JPH01181575A - Junction type photoelectric conversion device - Google Patents

Abstract

PURPOSE:To improve a physical strength and the photoelectric conversion characteristics by a method wherein thin layer parts whose thicknesses are thinner than the diffusion length of minority carriers emitting lights in a semiconductor substrate and thick layer parts which have physically sufficient strength are made to coexist and the widths of the thick layer parts are less than two times of the minority carrier diffusion length. CONSTITUTION:Relatively thin thin layer parts 11 and relatively thick thick layer parts 12 coexist in a semiconductor substrate 10. One thick layer part 12 is positioned between a pair of the thin layer parts 11 and 11 facing each other laterally. Each thin layer part 11 has a thickness (d) which is smaller than the diffusion length of minority carriers which emit lights at that part in the semiconductor. On the other hand, the thickness of each thick layer part 12 is larger than the thickness of the thin layer part 11 and large enough to support the device structure mechanically. Further, the dimension W of the thick layer part 12 along the direction parallel to the surface of the semiconductor substrate 10, i.e. the width W, is less than two times of the minority carrier diffusion length. With this constitution, the reciprocity between the photoelectric conversion characteristics and mechanical strength characteristics can be harmonized and a high efficiency can be obtained while a sufficient strength is maintained.

Description

[Detailed Description of the Invention] (Field of Industrial Application) The present invention relates to a junction type photoelectric conversion device such as a solar cell, and in particular, the present invention relates to a junction type photoelectric conversion device such as a solar cell, and in particular, the open-circuit electromotive voltage can be reduced without causing physical strength problems of such device elements. V. This invention relates to improvements that can increase C.

(Prior art) Open-circuit electromotive voltage V in junction-type photoelectric conversion devices such as solar cells
. C begins to increase when the thickness of the semiconductor layer that primarily absorbs light becomes thinner than the diffusion length of the minority carriers generated by the light, and increases as the surface recombination rate decreases and becomes thinner.

However, in general, it is difficult to make a large-area semiconductor crystal as a whole into an extremely thin order, such as the diffusion length of minority carriers, as described above, and to further process it into a device, even when considering only mechanical strength. It's impossible.

Therefore, what was previously proposed as a possible structure was !
A thin semiconductor crystal layer is grown on a type A substrate, and this is used as a photoelectric conversion functional layer.

(Problems to be Solved by the Invention) However, the above-mentioned conventional structure suffers from mismatching of crystal lattice spacing with the substrate, difference in thermal expansion coefficient, etc., and tends to generate crystal defects and patterns. However, it is actually extremely difficult to obtain high-quality semiconductor thin films due to problems such as the diffusion of impurities from the substrate into the crystalline thin film, and the advantage of improved photoelectric conversion characteristics due to thinner films has not been utilized.

The present invention has been made in view of these points, and it is possible to construct a semiconductor substrate from a single type of semiconductor substrate without using the technique of crystal growth on a different type of substrate, but it also creates the problem of physical fragility. Instead, we aim to propose a new configuration that can improve photoelectric conversion characteristics.

<Means for Solving the Problems> In order to achieve the above object, the present invention has the following features: Processed so that it has a mixed structure of a thin layer part and a thick layer part that is thicker than the thin layer part and has sufficient physical strength: and a junction forming layer that collects the minority carriers along one surface of the semiconductor substrate, and a barrier forming layer that repels the minority carriers along the other surface; Either the semiconductor surface with the junction forming layer or the barrier forming layer is used as a light-receiving surface, and the other surface opposite to the light-receiving surface is a reflective surface with a reflective layer; Provided is a junction-type photoelectric conversion device characterized in that the minority carrier diffusion length is less than or equal to twice the minority carrier diffusion length.

(Operations and Effects) Among the above components of the present invention, there is a bonding layer formed by thinning the semiconductor substrate, which collects photogenerated minority carriers along one side of both surfaces, and a bonding layer that collects photogenerated minority carriers along one side of both surfaces. The thickness of the thin semiconductor layer is less than the diffusion length of the minority carriers that can be photogenerated in this part, so that The open circuit voltage obtained in the thin layer portion is sufficiently high.

The barrier forming layer that electrically repels minority carriers toward the junction forming layer has the function of increasing the carrier concentration itself even in this thin layer portion, and therefore,
The open-circuit electromotive force can be further improved than when the thin layer portion is simply configured to be shorter than the minority carrier diffusion length.

On the other hand, such thin layer portions alone cannot be expected to have mechanical or physical strength as a device element, but in a photoelectric conversion device constructed according to the present invention, the entire device element is In order to obtain sufficient physical strength, there is a thick layer (although it can generally be the thickness of the semiconductor substrate as a supplied starting material), and this problem can be solved. It's resolved.

