JP3423280B2 - Organic / inorganic composite thin film solar cell - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a solid-state solar cell that converts solar energy into electricity using an organic semiconductor.

[0002]

2. Description of the Related Art Conventionally, a solid-state solar cell using an organic semiconductor utilizes a photovoltaic effect in a simple junction in which a single layer of an organic semiconductor and a single layer of a metal, an inorganic semiconductor, or an organic semiconductor are laminated to make use of a photovoltaic effect. It was customary to convert energy. Conventional technologies include organic semiconductor / metal bonding (eg DL Morel, AK Ghosh, T. Feng, EL
Stogryn, PE Purwin, RF Shaw, C. Fishman, App
lied Physics Letters, 32, 495 (1978)), heterogeneous organic semiconductor junctions (eg CW Tang, Applied Physics
Letters, 48, 183 (1986)) and organic / inorganic semiconductor junctions (eg AM Hor, RO Loutfy, Can).
There is an organic solar cell having adian Journal of Chemistry, 61, 901 (1983)).

[0003]

In the above-mentioned conventional simple junction organic solar cell, the width of the active region of the organic semiconductor formed near the junction for generating photocarriers is very narrow, and the active region near the junction ( The organic semiconductor layers other than several tens of nm are usually dead layers that do not generate carriers even if they absorb light, and the optical carrier generation capacity of the organic semiconductor is not high, so the optical carrier generation efficiency of the entire organic thin film is high. Becomes a very low value, resulting in a small photocurrent,
That is, there is a drawback that only low photoelectric energy conversion efficiency can be obtained. Then, this invention aims at increasing the photocurrent of the solid-state solar cell using an organic semiconductor, and raising photoelectric energy conversion efficiency.

[0004]

A solid-state solar cell of the present invention is provided with a composite thin film in which an organic semiconductor and an inorganic semiconductor are mixed to form a composite film, and the composite thin film is provided on both sides of the composite thin film with the built-in electric field. Semiconductor or metal to give
Or a composite thin film having an electrode part composed of both of them
The whole of the
This is an organic-inorganic composite thin film solar cell having a structure in which a pn junction is present, and is one that performs high-efficiency solar energy conversion by utilizing the high photocarrier generation ability of the organic-inorganic composite thin film.

In contrast to the conventional simple junction organic solar cell described above, the organic-inorganic composite thin-film solar cell of the present invention mixes an organic semiconductor and an inorganic semiconductor to form a composite, and an organic / inorganic semiconductor junction is formed over the entire thin film. By forming an extremely fine structure that stretches around, the entire film acts as an active layer for the generation of photocarriers, and also due to the effect of electron transfer at the organic / inorganic interface and the high dielectric constant of the inorganic semiconductor. Due to the effect of promoting the separation of photogenerated electron-hole pairs, a large photocurrent and high efficiency photoelectric energy conversion efficiency are achieved.

[0006]

BEST MODE FOR CARRYING OUT THE INVENTION The electrode portion in the present invention can be composed of a semiconductor, a metal, or both.

One preferred method of applying a built-in electric field to the composite thin film is to construct one of the electrode portions containing a metal having a large work function and the other containing a metal having a small work function. Metals with a high work function include platinum (Pt), gold (Au), ITO (indium tin oxid)
e), osmium (Os), palladium (Pd) and the like can be used, and examples of the metal having a small work function include magnesium-silver (Mg-Ag) alloy and calcium (C).
a), lithium (Li), aluminum (Al), silver (Ag), magnesium (Mg), indium (In)
Etc. can be used. As one combination, Pt can be used as a metal having a large work function, and Mg-Ag alloy can be used as a metal having a small work function.

