the impurity layer, the short-circuit current (I.sub.sc) is reduced. In addition, since the film formed in the first stage is amorphous, the V.sub.oc is reduced.

In order to microcrystallize an impurity doped layer of limited thickness, a special apparatus, such as an apparatus for producing semiconductors by an ECR plasma, is indispensable. However, this is a great hindrance for practical use.

The present invention has been achieved for solving the above described problems and it is a main object of the present invention to provide a photovoltaic device in which a semiconductor film is microcrystallized by first forming a film having an element which reacts with a semiconductor film laid on a conductive surface of a substrate to accelerate crystallization of the semiconductor film on a part or all of said conductive surface of the substrate or by adding the element to the conductive surface to accelerate crystallization of the semiconductor film during the first stage of its formation thereby to microcrystallize the semiconductor.

The above and further objects and features of the invention will more fully be apparent from the following detailed description taken with the accompanying drawings.

FIG. 1 is a schematic sectional view showing the construction of a photovoltaic device of the present invention,

FIGS. 2-5 are schematic sectional views showing constructions of photovoltaic devices of the other preferred embodiments of the present invention, and

FIGS. 6a-6c are diagrams showing the distribution of concentration of a crystallization accelerating element in a light-receiving-side electrode.

FIG. 1 is a schematic sectional view showing the construction of the first example of the photovoltaic device of the present invention. Referring now to FIG. 1, reference numeral 1 designates a substrate formed of a transparent material such as glass. On said substrate 1 is a light-receiving-side electrode 2 made from TCO, such as SnO.sub.2 or ITO, over the entire surface thereof. On said light-receiving-side electrode 2 is a Si.sub.x N.sub.y film 3a made from silicon containing nitrogen (hereinafter denoted as Si.sub.x N.sub.y for short: wherein x=1 to 3, y=1 to 4). On said Si.sub.x N.sub.y layer 3a is a p-type impurity doped layer (hereinafter denoted as p-type layer for short) 4 formed of a hydrogenated microcrystallized silicon carbide (hereinafter denoted as .mu.c-SiC:H for short) film doped with an element of group III of the periodic table, such as B or Al. On said p-type layer 4 is an intrinsic or a substantially intrinsic photo-active layer 5 formed of a hydrogenated amorphous silicon (hereinafter denoted as a-Si:H for short) film. On said photo-active layer 5 is an n-type impurity doped layer (hereinafter denoted as n-type layer for short) formed of an a-Si:H film doped with an element of group V of the periodic table, such as P or As. On said n-type layer 6 is a back electrode film 7 made from Al, Ag, Cr, TCO/Ag, TCO/Cr, Al/Ti, Al/TiAg, Al/Ag, or the like.

The Si.sub.x N.sub.y film 3a, p-type layer 4, photo-active layer 5 and n-type layer 6 are formed by the plasma decomposition method silicon compound gas as the main raw material gas. The basic film-forming conditions for obtaining these layers by the plasma decomposition method are shown in Table 1.

It has been confirmed by IMA that the value of x of the Si.sub.x N.sub.y film 3a obtained under the above described film-forming conditions is 3 and the value of y is 4. This Si.sub.x N.sub.y film 3a is formed prior to the p-type layer 4 and acts like a nucleus for microcrystallizing the p-type layer 4. The p-type layer 4 is microcrystallized from the first stage of film-formation, coupled with a high concentration of hydrogen in the raw material gas during the film-formation.

FIG. 2 is a schematic sectional view showing the construction of a second example of a photovoltaic device of the present invention. In this example, Si.sub.x N.sub.y film 3b is arranged like islands at the boundary between the light-receiving-side electrode 2 and the p-type layer 4 at intervals of about 5 .mu.m, for instance, in place of the Si.sub.x N.sub.y film 3a in the first example. The other layers are the same as in the first example. The islands of the Si.sub.x N.sub.y film 3b are, for example, regular squares where one side is about 5 .mu.m in length, and the adjacent islands are separated from each other. The light-receiving-side electrode 2 is in direct contact with the p-type layer 4 in the spaces between the adjacent islands of the Si.sub.x N.sub.y film 3b. Since Si.sub.x N.sub.y is essentially an insulating material, in such a construction in which the entire surface of the light-receiving-side electrode 2 is covered with Si.sub.x N.sub.y as shown in the first e>:ample, the series resistance is increased. In the second example, the p-type layer 4 is partially in direct contact with the light-receiving-side electrode 2 thereby preventing an increase in the series resistance. The p-type layer 4 reacts by contacting the Si.sub.x N.sub.y film 3b and thus is microcrystallized immediately after starting the film-formation thereof.