However, such thick layer parts themselves inherently tend to reduce the open-circuit electromotive force, so simply providing a thick layer part as described above does not improve the photoelectric conversion efficiency by providing a thin layer part. The effect may be greatly impaired, and in some cases, it may not be possible to exhibit any effect at all.

Therefore, in the present invention, the width of this thick layer part is further limited to "less than twice the minority carrier diffusion length" to eliminate such concerns.

Simply put, as long as the width of the thick layer section is kept within that range, at least the barrier forming layer provided along the surface of one of the pair of thin layer sections on both sides of the thick layer section can be This is because it can also act on this thick layer portion from the lateral direction.

In other words, the electrical carrier repulsion effect due to the presence of the barrier forming layer is not limited to the vertical direction, but only in the vertical direction, as long as the distance is about the minority carrier diffusion length from the edge of the barrier forming layer.
It can extend in all directions including the lateral direction.

Therefore, if the width of each thick layer is set to be less than twice the minority carrier diffusion length as described above, the barrier forming layer will function by allowing carriers to flow in the lateral and diagonal upward directions. Minority carriers that may exist therein also receive a desirable repulsion effect due to the influence of the barrier-forming layer portion. However, depending on the embodiment, the barrier forming layer provided in the thick layer part itself may exert its effect by directly passing through carriers from a closer position.

In any case, in this way, even though a thick layer is provided, the drop in open circuit voltage due to its presence can be ignored, and even in the worst case, it can be reduced to a sufficiently high value compared to the conventional one. Can be maintained.

In summary, the effects of the present invention can be summarized as: well harmonizing the contradiction between photoelectric conversion characteristics and mechanical strength characteristics;
It can be said that it is possible to provide a highly efficient photoelectric conversion device while being sufficiently durable.

Conversely, if the high efficiency effect is somewhat suppressed but still sufficient, the semiconductor material used in the device of the present invention may be of lower quality than conventional ones. In some cases, this is actually an extremely desirable effect.

(Example) FIG. 1 schematically shows a cross-sectional configuration of a main part of a junction type photoelectric conversion device which is an example constructed according to the present invention.

Since the state shown in the figure is a completed state of the device, it is of course shown in a state in which a semiconductor wafer made of a general appropriate material as a starting material has already been processed into a geometric shape.
The semiconductor substrate IO has a relatively thin thin layer portion 11 and a relatively thick thick layer portion 12 in a mixed state, and in particular,
In accordance with the subject matter of the invention, the thicker section 12 is located between at least a pair of laterally opposed thinner sections 11.11.

In the illustrated embodiment, the upper surface of the semiconductor substrate 10 in the drawing is a series of flat surfaces, and the opposite surface (back surface) has a thickness of the thin layer portion 11 and the thick layer portion 12. The surface is uneven to accommodate different tastes.

Each thin layer portion 11 has a thickness d that is thinner than the diffusion length of minority carriers photogenerated inside the semiconductor in that portion.
On the other hand, the thickness of the thick layer portion 12 is not only thicker than the thin layer portion 11 but also set to a sufficient thickness so as to mechanically support the illustrated element structure.

However, in accordance with the spirit of the present invention, the dimension W of the thick layer portion 12 in the direction parallel to the semiconductor substrate surface, that is, the width W, is set to be twice or less the minority carrier diffusion length.

In the photoelectric conversion device of the present invention, the light-receiving surface can be set on either of the two surfaces of the semiconductor substrate, but in the embodiment shown here, the light-receiving surface is set on the flat surface located above in the figure. It is said that

A bonding layer 13 is provided on this surface side,
This may be a thin layer made of the same material as the semiconductor substrate IO and containing impurities of the opposite conductivity type, or may be made of a material different from the semiconductor substrate and having a relationship to form a Peter junction.

In short, it is sufficient to show the function of collecting minority carriers photogenerated in the thin layer portion 11 of the semiconductor substrate IO, and in some cases, any suitable metal may be used as the junction forming layer 13 as long as it is thin enough to transmit light. It is also possible to select layers.

In the case shown in the figure, an antireflection film 31 that also serves as a protective film is further provided on the bonding layer 13, but this may be provided as necessary, and may be used as a layer that performs an antireflection function. The layer that performs the protective function may be a separate layer, resulting in a multilayer structure.

For convenience, the surface of the semiconductor substrate on which the bonding layer 13 is located is simply referred to as the "front surface" in the illustrated embodiment, and the other surface is referred to as the "back surface".

