JP2007258263A - Organic solar cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an organic solar cell which is appropriate in photoelectric conversion efficiency by designing an organic electron donor layer forming the organic solar cell at an appropriate thickness. <P>SOLUTION: The organic solar cell is provided with at least a photoelectric conversion layer with hetero junction that is formed of an organic electron donor layer and an electron receptor layer, and a pair of electrodes pinching the photoelectric conversion layer. In this case, the organic electron donor layer is made two to five times in thickness the exciton diffusion length of the organic electron donor forming the organic electron donor layer. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an organic solar cell.

  Solar cells using organic semiconductors (organic solar cells) are diverse, have low toxicity, have good processability and productivity, can be reduced in cost by mass production, and are easily flexible It has excellent features such as a wide range of use and is currently under active development.

  Here, since an organic solar cell is generally inferior in photoelectric conversion efficiency as compared with a solar cell using an inorganic semiconductor, various developments have been made to improve the photoelectric exchange efficiency (for example, Patent Document 1).

JP2004-319705

  As described above, various developments have been made to improve the photoelectric exchange efficiency of organic solar cells, but there are few that have reached a practical level yet. Even in the present inventor, as a result of repeated studies to improve the photoelectric exchange efficiency of the organic solar cell, attention has been paid to the thickness of the organic electron donor layer constituting the organic solar cell.

  That is, in an organic solar cell including a photoelectric conversion layer having a heterojunction formed from an organic electron donor layer and an electron acceptor layer, in order to improve the photoelectric conversion efficiency, light to the organic electron donor layer is It is conceivable to maximize absorption, and in order to realize this, it is necessary to make the organic electron donor layer as thick as possible. However, since the organic electron donor layer has a large resistance, if the layer is designed to be thick considering only light absorption, current does not flow easily, resulting in poor device characteristics. Conversely, when only the current flow of the organic electron donor layer is taken into consideration, the thickness of the organic electron donor layer is designed to be as thin as possible, and this cannot sufficiently absorb light. Therefore, it is difficult to improve the photoelectric exchange efficiency of the organic solar cell by designing the thickness of the organic electron donor layer only by absorbing light into the organic electron donor layer and the current flow of the layer.

  In addition to the photoelectric exchange efficiency described above, parameters representing the performance of the organic solar cell include a short-circuit current value (Jsc), an open-circuit voltage value (Voc), a fill factor value, and the like. It is known that it depends on the thickness of the electron donor layer.

  Therefore, it is conceivable to design the optimum thickness of the organic electron donor layer from the values of these parameters, but the values of these parameters depend independently on the thickness of the electron donor layer. Therefore, there is a trade-off relationship, and it is difficult to improve the photoelectric exchange efficiency of the organic solar cell by designing the optimum thickness of the organic electron donor layer only from the values of these parameters.

  The present application has been made under such circumstances, and the main object is to improve the photoelectric exchange efficiency of the organic solar cell, and more specifically, the thickness of the organic electron donor layer constituting the organic solar cell. The main object is to provide an organic solar cell with good photoelectric exchange efficiency by being appropriately designed.

  The invention according to claim 1 for solving the above-described problem includes at least a photoelectric conversion layer having a heterojunction formed of an organic electron donor layer and an electron acceptor layer, and a pair of electrodes sandwiching the photoelectric conversion layer The thickness of the organic electron donor layer is 2 to 5 times the exciton diffusion length of the organic electron donor constituting the organic solar cell.

  Below, the organic solar cell of this application is concretely demonstrated using drawing.

  FIG. 1 is a schematic cross-sectional view for explaining the configuration of the organic solar battery of the present application.

  As shown in FIG. 1, the organic solar cell 10 of the present application includes at least a photoelectric conversion layer 13 having a heterojunction formed of an organic electron donor layer 11 and an electron acceptor layer 12, and the photoelectric conversion layer 13. A pair of electrodes 14 and 15 are provided. The solar cell 10 is usually formed on the substrate 16.