FIG. 3 is a schematic sectional view showing the construction of a third example of a photovoltaic device of the present invention. In the first and second examples, the substrate is formed of a transparent material, such as glass, and the impurity doped layer on the light receiving side is microcrystallized. In the third example, a substrate 10 is formed of metal, such as stainless steel, or metal-coated glass. Light enters the device from the side opposite of the substrate 10. In this embodiment, the impurity doped layer on the side opposite the light incident side is to be microcrystallized. Islands of Si.sub.x N.sub.y film 3b formed on the surface of a metal substrate 10 or on the metal-coated surface of a substrate 10 in the same manner as in the second example. On the surface of the substrate 10 where the Si.sub.x N.sub.y film 3b is formed, an n-type layer 11 formed of a hydrogenated microcrystallized silicon (hereinafter denoted as .mu.c-Si:H for short) film, a photo-active layer 5, a p-type layer 13 formed of an a-SiC:H film, and a light-receiving-side electrode 14 made from TCO are formed in this order. The n-type layer 11 reacts by contacting the Si.sub.x N.sub.y film 3b and thus is microcrystallized immediately after starting the film-formation thereof.

FIG. 4 is a schematic sectional view showing the construction of a fourth example of a photovoltaic device of the present invention. In this example, polycrystalline silicon, obtained by the thermal CVD method, is used as a semiconductor film. Referring to FIG. 4, reference numeral 20 designates a substrate formed of stainless steel or the like, enduring temperatures (600 to 1,000.degree. C.) applied for the thermal CVD and being conductive. On the substrate 20 the Si.sub.x N.sub.y film 3b is formed thereon as islands as in the third example by the plasma decomposition method. An n-type layer 21 formed of polycrystalline silicon is then formed on the surface of the substrate 20 where the Si.sub.x N.sub.y film 3b is formed. The n-type layer 21 is brought into direct contact with a conductive surface of the substrate 20. On the n-type layer 21, a p-type layer 22 formed of polycrystalline silicon and forming a semiconductor junction with the n-type layer 21 is deposited. A light-receiving-side electrode 14 made from TCO is formed on the p-type layer 22. The n-type layer 21 reacts by contacting the Si.sub.x N.sub.y film 3b during the film-formation by the thermal CVD method, thus providing a polycrystalline film with large particle diameters commencing from the first stage of film-formation.

Table 2 shows the characteristics of the photovoltaic devices according to the first through fourth examples and the first through third comparison devices, which are partially different in construction from the examples, measured under the irradiation of AM-1 and 100mW/cm.sup.2.

The first comparison photovoltaic device has the same construction as in the first and second examples without the Si.sub.x N.sub.y films 3a, 3b; the second comparison photovoltaic device has the same construction as in the third example without the Si.sub.x N.sub.y film 3b; and the third comparison photovoltaic device has the same construction as in the fourth example without the Si.sub.x N.sub.y film 3b.

As is obvious from Table 2, in the first through fourth examples of the present invention, the open-circuit voltage is larger than that of the first through third comparisons. Also, the conversion efficiency is improved as a result of the larger opencircuit voltage. In the photovoltaic devices of the present invention, the impurity layers brought into contact with the Si.sub.x N.sub.y films are microcrystallized or polycrystallized from the first stage of the film-formation. Accordingly, the reduction of the built-in potential due to the existence of amorphous portions as in the comparisons is reduced, and as a result, the open-circuit voltage is increased.