On the surface side of the semiconductor substrate, a lead-out terminal 14 is also appropriately provided for establishing electrical continuity with the bonding layer 13 formed along the surface. Each location is located in the same location.

This also means that the area of the thin layer section 11 that provides the best results in terms of photoelectric conversion characteristics can be used as effectively as possible, and that the entire area can be used as the light incident area. Even if a large external force is applied in the process of connecting external circuits to these terminal groups after the manufacturing process of the extraction terminals, as long as the thick layer part 12 has high mechanical strength, This is also with the intention of being able to resist this as much as possible.

As mentioned earlier, the back surface of the semiconductor substrate 10 has the thin layer portion 1.
Since the bottom of the thick layer portion 12 is continuous with the bottom of the thick layer portion 12, the resulting surface is uneven, but the barrier forming layer 20 is formed along this back surface (therefore, it is also uneven). It is provided.

This barrier forming layer 20 may be formed as an impurity layer having the same conductivity type as the semiconductor substrate IO and having a higher concentration, or may be a layer of a different material.

The barrier forming layer 20 is also expected to have the function of forming a barrier against the minority carriers generated in the thin layer section 11, and electrically repelling the minority carriers flowing toward this layer. For example, a semiconductor or insulating film with a wider gap than the thin layer portion 11 can be selected as a different material layer for the thin layer portion 11 so as to form a barrier against the minority carriers. .

In particular, when a suitable insulating film is selected as the barrier forming layer 20 as described above, the protective layer 41 provided to cover it in this embodiment may also be used as the insulating film itself. can.

Further, on the protective layer 41 which is desirably provided (although it is located further down in the figure), a reflective layer 42 generally made of a suitable metal layer is also provided, and this embodiment Again, for the same reason as stated regarding the terminal 14, the reflective layer 42 penetrates the protective layer 41 at a certain point in the thick layer portion 12 and constitutes an electrical contact portion 21 with respect to the semiconductor substrate IO.

In such a structure, considering the significance of the present invention, the thickness d of the semiconductor thin layer portion 11 is thinner than the minority carrier diffusion length as described above.

The open circuit voltage VOC generated in this portion can be made sufficiently high as desired. Even compared to photoelectric conversion devices obtained by peeling a semiconductor substrate to its mechanically durable limit while maintaining uniformity of thickness, the value is still sufficiently high. Become.

In other words, in conventional photoelectric conversion devices, especially when a low-quality semiconductor substrate is used, the total area of the substrate is
While it was impossible to reduce the thickness to less than the minority carrier diffusion length, in the photoelectric conversion device constructed according to the present invention, even if the thickness is made to be less than the minority carrier diffusion length as described above, Due to the presence of the thick layer portion 12, the overall mechanical strength can be maintained to a satisfactory level.

However, if the presence of the thick layer portion 12 provided to increase mechanical strength greatly impairs the photoelectric conversion characteristics that could be improved, we would like to apologize for any inconvenience caused by removing the thin layer portion 11.
However, in the case of the present invention, an important limitation is imposed on the width W of the thick layer portion 12 to be less than or equal to twice the minority carrier diffusion length. Such concerns can be avoided.

The reason for this has already been stated in the operation section, but just to be sure, I will mention it in connection with this embodiment.In the case of the illustrated embodiment, the semiconductor substrate! The barrier forming layer 20 provided on the back side of the semiconductor layer 20 has the function of increasing the carrier concentration of the thin layer portion 11 itself, and therefore simply makes the semiconductor thin layer portion 11 less than the diffusion length of minority carriers. It can be expected that the open-circuit electromotive force of the negative layer will be further improved compared to when the structure is simply configured.

On the other hand, the electrical carrier suitability effect due to the existence of this barrier forming layer 20 is not limited to the vertical direction but is also effective in the lateral direction as long as the distance of the minority carrier diffusion length from the edge of the barrier forming layer 20 is approximately the same. It extends in all directions including (however, of course, excluding areas where there is no semiconductor region).

Therefore, if the width W of each thick layer section 12 is set to twice the minority carrier diffusion length or less as described above, this thick layer section 12
As is clear from looking at both ends in the left and right direction in the figure, there are thin layer parts 11.1 on the right and on the left at both ends.
The folded end of the barrier forming layer portion provided on the lower surface of 1 is facing, and therefore, from here, the thick layer portion 12
The end portions of the barrier forming layer 20 can also have an influence in the lateral direction and diagonally upward direction over half of the width W of the barrier forming layer 20 .