Another preferred method of applying a built-in electric field to this composite thin film is to have a structure in which a metal electrode is arranged in each of the electrode parts via a semiconductor layer in contact with the composite thin film, and the semiconductor layer of one of the electrode parts is a composite thin film. Is a thin film of the same organic semiconductor as the organic semiconductor constituting the above, and the semiconductor layer of the other electrode part is a thin film of the same inorganic semiconductor as the inorganic semiconductor constituting the composite thin film. The metal electrode in the electrode portion is provided on the semiconductor layer when the semiconductor layer is included, and is provided so as to be in direct contact with the composite thin film when the semiconductor layer is not included.

Any kind of organic semiconductor may be used as the organic semiconductor as long as it is a photoconductive organic semiconductor (organic semiconductor capable of generating carriers by light irradiation). The organic semiconductor includes an organic semiconductor exhibiting p-type property (p-type organic semiconductor) and an organic semiconductor exhibiting n-type property (n-type organic semiconductor). The main organic semiconductors used in the present invention are illustrated in FIG.

For n-type organic semiconductors, various types of perylene pigments and their derivatives (derivatives having different substituents on the nitrogen atom) are known. For example, Me-PTC, t-BuP.
There are h-PTC, PhEt-PTC and the like, and also Im-PTC having a high photoelectric conversion ability. ), A naphthalene derivative (wherein the perylene skeleton of the perylene pigment is naphthalene, such as NTCDA), and C60 (also called fullerene).

The p-type organic semiconductor includes phthalocyanine pigments and their derivatives (MPc having various metals at the center, H ₂ Pc having no metal, and various substituents around it), quinacridone pigment ( DQ), porphyrin, merocyanine and the like and derivatives thereof.

Inorganic semiconductors include n-type and p-type semiconductors as well as some of them. As the n-type inorganic semiconductor, Cd
S, ZnO, TiO ₂ , SnO ₂ , WO ₃ , Fe ₂ O ₃ , S
i, SiC, GaAs or the like can be used. Further, as the p-type inorganic semiconductor, CdTe, Si, SiC,
GaAs or the like can be used. This composite thin film
An organic semiconductor and an inorganic semiconductor can be formed by co-evaporation to form a composite. It may be formed by another film forming means, for example, a sputtering method.

It is necessary to optimize the film thickness of this composite thin film so that it can absorb most of incident sunlight and the photocarrier generation quantum yield does not decrease. When the film thickness is increased, all the light can be absorbed and the light utilization efficiency (absorptivity) is improved, but the built-in electric field strength is reduced, so there is a high possibility that the photocarrier generation efficiency will be reduced. Therefore, optimization is necessary. It is also desirable to select the combination of semiconductors that make up the composite thin film so that the composite thin film has absorption over the entire solar spectrum.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, the present invention will be described with reference to the drawings. FIG. 1 is a sectional view of an embodiment of the present invention. 3 is a co-deposited composite thin film composed of a p-type organic semiconductor and an n-type inorganic semiconductor, and sandwiches the composite thin film 3 between the p-type organic semiconductor thin film 2 and the n-type inorganic semiconductor thin film 4 to form a sandwich-like laminated body. . A metal electrode 1 is formed on the p-type organic semiconductor thin film 2, and a transparent electrode 5 is formed on the n-type inorganic semiconductor thin film 4.

When sunlight is radiated from the transparent electrode 5 side to the battery having the above structure, a photovoltaic voltage is generated in which the electrode 1 is positive with respect to the electrode 5, and electrons are emitted from the electrode 1 toward the electrode 5 inside the battery. Photocurrent is generated in the flowing direction.

Next, the operating principle of the present invention will be described with reference to this embodiment. FIG. 3 is a diagram schematically showing the energy structure of an organic-inorganic composite thin film having a composite structure in which an organic semiconductor and an inorganic semiconductor are finely mixed. 6 is a p-type organic semiconductor, 7 is an n-type inorganic semiconductor, 8 is sunlight, 9 is an electron-hole pair photogenerated by an organic / inorganic pn junction, and 10 is photogeneration generated in the p-type organic semiconductor 6. Holes, 11 photogenerated electrons transported in the n-type inorganic semiconductor 7, 12 conduction band,
13 is a valence band, and 14 is electron energy.

