Photoelectric converter with a multi-layered quantum dot film

A photoelectric converter includes two semiconductor layers forming a p/n junction as a photoelectric conversion layer. At least one semiconductor layer of the two semiconductor layers is a quantum dot integrated film, and the quantum dot integrated film includes two or greater quantum dot layers having different energy levels. In a case that the quantum dot integrated film is a p-type, a quantum dot layer having a large difference between an energy level (Bv) of a valence band and a Fermi level (Ef) is disposed closer to a p/n junction surface.

TECHNICAL FIELD

The present disclosure relates to a photoelectric converter.

BACKGROUND ART

Development of solar cells as a clean energy source that provides energy and resource savings is actively progressing. Photoelectric converters such as solar cells are power devices that utilize the photovoltaic effect to convert light energy directly into electric power. In recent years, based on the anticipation that a conversion efficiency of 60% or greater can be theoretically achieved, photoelectric converters using integrated films in which semiconductive nanoparticles (quantum dots) are integrated as photoelectric conversion layers have been considered as next-generation photoelectric converters (for example, Patent Literature 1 to 4).

Incidentally, as can be understood from the examples of Patent Literature 1 to 4, in photoelectric converters disclosed up to this point, the photoelectric conversion layer has a configuration made up of quantum dots having the same shape.

CITATION LIST

Patent Literature

Patent Literature 1: JP 2013-229378 A

Patent Literature 2: JP 2011-249579 A

Patent Literature 3: JP 2011-121862 A

SUMMARY OF INVENTION

The present disclosure relates to the recognition that by controlling the band structure of quantum dots included in a semiconductor layer constituting the photoelectric conversion layer and applying a change to energy levels in the photoelectric conversion layer, the mobility of generated carriers (electrons e, holes h) can be improved. A photoelectric converter is configured including two semiconductor layers forming a p/n junction as a photoelectric conversion layer. At least one semiconductor layer of the two semiconductor layers is a quantum-dot integrated film, and the quantum dot integrated film includes two or greater quantum dot layers having different energy levels.

DESCRIPTION OF EMBODIMENTS

In cases where the photoelectric conversion layer is occupied by quantum dots having the same band structure as in the known photoelectric converter described above, the energy levels formed in the photoelectric conversion layer have almost no inclination in the thickness direction of the photoelectric conversion layer, and the mobility of carriers generated in the semiconductor layer is low. As a result, the carrier collection efficiency is low, and an improvement in the photoelectric conversion efficiency cannot be expected.

FIG. 1Ais a schematic cross-sectional view partially illustrating a photoelectric converter according to a first embodiment of the present disclosure, where the semiconductor layer including quantum dots is p-type, and the thickness of the barrier layer surrounding the quantum dots is modified.FIG. 1Bis a schematic diagram illustrating the energy levels in the photoelectric converter ofFIG. 1A. InFIG. 1AandFIG. 1B, the semiconductor layer5is n-type and the semiconductor layer7is p-type. In addition, the symbol Ef(denoted by an alternate long and two short dashed line) is a Fermi level, BC(denoted by a solid line) is the energy level of the conduction band, and BV(denoted by a solid line) is the energy level of the valence band. As illustrated inFIG. 1B, the energy level BVof the valence band and the energy level BCof the conduction band change such that the energy levels in the semiconductor layer7including the quantum dots9become higher than the energy levels in the semiconductor layer5.

In the photoelectric converter (for example, a solar cell) illustrated inFIG. 1AandFIG. 1B, the photoelectric conversion layer1includes electrode layers3A and3B on both sides thereof. In the photoelectric conversion layer1, the two semiconductor layers5and7forms a p/n junction, and in this case, the interface denoted by reference numeral8indicates a p/n junction surface.

Among the semiconductor layers5and7that constitute the photoelectric conversion layer1, for example, the semiconductor layer7illustrated inFIG. 1Ais the quantum dot integrated film11.

The quantum dot integrated film11is formed by quantum dot composite particles14, each including a barrier layer13around the quantum dot9. In this case, the quantum dot integrated film11has a configuration in which two kinds of quantum dot composite particles14aand14bhaving different thicknesses of the barrier layer13are layered. This allows the semiconductor layer7which serves as the quantum dot integrated film11to have a structure in which two quantum dot layers7A and7B having different energy levels are layered.

