JP2012033758A - Solar battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To achieve good characteristics by a field passivation effect while avoiding occurrence of a parasitic shunting phenomenon, in a rear face passivation type solar battery.SOLUTION: A solar battery has a pn junction, and configures a photoelectric conversion element. An insulation film and a rear face electrode are formed on a front face of a p-type semiconductor layer on a side opposite to a light reception side. A value of an interface fixed charge density Qon an interface between the p-type semiconductor layer and the insulation film is in a range of 2e11 cm≤Q≤4e12 cm. Thereby, good characteristics can be obtained while avoiding occurrence of a parasitic shunting phenomenon.

Description

  The present invention relates to a solar cell constituting a photoelectric conversion element, and more specifically to, for example, a back surface passivation type solar cell.

  A solar cell which is a kind of photoelectric conversion element basically has a pn junction of a p-type semiconductor layer that collects positively charged holes generated by light reception and an n-type semiconductor layer that collects negatively charged electrons. Specifically, the pn junction is formed by forming an n-type impurity diffusion layer on the light-receiving surface side of the p-type silicon substrate, and electrodes are respectively formed on the light-receiving surface side and the back surface side of the p-type silicon substrate. Is provided.

  The electrode formed on the back surface generally has a two-layer structure of a p + layer formed on a p-type silicon substrate and a metal layer formed on the layer.

  Conventionally, aluminum is used as a material for forming the metal layer. When heat of 700 ° C. or higher is applied in a state where aluminum is in contact with the p-type silicon substrate, a diffusion region in which aluminum diffuses in the p-type silicon substrate is formed, and an alloy of aluminum and silicon is generated. Since aluminum serves as a dopant for supplying holes to silicon, a p + layer is formed in the diffusion region. Thus, a part of aluminum contributes to p + layer formation, and the remaining aluminum becomes a metal electrode as it is.

  In this type of solar cell, an electric field derived from a potential difference is formed at the interface (p / p + interface) between the p-type silicon substrate and the p + layer. This electric field generated by the p + layer is mainly generated in the p-type silicon substrate and diffused to the back surface, and reflects the electrons into the p-type silicon substrate and selectively passes the holes to the p + layer. It has a function to make it. That is, this action brings about an effect (field passivation effect) of eliminating carriers and reducing carrier loss due to recombination of holes and electrons at the back surface interface of the solar cell.

  This is because the electric field generated at the p / p + interface spatially separates the distributions of holes and electrons in the vicinity of the back surface and causes a density difference on the back surface, thereby significantly reducing the probability of coupling between the two. Because. As a result, this contributes to improvement of the conversion efficiency of the solar cell.

  A conventional solar cell having such an aluminum alloy layer on the back surface is widely used as a mass production technique because it is relatively easy to manufacture. However, this method has the following problems in view of the ultimate high efficiency of solar cells.

  That is, as described above, most of the electrons are rejected by the electric field created by the p + layer, but still a few electrons with kinetic energy higher than the potential difference generated at the p / p + interface will overcome this potential difference. This leads to the p + layer / back electrode interface. However, in the conventional method, no deactivation treatment is performed on the p + layer / back electrode interface to reduce the interface recombination rate. In addition, since the p + layer itself is formed by highly doped aluminum to form recombination centers, the density of recombination centers is high and the quality as a semiconductor is poor.

  Therefore, a small number of electrons that have overcome the barrier are recombined with high probability and disappear in the p + layer or at the p + layer / back electrode interface. This electron annihilation causes the loss of the electrical output of the solar cell. Become.

  Further, it is desirable that a part of the light incident from the front surface of the solar cell and reaching the back surface is reflected on the back surface to return to the inside of the solar cell again and contribute to power generation. However, in the conventional method, the light absorption rate in the p + layer is large. For this reason, most of the light reaching the back surface of the solar cell is absorbed by the p + layer, and the electron-hole pairs generated at that time immediately recombine and disappear, so that they do not contribute to the electrical output. . Furthermore, since the physical shape of the interface between the p + layer and the electrode is generally uneven and is not a smooth flat surface, the reflectance of the reaching light decreases due to this uneven shape.

  As described above, the conventional solar cell has a problem that the light reaching the back surface of the solar cell cannot be effectively used for power generation.

  In response to this problem, development of backside passivation solar cells is underway with the aim of future replacement of the conventional solar cells. The present solar cell is different from the conventional solar cell. Specifically, by covering the back surface of the solar cell with a passivation film, the dangling bonds that inherently exist at the interface between the substrate and the passivation film and cause recombination are terminated. That is, the back surface passivation type solar cell does not attempt to reduce the carrier recombination velocity by the electric field generated at the p / p + interface, but rather reduces the density of the recombination centers on the back surface itself and reduces the carrier recombination. Based on the idea.

  Therefore, in this solar cell, when the performance of the back surface passivation film is sufficiently high, a low-quality p + layer as a semiconductor becomes unnecessary, and recombination loss caused by the p + layer and large light absorption in the p + layer are eliminated. be able to. Furthermore, the interface between the passivation film and the substrate can be produced smoothly, and the light reflectance at the interface can be increased.

  The back surface passivation film used for the back surface passivation type solar cell contains silicon and hydrogen. The dangling bonds that are inherently present at the interface between the p-type silicon substrate and the back surface passivation film and cause recombination are terminated by the silicon element and the hydrogen element of the back surface passivation film, so that they are not recombination centers. Therefore, the holes and electrons that have reached the back surface are not captured by dangling bonds, but are reflected at the interface as if nothing happened, and return to the p-type silicon substrate.

