JPS6162583A - El element - Google Patents

Abstract

PURPOSE:An inexpensive EL element, easily producible, capable of providing emission with high luminance at low voltage, wherein a luminous layer having a multi-layer structure of thin layer comprising electrically luminous organic compound having different electronegativity based on neighboring other layers, and the luminous layer is sandwiched in between electrode layers. CONSTITUTION:In the luminous layer 2 obtained by laminating alternately the first and the second layers repeatedly >=4 layers, the first layer is a molecule piled layer consisting of a mixture comprising one or more electrically luminous organic compounds 5 having electron-accepting properties relatively based on the second layer and one or more organic compounds 5' having electron-donating properties based on the compounds, and the second layer is a monomoleclar layer consisting of a mixture comprising one or more compounds 6 having electron-donating properties relatively based on the first layer and one or more organic compounds 6' having electron-accepting properties relatively based on these compounds or its accumulated layer, and the luminous layer 2 consisting of the first and the second layers is sandwiched in between the transparent electrode 1 and the back electrode 3, to give the aimed EL element.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to an EL device using electrical light emission, that is, EL, and more specifically, the present invention relates to an EL device that uses electrical light emission, that is, EL, and more specifically, a light emitting layer having a multilayer structure,
The present invention relates to an EL device comprising a thin film of at least one electroluminescent organic compound, each layer having a different electronegativity relative to other adjacent layers.

(Prior art) Conventional EL elements are made of Mn, Cu or Re F3.
ZnS containing (Re; rare earth ion) etc. as an activator
It consists of a light-emitting layer having a light-emitting base material, and the light-emitting layer is broadly classified into powder type EL and thin film type EL depending on the basic structure of the light emitting layer.

Among the elements in practical use, thin 11QEL generally has higher brightness than powder type EL, but thin II! Since the light-emitting layer of EL is formed by vapor-depositing a light-emitting base material onto a substrate, it has drawbacks such as difficulty in manufacturing large-area devices and extremely high manufacturing costs.

For this reason, attention has been paid to powder-type EL, in which a light-emitting base material, that is, ZnS, is dispersed in organic paint, which is most easily mass-produced and costs only a few tenths of the cost of thin-film devices. Generally, in full EL light emission, the thinner the light emitting layer is, the better the light emitting characteristics will be. However, in the case of the powder type EL, since the luminescent base material is a discontinuous powder, pinholes are likely to occur in the luminescent layer when the luminescent layer is thinned.
It has a major drawback in that it is difficult to reduce the layer thickness, and therefore sufficient brightness characteristics cannot be obtained. Recently, an improved type of element in which an intermediate dielectric layer made of a vinylidene fluoride polymer is disposed within the light emitting layer of the powder type EL has been disclosed in JP-A-58-172891.
Although sufficient performance in terms of luminance and power consumption has not yet been achieved, recent research and development efforts have been actively conducted to control the chemical structure and higher-order structure of organic materials to create new materials for optical and electronics applications. EC element,
Organic materials, such as piezoelectric elements, pyroelectric elements, non-radiometer optical elements, and ferroelectric liquid products, have been announced that are comparable to or superior to metals and inorganic materials. As described above, there is a demand for the development of functional organic materials as new functional materials that surpass inorganic materials. An EL element formed on an electrode substrate was disclosed in Japanese Patent Application Laid-Open No. 52-35587.
It is proposed in the Publication No. However, these EL devices have not yet achieved sufficient performance in terms of brightness, power consumption, and actual EL devices.Furthermore, in the case of such aEL devices, the density of carrier electrons or holes is extremely low. The probability of excitation of functional molecules due to carrier recombination or the like becomes extremely small, and efficient light emission cannot be expected.

(Disclosure of the Invention) Therefore, one object of the present invention is to eliminate the drawbacks of the prior art as described above, and to provide an EL element that can emit light with sufficiently high brightness even when driven at a low voltage, is inexpensive, and is easy to manufacture. It is to provide.

The above object of the present invention has been achieved by forming a light emitting layer of an EL element by combining specific materials and having a specific configuration.

That is, the present invention provides an EL device comprising a multilayered light emitting layer and two electrode layers sandwiching the light emitting layer, at least one of which is transparent. The above-mentioned EL device is characterized in that it is formed by repeatedly laminating four or more layers of two layers alternately.