The organic-inorganic composite thin film is an n-type inorganic semiconductor 7
And the p-type organic semiconductor 6 have an intricate microstructure, and the pn junction formed at the interface between the two has an energy structure extending over the entire composite thin film. The irradiated sunlight is absorbed by the organic semiconductor 6 or the inorganic semiconductor 7, and the pn
Electron-hole pairs 9 are photogenerated at the junction interface. The photo-generated electrons 11 are separated to the n-type inorganic semiconductor 7 side, and the photo-generated holes 10 are separated to the p-type organic semiconductor 6 side and transported separately.

The principle that the organic / inorganic composite thin film exhibits high photoelectric conversion capability will be described below. First, the entire composite thin film is active in photocarrier generation. That is, in a conventional organic solar cell having a simple junction between a metal or inorganic semiconductor and an organic semiconductor, photocarrier generation of the organic semiconductor occurs only in the active layer in the immediate vicinity of the junction,
The organic semiconductor layer far away from the junction does not generate carriers even if it absorbs light, so it becomes a dead layer, and as a result, the photocarrier generation efficiency of the thin film as a whole is very low. In the inorganic composite thin film, since the pn junction is spread over the entire film, d
Since there is no ead layer, the entire film works actively for photocarrier generation, and all the light absorbed in the film contributes to carrier generation, so that a large photocurrent can be obtained.

Secondly, there is an effect that the photocarrier generation efficiency is improved by electron transfer at the organic / inorganic interface described below. FIG. 4 shows the energy relationship between an n-type inorganic semiconductor and a p-type organic semiconductor that are expected to give a high photocarrier generation ability. 16, 17 are valence band and conduction band of n-type inorganic semiconductor, 19 and 20 are HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) levels of p-type organic semiconductor,
21 is photoexcitation from HOMO to LUMO, 22 is photoexcited electron, 23 is photogenerated hole, 24 is electron transfer of excited electron 22 to inorganic semiconductor conduction band 17, 25
Indicates the energy of an electron. In the case of a single organic semiconductor film,
Excited states are localized in the excited molecule, and excited electrons and holes form Frenkel-type excitons bound by strong Coulomb attraction, so most excited states are usually deactivated and The efficiency of generating free electrons and holes that become electric current is very low. On the other hand, when the inorganic semiconductor and the organic semiconductor are mixed, the electrons 22 generated by photoexcitation move to the conduction band 17 of the inorganic semiconductor having lower energy and are separated from the holes, so that free electrons and holes are generated. Greatly improves efficiency. In order for this effect to occur, the LUMO level 20 of the organic semiconductor needs to be at a higher energy position than the lower end of the conduction band 17 of the inorganic semiconductor, as shown in FIG. A combination of related organic and inorganic semiconductor materials is desired.

Thirdly, the effect of improving the photocarrier generation ability due to the high dielectric constant of the inorganic material described below can be mentioned. Figure 5
FIG. 3 is a diagram schematically showing an inorganic / organic interface. 26 is an inorganic semiconductor, 27 is an organic semiconductor, 28 is an organic / inorganic interface,
29 is a strongly bound Frenkel exciton (illustrated as an exciton bound at a distance of 1 nm in the figure) generated by photoexcitation of an organic semiconductor, 30 is a hole of an exciton reaching an organic / inorganic interface, 31 Are electrons of excitons that have reached the organic / inorganic interface. The exciton orbital radius strongly depends on the relative permittivity of the material. The dielectric constant of organic materials is usually 3 to
Photon-generated electrons and holes, which are small at about 4, form Frenkel excitons of about the size of a strongly bound molecule (~ 1 nm). On the other hand, the relative permittivity of inorganic materials is considerably larger than that of organic materials, and electrons and holes form Wannier-type excitons spread in a weakly bound and considerably large space.
Here, when the Frenkel excitons 29 made of the organic material reach the organic / inorganic interface 28, the electrons 31 move to the inorganic side as described in FIG. 4, so the excitons are inorganic materials having a large relative dielectric constant. Feeling the inside field, the bond with the hole 30 is greatly weakened, that is, the distance between the hole and the electron is significantly increased, and dissociation into free carriers is promoted (in the figure, as an example, GaAs (ratio An exciton orbital radius with a dielectric constant of 12.9 and an exciton orbital radius of 16 nm is drawn). As described above, it is considered that the photocarrier generation efficiency in the organic semiconductor can be improved by utilizing the high relative dielectric constant of the inorganic material. If a dielectric material having a very high dielectric constant (for example, barium titanate (relative dielectric constant 2900 or the like)) is used as the inorganic material, the above effect may be significantly exhibited.