  This idea looks very promising from one point of view of interface deactivation. However, there is a problem when applied to actual solar cells. A portion covered with a passivation film and an electrode are adjacent to each other on the front or back surface of the solar cell. When an interaction between two parties is considered, a simple scenario as described above is not necessarily established. That is, in the backside passivation type solar cell, there is a problem that a characteristic deterioration phenomenon known as parasaimatic shunting occurs in the world, and the improvement in characteristics is limited.

  Here, the principle of occurrence of the parasitic shunting phenomenon will be described with reference to FIGS.

  FIG. 11 is a schematic view showing a structural example of a back surface passivation type solar cell. In the figure, an n + layer 82 and an antireflection film 83 are formed on the surface of a P-type silicon substrate 81. In order to take out an electrical output also in the back surface passivation type solar cell, the back surface electrode layer 85 is required in addition to the front surface electrode layer 84. In this figure, the back electrode layer 85 is in contact with the P-type silicon substrate 81, and the opening region of the passivation film 86 for taking out electrical output from the P-type silicon substrate 81 is referred to as a back electrode 89.

  Since it is necessary to take out an electrical output from the P-type silicon substrate 81 at a portion where the back electrode 89 is formed, a passivation film 86 (in many cases, an insulator film) cannot be deposited. Since the back surface electrode 89 is provided in a dot shape or a stripe shape on a part of the back surface, it occupies several percent of the entire back surface. As a result, the portion covered with the passivation film and the back electrode portion are adjacent to each other on the back surface of the back surface passivation type solar cell.

  At this time, in the case of the back surface passivation type solar cell using the P type silicon substrate 81, if a positive fixed charge is present at the interface between the passivation film 86 and the P type silicon substrate 81 (hereinafter referred to as the back surface interface). For example, in the P-type silicon substrate 81 immediately below the passivation film 86, an electric field is generated by this fixed charge. As a result, holes are expelled from the interface and electrons are attracted to the interface. As a result, an inversion layer 87 in which electrons are higher than the hole density appears although it is p-type silicon immediately below the passivation film 86.

The appearance of the inversion layer 87 has the following two effects.
(1) The movement of electrons in the back surface interface in the lateral direction is facilitated.
(2) The electron density is increased in the contact region between the back electrode 89 and the passivation film 86.

  As described above, since the holes selectively passed from the P-type silicon substrate 81 to the p + layer gather in the contact region described in the above (2), that is, the peripheral portion of the back electrode 89, The hole density is higher than that inside the substrate.

  In the contact region between the back electrode 89 and the passivation film 86, the electron density is increased by the above (2), so that both the electron and hole densities are increased. Further, since this contact region is a materially discontinuous region in which the electrode material and the passivation film material are adjacent to each other, a high inaccurate level region 88 is generated due to the discontinuity.

  Therefore, as shown in FIG. 12 (schematic diagram showing an enlarged contact region between the passivation film and the electrode portion in the structural example of FIG. 11), in the vicinity of the boundary between the back electrode 89 and the passivation film 86, electrons 91 and In addition to an increase in the value of the density product with the holes 92 and an increase in the probability that they will collide and recombine, the illegal level density in the high illegal level region 88 also increases. Occurs.

  According to the above (1), electrons that are commensurate with such carrier loss are supplied by the inversion layer at the interface between the passivation film 86 and the p-type silicon substrate 81.

  That is, (2) creates a soil with high carrier recombination loss, and (1) establishes a transport path for efficiently transporting electrons to compensate for the lost electrons. This phenomenon is known as a parasite shunting phenomenon, and causes a serious deterioration in various characteristics of the back surface passivation type solar cell.

In the back side passivation solar cell, in order to suppress the occurrence of para Sai tick shunting phenomenon reduces the positive interface fixed charge density Q _f at the interface between the passivation film and the silicon substrate, preferably a negative value Is required.

As a specific countermeasure, Non-Patent Document 1 discloses a technique for suppressing the occurrence of the parasitic shunting phenomenon by devising the passivation film material and setting the interface fixed charge density _Qf to a negative value. Has been. In the method according this publication, as the passivation film material, by typically employing alumina interfacial fixed charge density Q _f at the interface between the passivation film and the silicon substrate becomes a negative value, the interface fixed charge density Q _f attracts holes in the silicon to the interface. Then, since the high-density carriers are the same type (holes) in both the vicinity of the electrode part where holes are collected in the p + part immediately below and the passivation film part, recombination of electrons and holes is caused. It becomes difficult to occur, and it is possible to avoid the occurrence of the parasitic shunting phenomenon and to suppress the deterioration of the characteristics of the back surface passivation type solar cell.

B. Hoex, J. Schmidt, R. Bock, PP Altermatt, MCM van de Sanden, and WMM Kessels, “Excellent passivation of highly doped p-type Si surfaces by the negative-charge-dielectric Al2O3”, Appl. Phys. Lett 91, 112107 (2007)

However, as described above, if alumina is used as the passivation film material, even if it is possible to avoid the occurrence of the parasite shunting phenomenon by setting the interface fixed charge density _Qf to a negative value, the alumina is a solar cell. However, it is necessary to deposit using a special method such as ALD (Atomic Layer Deposition). In general, the deposition rate of alumina by ALD is extremely slow, so the throughput is low and it is not suitable for mass production as a passivation film.