First layer: from a mixture of at least one electroluminescent organic compound that is electron-accepting relative to the second layer and at least one organic compound that is electron-donating relative to the second layer. The molecular salt ms.

Second layer; from a mixture of at least one electroluminescent organic compound that is electron-donating relative to the first layer and at least one organic compound that is electron-accepting relative to the compound; monomolecular film or its cumulative film.

To explain the present invention in detail, 1. The electroluminescent organic compound used in the present invention and which mainly characterizes the present invention has a high luminescence quantum efficiency and is easily susceptible to external perturbation.
It is a compound that has an electronic system and can be electrically excited. For example, it is basically a condensed polycyclic aromatic hydrocarbon, P-terphenyl.

2,5-diphenyloxazole, 1,4-bis(2-
methylstyryl) - benzene, xanthine, coumarin,
Examples include acridine, cyanine dyes, benzophenone, phthalocyanine and metal complexes thereof, porphyrin and metal complexes thereof, 8-hydroxyquinoline and metal complexes thereof, organic ruthenium complexes, organic rare earth complexes, and derivatives of these compounds. Furthermore, compounds that can serve as electron acceptors or electron donors for the above compounds include heterocyclic compounds other than those mentioned above and derivatives thereof, aromatic amines and aromatic polyamines, compounds with a quinone structure, and tetracyanoquinodimethane. and tetracyanoethylene.

In the present invention, compounds useful for forming the first light-emitting layer are selected from the above-mentioned compounds.

In addition, in the present invention, the organic compound useful for forming the second light-emitting layer can be obtained by chemically modifying the electroluminescent compound as described above by a known method as necessary, and at least A compound having one hydrophobic part and at least one hydrophilic part (all of these are in a relative sense), for example, represented by the following general formula (I) Compounds and other compounds.

[(X-R,), LZl, -φ-R, (I) In the above formula, X is a hydrogen atom, a halogen atom, an alkoxy group, an alkyl ether group, a nitro group; a carboxyl group, a sulfonic acid group, a phosphorus Acid group, silicic acid group, 1st to 3rd amino group;
These metal salts.

Primary to tertiary amine salts, acid salts; ester groups, sulfamide groups, amide groups, imino groups, quaternary amino groups and their salts, hydroxyl groups, etc.; R1 has 4 to 30 carbon atoms, preferably 10 to 25 carbon atoms. m is 1 or a linking group such as -S O, N R, -CO-, -COO-, etc. (R is a hydrogen atom, an alkyl group, an aryl group, etc.); ); φ is a residue of an electroluminescent compound as exemplified later; R2, like X, is a hydrogen atom or any other substituent; one or more X of
, φ and R2 (!I is a hydrophilic moiety, and at least one is a hydrophobic moiety.

Preferred examples of basic skeletons of compounds useful for forming the first calendar and φ of the compound of general formula (I) include:
It is as follows. (However, φ (basic skeleton) illustrated below is an alkyl group having 1 to 4 carbon atoms, an alkoxy group, an alkyl ether group, a halogen atom, a nitro group,
It may have general substituents such as a class amine group, a hydroxyl group, a carboxamide group, a sulfoamide group, etc. ) (below margin)
jZ=NH10, S Z=CO, NHZ=C
O1NH10,SZ = NH,01S Z=NH10,5Z=NH10,5 Z=S, Se Z=S%Se
Z=S, SeZ=NH, OlS Z=NH,
αS Z=NH, O, SM=Mg, Zn, Sn, At
CtM=Hz%Be, Mg5Ca%CdSn, AtC4
YbC2 M= Er, Tm Sm, Eu, Tb,
Z = O, N true M = A4Ga, Ir, Ta, a = 3
M=Er, Sm, EuM=Zn%Cd, Mg,
pb, a=2 Gd, Tb, Dymt Yb M = E r %Sm * Eu * Gd
M=E r s Sm, E uTb, Dy, Tm,
Yb' Gd, Tb, Dy 7m% Yb Z=O, S, Se O≦p≦2 The above luminescent compounds can be used alone or as a mixture in each luminescent layer in the present invention. In addition,
These compounds are examples of preferred compounds, and it goes without saying that other derivatives or other compounds may be used as long as the same purpose is achieved.

