JP5784358B2 - Photoelectric conversion element and solar cell - Google Patents

Description

  Embodiments described herein relate generally to a photoelectric conversion element and a solar cell.

For example, in a solar cell, a compound thin film photoelectric conversion element using a semiconductor thin film as a light absorption layer has been developed. Among compound semiconductors having a chalcopyrite structure composed of Ib group, IIIb group and VIb group, Cu Attention has been focused on photoelectric conversion elements such as thin-film solar cells using Cu (In, Ga) Se ₂ , which is made of In, Ga and Se, so-called CIGS as a light absorption layer. In order to improve the conversion efficiency, various attempts have been made such as promoting crystal growth and improving film quality by reducing defects, one of which is a band gap distribution forming technique. The band gap distribution is formed by changing the composition ratio of Ga and In in the film thickness direction of the CIGS light absorption layer. When the composition ratio of Ga and In is changed by CIGS, the band gap is widened with a composition having a large Ga composition ratio, and the band gap is narrowed with a composition having a large In composition ratio. In adjustment of the band gap by modulation of the composition ratio of Ga and In, since the conductor level mainly changes, the Ga composition ratio on the back electrode side of the CIGS light absorption layer is high, and the Ga composition ratio on the CIGS light absorption layer surface is set to be high. By making it low, an internal electric field is generated, and the photoexcited carriers move from the back electrode side toward the pn junction interface, improving the carrier absorption efficiency. The light absorption layer of the photoelectric conversion element absorbs short wavelength light on the pn junction interface side and absorbs long wavelength light on the back electrode side. However, in forming a band gap distribution by changing the composition ratio of Ga and In, the conductor level mainly changes and the valence band level hardly changes. Therefore, the band gap is narrow on the pn junction interface side, and the back electrode side As a result, the band gap becomes wider, and the wavelength of light that can be taken in is limited by the band gap on the pn junction interface side, so that light cannot be absorbed efficiently.

  The band gap is the same constituent element, and can be changed by changing the constituent ratio of the Ib group, IIIb group and VIb group. By replacing the group Ib element in the chalcopyrite structure with a group IIIb element and removing the group I element to form a hole-arranged chalcopyrite (OVC) structure, the group IIIb group compared to a compound semiconductor having a chalcopyrite structure The / Ib group molar ratio and the band gap both increase. Furthermore, with the OVC structure, both the conductor level and the valence band level become deeper from the vacuum level. In the chalcopyrite thin film photoelectric conversion element in which the VIb group element is S, an intermediate layer having a IIIb group / Cu molar ratio larger than that of the p-type light absorption layer is formed between the p-type light absorption layer and the n layer, Minority carriers generated by light irradiation are moved at high speed by drift due to an electric field generated at the interface of the intermediate layer, thereby suppressing carrier recombination. However, the conductor level and valence band level in the intermediate layer are flat, the generated carriers are likely to recombine, and the carriers generated by the long wavelength light inside the p-type light absorption layer are effectively It is difficult to move.

JP-A-8-97451

  An object of the embodiment is to provide a photoelectric conversion element and a solar cell with high conversion efficiency.

The photoelectric conversion element of the embodiment includes at least one group IIIb element selected from the group consisting of Cu, Al, In, and Ga, and at least one group IVb element selected from the group consisting of O, S, Se, and Te. A p-type absorber layer having a chalcopyrite structure is provided between the n-type compound semiconductor layer and the back electrode, and the IIIb group / Cu molar ratio, which is the molar ratio of the group IIIb element to the Cu, and having a gradient composition portion which decreases from the interface side between the n-type compound semiconductor layer at the interface of the back surface electrode side from the interface between at least the n-type compound semiconductor layer of the p-type light absorbing layer .
Moreover, the solar cell of other embodiment uses the photoelectric conversion element of the said embodiment, It is characterized by the above-mentioned.

