JPH04249374A - Photoelectric converting element - Google Patents

Abstract

PURPOSE:To contrive to increase an absorbing coefficient and enhance a light converting function by a method wherein a poly-crystal line silicon germanium thin film containing a specific amount or more of germanium atom is used in a photoelectric active region of a photoelectric converting element. CONSTITUTION:A metallic film 1-1 is formed on a surface of a glass substrate 1, and an n<+> type silicon and germanium layer 2 and an n type silicon and germanium layer 3 are formed on the surface thereof. At this time, a mixture ratio of phosphine (PH3) gas is same, and germanium (GeH4)gas other than silane (SiH4) gas is mixed as material gas at a mixture gas ratio of 10% or above to make the layers. A p<+> type silicon layer 4, a transparent conductive film 5, a collecting electrode 6, and the like are formed on the surface. Thereby, it is possible to enhance a photoelectric converting function.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to improvements in photoelectric conversion elements that convert light energy into electrical energy.

0002

[Prior Art] Looking at solar cells as an example of photoelectric conversion elements, in recent years, amorphous silicon-based solar cells and thin-film solar cells using compound semiconductors have been actively researched and developed. It has already been put into practical use as a power source for consumer electronics such as calculators. On the other hand, solar cells using wafers of crystalline (monocrystalline, polycrystalline) silicon, etc. have been put into practical use for space applications, lighthouses, and the like. However, in order to develop both of these into power generation systems that supply general electricity, which has the largest market size, (1) crystalline silicon using wafers, etc., requires the use of wafers, which are already expensive; It is difficult to compete with the power generation costs of general thermal power, hydropower, etc.

(2) Furthermore, thin-film solar cells have problems with photoelectric conversion efficiency and reliability, such as photodegradation, as seen in conventional amorphous silicon devices, and the degree of freedom in substrate selection is limited. , the advantages of low-cost processes are not fully utilized.

[0004]Recently, in order to solve the above-mentioned problems, there has been active research into growing a crystalline thin film, such as a polycrystalline silicon thin film, on a substrate such as glass to impart a photoelectric conversion function. These are attempts to bring out the inherent advantages of thin-film solar cells by depositing high-quality silicon thin films on the cheapest possible substrates without using expensive silicon wafers, and this direction has not been proven. Needless to say, it is effective not only for solar cells but also for general photoelectric conversion elements such as optical sensors and photocouplers.

[0005] A method for forming such a film is performed at low temperatures (room temperature to
400°C), plasma chemical vapor deposition method (p-
An amorphous film is deposited by CVD method), sputtering method, thermal CVD method, etc., and then deposited in a high temperature furnace (500°C to 1000°C).
A polycrystalline thin film is obtained by heat treatment or laser annealing.

FIG. 2 is a schematic cross-sectional view of an example of a conventional solar cell using a polycrystalline silicon thin film. This is manufactured as follows. First, a metal film 1 is placed on the surface of a glass substrate 1.
-1 is formed. Next, using a gas containing silane (SiH4) mixed with phosphine (PH3) gas in the range of 0.1% to 1%, the surface of the metal film 1-1 was coated with 1000 to 3000
An angstrom thin n+ type silicon layer 2-1 is formed. Next, an n- type silicon layer 3-1 is formed on the surface. This is the photoelectrically active layer. At this time, by setting the phosphine gas concentration to 0.1% or less, n-
A mold silicon layer 3-1 is obtained. Furthermore, p+
A mold silicon layer 4 is formed. At this time, silane (SiH
4) Diborane (B2 H6) gas is mixed in advance at a gas ratio of 0.1 to 1% to form a film of 100 to 1000 angstroms. Further, on the surface, a transparent conductive film 5
and a collector electrode 6 is formed.

[0007]

SUMMARY OF THE INVENTION The polycrystalline silicon thin film described above has the following problems. In other words, whether a polycrystalline silicon wafer or a polycrystalline silicon thin film is used, in order to sufficiently absorb sunlight inside the device, as long as the material is the same, the film thickness must be the same, that is, several tens of microns. It is natural that a film thickness greater than that is required. However, the film quality of the polycrystalline silicon thin films reported so far is much worse than that of polycrystalline wafers. This is partly because the crystal grain size is much smaller and the influence of poor quality crystal grain size cannot be avoided.

[0008] Therefore, when a polycrystalline silicon thin film is used in the photoelectrically active region and the film thickness is about the same as that of a polycrystalline wafer, carriers such as electrons or holes generated by sunlight will There is a problem in that the proportion collected by the electrodes becomes extremely low, and the photoelectric conversion function is significantly reduced compared to the case where a polycrystalline wafer is used.