In the present invention, the luminescent compounds as described above are divided into the first luminescent layer and the luminescent layer of ff12 of an EL element of fLll according to their electronegativity, and these calendars are alternately used in four layers. It is characterized in that a light emitting layer with a multilayer structure is formed by repeatedly laminating more than one layer.

That is, since the above-mentioned luminescent compounds have different electronegativities, among them, the compound that is relatively electron-accepting is selected as the luminescent compound for forming the first luminescent layer. And, the above-mentioned luminescent compound which is electron-donating relative to them may be selected as the compound for forming the second luminescent layer.Among such luminescent compounds, as the TL electron-donating compound, Particularly preferred compounds are those having an electron-donating group such as a primary to tertiary amine group, a hydroxyl group, an alkoxy group, an alkyl ether group, or a nitrogen heterocyclic compound,
The electron-accepting compounds are mainly compounds having an electron-withdrawing group such as a carbonyl group, a sulfonyl group, a nitro group, or a quaternary amino group.

The present invention further provides an organic compound (hereinafter referred to as a donor) that is relatively more electron-donating than the luminescent compound forming the first layer as described above.

Preferably, about 1/100 to 1/1 per mol of the former
In addition to adjusting the electron-accepting properties of the first layer and forming the second layer, a luminescent compound is added in a 0 molar proportion.
An organic compound (hereinafter referred to as an acceptor) that is relatively more electron-accepting than the above compound is preferably added at a ratio of about 1/10 to 17,100 mol per 1 mol of the former, and the second
It is characterized by adjusting the electron donating property of the layer.

The acceptor as described above may be selected from the above-mentioned luminescent compounds, or may be selected from other organic compounds with large electron-withdrawing properties other than the above-mentioned, and the donor may also be selected from the above-mentioned luminescent compounds. For example,
The acceptor can be selected from various compounds having a large electron-withdrawing substituent as described above, and the donor can be selected from a variety of organic compounds having a large electron-donating substituent as described above. It is preferable to do so.

The other elements forming the EL element of the present invention, that is, the two electrode layers sandwiching one light emitting layer, can use any conventionally known elements, but at least one of the layers must be transparent. There is a need. As the transparent 7tt electrode layer, any desired transparent electrode layer can be used as in the past, and preferred examples include transparent synthetic resins such as polymethyl methacrylate and polyester, and transparent films or sheets made of glass or the like. indium oxide, tin oxide,
The entire surface or pattern is coated with a transparent conductive material such as indium-tin-oxide (ITO). A conventionally known opaque back electrode may be used on one side.
A typical and preferred thickness is about 0.1 to 0.3 g.
It is a vapor-deposited film of aluminum, silver, gold, etc. of m. In addition, the shape of the transparent electrode layer or back electrode layer may be plate-like, belt-like,
It may have any shape, such as a cylindrical shape, and can be selected depending on the purpose of use. Further, the thickness of the transparent electrode layer is approximately 0.01
A thickness of about 0.2 gm is preferable. If the thickness is less than this range, the physical strength and electrical properties of the element itself will be insufficient, and if the thickness is more than the above range, transparency, lightness, compactness, etc. will be deteriorated. There's a problem.

The EL device of the present invention uses electroluminescent compounds (containing acceptors or donors) having relatively different electronegativities as described above between the two TrL pole layers, and It is obtained by forming a first layer and a second layer, and stacking the first layer and the second layer alternately to form a multilayer structure of 4 or more layers,
The first layer constituting the light emitting layer of the formed multilayer structure is the second layer.
A mixed molecule deposited film comprising a relatively electron-accepting compound containing a donor with respect to the first layer, and a second layer comprising a relatively electron-donating compound containing an acceptor with respect to the first layer. It is characterized by being a mixed monomolecular film or a cumulative film thereof arranged with highly ordered molecular orientation.

In the present invention, particularly preferred methods for forming the mixed molecule deposited film constituting the first light-emitting layer are resistance heating evaporation method and CVD method. As, fi of about 500A! A film can be formed.

For example, when using the resistance heating evaporation method, the material is placed in a tungsten board placed in a vacuum chamber, and the material is placed 30 cm from the substrate.
Otherwise, resistance heating is performed, and sublimable materials are set at the sublimation temperature, and meltable materials are vapor deposited at a temperature higher than the melting point. What is the pre-vacuum level? After reducing the temperature to below ×10-' Torr, closing it with a shutter before vapor deposition, heating the hoard, and letting it air for 2 minutes.

