JP3746711B2 - Method for manufacturing microcrystalline silicon solar cell - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for manufacturing a microcrystalline silicon solar cell including a microcrystalline silicon layer having a pin structure.
[0002]
[Prior art]
Conventionally, a microcrystalline silicon solar cell having a pin structure is manufactured as follows using a plasma CVD apparatus. First, after forming a transparent electrode on a transparent insulating substrate, a p-type microcrystalline silicon layer (p layer) is formed using SiH ₄ as a main gas, H ₂ as a dilution gas, and B ₂ H ₆ as a doping gas. To do. Next, a substantially intrinsic i-type microcrystalline silicon layer (i layer) is formed on the p layer using SiH ₄ as a main gas and H ₂ as a diluent gas. Further, after forming an n-type microcrystalline silicon layer (n layer) using SiH ₄ as a main gas, H ₂ as a dilution gas, and PH ₃ as a doping gas, it is made of a conductor such as indium tin oxide, zinc oxide, or aluminum. A back electrode is formed, which becomes a solar cell.
[0003]
By the way, in order to improve the power generation efficiency, the p-layer, which is a conductive semiconductor layer and window material, increases the output voltage by increasing the open circuit voltage, that is, the generated voltage at the time of light irradiation, and reducing the light absorption. It is desirable to have a wide band gap and high conductivity in order to reduce resistance loss.
[0004]
Note that although generally referred to as microcrystal, microcrystalline silicon generally represents a mixture of crystalline silicon and amorphous silicon. Here, what has a peak (520 cm ⁻¹ ) of a microcrystalline silicon spectrum by a Raman spectroscopic spectrum is included in “microcrystal”.
[0005]
[Problems to be solved by the invention]
However, in the conventional silicon-based solar cell, even if the p layer is simply widened, the conductivity of the p layer is greatly impaired and the short-circuit current is reduced, resulting in a decrease in power generation efficiency. . As described above, in the conventional solar cell, when the band gap is widened, the conductivity is decreased, and conversely, when the conductivity is increased, the band gap is narrowed. Therefore, the band gap and the conductivity are in a trade-off relationship. For this reason, the current situation is that the power generation efficiency remains at a low level. Improvement of the efficiency of solar cells has been strongly demanded by users, and it has become an urgent task to develop and put into practical use high-efficiency solar cells.
[0006]
The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a method for producing a highly efficient microcrystalline silicon solar cell having a wide band gap without impairing electrical conductivity.
[0008]
[Means for Solving the Problems]
When a carbon-containing silicon layer is inserted between the p layer and the i layer, only the vicinity of the interface has a wide band gap and the open circuit voltage increases. In addition, since the carbon-containing silicon layer is thin and the crystallinity is not significantly impaired, the conductivity is not significantly reduced as in the case where a silicon carbide film is formed as the p layer.
The method for producing a microcrystalline silicon solar cell according to the present invention includes a microcrystalline silicon layer in which at least a transparent electrode film, p-type, i-type, and n-type microcrystalline silicon layers and a back electrode film are stacked on a transparent insulating substrate. A method for manufacturing a crystalline silicon solar cell or a tandem-type thin film silicon solar cell including p-type, i-type, and n-type microcrystals, after forming a p-type microcrystalline silicon layer on a substrate having a transparent electrode film, The container is evacuated, a source gas containing at least a carbon-containing gas is introduced into the container, and high-frequency power of a predetermined frequency is applied to the discharge electrode to generate plasma between the discharge electrode and the substrate, The i-layer side surface of the p-type microcrystalline silicon layer is exposed to plasma, carbon in the plasma is allowed to act on the surface to carbonize the surface to form a carbon-containing silicon layer, and then the carbon-containing silicon layer An i-type microcrystalline silicon layer is formed on the substrate.
[0009]
The carbon-containing gas is preferably an unsaturated hydrocarbon gas such as ethylene (C ₂ H ₄ ) or acetylene (C ₂ H ₂ ). This is because unsaturated hydrocarbons have double bonds or triple bonds, so they are easily dissociated in plasma, and the reaction rate with the underlying silicon is high, so the productivity is remarkably increased.
[0010]
Further, hydrogen, argon, xenon, or any mixed gas thereof is used as a dilution gas for diluting the carbon-containing gas. Hydrogen promotes the reaction between silicon and carbon by cutting and activating the bond on the surface of the silicon film. Argon and xenon have the effect of injecting carbon into the film by its ion energy. That is, it has the effect of increasing the thickness of the silicon carbide layer.
