JP3229753B2 - Photovoltaic device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a photovoltaic device,
In particular, the present invention relates to a photovoltaic device capable of increasing conversion efficiency after light degradation.

[0002]

2. Description of the Related Art A photovoltaic device using an amorphous Si alloy in which dangling bonds are terminated by hydrogen or a halogen group element (hereinafter referred to as a-Si alloy) for a power generation layer is composed of silicon (Si) atoms. Since the optical gap can be controlled in a wide range by alloying with carbon (C), nitrogen (N), oxygen (O), germanium (Ge), or tin (Sn) atoms, the aim is to make effective use of the solar spectrum. Promising as such a stacked photovoltaic device.

[0003] As a method of forming an alloy type amorphous semiconductor for forming a power generation layer, a plasma decomposition method is frequently used, and in terms of equipment, a capacitively coupled reactor having parallel plate electrodes is often used. The film-forming gas is selected according to the components constituting the alloy with Si. For example, amorphous silicon (hereinafter referred to as a-Si) and germanium (Ge), which occupy a representative position in the alloy type amorphous semiconductor, are selected. The alloyed a-SiGe: H is a monosilane (Si
H ₄ ), germane gas (GeH ₄ ), and hydrogen (H ₂ ) are used as film-forming gases.

[0004] The a-Si alloy is made of amorphous Si (hereinafter a-Si alloy).
It is called Si. Since the electrical characteristics are inferior to those of (1), the optical gap is changed by changing the composition ratio of constituent atoms other than Si in the power generation layer. To improve the initial characteristics of the photovoltaic device.

As the grading, as the film thickness increases, the composition ratio of Ge to Si (C _Ge
/ C _Si ) increases, C _Ge / C _Si simply decreases as the film thickness increases, and C _Ge / C _Si increases as the film thickness increases halfway,
It is known that C _Ge / C _Si decreases thereafter (App
l. Phys. Lett. 5 June 1989 p2330-2332).

Conventionally, in order to change the composition ratio of constituent atoms other than Si in the power generation layer, the flow rate ratio (S
iH ₄ / GeH ₄ ratio) and the composition ratio thereof, and therefore, for example, as shown in FIG.
Ge composition ratio of SiGe: H to Si (C _Ge / C _Si )
Increases, the composition ratio (C _H / C _Si ) of hydrogen, which is a terminating element, to _Si decreases.

[0007]

By the way, in a photovoltaic device in which a power generation layer is composed of a-Si, SiH
₂ of the film quality is deteriorated when there is a coupling of higher order, because the conversion efficiency is lowered, it is recommended to control appropriately the amount of hydrogen C _H.

Therefore, in a photovoltaic device in which the power generation layer is composed of an a-Si alloy, for example, a photovoltaic device in which the power generation layer is composed of a-SiGe: H, the amount of hydrogen C
_{When the} effect of _H on the conversion efficiency was determined, the amount of hydrogen C
_It has been found that the conversion efficiency increases in proportion to the hydrogen amount C _H until _H reaches a certain amount, but the conversion efficiency sharply decreases when the amount exceeds a certain amount. Also, SiH ₄ / Ge
When the amount of Ge is changed corresponding to the growth of the power generation layer by changing the H ₄ ratio, C _H / C _Si also changes. However, by controlling the growth rate (depo rate) and pressure of the power generation layer, termination element amount over the entire film thickness, i.e., can be made uniform C _H / C _Si over the entire thickness, moreover, C _Ge /
They discovered that C _Si can be changed arbitrarily.

The present invention has been completed based on the above findings, and has as its object to provide a photovoltaic device capable of increasing the conversion efficiency.

[0010]

SUMMARY OF THE INVENTION The present invention relates to a photovoltaic device using an a-Si alloy in which dangling bonds are terminated by hydrogen or a halogen group element as a power generation layer. A power generation layer is provided in which the composition ratio of the terminating element to the silicon element is uniform over the entire thickness of the layer and the composition ratio of alloy constituent atoms other than silicon is changed.