In the photoelectric converter according to the first embodiment, as the thickness t of the barrier layer13constituting the quantum dot composite particles14(14aand14b) differs between the two quantum dot layers7A,7B constituting the semiconductor layer7, as illustrated inFIG. 1B, a difference occurs in the energy levels (reference symbol ΔE1inFIG. 1B) that change one-dimensionally in the photoelectric conversion layer1as a result of the difference in surface energy due to the difference in thickness t of the barrier layer13.

This allows the photoelectric converter of the first embodiment to form an inclination for the energy band such that the energy level increases from the quantum dot layer7A disposed closer to the p/n bonding surface8to the quantum dot layer7B farther from the p/n junction surface8in the semiconductor layer7. That is, when the quantum dot integrated film11is p-type, a quantum dot layer (7A, in this case) having a large difference between the energy level BVof the valence band and the Fermi level Efis disposed closer to the p/n bonding surface8.

As a result, carriers (electrons e, holes h) having different polarities formed in the photoelectric conversion layer1may be easily moved in the respective directions of the electrode layers3A and3B, whereby the collection efficiency of carriers in the electrode layers3A and3B improves, and the photoelectric conversion efficiency can be enhanced. Hereinafter, similar effects can be obtained for the photoelectric converters of the second embodiment to the eighth embodiment.

Here, the statement that the thickness t of the barrier layer13constituting the quantum dot composite particles14differs refers to a case in which, when a comparison is made between the one with the thicker average thickness of the barrier layer13between the quantum dot layers7A and7B taken as t1and the one with the lesser average thickness taken as t2, the average thickness ratio t1/t2is 1.5 or greater.

At this time, from the viewpoint that the mobility of the carriers (electrons e, holes h) in the photoelectric conversion layer1can be increased, it is preferable that no members other than the quantum dot composite particles14exist at the boundary between the quantum dot layer7A and the quantum dot layer7B. This similarly applies to the photoelectric converters of other embodiments described below.

The average thicknesses t1and t2of the barrier layer13are obtained by observing, for example, 5 to 20 quantum dot composite particles14existing in a predetermined range in each region of the quantum dot layers7A and7B, extracting a portion in each quantum dot composite particle14where the thickness of the barrier layer13is maximum, and acquiring the average value thereof.

The photoelectric converter of the first embodiment can be realized by specifically applying the following members. For example, a semiconductor material including silicon or zinc oxide as a main component is suitable for the semiconductor layer5. In this case, when silicon is used for the semiconductor layer5, a material including an n-type doping component is utilized. In contrast, at least one of silicon, lead sulfide (PbS), or indium phosphide can be used for the quantum dots9that constitute the semiconductor layer7(quantum dot integrated film11).

Either an inorganic material or an organic material can be applied to a material of the barrier layer13. In this case, in a case where an inorganic material is utilized for the barrier layer13, a photoelectric converter having a high degree of weather resistance can be obtained. In contrast, in a case where an organic material is applied to the barrier layer13, since the thickness of the barrier layer13can be modified by the molecular weight of the organic material, it is easy to control the band gap between the quantum dot layers7A and7B. This allows a quantum dot integrated film11(semiconductor layers7A and7B) having a high carrier confinement effect to be formed. Here, silicon carbide, silicon dioxide and zinc sulfide may be suitable as the inorganic material of the barrier layer13, and tetrabutylammonium iodide (TBAI) or 1,2-ethanedithiol (EDT) may be used as the organic material.

FIG. 2Ais a schematic cross-sectional view partially illustrating a photoelectric converter according to a second embodiment of the present disclosure, where the semiconductor layer including quantum dots is p-type, and the components of the barrier layer surrounding the quantum dots are modified.FIG. 2Bis a schematic diagram illustrating the energy levels in the photoelectric converter ofFIG. 2A.

With the exceptions that the thickness of the barrier layers13are the same and that the components of the barrier layer13provided around the quantum dots9are different, the photoelectric converter of the second embodiment illustrated inFIG. 2AandFIG. 2Bhas the same configuration as the photoelectric converter of the first embodiment illustrated inFIG. 1AandFIG. 1B.

In the photoelectric converter of the second embodiment, by making the components of the barrier layer13provided around the quantum dots9different between the quantum dot layers7A and7B, the surface energy of the quantum dot composite particles14is modified, and as illustrated inFIG. 2B, a difference in energy levels (reference symbol ΔE1inFIG. 2B) which changes one-dimensionally in the photoelectric conversion layer1is generated.

This allows an inclination of the energy band to be formed in the semiconductor layer7such that the energy level becomes higher from the quantum dot layer7A to the quantum dot layer7B.