In view of such a problem, the present inventor has intensively studied the performance improvement of the back surface passivation type solar cell, and as a result, the interface fixed charge density Q _{f at the} interface between the p-type semiconductor layer and the insulating film formed on the surface thereof. It has been found that the occurrence of the parasite shunting phenomenon can be suppressed by setting the value within a predetermined numerical range.

  In view of the above-described problems, an object of the present invention is to provide a solar cell that suppresses the occurrence of a parasite shunting phenomenon and realizes good characteristics.

In order to solve the above problems, a solar cell according to the present invention is a solar cell having a pn junction and constituting a photoelectric conversion element, on the surface of a p-type semiconductor layer opposite to the light receiving side And an interface fixed charge density Q _{f at} the interface between the p-type semiconductor layer and the insulating film is in the range of 2e11 cm ⁻² ≦ Q _f ≦ 4e12 cm ⁻² . It is characterized by being.

According to the above configuration, since the upper limit value of the interface fixed charge density Q _{f at} the interface between the p-type semiconductor layer and the insulating film formed on the surface thereof is 4e12 cm ⁻² , it is insulated from the back electrode. one generation of para Sai tick shunting phenomenon caused by the interaction of the membrane is avoided, also, the lower limit value of the surfactant fixed charge density Q _f is the field passivation effect can accurately reflect the fact that a 2E11cm ^-2 Will be.

  Thereby, the solar cell which has a favorable various characteristic is realizable.

  In order to solve the above problems, the solar cell according to the present invention is characterized in that the film material of the insulating film is SiNx.

  According to the above configuration, the SiNx that is easy to handle and widely used as a passivation film material can be used, and the occurrence of the parasite shunting phenomenon and the reflection of the field passivation effect can be accurately realized. Become.

  Thereby, the solar cell which has a favorable characteristic can be simply implement | achieved, without changing the conventional manufacturing process largely.

In the back-side passivation type solar cell, the value of the interface fixed charge density Q _f is in the range of 2e11 cm ⁻² ≦ Q _f ≦ 4e12 cm ⁻² , thereby avoiding the occurrence of the parasite shunting phenomenon and the field passivation There is an effect that various characteristics due to the effect can be realized.

It is a graph which shows typically the suitable range of the interface fixed charge density in the solar cell in the present invention. It is sectional drawing which shows typically the model structure of the solar cell used for numerical model calculation. Is a graph showing the characteristics calculation result of the solar cell when changing the interfacial fixed charge density Q _f, (a) shows the relationship between the interfacial fixed charge density Q _f and the open-circuit voltage (V _OC), FIG ( b) is the relationship between the interface fixed charge density Q _f and the short-circuit current density (J _SC ), and FIG. 10C is the relationship between the interface fixed charge density Q _f and the fill factor (FF), FIG. _Shows the relationship between the interface fixed charge density Q _f and the photoelectric conversion efficiency (E _FF ). In numerical model calculation, it is the energy band figure in the board _| substrate of a solar cell when changing interface fixed charge density _Qf to positive / negative, and the graph which showed the mode of the change of the hole density and electron density in the said board _| substrate. , (A) is an energy band and potential when Q _f > 0, (b) is a state when Q _f > 0, (c) is an energy band and potential when Q _f <0, ( d) shows the situation when Q _f <0. In numerical model calculations, a hole density and the graph showing the manner of change in the electron density at the back surface near the interface of the solar cell at the time of changing the interfacial fixed charge density Q _f. In numerical model calculations is a graph showing a state of a change in the surface recombination velocity (SRV) on the rear surface near the interface of the solar cell when changing the interfacial fixed charge density Q _f. Is a distribution diagram showing the absolute value of the electron current flows electron current (vector) (contours) in the back electrode vicinity of the solar cell when the Q f ₌ 6e12cm ^-2. A distribution diagram showing the SRH recombination probability distribution in the back electrode vicinity of the solar cell, (a) shows _the, state when the Q f = 6e12cm ^-2, the state of (b) _is Q f = 1E12 cm ^-2 It is shown. It is process drawing which shows the process of the first half in the manufacturing method of the solar cell which concerns on this invention in order. It is process drawing which shows the process of the latter half in the said manufacturing method in order. It is a schematic diagram which shows the structural example of a back surface passivation type solar cell. FIG. 12 is a schematic diagram showing an enlargement of a contact region between a passivation film and an electrode portion in the structural example of FIG. 11.

  Hereinafter, embodiments of the present invention will be described in detail with reference to FIGS.

[1. Features of the solar cell according to the present invention]
Based on the following considerations, the present inventor has determined the value of the interface fixed charge density Q _{f at} the interface between the p-type silicon substrate and the passivation film to be 2e11 cm in order to improve various characteristics of the back surface passivation type solar cell. It has been found that it is preferable to design such that ⁻² ≦ Q _f ≦ 4e12 cm ⁻² . Note that 2e11 represents 2 × 10 ¹¹ and 4e12 represents 4 × 10 ¹² .

When the preferred range of the interface fixed charge density Q _f shown schematically, it is shown in Figure 1.

As shown in the figure, when the interface fixed charge density Q _f is larger than 4e12 cm ⁻² , various characteristics of the back surface passivation type solar cell (represented in FIG. illustrating the photoelectric conversion efficiency E _FF) is while degradation, improvement in recombination rate SRV can not be sufficiently obtained by field passivation effect increases when the interface fixed charge density Q _f is smaller than 2E11cm ^-2 as specific properties Therefore, the numerical range of 2e11 cm ⁻² ≦ Q _f ≦ 4e12 cm ⁻² is a preferable appropriate range.