In other words, the effective thickness of the thick layer portion 12 can be considered to be equivalent to the thickness d, which is not much different from the thickness d of the thin layer portion 11 in terms of minority carriers. Also, the minority carriers that may exist therein are influenced laterally by the barrier-forming layer portions under the thin layer on both sides of the thick layer, particularly within the depth d from the surface, which is desirable. Uke begins to receive a repelling effect.

In this way, even though there is a thick layer part +2, the open circuit voltage V based on its existence. The decrease in C can be made negligible, and even in the worst case, it can be maintained at a sufficiently high value compared to the conventional case.

Furthermore, as in the present invention, the width of the thick layer portion 12 is defined as described above in relation to the barrier forming layer 20, and then the thin layer portion 1 is thinned to be less than the minority carrier diffusion length.
The existence of 1 and its effect of improving the open circuit voltage are of great significance.

In this example, the bonding layer 13 was provided on the light-receiving surface or light-incident surface side, and the barrier-forming layer 20 was provided on the back side, but these may be placed in a relatively reverse positional relationship. can.

Furthermore, as in the first illustrated embodiment, the flat surface of the semiconductor substrate as a whole is not used as the incident surface or the light-receiving surface, but the thickness difference between the thin layer portion 11 and the thick layer portion 12 is Since it is also possible to use the surface with the uneven surface as the light-receiving surface, an example of such a case will be described with reference to FIG. 2.

However, constructors in this figure with the same reference numerals as in Figure 1 have the same or similar functions as the corresponding constructors in Figure 1, and the previous explanation may be referred to in some cases. There are some cases in which the description below will be omitted.

As a result of the thin layer portion 11 and the thick layer portion 12 being mixed together, the uneven surface becomes the surface of the semiconductor substrate ]O, and bonding is performed via an anti-reflection film 31 provided as necessary on this surface side. A forming layer 13 is provided.

On the flat back side of the semiconductor substrate as a whole, as in the previous embodiment, a reflective layer 42 is provided with a barrier forming layer 20 and a protective layer 41, which is separately provided as required, sandwiched therebetween.

Of course, the thickness d of the thin layer portion H is set to be less than the diffusion length of minority carriers that can be photogenerated here, and the lateral dimension or width W of the thick layer portion 12 is 6, which is twice the minority carrier diffusion length. The following is considered.

This configuration provides a photoelectric conversion device that satisfies both mechanical strength and electrical photoelectric conversion characteristics for almost the same reason as explained above, but in the case of this embodiment, one Regarding each thick layer portion 12, the thin layer portions 11 on both sides of each thick layer portion 12, and the barrier forming layer under the layer 11, can not only carry out minority carrier functions from the lateral direction but also act as a barrier forming layer provided under itself. The cambium layer also performs the same function.

What is particularly desirable in this example is the thick layer! The side surface of the protruding portion 2 is a slope, and a reflective film 50 is provided on this slope.

In this case, as shown in the figure, even if the thick layer portion 12 protrudes into the light-receiving surface and some light hits this protrusion, this light is reflected by the oblique reflective film 50. , is reflected toward the surface of the thin layer portion 11, and can contribute to the generation of photoelectric conversion energy. In other words, even if there is a protrusion 12, it does not create a "shadow".

Furthermore, if this reflective film 50 is also used as the lead terminal 14 of the bonding layer 13 at one place on the left hand side in FIG. 2, as schematically shown by the phantom line, such a lead terminal can be created. The problem of wax “shadow” can also be solved.

However, in practice, it is more difficult to form the side surfaces of each thick layer part 12 as almost completely steep sides as shown in the first figure, so the flat side surface of the semiconductor substrate as shown in the first figure is Regardless of when the light-receiving surface is
As shown in the illustrated embodiment, when a concavo-convex surface is used as a light-receiving surface, it is practically desirable to adopt the above-mentioned configuration for applying the reflective film 50.

Of course, also in this second embodiment, the bonding layer 1
3 and barrier forming layer 20 are possible.

According to the present invention, the fact that the thin layer portion 11 can be made thinner also raises the fear that the thin layer portion 11 will be too thin and the light absorption will be insufficient. By forming the reflective layer 42 on the side surface, this point can be partially improved, and if a so-called textured structure or finely uneven surface shape is formed on the light receiving surface or reflective surface side, the thin layer portion 11 can be improved. Since the light incident path is oblique and the optical path becomes long, sufficient light absorption can be obtained.

FIG. 3 shows an example of the manufacturing process in the case where the embodiment structure shown in FIG. 1 or a structure similar to it is actually obtained.