[0009] Furthermore, there are process problems such as the fact that forming such a thin film usually requires a high temperature of 600° C. or higher, and that it takes a long time to form the film to obtain a thickness of several tens of microns.

0010

In the present invention, a polycrystalline silicon germanium thin film containing 10% or more of germanium (Ge) atoms is used in the photoelectric active region of a photoelectric conversion element.

[0011]

[Operation] It is a well-known fact that the absorption coefficient of crystalline germanium is generally more than an order of magnitude larger than that of crystalline silicon over the entire wavelength range. Therefore, by adding germanium to silicon, the absorption coefficient can be increased. Conversely, this means that the required film thickness for a photoelectric conversion element can be reduced by the proportion of the increase in the absorption coefficient, making it easier to collect photogenerated carriers to the electrodes and improving the photoelectric conversion function. improves.

0012

[Example] Figure 1 shows an example of an embodiment of the present invention, such as a solar cell (
FIG. 2 is a schematic cross-sectional view of a pn structure. This is manufactured as follows. First, as in the case of FIG. 2, a metal film 1-1 is formed on the surface of a glass substrate 1. Next, n+ on the surface
A type silicon germanium layer 2 and an n- type silicon germanium layer 3 are formed. At this time, phosphine (
The mixing ratio of PH3) gas is the same, and germane (GeH4) gas is mixed in, for example, at a mixed gas ratio of 10% in addition to silane (SiH4) gas as a raw material gas. Thereafter, a p+ type silicon layer 4, a transparent conductive film 5, a collector electrode 6, etc. are formed on the surface in the same manner as in FIG.

Generally, as the amount of germanium added increases, the band gap of the silicon-germanium thin film decreases, and as a result, in the case of solar cell elements, a decrease in open-circuit voltage appears. It goes without saying that the amount of germanium added should be determined by the balance of the rate of decrease in open circuit voltage.

In any of the above cases, p on the light incident side
In order to increase the bandgap as a type layer, CH4
It goes without saying that a p+ type SiC film may be used by mixing gas.

FIG. 3 shows the results of evaluating the absorption coefficient of a single layer thin film of the n- type silicon germanium layer 3 according to the present invention and the conventional n- silicon layer 3-1 with respect to the energy of light. It is a graph. When the device structure is as shown in FIG. 1 or 2, the photoelectrically active region is mostly n-
It is formed in the mold layer, and the amount of light absorbed and the photoelectric conversion function in this region greatly influences the device characteristics. As is clear from the figure, the characteristics of the polycrystalline n-type silicon germanium layer 3 of the present invention (shown by the solid line) are the same as those of the polycrystalline n-type silicon layer 3-1 of the conventional example (shown by the dotted line). The light absorption coefficient is large over the entire energy range, especially at low energy (1.5e
It can be seen that there is a difference of about 5 times in the absorption coefficient on the side (V or less) compared to the conventional example. This means that by using the polycrystalline silicon germanium thin film according to the present invention (in the case of this example, the germanium concentration in the film is about 30%), the film thickness of the device can be reduced by half an order of magnitude or less. That is,
The element configuration shown in FIG. 2 requires a film thickness of at least about 30 microns, but in the embodiment shown in FIG. 1 according to the present invention, it is possible to absorb enough light with a film thickness of about 6 microns. Ta. As a result, it has been difficult to collect photogenerated carriers due to the low quality film, which has always been a problem with polycrystalline silicon thin films. was found to be significantly improved. It can also be applied to pin structures.

[0016]

[Effects of the Invention] By adding germanium to silicon as in the present invention, the above-mentioned photoelectric conversion function can be expected to be improved. Further process advantages include:
(1) By adding germanium, it is possible to lower the crystallization temperature (600°C or higher for Si,
(550°C or less in the case of e), the degree of freedom in substrate selection increases, so cheaper substrates can be used, and (2) the device film thickness becomes thinner, which improves process throughput. can.

[Brief explanation of the drawing]

FIG. 1 is a schematic cross-sectional view of a solar cell that is an embodiment of the present invention;

[Fig. 2] A schematic cross-sectional view of an example of a conventional solar cell.

FIG. 3 is a graph showing a comparison of absorption coefficients of a polycrystalline silicon thin film and a silicon germanium thin film.

[Explanation of symbols]

1 Glass substrate 1-1 Metal film 2 N+ type silicon germanium layer 3 n-
Type silicon/germanium layer 4 P+ type silicon layer 5 Transparent conductive film 6 Collector electrode

Claims (1)

[Claims] Claim 1: Semiconductor thin films of different conductivity types are stacked on a substrate, and the layer that converts light energy into electrical energy is made of crystalline silicon containing 10% or more of germanium atoms.
A photoelectric conversion element characterized by using a germanium thin film.