It is a conceptual diagram of the photoelectric conversion element of embodiment. It is a conceptual diagram which shows the change of the IIIb group / Cu composition (mol) ratio in the film thickness direction of the light absorption layer of embodiment. It is a conceptual diagram which shows the change of the band gap in the film thickness direction of the light absorption layer of embodiment. It is a schematic diagram which shows the Fermi level of the Cu-In-Te thin film of embodiment, a conduction band lower end, and a valence band upper end. It is the surface SEM image of the Cu-In-Te thin film sputter-deposited using the target of In / Cu = 3 of embodiment. It is the surface SEM image of the Cu-In-Te thin film sputter-deposited using the target of In / Cu = 1 of embodiment. It is the figure which put together the short circuit current density of an Example and a comparative example.

Hereinafter, embodiments for carrying out the present invention will be described in detail with reference to the drawings.
The photoelectric conversion element 10 shown in the conceptual diagram of FIG. 1 includes a substrate 11, a back electrode 12 provided on the substrate, a first extraction electrode 13 provided on the back electrode 12, and the back electrode 12. A light absorption layer 14 provided thereon, a buffer layer 15 (15a, 15b) provided on the light absorption layer 14, a transparent electrode layer 16 provided on the buffer layer 15, and the transparent electrode layer 16 is provided with at least a second extraction electrode 17 provided on 16 and an antireflection film 18 provided on the transparent electrode layer 16.

The light absorption layer (p-type) 14 of the embodiment is at least one selected from the group consisting of Cu, at least one group IIIb element selected from the group consisting of Al, In and Ga, and O, S, Se and Te. It consists of two VIb group elements, and the kind of the constituent elements is preferably the same in the entire region of the light absorption layer 14, and the crystal structure is desired to have a chalcopyrite structure or a pore arrangement type chalcopyrite structure. It is preferable that a gradient composition part is included in at least a part of the light absorption layer 14.
In the embodiment, the chalcopyrite structure and the hole-arranged chalcopyrite structure are both described as a chalcopyrite structure unless otherwise described.

  Here, the graded composition part is a change in the IIIb group / Cu molar ratio measured at 11 points on the straight line in the cross-sectional film thickness direction, with 10 equal distributions of the interface between the n-type compound semiconductor layer and the back electrode. It is an area to do. In the embodiment, the shape of the slope is determined by the 11 point group IIIb / Cu molar ratio change. When the IIIb group / Cu molar ratio decreases from the pn interface side with the n-type compound semiconductor layer toward the interface with the back electrode, the valence band top (VBM) can be inclined, and carriers can move easily. It is preferable that recombination is easily suppressed. If the inclination is in the reverse direction, the carrier mobility at the pn interface does not increase, which is not preferable. It is desirable that the group IIIb / Cu molar ratio is constantly decreasing (continuous change) because carriers can move without stagnation, and it is allowed to have a flat region that does not change partially (intermittent change). The On the contrary, if a region where the IIIb group / Cu molar ratio increases from the pn interface toward the back electrode is included, it is not preferable because the movement of carriers is inhibited in the region. It is preferable that the opposite area is not included unless it is inevitable.

  From the viewpoint of suppressing carrier recombination, it is preferable that the gradient composition part is at least 2 parts on a straight line in the cross-sectional film thickness direction. Further, from the same point of view, the gradient composition portion that decreases from the pn interface side of the light absorption layer 14 toward the back electrode 12 side so as to draw a logarithmic function, an inverse proportional function type curve, or a similar type curve or a substantially curved line, is 1 It is preferable that more than part is contained.