  Below, the organic solar cell 10 of this application is demonstrated for every structure.

<Board>
The substrate 16 is not limited by the material or thickness as long as the electrode 14 can be held on the surface. Therefore, the substrate 16 may be in the form of a plate or a film, and materials such as metals such as aluminum and stainless steel, alloys, plastics such as polycarbonate and polyester can be used. Moreover, glass, transparent plastic, etc. can be used as a light transmissive material. Here, the light transmittance in the present application means a property of transmitting light in a predetermined wavelength region, for example, a visible region used in the organic solar cell with high efficiency (80% or more). In addition, when light enters from the cathode 15 side, light transmittance is not necessary.

<Electrode>
In the organic solar cell 10 of the present application, two electrodes 14 and 15 are required, one of which functions as an anode and the other functions as a cathode. In the organic solar cell shown in FIG. 1, the electrode 14 formed on the substrate 15 acts as an anode, and the electrode 15 located on the uppermost surface acts as a cathode.

The anode 14 is an electrode for efficiently collecting holes generated between the anode 14 and the cathode 15, and an electrode material made of a metal, an alloy, an electrically conductive compound, or a mixture thereof having a large work function is used. It is preferable to use one having a work function of 4 eV or more. Examples of such electrode materials include conductive transparent materials such as ITO (indium tin oxide), SnO ₂ , AZO, IZO, and GZO. The anode 14 can be produced, for example, by forming these electrode materials on the surface of the substrate 16 into a thin film by a method such as vacuum deposition, ion plating, or sputtering.

  On the other hand, the cathode 15 is an electrode for efficiently collecting electrons generated in the photoelectric conversion layer 13 and is formed of an electrode material made of a metal, an alloy, an electrically conductive compound, and a mixture thereof having a small work function. It is preferable to use a material having a work function of 5 eV or less. Examples of the electrode material of the cathode 15 include metal electrode materials typified by Al, Ca and the like. The cathode 15 can be formed into a thin film by a method such as a vacuum deposition method or a sputtering method using these electrode materials, for example.

<Photoelectric conversion layer>
The photoelectric conversion layer 13 constituting the organic solar cell 10 of the present application is configured by at least heterojunction between the organic electron donor layer 11 and the electron acceptor layer 12.

  The organic electron donor constituting the organic electron donor layer 11 is a conjugated electron-spread π-electron compound that has charge carriers as holes and a material that exhibits stable p-type semiconductor characteristics in air. There is no particular limitation.

  Specifically, oligomers and polymers having thiophene and its derivatives in the skeleton, oligomers and polymers having phenylenevinylene and its derivatives in the skeleton, oligomers and polymers having thienylene vinylene and its derivatives in the skeleton, vinylcarbazole and its derivatives Oligomers and polymers with backbones, oligomers and polymers with backbones of pyrrole and derivatives, oligomers and polymers with backbones of acetylene and derivatives, oligomers and polymers with backbones of isothiaphene and derivatives, heptadiene and derivatives thereof Polymers such as oligomers and polymers in the skeleton, metal-free phthalocyanines, metal phthalocyanines and their derivatives, diamines, phenyldiamines and their derivatives, pentacene, etc. Sens and derivatives thereof, porphyrin, tetramethylporphyrin, tetraphenylporphyrin, diazotetrabenzporphyrin, monoazotetrabenzporphyrin, diazotetrabenzporphyrin, triazotetrabenzporphyrin, octaethylporphyrin, octaalkylthioporphyrazine, octaalkylaminoporphyrin Low molecules such as metal-free porphyrins such as azine, hemiporphyrazine and chlorophyll, metalloporphyrins and derivatives thereof, quinone dyes such as cyanine dyes, merocyanine, benzoquinone and naphthoquinone can be used. As the central metal of metal phthalocyanine and metal porphyrin, magnesium, zinc, copper, silver, aluminum, silicon, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, tin, platinum, lead and other metals, metal oxides, Metal halides are used.