With regard to materials for making the photoelectric converter of the second embodiment, it is preferable that the semiconductor material that serves as the semiconductor layer5and the quantum dots9constituting the semiconductor layer7(quantum dot integrated film11) have the same configuration as in the case of the first embodiment, but for the material of the barrier layer13, the TBAI, which is an organic material, can be applied to the quantum dot composite particles14in the quantum dot layer7A, and the EDT, which is also an organic material, can be applied to the quantum dot composite particles14in the quantum dot layer7B.

FIG. 3Ais a schematic cross-sectional view partially illustrating a photoelectric converter according to a third embodiment of the present disclosure, where the semiconductor layer including the quantum dots is p-type, and the doping components included in the quantum dots are modified.FIG. 3Bis a schematic diagram illustrating the energy levels in the photoelectric converter ofFIG. 3A.

The quantum dot integrated film11that constitutes the photoelectric converter of the third embodiment is not formed as what is known as a core-shell structure as illustrated inFIGS. 1A and 1BandFIGS. 2A and 2Bin which the barrier layer13is provided around individual quantum dots9(9a,9b), but instead, as a representative example, has a structure in which the material that serves as the barrier layer13is a matrix13A, and the quantum dots9(9a,9b) are included in the matrix13A. Except for this, the photoelectric converter has the same configuration as the photoelectric converter of the first embodiment illustrated inFIG. 1A.

In the photoelectric converter of the third embodiment, by modifying the acceptor type doping component (element) or the concentration of the doping component included in the quantum dots9(9a,9b), a difference can be created in the surface energy of the quantum dots9between the quantum dot layers7A and7B. This allows, as illustrated inFIG. 3B, an inclination of the energy band to be formed in the semiconductor layer7such that the energy level becomes higher from the quantum dot layer7A to the quantum dot layer7B.

With regard to materials for making the photoelectric converter of the third embodiment, it is preferable that the semiconductor material that serves as the semiconductor layer5and the quantum dots9(9a,9b) that constitute the semiconductor layer7(quantum dot integrated film11) have the same configuration as in the case of the first embodiment. In contrast, with regard to the semiconductor layer7(quantum dot integrated film11), when silicon is applied to the quantum dots9(9a,9b), elements of Group12and Group13of the periodic table can be applied as doping components. In this case, a configuration is preferable in which the quantum dots9athat constitute the quantum dot layer7A are made to include a large number of elements belonging to Group13(any one of B, Al, Ga, or In), and the other quantum dots9bthat constitute the quantum dot layer7B are made to include a large number of elements belonging to Group12(for example, any one of Zn, Cd, or Hg).

In addition, when indium phosphide is applied to the quantum dots9(9a,9b), a configuration is preferable in which, for example, the quantum dots9aare made to include a large number of elements belonging to Group14(Sn), and the other quantum dots9bthat constitute the quantum dot layer7B are made to include a large number of elements belonging to Group7, Group11, and Group12(for example, any one of Mn, Cu, or Zn).

In cases where the concentration of the doping components is modified between the quantum dot layers7A and7B, it may be desirable for the concentration of the doping components included in the quantum dot layer7B to be made higher than the concentration of the doping components included in the quantum dot layer7A.

In cases where the atomic valence of the doping components is modified between the quantum dot layers7A and7B, it may be desirable to use elements having different atomic valences as the doping components of the quantum dots9aand9b.

In this case as well, any of the above-described silicon carbide, silicon dioxide, tetrabutylammonium iodide (TBAI), or 1,2-ethanedithiol (EDT) may be suitable for the matrix13A.

FIG. 4Ais a schematic cross-sectional view partially illustrating a photoelectric converter according to a fourth embodiment of the present disclosure, where the semiconductor layer including quantum dots is n-type, and the thickness of the barrier layer surrounding the quantum dots is modified.FIG. 4Bis a schematic diagram illustrating the energy levels in the photoelectric converter ofFIG. 4A.

In the photoelectric converter of the fourth embodiment illustrated inFIG. 4AandFIG. 4B, the semiconductor layer5is a p-type and the semiconductor layer7is n-type. In this case as well, as inFIGS. 1A and 1B, the interface denoted by reference numeral8is the p/n junction surface. In addition, the reference symbols Ef, BC, and BVare the same as those illustrated inFIGS. 1A and 1B.