Fourth, since the generated free electrons and holes are spatially separated and transported, the recombination of both is suppressed, and the final photocarrier generation efficiency of the thin film as a whole, that is, the photoelectric flow rate. The effect of increasing the.

FIG. 6 shows, as an example, a composite thin film of metal-free phthalocyanine (H ₂ Pc) which is a p-type organic semiconductor and cadmium sulfide (CdS) which is an n-type inorganic semiconductor (composition ratio is H ₂ Pc / molar ratio). CdS = 9/1, film thickness is 15
A small voltage (horizontal axis) is applied to
The photocarrier generation quantum yield (vertical axis) in the case of irradiating 0 nm monochromatic light to excite only H ₂ Pc is shown. Quantum yield is obtained of over 30% at a voltage of 0.4V, which is H ₂ P
This means that 30 or more free carriers were generated by 100 photons absorbed in c. this is,
This is a large value that cannot be obtained by H ₂ Pc alone, indicating that the organic / inorganic composite thin film has a large photocarrier generation ability.

In order to use the co-deposited film as a solar cell, a photocurrent is not obtained by externally applying a voltage as in the case of FIG. 6, but a photocurrent is obtained by a built-in electric field incorporated in the cell. It is necessary. First to get a built-in electric field
The method is to sandwich the same p-type organic semiconductor 2 and n-type inorganic semiconductor 4 used to form the co-deposited layer 3 as shown in FIG. Then, at the n-type inorganic semiconductor 4 / co-deposited layer 3 interface, a pn junction is formed only at the interface of the part of the co-deposited layer 3 where the p-type organic semiconductor is exposed, and movement of holes to the n-type semiconductor layer 4 is prevented. Thus, only electrons can be taken out from the interface of the portion of the co-deposited layer where the n-type inorganic semiconductor is exposed. Similarly, at the co-deposition layer 3 / p-type semiconductor 2 interface, an np junction is formed only at the interface where the n-type inorganic semiconductor of the co-deposition layer 3 is exposed, and movement of electrons to the p-type semiconductor layer 2 is blocked. Only holes can be taken out from the interface of the exposed portion of the p-type organic semiconductor of the vapor deposition layer. In this case, the built-in electric field comes from the pn junction formed at the interface between the p-type organic semiconductor and the n-type inorganic semiconductor. Note that the individual layers of the p-type organic semiconductor 2 and the n-type inorganic semiconductor 4 are made as thin as possible, and in consideration of the reflection of the metal electrode 1, almost all incident sunlight is absorbed by the co-deposited layer 3. It is desirable to design the film thickness of each layer of the cell.