FIG. 5 is a schematic sectional view showing a fifth example of a photovoltaic device of the present invention. Referring to FIG. 5, reference number 1 designates a substrate formed of a transparent material such as glass. A light-receiving-side electrode 32 formed of TCO, such as SnO.sub.2 or ITO, doped with nitrogen, phosphorus or the like is formed on the substrate 1. On the light-receiving-side electrode 32 are formed the same p-type layer 4, photo-active layer 5, n-type layer 6 and back electrode 7 as in the first and second examples, in this order. This example is characterized in that an element, such as nitrogen or phosphorus, reacting with Si, a constituent element of a .mu.c-SiC:H semiconductor film subsequently laid on the light-receiving-side electrode 32, is doped into the light-receiving-side electrode 32 to accelerate crystallization of the semiconductor film.

FIGS. 6a-c are diagrams showing the distribution of concentration of the crystallization accelerating element in the light-receiving-side electrode 32. FIG. 6(a) shows the distribution in the case where this element is doped uniformly in the direction of film-thickness. FIG. 6(b) shows the distribution in the case where this element is doped merely at the boundary with the p-type layer 4. FIG. 6(c) shows the distribution in the case where this element is doped from the middle of the direction of film-thickness so as to be at a maximum in quantity at the boundary with the p-type layer 4. In every case, the concentration at the boundary with the p-type layer 4 side is suitable at 1 to 10 at. %. The distribution of the crystallization accelerating element can be easily adjusted by controlling the quantity of materials used in the film-formation of the light-receiving-side electrode 32.

One method of forming the light-receiving-side electrode 32 formed of nitrogen-doped SnO.sub.2 is described below. The light-receiving-side electrode 32 having a film-thickness of about 3,000 to 10,000 .ANG. is obtained by the thermal CVD method by heating the substrate 1 to about 500 to 600.degree. C. and forming a film with SnCl.sub.4 at a flow rate of 1 to 10g/min, O.sub.2 at a flow rate of 0.1 to 10 .lambda./min and NH.sub.3 at a flow rate of 0.1 to 20 .lambda./min. In the case where the sheet resistance of the obtained light-receiving-side electrode 32 is high, the sheet resistance can be reduced by doping a suitable quantity of fluorine or antimony. The electron beam evaporation method, the reactive sputtering method or the like may be used as the film-forming method rather than the thermal CVD method.

On the resulting light-receiving-side electrode 32 the p-type layer 4, the photo-active layer 5 and the n-type layer 6 are formed by the plasma sputtering method under the film-forming conditions as shown in Table 3.

Subsequently, a back electrode 7 of Al of bout 5,000 .ANG. to 1 .mu.m in thickness is formed by evaporation. The photovoltaic characteristics of the device obtained in the above described manner and of the comparisons shown in Table 4 were measured under an irradiation of AM-1, 100 mW/cm.sup.2. The light-receiving-side electrode 32 in the fifth example is formed of SnO.sub.2 film of about 5,000 .ANG. in thickness doped with nitrogen as a crystallization accelerating element in a thickness of about 1,000 .ANG. at the boundary of the p-type layer 4. The light-receiving-side electrode 32 in the comparison is formed of SnO.sub.2 film of about 5,000 .ANG. in thickness without nitrogen doping. For exactness, a device, on which a semiconductor film is formed at the same time as the fifth example under the film-forming conditions such as shown in Table 3, is used as the comparison.

As shown in Table 4, in the fifth example, the open-circuit voltage is increased and the efficiency of photovoltaic conversion is improved compared with that of the comparison. It is presumed that .mu.c-SiC:H in the p-type layer 4 reacts with the element added to the light-receiving-side electrode 32 to be microcrystallized immediately from the boundary of the p-type layer 4 on the light-receiving-side electrode 32 side, thereby increasing the built-in potential of the p-type layer 4.

As this invention may be embodied in several forms without departing from the spirit of essential characteristics thereof, the present embodiment is therefore illustrative and not restrictive, since the scope of the invention is defined by the appended claims rather than by the description preceding them, and all changes that fall within the meets and bounds of the claims, or equivalence of such meets and bounds thereof are therefore intended to be embraced by the claims.