Here, the a-Si alloy means an amorphous semiconductor material composed of Si atoms and other atoms, for example, any one of C, N, O, Ge, and Sn.

[0012]

According to the present invention, since the composition ratio of the terminating element to the silicon element is uniform over the entire thickness of the power generation layer, the optimum uniformity that minimizes dangling bonds and high-order bonding of Si—H ₂ is minimized. It is possible to form an excellent film quality and reduce a decrease in conversion efficiency due to light degradation.

In the present invention, the composition ratio of the terminating element to the silicon element can be freely set, but it is not possible to increase the composition ratio of Si—H ₂ beyond the initial optimum value at which the highest initial characteristics are obtained. It is not preferable because the higher-order coupling increases and deteriorates the initial characteristics and the characteristics after light deterioration, and if the value is less than the optimum value after light deterioration at which the best characteristics are obtained after light deterioration, unbonded hands increase. This deteriorates the initial characteristics and the characteristics after light deterioration, which is not preferable. Most preferably, the optimum value after light degradation is smaller than the optimum value of the initial characteristics.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be specifically described below with reference to the drawings.

A photovoltaic device according to one embodiment of the present invention has a transparent electrode 1, a p-type a-Si layer 2, and a film thickness of 200.degree. a
A-Si layer 3, an a-SiGe layer 4 having a thickness of 1300 °, an a-Si layer 5 having a thickness of 200 °, an n-type a-Si layer 6 and a back electrode 7, and the a-Si layer 3 and the a-SiGe layer 4, a
-Si layer 5 forms an i-layer (power generation layer).

As shown in FIG. 2, Ge in the i-layer
The gradient profile is 0 to 20 growth
In the 0-degree a-Si layer 3, it increases almost linearly,
In the a-SiGe layer 4 between about 1300 ° and 1300 °, it is set to a substantially constant value (here, C _Ge / C _Si = 29%),
In the a-Si layer 5 during the period of 00 °, the shape is an isosceles trapezoid that decreases almost linearly. Further, the hydrogen composition ratio C _H / C _Si in the i-layer is constant over the entire film thickness (here, it is set to be 0.
098%).

The basic conditions for forming a-SiGe are as shown in Table 1. By controlling the reaction pressure for controlling the deposition and the RF power, the amount of the terminating element, that is, the amount of the terminating element over the entire film thickness, that is, C _H / C _Si is a constant value (0.
098%).

[0018]

[Table 1]

In the case where the optical gap determined by a-SiGe layer 4 (αhν) ^1/3 plot of this photovoltaic device constant at 1.32 eV, as shown in FIG. 3, and the hydrogen content C _H relationship between the initial conversion efficiency under the red filter is hydrogen amount C _H of about 9 atomic% or less is increased substantially in proportion to the conversion efficiency to the increase of the hydrogen content C _H, the conversion efficiency in excess of about 9 atomic% It can be seen that the value decreases and reaches a maximum value when C _H = about 9 at%. The decrease in the conversion efficiency in the region where the amount of hydrogen C _H is smaller than the maximum value is considered to be due to the presence of dangling bonds that are not terminated by hydrogen or a halogen group element, and the amount of hydrogen C _H is larger than the maximum value. The decrease in the conversion efficiency in the region is considered to be due to the occurrence of higher-order bonding of SiH _{2 and} the deterioration of the film quality.

[0020] Also, we determined the relationship between the initial conversion efficiency of the hydrogen amount after photodegradation C _H and a red filter under the photovoltaic device according to the conditions shown in Table 2, as shown in FIG. 4,
About the amount of hydrogen compared to the initial conversion efficiency C _H less 7
highest value obtained by at%, and satisfy the relations different optimum hydrogen content C _H is at about photodegradation 3 and 4, the optimum value after the light degradation is seen to be shifted to the low-hydrogen side.