  On the other hand, the electron donor constituting the electron acceptor layer 12 may be an organic substance or an inorganic substance in the present application, and the charge carrier is an electron, and exhibits stable n-type semiconductor characteristics in the air. If it is a material, it will not specifically limit.

  Specifically, organic electron acceptors include oligomers and polymers having pyridine and derivatives thereof as skeletons, oligomers and polymers having quinoline and derivatives thereof as skeletons, ladder polymers based on benzophenanthrolines and derivatives thereof, and cyanopolyphenylene. Small molecules such as polymers such as vinylene, fluorinated metal-free phthalocyanines, fluorinated metal phthalocyanines and derivatives thereof, perylene and derivatives thereof, naphthalene derivatives, bathocuproine and derivatives thereof may be used. Examples of the inorganic electron acceptor include modified or unmodified fullerenes and carbon nanotubes.

  Here, in the organic solar cell 10 of the present application, the thickness X of the organic electron donor layer 11 is 2 to 5 times the exciton diffusion length of the organic electron donor constituting the organic electron donor layer 11. have. By making the relationship between the thickness X of the organic electron donor layer 11 and the exciton diffusion length of the organic electron donor constituting the organic electron donor layer 11, that is, the thickness X of the organic electron donor layer 11 is By designing based on the exciton diffusion length, the organic electron donor layer 11 can be well balanced in both the light absorption amount of the organic electron donor layer 11 and the ease of current flow, and as a result, It is possible to provide an organic solar cell excellent in photoelectric exchange efficiency. In the organic solar cell 10 of the present application, as described above, the thickness X of the organic electron donor layer 11 can be set based only on the exciton diffusion length of the organic electron donor constituting the organic electron donor layer 11. Without measuring various parameters (for example, short circuit current value (Jsc), open circuit voltage value (Voc), fill factor value, etc.) representing the performance of the organic solar cell, the design of the organic solar cell is facilitated Become.

  FIG. 2 is a conceptual diagram for explaining “exciton diffusion length of an organic electron donor” in the present application.

As shown in FIG. 2, in the present application, the exciton diffusion length of the organic electron donor means that the exciton generated in the organic electron donor layer 11 is lost by recombination or the like by irradiating the organic electron donor layer 11 with light. It means the average distance (symbol L _{P in} FIG. 2) that moves before being used.

This exciton diffusion length L _P can be calculated by the following method (see FIG. 2).

(Calculation method of exciton diffusion length L _P )
(1) In consideration of optical interference, the optical intensity distribution inside the organic electron donor layer 11 is calculated by a matrix method.
(2) The exciton movement follows the diffusion equation.
(3) Assume that interface reaches excitons of the electron acceptor layer 12 is separated into electrons and holes with probability eta _ED.
(4) It is assumed that electrons and holes accumulated near the interface with the electron acceptor layer 12 are recombined with a probability η _R.
(5) It is assumed that all electrons and holes separated from the interface with the electron acceptor layer 12 are collected by the electrode.
(6) Based on the assumptions of (3) to (5), (number of extracted carriers) = (1−η _R ) × η _ED × (number of excitons reaching the interface), and (1−η _R ) × When the value of η _ED is collectively taken as a correction term and η _C , (the number of extracted carriers) = η _C × (the number of excitons reaching the interface).
(7) The photoelectric exchange efficiency (EQE (%)) when the wavelength of light irradiated to the organic electron donor 11 was changed was measured and obtained based on the above conditions (1) to (6). Using the exciton diffusion length L _P of the electron donor layer 11, the exciton diffusion length L _{n of} the electron acceptor layer 12 in the calculation formula, and the correction term η _C as parameters, the measured photoelectric exchange efficiency ( The value of each parameter is determined so that EQE (%)) matches the calculation formula obtained based on the above conditions (1) to (6).
(8) The exciton diffusion length L _P of the electron donor layer 11 determined in the above (7) is set as the exciton diffusion length defined by the present application.