[0011]
In the present invention, the carbon-containing silicon layer has a thickness of 1 to 20 nm. This is because if the film thickness is less than 1 nm, the effect of widening the band gap becomes insufficient, and if the film thickness exceeds 20 nm, the conductivity is lowered.
[0012]
The p-type microcrystalline silicon layer as the underlayer is preferably doped with boron at a concentration of 10 ¹⁹ to 10 ^{2 1} atom / cm ³ . When the B doping concentration is less than 10 ¹⁹ atoms / cm ³ , p-type conductivity is not exhibited. On the other hand, if the B doping concentration exceeds 10 ^{2 1} atom / cm ³ , the excess boron causes a disadvantage of inhibiting crystallization.
[0013]
It is preferable to apply a negative bias voltage of 1 kV or less to the substrate while the surface of the p-type microcrystalline silicon layer is exposed to the plasma containing carbon active species. This is because when the negative bias voltage exceeds 1 kV, the diluent gas penetrates into the deep portion of the silicon layer and remains, thereby inhibiting conductivity. The bias voltage may be direct current or alternating current. In the case of alternating current, it is sufficient that a negative bias of 1 kV or less can be periodically applied. The AC frequency of the bias voltage is set to one tenth or less of the plasma generation high frequency power frequency so as not to interfere with the plasma generation high frequency power.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, various preferred embodiments of the present invention will be described with reference to the accompanying drawings. First, a plasma CVD apparatus used for manufacturing the microcrystalline silicon solar cell of the present invention will be described with reference to FIG.
[0015]
The plasma CVD apparatus 1 includes a conductive plate 3, an insulating plate 4, a heater 5, and a plasma generation unit 6 in a vacuum container 10 through which a gas supply pipe 8 and an exhaust pipe 13 communicate. The conductive plate 3, the insulating plate 4, the heater 5, and the plasma generation unit 6 are electrically insulated from the grounded container 10, respectively. The exhaust pipe 13 communicates with the suction side of the vacuum pump 14 and can be exhausted until the internal pressure of the container 10 becomes a high vacuum state of 1 × 10 ⁻⁷ Torr or less.
[0016]
One end of the gas supply pipe 8 communicates with the gas supply unit 9 and the other end communicates with the plasma generation unit 6 in the container 10. The gas supply unit 9 has a plurality of gas supply sources that house different gas types. Each gas supply source includes an on-off valve, a pressure regulating valve, and a mass flow controller (MFC). The controller 12 controls the operation of these valves and the MFC. That is, when the controller 12 sends a signal to each valve and the power supply circuit of the MFC, the gas supply unit 9 sends silane (SiH ₄ ), hydrogen (H ₂ ), diborane (B ₂ H ₆ ), ethylene (C ₂ H ₄ ). A gas such as argon (Ar) is supplied to the gas supply pipe 8 at a predetermined combination and a predetermined flow rate. A plurality of types of gases join together in the gas supply pipe 8 and are introduced into the bottom of the plasma generation unit 6 through the gas supply pipe 8.
[0017]
In the vacuum vessel 10, a conductive plate 3, an insulating plate 4, a heater 5 and a plasma generation unit 6 are provided as substrate holders. The conductive plate 3 is attached to the heater 5 via the insulating plate 4 so that the substrate 2 is held by the conductive plate 3. The heater 5 is connected to a power source (not shown). The heater 5 has the ability to heat the substrate 2 up to a temperature of 400 ° C.
[0018]
The conductive plate 3 includes a mechanical clamp (not shown) for mechanically holding the substrate 2. The conductive plate 3 is connected to the negative electrode side of the DC power supply 15. When the controller 12 sends a signal to the power supply 15, a negative bias voltage of 100 V, for example, is applied to the conductive plate 3.
[0019]
A grid-like (ladder-type) discharge electrode 7 is attached to the upper part of the frame of the plasma generation unit 6. The discharge electrode 7 is disposed opposite to the substrate 2 held on the conductive plate 3. The gas from the gas supply unit 9 is introduced into the bottom of the plasma generation unit 6, passes between electrode bars constituting the discharge electrode 7, and flows toward the substrate 2.
[0020]
The discharge electrode 7 is connected to the high frequency power supply 11 through a plurality of feeding points. The high frequency power supply 11 is controlled by a controller 12 so that high frequency power having a predetermined frequency is applied to the electrode 7.
[0021]
Example 1
The solar cell of Example 1 will be described with reference to FIG.