The photoelectric converter of the fourth embodiment illustrated inFIG. 4AandFIG. 4Bdiffers from the photoelectric converter of the first embodiment illustrated inFIG. 1AandFIG. 1Bin that the polarity of the semiconductor layer5and the semiconductor layer7is reversed, the semiconductor layer5is p-type, and the semiconductor layer7formed of the quantum dot integrated film11is n-type. In this case, the energy level BVof the valence band and the energy level BCof the conduction band change such that the energy levels in the semiconductor layer5become higher than the energy levels in the semiconductor layer7formed by the quantum dot integrated film11, opposite to the case ofFIG. 1B.

In the photoelectric conversion device of the fourth embodiment, the quantum dot composite particles14that constitute the quantum dot layer7B closer to the semiconductor layer5have a lesser thickness of the barrier layer13than that of the quantum dot composite particles14that constitute the quantum dot layer7A, and the quantum dot layer7A has a greater increase in the Fermi level Efdue to the barrier layer13than that in the quantum dot7B. This allows an inclination of the energy band to be generated such that the energy level becomes lower from the quantum dot layer7B to the quantum dot7A.

FIG. 5Ais a schematic cross-sectional view partially illustrating a photoelectric converter according to a fifth embodiment of the present disclosure, where the semiconductor layer including quantum dots is n-type, and the components of the barrier layer surrounding the quantum dots are modified.FIG. 5Bis a schematic diagram illustrating the energy levels in the photoelectric converter ofFIG. 5A.

Also, in the photoelectric converter of the fifth embodiment illustrated inFIG. 5AandFIG. 5B, similarly to the photoelectric converter of the fourth embodiment described above, the semiconductor layer5is p-type, the semiconductor layer7is n-type, and the interface denoted by reference numeral8is a p/n junction surface. In addition, reference symbols Ef, BCand BValso indicate the levels illustrated inFIG. 5B.

The photoelectric converter of the fifth embodiment illustrated inFIG. 5AandFIG. 5Bdiffers from the photoelectric converter of the second embodiment illustrated inFIG. 2AandFIG. 2Bin that the polarity of the semiconductor layer5and the semiconductor layer7is reversed, the semiconductor layer5is p-type, and the semiconductor layer7formed of the quantum dot integrated film11is n-type. In this case, the energy level By of the valence band and the energy level BCof the conduction band change such that the energy levels of the semiconductor layer5become higher than the energy levels of the semiconductor layer7formed by the quantum dot integrated film11, opposite to the case ofFIG. 2B.

In the photoelectric converter of the fifth embodiment, by making the components of the barrier layer13provided around the quantum dots9different, a difference is created in the surface energy of the quantum dot composite particles14. This allows, as illustrated inFIG. 5B, a difference to occur in the energy levels (reference symbol ΔE1inFIG. 2B) that change one-dimensionally in the photoelectric conversion layer1. In this case, the barrier layer13has a configuration opposite to that of the photoelectric converter of the second embodiment in materials thereof. For example, EDT can be applied to the quantum dot composite particles14in the quantum dot layer7A, and TBAI can be applied to the quantum dot composite particles14in the quantum dot layer7B.

FIG. 6Ais a schematic cross-sectional view partially illustrating a photoelectric converter according to a sixth embodiment of the present disclosure, where the semiconductor layer including the quantum dots is n-type, and the doping components included in the quantum dots are modified.FIG. 6Bis a schematic diagram illustrating the energy levels in the photoelectric converter ofFIG. 6A.

In the photoelectric converter of the sixth embodiment illustrated inFIG. 6AandFIG. 6B, the semiconductor layer5is a p-type and the semiconductor layer7is an n-type. In this case as well, as inFIGS. 3A and 3B, the interface denoted by reference numeral8is the p/n junction surface. In addition, the reference symbols Ef, BC, and BVare the same as those illustrated inFIGS. 3A and 3B.

The photoelectric converter of the sixth embodiment illustrated inFIG. 6AandFIG. 6Bdiffers from the photoelectric converter of the third embodiment illustrated inFIG. 3AandFIG. 3Bin that the polarity of the semiconductor layer5and the semiconductor layer7is reversed, the semiconductor layer5is p-type, and the semiconductor layer7formed of the quantum dot integrated film11is n-type. In this case, the energy level By of the valence band and the energy level BCof the conduction band change such that the energy levels of the semiconductor layer5become higher than the energy levels of the semiconductor layer7formed by the quantum dot integrated film11, opposite to the case ofFIG. 3B.

With respect to the photoelectric converter of the sixth embodiment as well, similar to the case of the photoelectric converter of the third embodiment described above, by modifying the doping component (donor type element) included in the quantum dots9(9a,9b) or the concentration of the doping component, a difference can be created in the surface energy of the quantum dots9between the quantum dot layers7A and7B. This allows, as illustrated inFIG. 6B, an inclination of the energy band to be formed in the semiconductor layer7such that the energy level becomes higher from the quantum dot layer7A to the quantum dot layer7B.