To explain step by step, first, as shown in FIG. 3(A), the front and back surfaces of a (100)-plane silicon substrate 10 are oxidized to grow silicon oxide films each having a thickness of about 1 cm.

Next, the silicon oxide film 15 on the front surface is left as is, and the silicon oxide film on the back surface is formed into a thick layer to be formed in the future]2
The etching mask 16 is patterned by a photolithography process according to a two-dimensional plane pattern.

In the presence of this mask 16, surface index-dependent etching is performed using KOH or hydrazine.

After etching, silicon oxide film 16 used as a mask
Since there is a risk of contamination from alkaline solutions, it should be treated with buffered HF, etc.
When removed with an appropriate solution, a basic structure is obtained in which the thin layer portion 11 and the thick layer portion I2 are combined in a nested or cross-shaped pattern, as shown in FIG. 3(B), in accordance with the spirit of the present invention. .

As is clear, the thickness of the thick layer portion 12 is not intentionally adjusted, and unless changes in the thickness during the oxidation process etc. are taken into account, the thickness of the supplied semiconductor substrate itself is not adjusted. good. In fact, it seems to be more economical and numerically effective not to intentionally process the thick layer 12 to make it thicker, but if necessary, an appropriate etching method or slicing method can be used. By employing this, the thickness itself of the thick layer portion 12 may be adjusted to a desired value.

On the other hand, the thick layer 1 is less than twice the diffusion length of minority carriers.
The limitation regarding the width W of 2 is defined as the dimension at the portion where the thick layer portion 12 intersects with the adjacent thin layer portion U, and is of course realized depending on the mask size for the etching.

As is clear from this figure, the embodiment shown in Figures 1 and 2 is not limited to the case where thick layer portions and thin layer portions coexist only in one direction in the plane (although this is not the case). ),
This includes having similar cross-sectional structures in two mutually orthogonal directions on a two-dimensional plane. The structure shown in Fig. 3 (B) looks like a "board cholo" from a long time ago.

From the process shown in Figure 3 (B), the surface is textured if necessary as mentioned above, and then the process shown in Figure 3 (C) is performed.
As shown in FIG. 1, impurities having the same conductivity type as the semiconductor substrate IO are diffused onto the back surface with a diffusion mask 17 formed on the front surface by CVD or thermal oxidation technology, to form a barrier forming layer 20.

Instead of forming by such a diffusion technique, a semiconductor layer of the same conductivity type but with a wider gap may be formed by CVD, and this may be used as the barrier forming layer 20.

After this, as shown in FIG. 3(D), after providing a diffusion mask and passivation layer yA41 covering the barrier forming layer 20 by CVD or thermal oxidation, the surface side is of a conductivity type opposite to that of the semiconductor substrate. The impurity diffusion is performed to form the bonding layer 13.
form. However, regarding this bonding layer 13 as well,
If necessary, formation by CVD is also possible.

After this, if an antireflection film 31 and an extraction terminal (electrode) 11 are formed on the front surface, and an extraction terminal (electrode) 42 serving as a reflective layer is formed on the back surface by a known and existing appropriate method, the photoelectric conversion device of the present invention can be used. A basic structure similar to that of the first embodiment is completed.

[Brief explanation of the drawing]

Fig. 1 is a schematic block diagram of the main parts of a photoelectric conversion device constructed according to the present invention, Fig. 2 is a schematic block diagram of another embodiment of the present invention, and Fig. 3 is an embodiment of the present invention following the manufacturing process. FIG. In the figure, lO is a semiconductor substrate, 11 is a semiconductor thin layer portion 1, 12
13 is a semiconductor thick layer portion, 13 is a junction forming layer, 20 is a barrier forming layer, 42 is a reflective layer, and 50 is a reflective film provided on the protruding side surface of the thick layer portion. District 1, Figure 2

Claims (1)

[Claims] A semiconductor substrate having a predetermined area, with respect to its thickness,
Processed to have a mixed structure of a thin layer portion thinner than the diffusion length of minority carriers photogenerated inside the semiconductor substrate and a thick layer portion thicker than the thin layer portion and having sufficient physical strength. the thick layer portion is located between at least a pair of adjacent thin layer portions; and the junction forming layer that collects the minority carriers is placed along one surface of the semiconductor substrate and along the other surface. A barrier forming layer for repelling the minority carriers is provided, and the semiconductor surface of either the junction forming layer or the barrier forming layer is a light-receiving surface;
A junction photoelectric conversion characterized in that the other surface facing the light-receiving surface is a reflective surface with a reflective layer; and the width of the thick layer portion is not more than twice the minority carrier diffusion length. Device.