The solar cell 20 includes a transparent electrode film 21, a p-type microcrystalline silicon layer 22, a carbon-containing silicon layer 27 (hereinafter referred to as a carbon mixed layer) 27, a substantially intrinsic i-type microcrystalline silicon layer on a glass substrate 2. 23, an n-type microcrystalline silicon layer 24, a transparent electrode film 25, and a back electrode film 26 are laminated in this order.
[0022]
The transparent electrode film 21 has a two-layer structure of a tin oxide (SnO ₂ ) layer and a zinc oxide (ZnO) layer stacked on the glass substrate 2 directly or via a silicon dioxide (SiO ₂ ) barrier layer. is there. The zinc oxide (ZnO) layer has a function of preventing the tin oxide (SnO ₂ ) layer from being reduced when the next p-type microcrystalline silicon layer 22 is formed. The upper tin oxide (SnO ₂ ) layer is connected to a negative electrode of an external load (not shown).
[0023]
The layers 22, 27, 23, and 24 are portions corresponding to a pin structure having a power generation function. Among these, the p-type microcrystalline silicon layer 22 is made of a p-type microcrystalline silicon film (μc-p-Si), has a film thickness of 10 to 50 nm, and a boron (B) doping amount of 10 ¹⁹ to 10 ^{2 1} atom / cm ³ .
[0024]
The carbon mixed layer 27 contains a maximum of 50 atomic% of carbon and has a film thickness of 1 to 20 nm. The film thickness of the carbon-mixed layer 27 is controlled by the gas type, bias voltage, and processing time of the carbon-containing gas plasma exposure process, but is preferably in the range of 1 to 20 nm. This is because if the thickness of the carbon-mixed layer 27 is less than 1 nm, the effect of a wide band gap cannot be obtained due to the tunnel current. On the other hand, when the film thickness of the carbon-mixed layer 27 exceeds 20 nm, the conductivity decreases. In this example, ethylene (C ₂ H ₄ ) diluted with argon was used as a source gas, and the film thickness of the carbon-mixed layer 27 was finally set to 4 nm.
[0025]
The substantially intrinsic i-type microcrystalline silicon layer 23 is made of an impurity-doped microcrystalline silicon film (μc-Si) having an average crystal grain size of 0.1 μm, and has a thickness of 1.5 μm.
[0026]
The n-type microcrystalline silicon layer 24 has a thickness of 40 nm and a phosphorus (P) doping amount of 10 ¹⁹ to 10 ^{2 1} atom / cm ³ .
[0027]
The transparent electrode film 25 is made of indium tin oxide (ITO).
[0028]
The back electrode film 26 is made of aluminum (Al), is connected to an external load, and totally reflects light incident from the glass substrate 2 side.
[0029]
(Production method)
Next, the manufacturing method of the solar cell of Example 1 is demonstrated with reference to FIG.
[0030]
The solar cell 20 of this example was manufactured by the following procedures (1) to (14) using the plasma CVD apparatus shown in FIG. 1, a thermal CVD apparatus (not shown), and a sputtering apparatus.
[0031]
(1) As the transparent insulating substrate 2, colorless and transparent soda lime glass (blue plate glass) was used. On this glass substrate 2, a silicon dioxide (SiO ₂ ) barrier layer and a tin oxide (SnO ₂ ) first transparent electrode film are formed by thermal CVD, and subsequently zinc oxide (ZnO) second transparent electrode film is formed by sputtering. 21 was formed (step S1).
[0032]
(2) Next, the substrate 2 was set so as to be in close contact with the conductive plate 3 of the plasma CVD apparatus, and then evacuated by the vacuum pump 14 until the internal pressure of the container 10 became 5 × 10 ⁻⁷ Torr or less. The heater 5 was energized, and the substrate 2 was heated to a predetermined temperature, for example, to stabilize the temperature sufficiently. In addition, the heating temperature of the board | substrate 2 is not limited only to predetermined temperature, It is possible to change variously in the range of 100-300 degreeC according to the design conditions of a solar cell.
[0033]
(3) Next, silane (SiH ₄ ), hydrogen (H ₂ ), and diborane (B ₂ H ₆ ) were introduced into the container 10 as reaction gases for forming the microcrystalline p layer 22. The gas flow rates were set to SiH ₄ : _{3 to} 5 sccm, H ₂ : 300 to 500 sccm, and B ₂ H ₆ : 0.005 to 0.10 sccm. Here, in order to obtain the microcrystalline p layer 22 having excellent crystallinity, the flow rate of B ₂ H ₆ needs to be 0.1 sccm or less. The reaction gas may be separately introduced into the container 10 for each component, or may be mixed in advance outside the container 10 and introduced into the container 10 at once.