In the case of the photoelectric converter of the sixth embodiment, regarding the semiconductor layer7(quantum dot integrated film11), when silicon is applied to the quantum dots9(9a,9b), elements of Group15and Group16of the periodic table can be applied as the doping components. In this case, a configuration is preferable in which the quantum dots9athat constitute the quantum dot layer7B include a large number of elements of Group15(any one of P, AS, or Sb), and the other quantum dots9bthat constitute the quantum dot7A include a large number of elements of Group16(any one of S, Se, or Te).

In addition, when indium phosphide is applied to the quantum dots9aand9b,elements of each of Groups7,11,12, and14of the periodic table can be applied as doping components.

In cases where the concentration of the doping components is modified between the quantum dot layers7A and7B, it is preferable that the concentration on the quantum dot layer7B side is lower than that on the quantum dot layer7A side.

Also, when changing the atomic valence of the doping components between the quantum dot layers7A and7B, elements having different atomic valences may be used as the doping components of the quantum dots9aand9b.

In addition, the photoelectric converter of the present embodiment can have the same configuration as that of the photoelectric converter of the third embodiment in the materials of the semiconductor layer5and the matrix13A.

Herein, the photoelectric converters of the first embodiment to sixth embodiment have been described above, but with respect to the semiconductor layer7constituted by the quantum dot integrated film11, a semiconductor layer7can be formed such that the semiconductor layer7includes three or greater quantum dot layers within a range in which the thickness t and components of the barrier layer13, or alternatively the concentration or doping amount of the doping components included in the quantum dot9, can be modified in multiple steps. Note that when the number of quantum dot layers disposed in the semiconductor layer7is three or greater, it is preferable that the quantum dot layer having the largest difference between the energy level BVof the valence band and the Fermi level Ef, or alternatively between the energy level BCof the conduction band and the Fermi level Ef, is placed closer to the p/n junction surface8.

Next, a method of manufacturing the photoelectric converter of the present embodiment will be described with reference toFIGS. 7A to 7D. Here, the photoelectric converter of the first embodiment will be described as an example.

First, as illustrated inFIG. 7A, a transparent electrically conductive film23is formed on one main surface of a glass substrate21that serves as a support by using an electrically conductive material such as ITO as an electrode layer3A.

Next, as illustrated inFIG. 7B, a zinc oxide film25that serves as the semiconductor layer5is formed on the surface of the transparent electrically conductive film23.

Next, as illustrated inFIG. 7C, quantum dot composite particles14having different thicknesses of the barrier layer13are film-formed on the surface of the zinc oxide film25in a layered manner to form the quantum dot integrated film11that serves as the semiconductor layer7. Here, the quantum dot integrated film11can be densified by heating or pressurizing the quantum dot integrated film11or alternatively by performing heating and pressurizing simultaneously. A spin-coating method or the like is suitable for forming the quantum dot integrated film11.

Finally, as illustrated inFIG. 7D, an electrically conductive material such as gold is vapor-deposited on the upper surface side of the quantum dot integrated film11to form an electrical conductor film27that serves as the electrode layer3B. Next, as necessary, after forming a protective layer on the surface of the electrical conductor film, the protective layer is covered with a glass film or the like. Through such a process, the photoelectric converter of the first embodiment can be achieved.

Note that when PbS (lead sulfide) is used as the quantum dot9, the photoelectric converter can be manufactured by a method in which the oleic acid solution of the selected element (Pb) and the sulfur-containing solution (here, Bis (trimethylsilyl) Sulfide solution) are heated to approximately 125° C., and then the resultant is cooled.

When modifying the thickness of the barrier layer13formed around the quantum dots9, organic molecules having different molecular weights are used as organic molecules for forming the barrier layer13.

As described above, the photoelectric converter of the first embodiment has been described by way of example. However, with respect to the photoelectric converter of the second embodiment that uses quantum dot composite particles14in which the components of the barrier layer surrounding the quantum dots has been modified, as described above, different organic molecules such as TBAI and EDT are applied.

In cases of using quantum dots9in which the doping components included in the quantum dots9are modified, for example, silicon quantum dots9doped with elements having different atomic valences or atomic weights are used. Elements included in Group11to Group16of the periodic table are appropriately selected and applied as doping component elements. In this way, the photoelectric converters of the second embodiment to sixth embodiment can be manufactured in a similar fashion.

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