  According to the back surface passivation type solar cell of the present invention, in the back surface passivation type solar cell, it is possible to realize various characteristics due to the field passivation effect while avoiding the occurrence of the parasaimatic shunting phenomenon. Play.

[2. Optimization design of interface fixed charge density by numerical model calculation)
Then, the present inventors have found by numerical model calculations, the detailed procedure to derive the above preferred range of surface fixed charge density Q _f at the interface between the p-type silicon substrate and the passivation film of a solar cell.

  As described above, in the contact region between the passivation film and the electrode portion on the back surface, electrons and holes are mixed in a high density, and a region with a high illegal level density is generated. Causes carrier loss (parasite shunting phenomenon).

Here, as the film material of the passivation film, when used, for example a general SiNx, the surfactant fixed charge density Q _f at the interface between the passivation film and the p-type silicon substrate is known that in general to negatively not However, when a film material having such a positive interface fixed charge is adopted, what value the interface fixed charge density Q _f can be set to avoid the occurrence of the parasite shunting phenomenon. It is not known about.

Therefore, the present inventor has a numerical model calculations using a computer, when changing the interfacial fixed charge density Q _f under realistic design, in any interface fixed charge density Q _f, solar cells We examined whether the characteristics of the solar cell deteriorated as a result of the occurrence of the parasite shunting phenomenon.

[2-1. (Specific method of numerical model calculation)
The basic principle of the numerical model calculation is based on the Poisson equation that governs carrier transport inside the substrate and the current continuity formula. The solution is to solve the basic equation. In order to execute the numerical model calculation, the basic equation is linearized and then discretized to obtain simultaneous equations that can be numerically calculated. By solving the simultaneous equations, the potential distribution and carrier density distribution inside the device can be obtained, and the current flowing between the electrodes can be obtained.

  In the present embodiment, the model structure of the solar cell 10 shown in FIG. 2 is adopted as the structure for numerical model calculation of the back surface passivation type solar cell which is one form of the photoelectric conversion element having a pn junction. In the figure, an n layer 32 is formed on the surface of a p-type silicon substrate 31 on the surface side (light incident side) of the solar cell 10, while a back electrode forming portion 33 is formed on the back surface side of the solar cell 10. A P + layer 34 is formed immediately below, and an antireflection film 35 that also functions as a surface passivation film so as to cover the surface of the n layer 32, and covers a part of the surface of the p-type silicon substrate 31 to the P + layer 34. Thus, a back surface passivation film 36 is formed. As a result, the P + layer 34 and the interface between the p-type silicon substrate 31 and n-typed by the positive charge in the back surface passivation film 36 are in contact with the back surface of the solar cell 10.

  Furthermore, an electrode layer 37 is provided in the region from which the antireflection film 35 has been removed, and an electrode layer 33 is provided in the region from which the back surface passivation film 36 has been removed. The charge 38 in the figure schematically shows the state of the fixed charge that is fixedly present at the interface between the p-type silicon substrate 31 and the back surface passivation film 36.

  In performing the numerical calculation for the above model structure, it is necessary to consider the movement in two directions, the direction parallel to the interface between the back surface passivation film and the p-type silicon substrate, and the direction perpendicular to the interface for the carrier movement. Therefore, a general-purpose device simulator (for example, Sentaurus manufactured by Synopsys) that can handle carrier movement in two dimensions was used.

  In the specific calculation, the substrate thickness, impurity concentration, electron lifetime, surface recombination velocity, surface impurity concentration, backside passivation film contact width, solar spectrum / illuminance, and so on, corresponding to the above model structure. Parameters such as surface reflectance, surface internal reflectance, back surface internal reflectance, and shadow loss directly under the electrode were set to be appropriate as actual values.

  However, among the various parameters described above, the width (Wtot) of one section formed by the electrode layer 33 adjacent on the back surface side of the solar cell 10 is set to 200 μm, and the numerical model calculation uses the symmetry of the solar cell structure. In view of the properties, 100 μm was used. In addition, the thickness of the p-type silicon substrate 31 including the n layer 32 was set to a relatively small value of 50 μm in order to strongly reflect the influence of the back surface passivation structure in the calculation result.

[2-2. The preferable lower limit value of the surfactant fixed charge density Q _f]
First, the present inventors have found by numerical model calculations, of the above preferred ranges of surface fixed charge density Q _f at the interface between the p-type silicon substrate 31 and the back passivation film 36 of the solar cell 1, to derive the preferred lower limit value procedures Will be described with reference to FIGS.

3A to 3D show the interface fixed charge density Q _f existing at the interface between the back surface passivation film 36 and the p-type silicon substrate 31 in the numerical model calculation, −1e12 cm ⁻² ≦ Q _f ≦ + 5 e13 cm ^−. It is the graph which showed the calculation result about the various characteristics of the solar cell 10 when it changed in the range of ^2. FIG.

Specifically, FIG. 10A shows the relationship between the interface fixed charge density Q _f and the open circuit voltage (V _OC ), and FIG. 10B shows the interface fixed charge density Q _f and the short-circuit current density (J _SC ). (C) shows the relationship between the interface fixed charge density _Qf and the fill factor (FF), and (d) shows the relationship between the interface fixed charge density _Qf and the photoelectric conversion efficiency ( _EFF ). Showing the relationship. Of the above characteristics, the open circuit voltage refers to the output voltage when the terminal is opened during light irradiation, and the short circuit current density refers to the effective light reception when the terminal is shorted during light irradiation. It is the one divided by the area.