Open the yatterer and evaporate.

The speed of admission is measured using a crystal oscillator film thickness monitor, but the preferred speed is 0, l A/
sec to 100 A/sec.

The degree of vacuum at that time is 10-'To prevent oxidation etc.
This is carried out by maintaining the temperature below rr, preferably about 10 Torr.

In the present invention, a particularly preferred method for forming the second layer of mixed monomolecular film or its cumulative film is the Langmuir-Prodgett method (LB method).
In this method, when a molecule has a structure that has a hydrophilic group and a hydrophobic group in the molecule, and the balance between the two (balance of amphiphilicity) is maintained moderately, the molecule has a hydrophilic group on the water surface. This is a method of forming a monomolecular film or a cumulative film thereof by utilizing the fact that the film turns downward into a monomolecular layer. Specifically, in order to prevent the monomolecular film spread on the water layer from freely diffusing and spreading too much, a partition plate (or float) is provided to limit the spread area and control the aggregated state of the film material. is controlled, the surface pressure is gradually increased, and a surface pressure suitable for manufacturing a monomolecular film or a cumulative film thereof is set. A monomolecular film is transferred onto the substrate by vertically raising or lowering the pure substrate while maintaining this surface pressure M1. A monomolecular film is produced in the above manner, and a cumulative film of a monomolecular film is formed as a cumulative film with a desired degree of accumulation by repeating the above operations.

In addition to the above-mentioned vertical dipping method, the monomolecular film can be transferred onto the substrate by methods such as a horizontal deposition method and a rotating cylinder method. The horizontal deposition method is a method in which the substrate is brought into horizontal contact with the water surface and transferred, and the rotating cylinder method is a method in which a cylindrical substrate is rotated on the water surface to transfer the monomolecular film onto the surface of the substrate. In the vertical attachment method described above, when a substrate with a hydrophilic surface is lifted out of water in a direction across the water surface, a monomolecular film is formed on the substrate with the phyllophilic groups of the molecules facing the substrate. As mentioned above, when the substrate is moved up and down, one monomolecular film is overlapped in each process.The direction of the film-forming molecules is reversed between the lifting process and the dipping process, so according to this method, the molecules between each layer are A Y-type IQ is formed in which the hydrophilic groups of the molecule and the hydrophilic groups, and the hydrophobic groups of the molecule and the hydrophobic groups face each other. In contrast, in the horizontal deposition method, a single molecule 11 with the hydrophobic group of the crystallite facing the substrate is formed on the substrate. In this method, even if monomolecular films are accumulated, there is no change in the orientation of the film molecules, and in all layers,
An X-type film with hydrophobic groups facing the substrate is formed.On the other hand, a cumulative film with hydrophilic groups facing the substrate in all layers is Z-type.
The 0-rotation cylindrical method, called a molded membrane, is a cylindrical method in which the monomolecular film is transferred to the surface of the substrate by rotating the substrate on its water surface.

The method of transferring the monomolecular film onto the substrate is not limited to these methods; in other words, when using a large-area substrate, a method of extruding the substrate from a substrate roll into a water layer may also be used. Furthermore, the directions of the above-mentioned hydrophilic groups and hydrophobic groups toward the substrate are in principle, and can be changed by surface treatment of the substrate, etc.

As mentioned in the section of the prior art, it is known to form an EL element by the LB method, but the first known method does not provide an EL element with sufficient performance, and the present inventor conducted various research. As a result, one light-emitting layer has a multilayer structure, the first light-emitting layer is formed as a mixed molecule deposited film using a relatively electron-accepting compound containing a donor as described above, and ff
By forming the $2 layer as a mixed monomolecular film or a cumulative film thereof from a compound that is relatively electron-donating and contains an acceptor with respect to the first layer, the performance of the conventional EL device is significantly improved. This is what I found out.