  Composition analysis of IIIb group / Cu molar ratio is cross-sectional SEM / EDX measurement measurement point analysis (SEM: scanning electron microscope, EDX: energy dispersive X-ray spectroscopy) To do. The measurement is performed at the center of the photoelectric conversion element 10. The IIIb group / Cu molar ratio is a five-point average value at the same depth in the cross-sectional film thickness direction by cleaving the central portion of the photoelectric conversion element 10. The method of determining the five points is to observe a cross-sectional SEM image of 20,000 times, divide the cross-sectional SEM image into five equal parts in a direction orthogonal to the film thickness direction, and use it as the center point of the divided region. The eleven points are measured at the center point in the film thickness direction, and the amount of change in the group III b / Cu molar ratio is measured. The interface with the n-type compound semiconductor layer and the interface with the back electrode are defined as measurement positions of the p-type compound semiconductor that do not include the components of the n-type compound semiconductor layer and the back electrode in the cross-sectional SEM / EDX measurement.

  Furthermore, as shown in the graph of FIG. 2, the group IIIb / Cu molar ratio at the interface with the n-type compound semiconductor layer is x1, and the group IIIb / Cu molar ratio at the center point in the layer thickness direction of the p-type light absorption layer. X2 (t / 2) and the group IIIb / Cu molar ratio at the interface with the back electrode is x3 (t), it is preferable to satisfy at least x1 ≧ x2 ≧ x3 from the viewpoint of suppressing carrier recombination, It is more preferable to satisfy> x2> x3. When the group IIIb / Cu molar ratio is large (for example, larger than 1), the Fermi level is separated from the VBM. Further, when the group IIIb / Cu molar ratio is relatively small (for example, 1), the Fermi level is closer to VBM than the former. Therefore, since the probability of carrier recombination is high in the group IIIb excess region with respect to Cu, it is preferable to increase the carrier mobility in the region near the pn interface. Therefore, x1, x2, and x3 are more preferably (x1-x2)> (x2-x3).

  Further, as shown in FIG. 3, when the IIIb group / Cu molar ratio is large, the VBM is small and the band gap is large, and when the IIIb group / Cu molar ratio is small, the VBM is large and the band gap is small. The band gap of the light absorption layer at the interface with the n-type compound semiconductor layer is Eg1, the band gap at the center point of the layer thickness of the p-type light absorption layer is Eg2, and the band gap of the light absorption layer at the interface with the back electrode Is more preferably (Eg1-Eg2)> (Eg2-Eg3). Accordingly, short wavelength light is absorbed on the pn junction interface side, and long wavelength light is absorbed on the back electrode side, and it can be said that an efficient band gap distribution is formed.

Hereinafter, the characteristics will be described by taking a light absorption layer made of Cu, In and Te as an example.
Figure 4 shows the Fermi level _{E F} and conductor lower end of the semiconductor thin film as constituent elements Cu and In and Te (CBM) and valence band maximum (VBM). When the In / Cu molar ratio is increased, both the CBM and VBM change in a direction away from the vacuum level, so the In / Cu molar ratio decreases from the interface side with the n-type compound semiconductor layer toward the interface on the back electrode side. The gradient composition part is formed on at least a part thereof. Thereby, electrons generated by photoexcitation move toward the pn junction interface, and holes move toward the back electrode. On the other hand, in a thin film having a large In / Cu molar ratio, the Fermi level is separated from the VBM. Therefore, when a pn junction interface is formed, carrier recombination at the interface is reduced compared to a thin film having a small In / Cu molar ratio. It tends to happen. Further, comparing the surface SEM images of FIG. 5 and FIG. 6, in the thin film having a small In / Cu molar ratio, the crystal grain size becomes small, and accordingly, the crystal grain boundary increases and becomes a carrier recombination center. It becomes easy. This carrier recombination in the vicinity of the pn junction interface can be reduced by moving holes generated by photoexcitation early from the pn junction interface due to a change in the In / Cu molar ratio.

  Further, as can be seen from FIG. 4, it is also preferable for the above reason to satisfy (Eg1-Eg2)> (Eg2-Eg3) by increasing the In / Cu ratio of the light absorption layer from the pn interface toward the back electrode. .