Referring to the graphs of FIGS. 3 (a) to 3 (d), for all the characteristics, in the region where the interface fixed charge density _Qf is negative ( _Qf <0), the behavior is relatively simple. Thus, it can be seen that in the region where the interface fixed charge density Q _f is positive (Q _f > 0), very complicated behavior is exhibited.

Specifically, when the absolute value of the surfactant fixed charge density Q _f is focused on a relatively small area at the interface fixed charge density Q _f is negative region, properties with an increase in the absolute value of the surfactant fixed charge density Q _f V _{OC, J} SC, _FF, whereas _{E FF} increases largely at the interface fixed charge density _{Q f} is positive region, properties _V OC with an increase in the absolute value of the surfactant fixed charge density _{Q f, J SC,} FF and E _FF decrease. When Q _f = 1e11 cm ⁻² , the minimum value is obtained, and when the absolute value of the interface fixed charge density Q _f is further increased, various characteristics V _OC , J _SC , FF, E _FF When Q _f = 1e12 cm ⁻² , the maximum value is obtained, and when the absolute value of the interface fixed charge density Q _f is further increased, the characteristics V _OC , J _SC , FF, and E _FF rapidly increase. Decrease.

  In order to consider the cause of the above tendency, various parameters in the vicinity of the back surface interface (˜50 μm) of the solar cell 10 were calculated. 4A and 4C are energy band diagrams along the thickness direction of the p-type silicon substrate 31. FIG. The horizontal axis of the graph shows the displacement in the substrate thickness direction, and the vertical axis shows the change in energy level. The position of 50 μm at the right end of the horizontal axis of each graph is the back surface interface of the p-type silicon substrate 31 in FIG. It corresponds to.

When the interface fixed charge density Q _f is positive (Q _f > 0), as shown in FIG. 4A, the conduction band edge energy Ec and the valence band edge energy Ev are In the back surface interface, it is bent to the low level side. Note that the potential at this time is bent toward the high level side at the back surface interface.

On the other hand, when the interface fixed charge density Q _f is negative (Q _f <0), as shown in FIG. 4C, the conduction band edge energy Ec and the valence band edge energy Ev are Conversely, it is bent toward the high level side. Note that the potential at this time is bent toward the lower level at the back surface interface.

  The charge in the vicinity of the back surface interface (˜50 μm) moves so as to absorb the potential change at the back surface interface and not transmit the potential change to the inside of the silicon substrate.

FIGS. 4B and 4D show the holes in the vicinity of the back surface interface (˜50 μm) of the p-type silicon substrate 31 of the solar cell 10 when the interface fixed charge density Q _f existing at the back surface interface is changed. It is the graph which showed the mode of the change of a density and an electron density.

According to FIGS. 4B and 4D, the hole density and electron density in the vicinity of the back surface interface (˜50 μm) of the solar cell 10 are obtained when the interface fixed charge density Q _f is positive (Q _f > 0). In the vicinity, holes are eliminated and their density decreases, and conversely, the electron density increases. On the other hand, when the interface fixed charge density Q _f is negative (Q _f <0), electrons are rejected in the vicinity of the interface and the density decreases, and conversely, the hole density increases.

FIG. 5 is a graph showing how the hole density and the electron density change in the vicinity of the back surface interface of the solar cell 10 when the interface fixed charge density _Qf is changed in the numerical model calculation. And Figure 6 is a graph showing a state of a change in the surface recombination velocity (SRV) on the rear surface near the interface of the solar cell 10 in the case of changing the interfacial fixed charge density Q _f.

As shown in FIG. 5, when the absolute value of the long Q _f is negative there be positive becomes large, the density difference between the holes and electrons to expand. Due to the presence of this density difference, the density product of both becomes small. This reduces the probability of recombination of holes and electrons, i.e. the recombination rate at the back interface. In fact to Figure 6, the recombination rate when the absolute value of Q _f is increased is reduced, i.e., it can be seen that the interface characteristics are improved.

Conversely, the recombination rate increases when the density of holes and electrons at the back interface becomes close. This is because the probability of recombination of holes and electrons is relatively increased. As shown in FIG. 5, the density of holes and electrons at the back surface interface is approximately the same at Q _f = 1e11 cm ⁻² , but in FIG. 6, the recombination velocity is actually the maximum value at that time. That is, it is shown that the interface characteristics are deteriorated.

Thus, from the above calculation results, a by increasing the absolute value of the interface fixed charge density Q _f of the solar cell 10, to reduce the surface recombination velocity (SRV) to reduce carrier loss, the characteristics good It is understood that

Based on the calculation results shown in FIG. 5 and FIG. 6, the present inventor has made a positive interface to sufficiently reduce the surface recombination velocity (SRV) and reflect the field passivation effect on the solar cell characteristics. preferred lower limit value of the fixed charge density _{Q f, 2e11cm} ^-2 are suitable, more preferably from appropriate 3E11cm ^-2, more preferably discussed with 4E11cm ^-2 is appropriate.