[0034]
(4) After introducing the source gas, the controller 12 controlled the internal pressure of the container 10 to 0.5 Torr. Here, the control pressure in the container 10 is not limited to only 0.5 Torr, but can be variously changed in the range of 0.2 to 2.0 Torr according to the design conditions of the solar cell. After sufficiently stabilizing the temperature and pressure in the container 10, plasma is generated by supplying high-frequency power from the high-frequency power source 11 to the electrode 7 via an impedance matching device (not shown), and the microcrystalline p-layer 22. Was formed (step S2).
[0035]
(5) Here, when the frequency of the high-frequency power source 11 is less than 10 MHz, a problem arises that the film forming conditions for microcrystallization become extremely narrow. Therefore, the frequency is preferably 10 MHz or more. Subsequently, in this state, a film was formed for a predetermined time so that the film thickness of the microcrystalline p layer 22 was 30 nm.
[0036]
(6) Next, the supply of the high frequency power and the raw material gas is stopped, the inside of the container 10 is once evacuated, and then ethylene (C ₂ H) diluted with argon (Ar) as a carbon-containing gas for forming the carbon mixed layer 27 is formed. ₄ ) Gas was introduced into the container 10. Here, the flow rate of each _{gas, C 2 H 4: 2~20sccm,} Ar: was 0～200Sccm.
[0037]
(7) Next, after introducing the carbon-containing gas, the controller 12 controlled the internal pressure of the container 10 to 0.15 Torr. Here, the pressure in the container is not limited to only 0.15 Torr, and can be variously changed within a range where plasma is generated. Further, the conductive plate 3 was energized from the DC power source 15 and a negative bias voltage of 100 V was applied to the substrate 2. In this embodiment, the bias voltage is a negative voltage of 100 V. However, the effect of the present invention can be obtained by applying a high bias voltage (for example, 1 kV) exceeding the bias voltage. Further, the bias voltage may be an alternating current as long as a negative bias can be applied periodically. The AC frequency of the bias voltage is preferably set to one-tenth or less of the plasma generation high frequency frequency so as not to interfere with the plasma generation high frequency power. After sufficiently stabilizing the temperature of the substrate 2 and the pressure in the container 10, plasma is generated by supplying high-frequency power of a frequency of 10 MHz or more to the electrode 7 from the high-frequency power source 11 through the matching unit, and p-type microcrystal The surface of the silicon layer 22 was exposed to plasma containing carbon active species to be modified into a carbon-mixed layer 27 made of silicon containing carbon (step S3).
[0038]
(8) Subsequently, in this state, the carbon-mixed layer 27 was processed for a predetermined time so that the film thickness became 4 nm.
[0039]
(9) Next, the supply of the high-frequency power and the carbon-containing gas is stopped, the inside of the container 10 is once evacuated, and then silane (SiH ₄ ) and hydrogen ( H ₂ ) was introduced, and a microcrystalline i layer 23 having a thickness of 1500 nm was formed in substantially the same manner as the microcrystalline p layer 22 (step S4).
[0040]
(10) Thereafter, the supply of high-frequency power and source gas is stopped, the inside of the container 10 is evacuated, and then silane (SiH ₄ ), hydrogen (H ₂ ) and phosphine (PH ₃ ) are used as source gases for the microcrystalline n layer. ) And a microcrystalline n layer 24 having a thickness of 40 nm was formed in substantially the same manner as the microcrystalline p layer 22 (step S5).
[0041]
(11) The laminate obtained up to step S5 is unloaded from the plasma CVD apparatus 1 and loaded into the sputtering apparatus, and is made of indium tin oxide (ITO) having a thickness of 100 nm on the microcrystalline n layer 24. A transparent electrode film 25 and a back electrode film 26 made of aluminum (Al) with a film thickness of 300 nm were formed (step S6). Thereby, a microcrystalline silicon solar cell 20 as a product was obtained.
[0042]
(Comparative example)
As Comparative Example 1, using the plasma CVD apparatus shown in FIG. 1, the step of exposing to carbon-containing gas plasma (S3 in FIG. 2) is not performed on Example 1, and the other conditions are the same as in Example 1. Thus, a sample of Comparative Example 1 was produced.
[0043]
As Comparative Example 2, a p-Si layer was formed in Example 1 and carbon-containing p layer (p-SiC layer) was replaced with the following in place of carbon-containing gas plasma exposure (S3 and S4 in FIG. 2). The film was formed under the conditions, and the sample of Comparative Example 2 was prepared under the same conditions as in Example 1 in the other steps.