One embodiment of the present invention is an embodiment in which the first layer is a mixed molecule deposited film as described above, and the t52 luminescent layer is a mixed monomolecular film made of the luminescent material containing an acceptor. In the EL device of this embodiment, first, a first layer is formed on a transparent electrode, and then a mixture consisting of a material that is electron-donating relative to the first layer and seven receptors is formed. , dissolved in a suitable organic solvent such as chloroform, dichloromethane, dichloroethane, etc., to a concentration of about 1O to IOM, and the solution at a suitable pH (for example, pH about 1 to 100%) which may contain various metal ions. 8) is developed on the aqueous phase and the solvent is removed by evaporation to form mixed monomolecules,
By the LB method as described above, the mixed molecule deposited film is transferred onto a transparent substrate on which it has already been formed to form a second calendar. This operation is repeated two or more times to obtain a multilayered light emitting layer. Next, an electrode material such as aluminum, silver, gold, etc. is deposited on the surface of the light emitting layer thus formed, preferably by vapor deposition. This is obtained by forming a back electrode layer.

The number of laminated layers of the light-emitting layer consisting of the molecular deposited film and the monomolecular film of the EL device obtained in this way is generally about 4 to 400.
is preferable, and the thickness of the entire luminescent layer varies depending on the type of material used, but is generally about 0.01 to lp.
A thickness of m is preferred.

Another important aspect is at least one of the second layers constituting the light emitting layer of the EL device of the present invention.

Preferably, all the ff12Gs are cumulative films of the above-mentioned monolayers. This aspect can be obtained by using the LB method described above, and by accumulating the required number of times in various ways for each formation of the second layer as described above.

The number of stacked layers of each layer of the light-emitting layer of the EL device obtained in this way is preferably about 4 to 400, and the cumulative number of each layer is preferably about 2 to 200, and the total thickness can be set as desired. However, in the present invention, the total amount is approximately 0.
．． 01-1 m is preferred.

Note that one or both of the electrode layers used as a substrate 8! The adhesion between the layer and the light-emitting layer is sufficiently strong in the single molecule deposition method and the LB method, and the light-emitting layer does not peel or fall off. may be treated in advance, or a suitable adhesive layer may be provided between the substrate and the light emitting layer.
Furthermore, in the LB method, adhesion can be improved by adjusting various conditions such as the type of material for forming the emissive layer, the pH of the water layer used, ionization, water temperature, transfer rate of the monomolecular film, or surface pressure of the monomolecular film. The performance of the EL element formed as described above may deteriorate due to the influence of moisture and oxygen in the air if left as it is, so it has to be made moisture-resistant and oxygen-resistant by conventionally known means. It is desirable to have a sealed structure.

In the EL device of the present invention as described above, the structure of the light emitting layer is as follows:
It is an ultra-thin film, the second layer has a high degree of molecular order and functionality necessary for the operation of the EL device, and the first layer and the second layer have various electrical interactions. By doing so, excellent light emitting performance can be achieved.

Furthermore, as schematically shown in FIG. 1, the light-emitting layer of the EL device of the present invention is different from the light-emitting layer consisting of a single layer in the prior art, as schematically shown in FIG. light emitting layer and second
has a uniform interface with the light-emitting layer, and these layers have multiple *
v1r, so 2 with different electronegativities
Various interactions between the layers are extremely easy, and it exhibits excellent light-emitting performance that cannot be achieved with conventional techniques. That is, by variously changing the difference in electronegativity between the first light-emitting layer and the second light-emitting layer, the light emission intensity can be improved or the light emission color can be changed arbitrarily, and the service life can also be changed. It can be significantly extended.

Furthermore, in the conventional technology, it was virtually impossible to use materials that had excellent luminescence but insufficient film formability or film strength; however, in the present invention, materials with poor film formability or film strength could not be used. is 1
Even if a material has excellent luminescence properties, by using a material with excellent film-forming properties for one of the layers, a light-emitting layer with excellent luminescence properties, film-forming properties, and film strength can be obtained.

The EL device of the present invention described above can achieve excellent EL emission by applying electrical energy such as alternating current, pulse, or direct current between the electrode layers so that electrical energy such as a suitable electric field acts on the light emitting layer. This shows that.

Next, the present invention will be explained in more detail with reference to Examples.

Example 1 An ITO layer with a thickness of 1,500 Å was deposited on the surface of a 50-square glass plate by sputtering.

A transparent electrode was formed.