The method for calculating the exciton diffusion length L _P will be described below using a specific example.

For example, in the case where iron phthalocyanine is used as the organic electron donor constituting the organic electron donor layer 11 and fullerene is used as the electron acceptor constituting the electron acceptor layer 12, the above calculation methods (1) to ( 6), and then according to the calculation method (7), an organic electron donor comprising iron phthalocyanine so that the obtained calculation formula and the photoelectric exchange efficiency (EQE (%)) measured in advance agree with each other. The values of the exciton diffusion length L _{FEPC of} the layer 11, the exciton diffusion length L _{C60 of} the electron acceptor layer 12 made of fullerene, and the correction term η _C were determined.

  The result is shown in FIG.

Here, in this specific example, the exciton diffusion length L _C60 of the electron acceptor layer 12 made of fullerene is not a variable, but is fixed at 40 nm, and the other two (organic electron donor layer 11 made of iron phthalocyanine). The exciton diffusion length L _FEPC and the correction term η _C ) were used as variables. As a result, as shown in FIG. 3A, when the exciton diffusion length _LFEPC of the organic electron donor layer 11 made of iron phthalocyanine is 1.0 nm and the correction term η _C is 0.14, the calculation formula ( The measured values (solid line in the figure) of the photoelectric conversion efficiency (EQE (%)) coincided with each other. Therefore, the exciton diffusion length L _FEPC of the organic electron donor layer 11 in this specific example is 1.0 nm, so that iron phthalocyanine is used as the organic electron donor constituting the organic electron donor layer 11 while the electron acceptor In the organic solar battery of the present application in the case where fullerene is used as the electron acceptor constituting the layer 12, the thickness of the organic electron donor layer 11 is designed to be 2 to 5 nm. By setting the thickness to such a level, it becomes possible to obtain an organic electron donor layer 11 having a good balance in both the amount of light absorption and the ease of current flow. A battery can be provided.

  Yet another specific example is shown.

Exciton diffusion of the organic electron donor layer 11 made of cobalt phthalocyanine is exactly the same as the case of using the iron phthalocyanine except that cobalt phthalocyanine is used as the organic electron donor constituting the organic electron donor layer 11. The length L _COPC was calculated.

  The result is shown in FIG.

As shown in FIG. 3B, when the exciton diffusion length _LFEPC of the organic electron donor layer 11 made of cobalt phthalocyanine is 1.6 nm and the correction term η _C is 0.17, the calculation formula (in the figure) The measured value (solid line in the figure) of the plot) and the photoelectric exchange efficiency (EQE (%)) coincided. Therefore, the exciton diffusion length L _COPC of the organic electron donor layer 11 in this specific example is 1.6 nm, so that cobalt phthalocyanine is used as the organic electron donor constituting the organic electron donor layer 11 while the electron acceptor. In the organic solar battery of the present application when fullerene is used as the electron acceptor constituting the layer 12, the thickness of the organic electron donor layer 11 is designed to be 1.6 to 8.0 nm. .

In the same manner as described above, nickel phthalocyanine, copper phthalocyanine, zinc phthalocyanine, and metal-free phthalocyanine are used as organic electron donors constituting the organic electron donor layer 11, and excitation of each organic electron donor layer 11 is performed. The child diffusion lengths L _NiPC , L _CuPC , L _ZnPC , and L _H2PC were calculated.

  The results are shown in FIGS.

As shown in FIG. 3C, when the exciton diffusion length L _NiPC of the organic electron donor layer 11 made of nickel phthalocyanine is 9.0 nm and the correction term η _C is 0.21, the calculation formula (in the figure) The measured value (solid line in the figure) of the plot) and the photoelectric exchange efficiency (EQE (%)) coincided. Therefore, the exciton diffusion length L _NiPC of the organic electron donor layer 11 in this specific example is 9.0 nm, so that nickel phthalocyanine is used as the organic electron donor constituting the organic electron donor layer 11 while the electron acceptor In the organic solar battery of the present application when fullerene is used as the electron acceptor constituting the layer 12, the thickness of the organic electron donor layer 11 is designed to be 18.0 to 45.0 nm. .