Hereinafter, configurations other than the light absorption layer 14 used in the photoelectric conversion element will be described.
As the substrate 11, it is desirable to use blue plate glass, and it is also possible to use a metal plate such as stainless steel, Ti or Cr, or a resin such as polyimide.

  As the back electrode 12, a conductive metal film such as Mo or W can be used. Among these, it is desirable to use a Mo film.

  As the extraction electrodes 13 and 17, for example, a conductive metal film such as Al, Ag, or Au can be used. Furthermore, in order to improve the adhesion with the transparent electrode 15, after depositing Ni or Cr, Al, Ag or Au may be deposited.

As the buffer layer 15, CdS, Zn (O, S, OH), or ZnO added with Mg can be used. The chalcopyrite compound semiconductor that is the light absorption layer 14 functions as a p-type semiconductor, the buffer layer 15a typified by CdS or ZnO: Mg functions as an n-type semiconductor, and the buffer layer 15b typified by ZnO functions as an n ⁺ -type layer. I think that. By selecting the material of the buffer layer 15a so as to induce a conduction band discontinuity (CBO) ΔEc at the pn junction interface, carrier recombination can be reduced.

The transparent electrode layer 16 is required to transmit light such as sunlight and to have conductivity, for example, ZnO: Al containing 2 wt% of alumina (Al ₂ O ₃ ) or B from diborane as a dopant. ZnO: B can be used.
For example, MgF ₂ is desirably used as the antireflection film 18.

As a manufacturing method of the photoelectric conversion element 10 of FIG.
In addition, it is an example of the following manufacturing method, You may change suitably. Therefore, the order of the steps may be changed, or a plurality of steps may be combined.

[Step of depositing back electrode on substrate]
A back electrode 12 is formed on the substrate 11. Examples of the film forming method include a thin film forming method such as a sputtering method using a sputtering target made of a conductive metal.

[Step of depositing light absorption layer on back electrode]
After the back electrode 12 is deposited, a compound semiconductor thin film that becomes the light absorption layer 14 is deposited. Since the light absorption layer 14 and the first extraction electrode 13 are deposited on the back electrode 12, the light absorption layer 14 is deposited on a part of the back electrode 12 excluding at least the portion where the first extraction electrode 13 is deposited. To do. As a film forming method, Cu, a group IIIb element and a group VIb element are independently supplied by sputtering. A gradient composition can be formed by adjusting the output so that the IIIb group / Cu molar ratio increases from the back electrode side. If the IIIb group / Cu molar ratio can be adjusted, two-way simultaneous sputtering may be used instead of three-way. In order to increase carrier mobility and suppress carrier recombination, the number of output adjustments is preferably at least two, and it is preferable that the number of adjustments is large. It is also preferable to continuously adjust the output and adjust the output so that the IIIb group / Cu molar ratio changes linearly or curvedly. At that time, the VIb group element may be excessively supplied so as not to be deficient. In order to greatly change the energy output for sputtering and widen the composition modulation range, the Cu and IIIb group elements are preferably DC sputtering that can be sputtered with high energy. The supply method of the VIb group element is not limited to the sputtering method, and can be supplied by a vapor deposition method.

[Step of depositing buffer layer on light absorption layer]
Buffer layers 15 a and 15 b are deposited on the obtained light absorption layer 14.
Examples of the method for forming the buffer layer 15a include a vacuum process sputtering method, a vacuum deposition method or metal organic chemical vapor deposition (MOCVD), and a liquid phase chemical deposition (CBD) method.
Examples of the method for forming the buffer layer 15b include sputtering in a vacuum process, vacuum deposition, or metal organic chemical vapor deposition (MOCVD).

[Step of depositing transparent electrode on buffer layer]
Subsequently, the transparent electrode 16 is deposited on the buffer layer 15b.
Examples of the film forming method include sputtering in a vacuum process, vacuum vapor deposition, or metal organic chemical vapor deposition (MOCVD).