Next, using a resistance heating 7-beading device, the above-mentioned transparent 5°C)
and anthraquinone (B) (mp, 286° C.) were deposited to a thickness of 250 μm to form the first layer. This vapor deposition is performed by first reducing the pressure in the vapor deposition tank to a vacuum level of 10 Torr, and gradually increasing the temperature of the resistance heating board (Mo) so that the anthraquinone vapor deposition rate is approximately IA/sec. Keeping the current constant, the total deposition rate was 5λ/
A vapor deposited film was formed by adjusting the current flowing through the board containing carbazole so that the temperature was sec. The degree of vacuum during evaporation was 9×10 Torr, and the temperature of the substrate holder was kept constant by circulating 20° C. water.

Next, a chloroform solution in which CHmCHtCOOCHS CD was dissolved at a molar ratio of 10:1 was added to P
Ja adjusted to H6.5! ce-Loebe1 company'
n's 1, Antmuir -Trr+ogh4's Sowayama Lm beat 1! D1. After the solvent chloroform was removed by evaporation, the surface pressure was increased (30 dyne/cm) to precipitate the above mixed molecules in the form of a film. Thereafter, while keeping the surface pressure constant, the film-forming substrate was gently moved up and down in a direction across the water surface (vertical speed 2 cm/sin), and the mixed monomolecular film was transferred onto the substrate and accumulated into 6 layers. A mixed single molecule cumulative layer was prepared to form the second layer. In this accumulation process, each time the substrate was pulled out of the water tank, it was left to stand for 30 minutes or more to evaporate and remove the water adhering to the substrate.

By repeating all the above operations three more times, a total of 8
A light emitting layer with a layered structure was formed.

Finally, the substrate with the thin film formed as described above is placed in a vapor deposition tank, and the nuclide is once depressurized to a vacuum level of 10 Torr, and then the vacuum level is adjusted to 10 Torr, and the base rate is 2.
An EL element made by evaporating Al with a film thickness of 1500λ on the thin film at 0 people/sec and using it as a back electrode was subjected to 7fS.
As illustrated in Figure a, after sealing with a sealing glass, purified, degassed, and dehydrated silicone oil was sealed in accordance with the conventional method. IOV to these EL light emitting cells
, when an AC voltage of 400Hz was applied, the current density was 0.
．． It exhibited EL light emission with a brightness of 30 ft-L at 11 mA/cm''.The EL device of the present invention has a lower driving voltage than the conventional EL device using ZnS as a light emitting matrix. It was an EL element with good luminance characteristics.

Comparative Example 1 A comparative EL element was obtained in the same manner as in Example 1 except that ff1lW was not formed, and evaluated in the same manner as in Example 1. As a result, the current density was 0.
．． The luminance was 15 f t-L or less at 09 mA/cm'.

Example 2 In place of compounds A and C in Example 1.

Using the following compounds E and F, ``E
The number of stacked layers was 4 and the number of cumulative layers was 6), and when evaluated under the same conditions as in Example 1, the current density was 0.12 mA/crn' and the brightness (Ft-L) was z9.

Example 3 An EL device was obtained in the same manner as in Example 1 except that the second layer in Example 1 was a monomolecular film. Example 1
When evaluated under the same conditions, the current density was 0.12 mA/
cm', and the luminance (Ft-L) was 28.

[Brief explanation of drawings]

FIG. 1 schematically shows an EL device using the LB method of the prior art, FIG. 2 schematically shows an EL device according to the present invention, and FIG. 3 schematically shows an EL device according to the present invention. It is a diagram schematically showing a cross section of an EL element. l; transparent electrode 2; luminescent layer 3; back electrode 4; luminescent compound 6; luminescent compound 6'; acceptor 7; seal glass
8; Silicone insulating oil 9; Glass plate

Claims (1)

[Claims] In an EL device comprising a multilayered light emitting layer and two electrode layers sandwiching the light emitting layer, at least one of which is transparent, the above light emitting layer has the following first layer and the following first layer. The above-mentioned EL device is characterized in that it is formed by repeatedly laminating four or more layers of two layers alternately. First layer: from a mixture of at least one electroluminescent organic compound that is electron-accepting relative to the second layer and at least one organic compound that is electron-donating relative to the second layer. A molecular deposition film. Second layer; from a mixture of at least one electroluminescent organic compound that is electron-donating relative to the first layer and at least one organic compound that is electron-accepting relative to the compound; monomolecular film or its cumulative film.