[0044]
1) Film formation condition pressure of p-SiC layer; 0.5 Torr
Gas flow rate; SiH ₄ / H ₂ / B ₂ H ₆ / C ₂ H ₄
3 sccm / 300 sccm / 0.02 sccm / 1 sccm
Film thickness: 20nm
B doping concentration; 10 ^{2 0} atoms / cm ³
(Evaluation results)
The solar cell 20 of Example 1 was irradiated with simulated sunlight (spectrum: AM1.5, irradiation intensity: 100 mW / cm ² , irradiation temperature: room temperature), and the power generation characteristics were evaluated. The solar cell of Example 1 improved the open circuit voltage by 1.10 times and the power generation efficiency by 1.16 times compared with Comparative Example 1. On the other hand, in Comparative Example 2, the open-circuit voltage was improved by 1.03 times compared to Comparative Example 1, but the power generation efficiency was reduced by 0.76 times. The reason why the open circuit voltage was improved as compared with Comparative Example 1 was that the i-layer side interface of the p layer was carbonized and widened by exposure to the carbon gas-containing plasma. Compared to Comparative Example 2, the efficiency increased in both cases where the p layer had a wide band gap, but because Comparative Example 2 had low p layer conductivity, the series resistance increased and the short circuit current decreased. On the other hand, Example 1 had a high conductivity. In addition, although Example 1 demonstrated the case of the solar cell of a single pin structure, this invention is not limited only to this, The tandem-type solar cell which laminates | stacks an amorphous cell and a microcrystal cell in series is demonstrated. It can also be applied to batteries.
[0045]
In the above embodiment, the case of a solar cell having a microcrystalline silicon pin structure has been described. However, the present invention is not limited to this, and a part of the p layer is replaced with microcrystalline silicon carbide (SiC). The present invention can also be applied to solar cells.
[0046]
【The invention's effect】
According to the present invention, there is provided a method for manufacturing a microcrystalline silicon solar cell having a wide band gap and a high conductivity.
[Brief description of the drawings]
FIG. 1 is an internal perspective sectional view schematically showing a plasma CVD apparatus.
FIG. 2 is a flowchart showing a method for manufacturing a microcrystalline silicon solar cell according to an embodiment of the present invention.
FIG. 3 is a schematic cross-sectional view showing a solar cell.
[Explanation of symbols]
1 ... Plasma CVD apparatus,
2 ... substrate,
3 ... conductive plate,
4 ... Insulating plate,
5 ... Heater,
6 ... Plasma generation unit,
7 ... discharge electrode,
8 ... Gas supply pipe,
9: Gas supply unit,
10 ... Vacuum container,
11 ... High frequency power supply,
12 ... Controller,
13 ... exhaust pipe,
14 ... pump,
15 ... DC power supply,
20 ... solar cell,
21, 25 ... Transparent electrode film,
22 ... p-type microcrystalline silicon film (p layer),
23 ... i-type microcrystalline silicon film (i layer),
24 ... n-type microcrystalline silicon film (n layer),
26: Back electrode film (Al),
27: a carbon-containing silicon layer.

Claims (4)

A microcrystalline silicon solar cell in which at least a transparent electrode film, a p-type, an i-type, and an n-type microcrystalline silicon layer and a back electrode film are laminated on a transparent insulating substrate, or a p-type, i-type, and n-type A method of manufacturing a tandem-type thin film silicon solar cell containing microcrystals, wherein after forming a p-type microcrystalline silicon layer, the container is evacuated and a source gas containing at least a carbon-containing gas is introduced into the container Then, a high frequency power of a predetermined frequency is applied to the discharge electrode to generate plasma between the discharge electrode and the substrate, and the p-type microcrystalline silicon layer i The surface on the layer side is exposed to plasma, and carbon in the plasma is allowed to act on the surface to carbonize the surface to form a silicon layer containing carbon, and then an i-type microcrystal on the carbon-containing silicon layer A method for manufacturing a microcrystalline silicon solar cell or a tandem-type thin film silicon solar cell including p-type, i-type, and n-type microcrystals, wherein a silicon layer is formed. 2. The method of claim 1, wherein a negative bias voltage is applied to the substrate while the surface of the p-type microcrystalline silicon layer is exposed to a plasma containing carbon active species. The method according to claim 1, wherein the carbon-containing gas comprises an unsaturated hydrocarbon gas. The p-type microcrystalline silicon layer has a concentration of 10 ¹⁹ -10 ²¹ Atom / cm ³ The method of claim 1, wherein the boron is doped.

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