Further, as shown in FIG. _3D , when the exciton diffusion length L _CuPC of the organic electron donor layer 11 made of copper phthalocyanine is 15.4 nm and the correction term η _C is 0.33, the calculation formula (FIG. The measured value (solid line in the figure) of the photoelectric conversion efficiency (EQE (%)) coincided. Therefore, the exciton diffusion length L _CuPC of the organic electron donor layer 11 in this specific example is 15.4 nm. Then, copper phthalocyanine is used as the organic electron donor constituting the organic electron donor layer 11, while the electron acceptor In the organic solar battery of the present application in the case where fullerene is used as the electron acceptor constituting the layer 12, the thickness of the organic electron donor layer 11 is designed to be 30.8 to 77.0 nm. .

Further, as shown in FIG. 3E, when the exciton diffusion length L _ZnPC of the organic electron donor layer 11 made of zinc phthalocyanine is 15.0 nm and the correction term η _C is 0.33, the calculation formula (FIG. The measured value (solid line in the figure) of the photoelectric conversion efficiency (EQE (%)) coincided. Accordingly, the exciton diffusion length L _ZnPC of the organic electron donor layer 11 in this specific example is 15.0 nm, so that zinc phthalocyanine is used as the organic electron donor constituting the organic electron donor layer 11, while the electron acceptor In the organic solar battery of the present application when fullerene is used as the electron acceptor constituting the layer 12, the thickness of the organic electron donor layer 11 is designed to be 30.0 to 75.0 nm. .

Further, as shown in FIG. _3F , when the exciton diffusion length L _H2PC of the organic electron donor layer 11 made of metal-free phthalocyanine is 11.9 nm and the correction term η _C is 0.34, the calculation formula ( The measured values (solid line in the figure) of the photoelectric conversion efficiency (EQE (%)) coincided with each other. Therefore, the exciton diffusion length L _H2PC of the organic electron donor layer 11 in this specific example is 11.9 nm. Then, metal-free phthalocyanine is used as the organic electron donor constituting the organic electron donor layer 11 while In the organic solar battery of the present application in the case where fullerene is used as the electron acceptor constituting the body layer 12, the thickness of the organic electron donor layer 11 is designed to be 23.8 to 59.5 nm. Become.

  Here, the effect of the present invention will be described with another specific example.

  Using ITO as the anode 14 and silver as the cathode 15, further using the copper phthalocyanine as the organic electron donor constituting the organic electron donor layer 11, and using fullerene as the electron acceptor constituting the electron acceptor layer 12, An organic solar cell 10 as shown in FIG. 1 was actually formed. At this time, the thickness of the anode 14 is 110 nm, the thickness of the cathode 15 is 50 nm, the thickness of the electron acceptor layer 12 is fixed to 30 nm, and only the thickness of the organic electron donor layer 11 made of copper phthalocyanine is 5 The photoelectric conversion efficiency, the short-circuit current value (Jsc), the open-circuit voltage value (Voc), and the fill factor value are measured for each organic solar cell 10 formed by changing the thickness to 10, 20, 40, 60, and 80 nm. Thus, the influence of the thickness of the organic electron donor layer 11 was examined.

  The result is shown in FIG.