[Step of taking out electrode film on back electrode and transparent electrode]
The first extraction electrode 13 is deposited on a portion excluding at least the portion where the light absorption layer is formed on the back electrode 12.
The second extraction electrode 17 is deposited on a portion excluding at least a portion where the antireflection film is formed on the transparent electrode 16.
Examples of the film forming method include a sputtering method and a vacuum deposition method.
The film formation of the first and second extraction electrodes may be performed in one step, or may be performed after any step as a separate step.

[Step of depositing antireflection film on transparent electrode]
Finally, an antireflection film 18 is deposited on the transparent electrode 16 at least on the part excluding the part where the second extraction electrode 17 is formed.
Examples of the film forming method include a sputtering method and a vacuum deposition method.
Through the above steps, the photoelectric conversion element shown in the conceptual diagram of FIG. 1 is manufactured.
When producing a module of a photoelectric conversion element, after the step of forming the back electrode on the substrate, the step of dividing the back electrode with a laser, the step of forming a buffer layer on the light absorption layer, and the buffer layer After the step of forming the transparent electrode, integration can be performed by sandwiching a step of dividing the sample by mechanical scribing.
Hereinafter, the present invention will be described in detail by way of examples.

Example 1
A blue glass substrate was used as the substrate 11, and a Mo thin film serving as the back electrode 12 was deposited by about 700 nm by sputtering. Sputtering was performed by applying 200 W by RF in an Ar gas atmosphere using Mo as a target.
After the Mo thin film to be the back electrode 12 was deposited, a Cu—In—Te thin film to be the light absorption layer 14 was deposited to about 2 μm. The substrate temperature during film formation was 550 ° C. Film formation was performed by ternary simultaneous sputtering using Cu, In, and Te targets. The RF output for the Cu target was monotonously changed from 200 W to 100 W in an Ar gas atmosphere, and the outputs of the other two targets were fixed at 200 W.
A ZnO thin film to which Mg was added as a buffer layer 15a was deposited on the obtained light absorption layer 14 to a thickness of about 50 nm. The film formation was performed by RF sputtering, but was performed at an output of 50 W in consideration of plasma damage at the interface. A ZnO thin film was deposited as a buffer layer 15b on the buffer layer 15a, and then ZnO: Al containing 2 wt% of alumina (Al ₂ O ₃ ) to be the transparent electrode 16 was deposited to a thickness of about 1 μm. As the extraction electrodes 13 and 17, Al was deposited by an evaporation method. The film thickness was 100 nm and 300 nm, respectively. Finally, MgF ₂ was deposited as the antireflection film 18 by sputtering to obtain a photoelectric conversion element in which the In / Cu ratio of the light absorption layer increased monotonously from the back electrode side.

  A short-circuit current density (Jsc) was obtained by measuring a current when no voltage was applied using a voltage source and a multimeter under irradiation of AM1.5 simulated sunlight by a solar simulator.

(Example 2)
In the process of changing the RF output for the Cu target from 200 W to 100 W in the formation of the Cu—In—Te thin film to be the light absorption layer 14, the same as Example 1 except that the process of decreasing the RF output is performed 10 times. The photoelectric conversion element was manufactured by the method.

(Reference Example 1)
In the process of changing the RF output for the Cu target from 200 W to 100 W in the formation of the Cu—In—Te thin film to be the light absorption layer 14, the same as Example 1 except that the process of decreasing the RF output is performed five times. The photoelectric conversion element was manufactured by the method.

(Reference Example 2)
In the process of changing the RF output for the Cu target from 200 W to 100 W in the formation of the Cu—In—Te thin film to be the light absorption layer 14, the process is the same as in Example 1 except that the process of decreasing the RF output is performed three times. The photoelectric conversion element was manufactured by the method.

(Reference Example 3)
In the process of changing the RF output for the Cu target from 200 W to 100 W in the formation of the Cu—In—Te thin film to be the light absorption layer 14, the same as Example 1 except that the process of decreasing the RF output is performed twice. The photoelectric conversion element was manufactured by the method.