[2-3. Preferred upper limit of the interface fixed charge density Q _f]
Then, the present inventors have found by numerical model calculations, of the above preferred ranges of surface fixed charge density Q _f at the interface between the p-type silicon substrate 31 and the passivation film 36 of the solar cell 1, to derive the preferred upper limit procedure Will be described with reference to FIGS. 3 and 7 to 8.

From the findings described in 2-2, at first glance, it tend considered good effect on solar cell characteristics appear because recombination rate is reduced the higher the interfacial fixed charge density Q _f. However, referring to FIGS. 3A to 3D described above, in the region where the interface fixed charge density Q _f > 2e12 cm ⁻² , the characteristics V _OC , J _SC , FF, and E _FF of the solar cell 10 are abrupt. It turns out that it deteriorates. This rapid decrease in solar cell characteristics cannot be explained by the theory of field passivation based on the density difference between holes and electrons described in 2-2.

In order to explain this, the concept of the above-mentioned parasaimatic shunting is introduced. When Q _f (> 0) increases, an inversion layer 87 (see FIG. 11) having a high electron density appears at the back surface interface, and this fact produces the following two effects. As described above, the interaction between these two effects causes parasite shunting.
(1) The movement of electrons in the lateral direction at the back interface is facilitated.
(2) The electron density is increased in the contact region between the back electrode and the passivation film.

These two effects are individually verified by numerical calculation. FIG. 7 shows the electron current in the vicinity of the electrode layer 33 on the back surface of the solar cell 10 (see FIG. 2) when Q _f = 6e12 cm ⁻² where the degradation of the solar cell characteristics is observed due to the occurrence of parasite shunting. It is the distribution map which showed the flow (vector) and the absolute value (contour: Acm ^<-2 >) of an electronic current. The arrow in the figure means the direction of electrons.

Note that x = 0 is the center of the electrode layer 33 (back electrode) shown in FIG. 2, and the contact point between the end of the electrode layer 33 and the passivation film 36 is at a position x = 10 μm. The y axis indicates the thickness direction of the solar cell 10, and the position of y = 50 corresponds to the position of the back surface of the p-type silicon substrate 31. The value indicated by the color distribution is the electron current density and has a dimension of Acm ⁻² .

  In FIG. 7, in a region where x> 15 μm, a state in which electrons flow toward the back surface passivation film 36 appears. A part of this current disappears due to recombination with holes in the back surface passivation film 36, but most of the current that remains without disappearing is applied to the inversion layer, which is a very thin portion near the back surface of the cell, as an electrode. It will flow in the horizontal direction in the figure.

  In the figure, the state of the current as described above is represented as a large arrow in the x <0 direction, which exists at the interface between the back surface passivation film 36 and the p-type silicon substrate 31 (see FIG. 2). x <0 direction, that is, toward the electrode. Thus, it was shown that the above effect (1) actually occurs.

It was assumed that such an electron flow reaches the vicinity of the electrode and disappears by recombination with holes in a region sandwiched between the P + layer 34 (see FIG. 2) and the passivation film 36. To verify this _assumption, the distribution diagram of SRH recombination probability in the back electrode near the time of ^{Q f = 6e12cm -2 (cm -3} s -1) is shown in FIG 8 (a). The definition of the x-axis and the y-axis is the same as in FIG. In the region of 10 μm <x <12 μm in FIG. 9A, the electron flow reaches the vicinity of the electrode and recombines with holes in the region sandwiched between the P + layer 34 and the back surface passivation film 36 and disappears. As can be seen, the recombination velocity with holes increases remarkably, and the SRH recombination probability approaches 1.0E + 20 (cm ⁻³ s ⁻¹ ), which is the electron absorption edge. Thus, it was shown that the effect (2) actually occurs.

  Since a large amount of electrons disappear at the absorption edge, electrons that are commensurate with the disappearance are supplied by the lateral transport of electrons at the interface between the back surface passivation film 36 and the p-type silicon substrate 31. As described above, these two calculation results are considered to indicate the occurrence of the parapsychic shunting phenomenon.

On the other hand, FIG. 8B is a distribution diagram showing the state of recombination at Q _f = 1e12 cm ⁻² in the region where the deterioration of various characteristics was not observed (see FIG. 3).

  In FIG. 8 (b), there is no electron absorption edge as seen in FIG. 8 (a), that is, no recombination increasing region near the electrode, and no parasite shunting phenomenon has occurred. I understand.

Thus, by comparing the calculation results of various characteristics in the solar cell 10 (see FIG. 3) with the distribution diagrams of FIGS. 7 and 8A and 8B, the above characteristics V _OC , J _SC , It has been confirmed that there is a correlation between the deterioration of _FF and EFF and the occurrence of the parasite shunting phenomenon.

That is, when the interface fixed charge density _Qf is increased, the occurrence of the parasitic shunting phenomenon becomes conspicuous, and it is considered that the deterioration of various characteristics of the solar cell 10 is greatly affected. to avoid to be made good the characteristics of the solar cell 10, it can be seen that it is preferable that the interfacial fixed charge density Q _f equal to or less than a predetermined value.

Considering based on the calculation results shown in FIG. 3, the occurrence of the parasite shunt phenomenon becomes remarkable, and the characteristics of the solar cell are greatly deteriorated in the region of Q _f > 4e12 cm ⁻² . It is believed that there is. Therefore, the inventor of the present application avoids the occurrence of the parasitic shunting phenomenon, and makes various characteristics of the solar cell favorable, so that a preferable upper limit value of the interface fixed charge density Q _f is 4e12 cm ^−2. are suitable, more preferably 3E12cm ^-2 are suitable, more preferably it determines that 2E12cm ^-2 is appropriate.
[2-4. (Summary of preferred range)
As described above, the present inventors in order to make good characteristics of solar cell 10, the value of the interface fixed charge density Q _f at the interface between the passivation film 36 and the p-type silicon substrate 2, 2e11cm ^- It has been found that it is preferable to design so that ² ≦ Q _f ≦ 4e12 cm ⁻² .