The second method of forming the built-in electric field in the battery is to utilize the work function difference of the metals used for the electrodes. That is, in order to further increase the extraction efficiency of the photogenerated free carriers, a combination of metals having a large work function difference is used for the metal pairs 1 and 5 used for the electrodes, and a built-in electric field derived from the work function difference is applied to the entire cell. It is considered effective to add them. Specifically, when a metal having a high work function is used as the metal of the electrode 1 and a metal having a low work function is used as the metal of the electrode 5, electrons generated in the co-deposition layer are generated in the electrode 5
, An internal electric field is applied in the direction of moving the hole to the electrode 1,
Since both can be efficiently transported in different directions, the obtained photoelectric flow rate can be expected to be greatly improved. In the case of sandwiching with a metal, the electrode metal on the side that radiates sunlight is preferably a transparent electrode that does not absorb sunlight, such as ITO. Further, as a method of applying a built-in electric field to the composite thin film, a cell structure of a type in which the two semiconductor single layers 2 and 4 are omitted and a metal having a different work function is directly sandwiched may be considered.

In FIG. 7, platinum (Pt) having a large work function is used as the metal of the electrode 1, and p-type H is used as the p-type organic semiconductor thin film 2.
₂ Pc (film thickness 75 nm), H ₂ Pc as co-deposited composite film 3
And CdS co-deposited film (composition ratio is molar ratio of H ₂ Pc / Cd
S = 9/1, film thickness 150 nm), n-type CdS (film thickness 55 nm) as the n-type inorganic semiconductor thin film 4, and magnesium-silver (Mg-Ag) having a small work function as the metal of the electrode 5.
It is the figure which showed the irradiation light wavelength (horizontal axis) dependence of the photocurrent quantum yield (vertical axis) of the short circuit photocurrent of the solar cell produced using the alloy.

This organic / inorganic composite thin film solar cell has a thickness of 0.6
A photovoltage of V can be generated. Also, as shown in FIG.
Up to 60% (400 nm) in the absorption region of CdS, H ₂ P
A maximum quantum yield of 40% (600 nm) was obtained in the absorption region of c. This is a result showing the feasibility of an organic-inorganic composite thin-film solar cell having a practical level of photocurrent and photoelectric energy conversion efficiency.

As the organic material and the inorganic material used in the present invention, a combination of a p-type inorganic semiconductor and an n-type organic semiconductor, which is a combination opposite to that described above, may be used. Further, in order to obtain high photoelectric energy conversion efficiency, it is desirable to use a combination in which two materials have complementary absorption wavelength regions that do not overlap each other and can cover the entire sunlight spectrum. It is also possible to use a material that does not absorb visible light as the inorganic semiconductor and cover the entire solar spectrum with only the organic semiconductor. In that case, it is also possible to co-evaporate an inorganic semiconductor and two or more kinds of other organic semiconductors.

The organic-inorganic composite thin film is produced by simultaneously evaporating the organic material and the inorganic material from different vapor deposition sources.
It is thought that a composite film in which both are mixed in a fairly fine manner. The microstructure inside the co-deposited layer, that is, the size of the inorganic semiconductor and the organic semiconductor mixed with each other, depends on the mixture of particles with a considerably large submicron order particle size and a very small particle size of nanometer order. Mixing of particles of size, and in the extreme case mixing at the molecular level is also considered, which is expected to be strongly related to the efficiency of the cell. For controlling the grain size, it is expected that it is important to appropriately control the substrate temperature by cooling or heating the substrate during vapor deposition. Note that an organic material can be vapor-deposited by resistance heating, but in the case of an inorganic material, almost all inorganic materials including oxides can be vapor-deposited by using an electron beam vapor deposition source in addition to resistance heating.

[0029]

As described above, according to the present invention, by forming a thin film structure in which ultrafine organic / inorganic semiconductor junctions are spread over the entire thin film by co-evaporation, the entire film is active for photocarrier generation. By operating as a layer and utilizing the photogenerated electron-hole separation promoting effect at the organic / inorganic interface, it has the effect of producing an organic solid-state solar cell having high efficiency photoelectric energy conversion efficiency.

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view showing an example of an organic / inorganic composite thin film solar cell of the present invention.