(Comparative Example 1)
In the process of changing the RF output for the Cu target from 200 W to 100 W in the formation of the Cu—In—Te thin film to be the light absorption layer 14, except that the process of decreasing the RF output is performed only once. The photoelectric conversion element was manufactured by the same method.

(Comparative Example 2)
A photoelectric conversion element was manufactured in the same manner as in Example 1 except that the Cu-In-Te thin film serving as the light absorption layer 14 was formed and the RF output for the Cu target was fixed to 200 W.

(Comparative Example 3)
A photoelectric conversion element was manufactured in the same manner as in Example 1 except that the Cu-In-Te thin film serving as the light absorption layer 14 was formed and the RF output for the Cu target was fixed to 100 W.

(Comparative Example 4)
A photoelectric conversion element was manufactured by the same method as in Example 1 except that the Cu output of the Cu target was monotonously changed from 100 W to 200 W in forming a Cu—In—Te thin film to be the light absorption layer 14.

(Example 3)
In the process of changing the RF output for the Cu target from 200 W to 100 W in the formation of the Cu—In—Te thin film to be the light absorption layer 14, the change rate of the RF output in the latter half of the film formation is changed to the RF output in the first half of the film formation. A photoelectric conversion element was produced by the same method as in Example 1 except that the change rate was doubled. The group IIIb / Cu molar ratio of the p-type light absorbing layer at the interface between the p-type light absorbing layer and the n-type compound semiconductor layer is x1, and the group IIIb / Cu mole is 1/2 of the layer thickness of the p-type light absorbing layer. When the ratio was x2 and the group IIIb / Cu molar ratio at the interface with the back electrode was x3, x1 = 2.8, x2 = 1.7, and x3 = 1.1.

Example 4
In the process of changing the RF output for the Cu target from 200 W to 100 W in the formation of the Cu—In—Te thin film to be the light absorption layer 14, the change rate of the RF output in the latter half of the film formation is changed to the RF output in the first half of the film formation. A photoelectric conversion element was produced by the same method as in Example 1 except that the rate of change was 3 times. The group IIIb / Cu molar ratio of the p-type light absorbing layer at the interface between the p-type light absorbing layer and the n-type compound semiconductor layer is x1, and the group IIIb / Cu mole is 1/2 of the layer thickness of the p-type light absorbing layer. When the ratio was x2 and the group IIIb / Cu molar ratio at the interface with the back electrode was x3, x1 = 2.8, x2 = 1.6, and x3 = 1.2.

(Example 5)
In the process of changing the RF output for the Cu target from 200 W to 100 W in the formation of the Cu—In—Te thin film to be the light absorption layer 14, the change rate of the RF output in the latter half of the film formation is changed to the RF output in the first half of the film formation. A photoelectric conversion element was produced in the same manner as in Example 1 except that the rate of change was 5 times. The group IIIb / Cu molar ratio of the p-type light absorbing layer at the interface between the p-type light absorbing layer and the n-type compound semiconductor layer is x1, and the group IIIb / Cu mole is 1/2 of the layer thickness of the p-type light absorbing layer. When the ratio was x2 and the group IIIb / Cu molar ratio at the interface with the back electrode was x3, x1 = 2.9, x2 = 1.4, and x3 = 1.1.

(Comparative Example 5)
In the process of changing the RF output for the Cu target from 200 W to 100 W in the film formation of the Cu—In—Te thin film to be the light absorption layer 14, the change rate of the RF output in the first half of the film formation is changed to the RF output in the second half of the film formation. A photoelectric conversion element was produced by the same method as in Example 1 except that the change rate was doubled.

FIG. 7 shows the shape of the VBM in the film thickness direction when Cu—In—Te thin films were formed in Examples 1 to 5, Reference Examples 1 to 3 and Comparative Examples 1 to 5. In the figure, ◎, ○, Δ, × indicate whether the short-circuit current density is good or bad. From ◎ to ×, the short circuit current density decreases.