That is, when the interface fixed charge density Q _f is larger than 4e12 cm ⁻² , various characteristics of the back surface passivation type solar cell are deteriorated due to an increase in recombination in the vicinity of the electrode portion due to the parasite shunt phenomenon, while the interface fixed charge density Q _{If f} is smaller than 2e11 cm ^{−2, the} recombination rate is not sufficiently improved by increasing the field passivation effect. Therefore, the numerical range of 2e11 cm ⁻² ≦ Q _f ≦ 4e12 cm ⁻² is a preferable appropriate range.

Incidentally, as a lower limit and an upper limit of the interface fixed charge density Q _f, preferred than the above, and also the range obtained by any combination of more preferred values, it is needless to say that within the scope of the present invention.
[3. Specific example of manufacturing process)
The following description shows an example of a manufacturing process of the solar cell 10 according to the present embodiment.

  As shown in FIG. 9, first, a p-type polycrystalline silicon substrate 20 (vertical and horizontal 10 cm × 10 cm, thickness 200 μm, resistivity 1 Ωcm) was cleaned by an RCA cleaning method developed by RCA. Subsequently, by performing texture etching using a mixed solution of NaOH aqueous solution and isopropyl alcohol at a liquid temperature of about 90 ° C., as shown as S1 in FIG. A micro pyramid 21 having a height of several μm was formed on the surface (abbreviated as 20) (light receiving surface, light incident surface).

  The silicon substrate 20 is obtained by adding a trace amount of a trivalent element such as boron, aluminum, or gallium to a polycrystalline silicon substrate. A p-type silicon substrate using single crystal silicon is also an application target of the present invention.

  Moreover, you may use the reactive ion etching method for the said texture etching. By forming a fine concavo-convex structure on the surface of the silicon substrate, reflection of light on the surface of the silicon substrate can be suppressed, so that the light use efficiency of the solar cell 10 can be increased.

Second, as shown as S2 in FIG. 9, the silicon substrate 20 is placed in a high-temperature gas containing POCl ₃ to thermally diffuse phosphorus, thereby having a thickness of 1.0 μm and an impurity concentration of 1.2 × 10 ²⁰ cm ^{−. 3} n-type silicon layers 22 and 23 were formed on the light-receiving surface side and the back surface side. The temperature of the silicon substrate 20 and the temperature of the diffusion furnace during thermal diffusion were set to 850 ° C., and the diffusion time was set to 10 minutes.

As a method for diffusing phosphorus on the surface of the silicon substrate 20, for example, in addition to the above-mentioned vapor phase diffusion method using POCl ₃ , a coating diffusion method using P ₂ O ₅ , ion implantation for directly diffusing P ions. There are laws.

  Third, as shown as S3 in FIG. 9, an SiNx film 24 as an inactive film and an antireflection film is formed on the light-receiving surface side of the silicon substrate 20 by plasma CVD (Chemical Vapor Deposition). Deposited 80 nm.

  Here, as the antireflection film that also functions as an inactivating film exhibiting a passivation effect, for example, an aluminum oxide film, a silicon oxide film, a titanium oxide film, or the like is used in addition to the SiNx film. In particular, in the case of a solar cell using a polycrystalline silicon substrate, it is preferable to use SiN containing hydrogen from the viewpoint of improving the conversion efficiency.

In addition to the plasma CVD method described above, as a method of forming an antireflection film, catalytic CVD
A CVD method such as an atmospheric pressure thermal CVD method, a low pressure thermal CVD method or a photo CVD method, or a PVD (Physical Vapor Deposition) method such as a vacuum evaporation method or a sputtering method can be used. In the case where a SiNx film is used as the antireflection film, the plasma CVD method is preferable from the viewpoint of easy control of the film thickness.

  Fourth, as shown as S4 in FIG. 9, a protective tape was applied to the light receiving surface, and immersed in a solution of nitric acid: hydrofluoric acid = 3: 1 for about 4 minutes. Thereby, the n-type silicon layer 22 existing on the back surface is removed, so that the p-type silicon surface is exposed.

  Fifth, as shown as S5 in FIG. 9, a SiNx film 25 was deposited to a thickness of about 100 nm on the back surface using a plasma CVD method. In the present embodiment, in the case of a solar cell using a polycrystalline silicon substrate, SiNx containing hydrogen is used from the viewpoint of improving conversion efficiency. When a SiNx film is used as the antireflection film, the plasma CVD method is preferable from the viewpoint of easy control of the film thickness.

In the plasma CVD method of the present embodiment, for forming the SiNx film 25, two types of gases, monosilane (SiH ₄ ) and ammonia (NH ₃ ), are used as source gases in addition to nitrogen (N ₂ ) as a dilution gas. However, the process conditions at the time of the film formation are the same as the value of the interface fixed charge density Q _{f at} the interface between the insulating film and the n-type silicon layer 22 as described above, 2e11 cm ⁻² ≦ Q _f ≦ 4e12 cm ^−. It is controlled to be within the numerical range of ² .