As is clear from FIG. 4, the photoelectric exchange efficiency, the short-circuit current value (Jsc), the open-circuit voltage value (Voc), and the fill factor value are all functions of the thickness of the organic electron donor layer 11. It became clear that each behaved independently. Here, paying attention to the photoelectric exchange efficiency, when the thickness of the organic electron donor layer 11 is 30 to 70 nm, the photoelectric exchange efficiency of the organic solar cell shows the maximum value, and this thickness indicates the organic electron donor. _Exciton diffusion length of layer 11 _Exciton diffusion length L _This is almost the same as the value calculated from CuPC (30.8-77.0 nm). Also from this, according to the organic solar cell of the present application, by designing the thickness of the organic electron donor layer 11 based on the exciton diffusion length, the light absorption amount and current of the organic electron donor layer 11 can be reduced. It can be seen that the organic electron donor layer 11 having a good balance in both flowability can be obtained, and as a result, an organic solar cell excellent in photoelectric exchange efficiency can be provided. Moreover, in the organic solar cell 10 of the present application, as described above, the thickness of the organic electron donor layer 11 can be set based only on the exciton diffusion length of the organic electron donor constituting the organic solar donor layer 11, Without measuring various parameters representing the performance of the organic solar cell (for example, short circuit current value (Jsc), open circuit voltage value (Voc), fill factor value, etc.), the design of the organic solar cell is facilitated. I understand that.

  The same method as described above was used to confirm the case where the nickel phthalocyanine was used as the organic electron donor constituting the organic electron donor layer 11.

  The result is shown in FIG.

If attention is paid to the photoelectric exchange efficiency as described above, the photoelectric exchange efficiency of the organic solar cell shows the maximum value when the thickness of the organic electron donor layer 11 is 20 to 50 nm. The exciton diffusion length of the body layer 11 is substantially the same as the value (18.0 to 45.0 nm) calculated from the exciton diffusion length _LNiPC , and the same can be said for the copper phthalocyanine.

As described above, in the organic solar cell of the present application, by setting the thickness of the organic electron donor layer to 2 to 5 times the exciton diffusion length of the organic electron donor constituting the organic solar cell, In other words, by designing the thickness of the organic electron donor layer based on the exciton diffusion length, the organic electron donor layer has a good balance between the light absorption amount and the current flowability of the organic electron donor layer. As a result, it is possible to provide an organic solar cell excellent in photoelectric exchange efficiency.

  In the organic solar cell of the present application, as described above, the thickness of the organic electron donor layer can be set based only on the exciton diffusion length of the organic electron donor constituting the organic solar donor layer. Without measuring various parameters representing the performance of the battery (for example, short circuit current value (Jsc), open circuit voltage value (Voc), fill factor value, etc.), the design of the organic solar cell is facilitated.

It is a schematic sectional drawing for demonstrating the structure of the organic solar cell of this application. It is a conceptual diagram for demonstrating the exciton diffusion length of the organic electron donor in this application. It is a figure (graph) used in order to calculate the exciton diffusion length of the organic electron donor layer in this application. The figure which shows the relationship between the thickness of the organic electron donor (copper phthalocyanine) layer in the organic solar cell of this application, photoelectric exchange efficiency, a short circuit current value (Jsc), an open circuit voltage value (Voc), and a fill factor value (graph) It is. The figure which shows the relationship between the thickness of an organic electron donor (nickel phthalocyanine) layer in the organic solar cell of this application, photoelectric exchange efficiency, a short circuit current value (Jsc), an open circuit voltage value (Voc), and a fill factor value (graph) It is.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Organic solar cell 11 ... Organic electron donor layer 12 ... Electron acceptor layer 13 ... Photoelectric conversion layer 14 ... Electrode (anode)
15 ... Electrode (cathode)
16 ... Board

Claims (4)

An organic solar cell comprising at least a photoelectric conversion layer having a heterojunction formed from an organic electron donor layer and an electron acceptor layer, and a pair of electrodes sandwiching the photoelectric conversion layer,
An organic solar cell characterized in that the thickness of the organic electron donor layer is 2 to 5 times the exciton diffusion length of the organic electron donor constituting the organic electron donor layer.   The organic solar cell according to claim 1, wherein the organic electron donor constituting the organic electron donor layer is metal phthalocyanine.   The organic solar cell according to claim 2, wherein the electron acceptor constituting the electron acceptor layer is fullerene.   The organic solar cell according to claim 2, wherein the electron acceptor constituting the electron acceptor layer is perylene.

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