FIG. 2 is a chemical structural formula illustrating an organic semiconductor that can be used in the present invention.

FIG. 3 is a diagram schematically showing an energy structure of an organic / inorganic composite thin film.

FIG. 4 is expected to give a high photocarrier generation ability.
It is a figure showing the energy relation of a p-type organic semiconductor and a p-type organic semiconductor.

FIG. 5 is a diagram schematically showing an inorganic / organic interface.

FIG. 6 is a diagram showing applied voltage dependency of photocarrier generation quantum yield in a composite thin film of H ₂ Pc and CdS.

FIG. 7: Pt / H ₂ Pc (75 nm thickness) / H ₂ Pc and Cd
Co-deposited film of S (150 nm thickness) / CdS (55 nm thickness)
It is the figure which showed the irradiation light wavelength dependence of the photocurrent quantum yield of the short circuit photocurrent of the organic-inorganic composite thin film solar cell of the composition of / Mg-Ag.

[Explanation of symbols]

1 metal electrode 2 p-type organic semiconductor thin film 3 co-deposited composite film composed of p-type organic semiconductor and n-type inorganic semiconductor 4 n-type inorganic semiconductor thin film 5 transparent electrode 6 p-type organic semiconductor 7 n-type inorganic semiconductor 8 sunlight 9 organic / Electron-hole pair photogenerated in inorganic pn junction 10 Photogenerated hole transported in p-type organic semiconductor 11 Photogenerated electron transported in n-type inorganic semiconductor 12 Conduction band 13 Valence band 14 Electron energy 15 n -Type inorganic semiconductor 16 Valence band of n-type inorganic semiconductor 17 Conduction band of n-type inorganic semiconductor 18 p-type organic semiconductor 19 HOMO level of p-type organic semiconductor 20 LUMO level of p-type organic semiconductor 21 Photoexcitation from HOMO to LUMO 22 Photo-excited electron 23 Photo-generated hole 24 Electron transfer of excited electron to conduction band 17 of inorganic semiconductor 25 Electron energy 26 Inorganic semiconductor 27 Yes Semiconductor 28 Organic / inorganic interface 29 organic semiconductor in strongly constrained Frenkel exciton 30 organic / inorganic interface excitons reach the hole 31 organic / inorganic interface electrons excitons reached

─────────────────────────────────────────────────── ─── Continuation of front page (58) Fields surveyed (Int.Cl. ⁷ , DB name) H01L 31/04-31/078

Claims (5)

(57) [Claims] 1. A composite thin film in which an organic semiconductor and an inorganic semiconductor are mixed to form a composite, and a semiconductor or a metal, or both of them, which are provided on both sides of the composite thin film so as to provide an internal electric field to the composite thin film. comprising an electrode portion, before
The composite thin film as a whole actively works for photocarrier generation.
Even if the structure has a pn junction throughout,
Organic-inorganic composite thin film solar cells Tsu. 2. The organic-inorganic composite thin film solar cell according to claim 1 , wherein one of the electrode portions contains a metal having a large work function and the other contains a metal having a small work function. 3. The organic / inorganic composite thin film solar cell according to claim 2 , wherein platinum is used as a metal having a large work function and magnesium-silver is used as a metal having a small work function in the electrode portion. 4. Each of the electrode portions has a structure in which a metal electrode is arranged via a semiconductor layer in contact with the composite thin film, and the semiconductor layer of one electrode portion is an organic material forming the composite thin film. a thin film of the same organic semiconductor as the semiconductor, the first aspect of the semiconductor layer of the other electrode portion is a thin film of the same inorganic semiconductors and inorganic semiconductors constituting the composite thin film 3
The organic / inorganic composite thin film solar cell according to any one of 1. Wherein said composite thin film, an organic-inorganic composite thin film solar cell according to any one of claims 1 to 4, the organic semiconductor and an inorganic semiconductor is codeposited composite films with by co-evaporation.