Generally, the interface fixed charge density Q _f is small in the interface between the silicon film and a silicon nitride containing a large amount of nitrogen, it is known to increase at the interface between the silicon film and silicon nitride containing a large amount of silicon. The most basic and intuitive method for controlling the interface fixed charge density Q _f is monosilane (SiH ₄ ) and ammonia (NH ₃ ) used in the vapor phase growth method using plasma in the insulating film formation stage. The composition ratio of nitrogen and silicon in the silicon nitride film is changed by changing the flow ratio or the mixing ratio (SiH ₄ / NH ₃ ). Thus the value of the interface fixed charge density Q _f can be adjusted or controlled so as to be within the above range.

In addition to the above method, the composition ratio is changed by changing the pressure, RF power, discharge gap, substrate temperature during vapor phase growth using plasma, and adding hydrogen to the source gas, thereby fixing the interface. it is possible to control the value of the charge density Q _f.

Further, prior to the formation of the silicon nitride film, the silicon surface is exposed to ammonia (NH ₃ ) plasma, oxygen (O ₂ ) plasma, or N ₂ O plasma, and a surface layer of 20 nm or less is nitrided or oxidized, Different atomic structures can be created inside the silicon substrate to form an interface. It is known that can control the value of the interface fixed charge density Q _f by this method. In particular, these methods are promising as methods capable of obtaining a low Q _f value of 10 ¹¹ cm ⁻² .

In the above description, the SiNx film is used as the antireflection film. However, the type of the insulating film used in the present invention is not limited to this, and other film materials such as a SiO ₂ film may be used. Good.

In the above description, as a method for controlling the value of the interface fixed charge density Q _f, although the method of changing the flow rate ratio or mixing ratio of the mixed gas used in the vapor deposition method, an interfacial fixed charge in the insulating film density how to control the value of Q _f is not limited to this, the annealing temperature and annealing time in S6, such an atmosphere set during annealing, by adjusting the other process conditions, the value of the interface fixed charge density Q _f May fall within the above range. Further, this annealing may be performed at the time of formation other than after the formation of the SiNx film 25.

Hence, such as adjusting the flow rate ratio or mixing ratio of each raw material gas used for vapor phase growth method, a value of the interface fixed charge density Q _f If it is difficult to the numerical range by adjusting the other manufacturing processes even, by the practice or combination of annealing, the value of the interface fixed charge density Q _f can be of the numerical range.

  Sixthly, as shown as S6 in FIG. 10, a hole corresponding to the back electrode forming portion 4 was formed in the SiNx film 25 by using a method such as photolithography.

  Seventh, as shown as S7 in FIG. 10, an aluminum film 26 was deposited on the entire back surface by 2 μm. As a method for forming the aluminum film 26, a printing method using a paste material mainly composed of aluminum and glass frit is preferable from the viewpoint of cost. In addition to the vacuum deposition method, a sputtering method can also be used.

  Eighth, annealing was performed as shown as S8 in FIG. By this annealing, aluminum diffuses from the aluminum film 26 to the silicon substrate 20 as a p-type impurity, and the aluminum alloy portion 6 is formed, and at the same time, the SiNx film is annealed.

  Ninthly, as shown as S9 in FIG. 10, the main surface electrode 27 was printed on the light receiving surface side using a conductive paste and annealed. Due to the fire-through phenomenon that occurs at this time, the principal surface electrode 27 penetrates the SiNx film 24 and reaches the n-type silicon layer 23. As a result, an electrical output can be extracted from the main surface electrode 27.

  The material which comprises the said main surface electrode 27 is not specifically limited, For example, materials, such as aluminum, silver, titanium, palladium, or gold conventionally used in the field of a solar cell, can be used. Among them, silver is most preferable as a material that causes a fire-through phenomenon. Further, the method for forming the main surface electrode 27 is not particularly limited, and for example, a screen printing method or a vacuum deposition method can be used. However, the screen printing method is used from the viewpoint of improving the mass productivity and reducing the manufacturing cost. Is preferred.

  By the above process, the aluminum alloy part 6 as a good p + layer having excellent back surface passivation characteristics and high hole density was generated, and the solar cell 10 having high photoelectric conversion efficiency could be manufactured.

  Thereby, according to the solar cell 10 of the present embodiment, an increase in recombination in the vicinity of the electrode due to the parasitic shunting phenomenon is suppressed, and an improvement effect of the recombination speed due to an increase in the field passivation effect is ensured. Good characteristics can be obtained.

  The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope of the claims, and other embodiments obtained by appropriately combining the technical means disclosed in the above-described embodiments. The form is also included in the technical scope of the present invention.

  The present invention relates to a solar cell constituting a photoelectric conversion element, and more specifically, can be suitably used for, for example, a back surface passivation type solar cell.

10 Solar cell (photoelectric conversion element)
31 p-type silicon substrate (p-type semiconductor layer)
33 Electrode layer (back electrode)
36 Back surface passivation film (insulating film)

Claims (2)

A solar cell having a pn junction and constituting a photoelectric conversion element,
An insulating film and a back electrode are formed on the surface of the p-type semiconductor layer opposite to the light receiving side, and the value of the interface fixed charge density Q _{f at} the interface between the p-type semiconductor layer and the insulating film Is within the range of 2e11 cm ⁻² ≦ Q _f ≦ 4e12 cm ⁻² .   The solar cell according to claim 1, wherein a film material of the insulating film is SiNx.