[0021]

[Table 2]

Next, the optical gap of the a-SiGe layer 4 of this photovoltaic device was changed to 1.50 eV, 1.43.
The same experiment was performed for the case of eV and 1.36 eV,
When the optimum hydrogen amount after deterioration in each optical gap was determined, the result shown in FIG. 5 was obtained. The vertical axis of FIG. 5 shows the hydrogen amount C _H, and when this is replaced by the hydrogen composition ratio C _H / C _Si , the result becomes as shown in FIG. 6, where C _H / C _Si = 0.098.
It can be seen that the optimum composition after light degradation of each optical gap exists in (%).

Based on the above results, the Ge composition ratio was changed as shown in FIG.
The hydrogen composition ratio C _H / C _{Si is changed} to a constant value (0.098%) to improve the carrier transport characteristics at the same time. When the solar cell characteristics were examined, the results shown in Table 3 were obtained. For comparison, the solar cell characteristics of the conventional photovoltaic device having an optical gap graded structure shown in FIG. 7 were examined. The results shown in Table 4 were obtained.

[0024]

[Table 3]

[0025]

[Table 4]

As is clear from Tables 3 and 4, according to the above-described embodiment, the deterioration rate is greatly improved to 4%, and the conversion efficiency (Ef) under the red filter after the deterioration.
f), a very high solar cell characteristic of 3.45% was obtained.

According to this embodiment, since the hydrogen amount C _H is set to the optimum value after photodegradation, the probability that the carriers injected into the i-layer are quickly collected by the electrode and recombine is probably high. It is considered that the photo-deterioration was reduced because the carrier injected into the i-layer is less likely to induce defects when it is recombined and deteriorate the film quality of the i-layer.

In the above-described embodiment, the Ge gradient profile has an isosceles trapezoidal shape, but the profile may be unequally shaped trapezoidal or mountain-shaped.

In the description of the above embodiment, a-SiG
e: H was described only, but similar results were obtained for alloys of Si and C atoms, alloys of Si and N atoms, alloys of Si and O atoms, and alloys of Si and Sn atoms. Have been obtained.

[0030]

As described above, according to the present invention, since the amount of the terminating element is fixed at a constant value over the entire film thickness, dangling bonds and Si not terminated by hydrogen or a halogen group element can be obtained. higher binding -H ₂ can form a uniform film quality of optimal minimized, it is possible to reduce the lowering of conversion efficiency due to photo-deterioration.

[0031] In the present invention, in particular it is possible to further enhance a conversion efficiency after light degradation in the case of setting the optimum value or less after the photodegradation than the initial optimum amount of hydrogen C _H, practically more preferable You can design.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view of an embodiment of the present invention.

FIG. 2 is a composition distribution diagram of a power generation layer according to an example of the present invention.

FIG. 3 is a characteristic diagram showing a relationship between an initial hydrogen amount and a conversion efficiency in an example of the present invention.

FIG. 4 is a characteristic diagram showing a relationship between a hydrogen amount after photodegradation and a conversion efficiency according to an example of the present invention.

FIG. 5 is a characteristic diagram illustrating a relationship between an optical gap and an optimum hydrogen amount after deterioration according to the example of the present invention.

FIG. 6 is a characteristic diagram showing a relationship between an optical gap and an optimum composition ratio after deterioration according to an example of the present invention.

FIG. 7 is a composition distribution diagram of a conventional power generation layer.

[Explanation of symbols]

The composition ratio of 3 a-Si layer 4 a-SiGe layer 5 a-Si layer C _H / C _Si hydrogen composition ratio C _Ge / C _{Si Ge}

Continuation of the front page (56) References JP-A-4-286167 (JP, A) JP-A-4-85816 (JP, A) JP-A-2-260662 (JP, A) JP-A-7-221337 (JP) , A) (58) Field surveyed (Int. Cl. ⁷ , DB name) H01L 31/04-31/078 H01L 21/205

Claims (1)

(57) [Claims] In a photovoltaic device using an amorphous silicon alloy terminated by dangling atoms of hydrogen or a halogen group for a power generation layer, the composition ratio of a terminating element to a silicon element over the entire thickness of the power generation layer is increased. A photovoltaic device comprising a power generation layer which is uniform and has a different composition ratio of alloy constituent atoms other than silicon.