In Example 1 and 2 Reference Examples 1 to 3 , there are two or more inclined parts of the VBM, which has the effect of increasing the mobility of carriers (holes) and suppressing carrier recombination. In Comparative Example 1, There is only one inclined part of the VBM, and carrier (hole) movement is insufficient.
Carriers (holes) generated by photoexcitation move from the pn junction interface side to the back electrode side. At that time, it is desirable that the VBM changes to the right shoulder. In the first embodiment, the VBM changes in a way that rises to the right, whereas in the comparative example 4, the VBM changes in the opposite direction, which is not preferable because the movement of the carrier (hole) is hindered. In Comparative Examples 2 and 3, the VBM does not change, and the mobility of carriers (holes) is low, which is not preferable because the recombination probability increases.
In Examples 3 to 5 , it is more preferable to use a VBM shape that can quickly move carriers (holes) from the vicinity of the pn junction interface where interface recombination easily occurs. On the other hand, Comparative Example 5 is not preferable because the carrier (hole) movement in the vicinity of the pn junction interface is slow and the recombination probability increases.
By using the photoelectric conversion element of the present invention for a solar cell, a solar cell with high conversion efficiency can be obtained.

  The embodiment of the present invention has been described above. However, the present invention is not limited to the above-described embodiment as it is, and can be embodied by modifying constituent elements without departing from the scope of the invention in the implementation stage. Moreover, various inventions can be formed by appropriately combining a plurality of components disclosed in the embodiment. For example, as in the modification, the constituent elements over different embodiments may be appropriately combined.

DESCRIPTION OF SYMBOLS 10 ... Photoelectric conversion element, 11 ... Board | substrate, 12 ... Back electrode, 13 ... 1st extraction electrode, 14 ... Light absorption layer, 15a ... Buffer layer, 15b ... Buffer layer, 16 ... Transparent electrode layer, 17 ... 2nd Extraction electrode, 18 ... antireflection film

Claims (6)

P having at least one group IIIb element selected from the group consisting of Cu, Al, In and Ga and at least one group IVb element selected from the group consisting of O, S, Se and Te and having a chalcopyrite structure A light absorption layer is provided between the n-type compound semiconductor layer and the back electrode, and a group IIIb / Cu molar ratio, which is a molar ratio between the group IIIb element and the Cu, is the same as that of the n-type compound semiconductor layer. the photoelectric conversion element characterized by having a graded composition portion decreases towards the interface between the back surface electrode side from the interface side from an interface between at least the n-type compound semiconductor layer of the p-type light absorbing layer.   The IIIb group / Cu molar ratio of the p-type light absorbing layer is x1 at the IIIb group / Cu molar ratio at the interface with the n-type compound semiconductor layer, and is 1/2 of the layer thickness of the p-type light absorbing layer. 2. The relation (x1-x2)> (x2-x3) is satisfied, where the group IIIb / Cu molar ratio is x2 and the group IIIb / Cu molar ratio at the interface with the back electrode is x3. The photoelectric conversion element as described.   The band gap of the p-type absorber layer is Eg1 (eV) at the interface with the n-type compound semiconductor layer, and Eg2 (1/2) of the layer thickness of the p-type absorber layer. eV), where Eg3 (eV) is a band gap at the interface with the back electrode, (Eg1-Eg2)> (Eg2-Eg3). Photoelectric conversion element.   4. A p-type light absorption layer containing at least one element selected from the group consisting of Cu, Al, In, and Ga, and O to Te and having a chalcopyrite type structure. Item 1. The photoelectric conversion element according to item 1.   5. The photoelectric conversion element according to claim 1, wherein x1, x2, and x3 satisfy a relationship of (x1−x2) / (x2−x3)> 1.8.   A solar cell comprising the photoelectric conversion element according to any one of claims 1 to 5.