WO2006052246A1

WO2006052246A1 - Light-emitting materials and devices

Info

Publication number: WO2006052246A1
Application number: PCT/US2004/037247
Authority: WO
Inventors: Chuanjun Xia; Kwang-Ohk Cheon
Original assignee: BTG International Inc
Current assignee: BTG International Inc
Priority date: 2004-11-09
Filing date: 2004-11-09
Publication date: 2006-05-18
Anticipated expiration: 2007-05-09

Abstract

A light-emitting material comprises an organic material, in which the material includes an evaporable small molecule. The performance of organic light-emitting devices (OLEDs) may be improving by the use of such additives.

Description

LIGHT-EMITTING MATERIALS AND DEVICES

This invention relates to light-emitting materials and devices, particularly to materials for use in organic light-emitting devices. The invention also includes methods of making the materials and devices, as well as to improving the perform- ance of organic light-emitting devices (OLEDs) by the use of additives.

Background to the invention

Organic light-emitting device (OLED) structures consist of a stack of organic layers sandwiched between two electrodes, an anode and a cathode. When an appropriate current is supplied to the device, positive charges (i.e. holes) are injected from the anode and negative charges (i.e. electrons) from the cathode. Under the influence of the field the charges migrate through the semiconducting organic material towards each other. The charges recombine forming unstable species which decay generating light in the process. This process is called electroluminescence. At least one of the electrodes, either the anode or the cathode should be transparent to pass or allow the light generated in the organic layers to be seen. Typically, transparent conducting indium tin oxide (ITO) on a glass or flexible plastic substrate is used as an anode, and a low work-function metal is used as a cathode. Both conjugated polymers and small molecules have been used as light-emitting layers in OLEDs.

To improve charge injection, a hole transport layer (HTL) is introduced between the anode and organic light-emitting layer, and sometimes an electron transport layer (ETL) is inserted between the light-emitting layer and the low work- -function cathode. For a polymer light-emitting device (PLED), poly(3,4-ethylene- dioxythiophene) : polystyrenesulfonic acid (PEDOT : PSS), or other suitable material such as polyaniline, coated on the ITO can be used to improve hole injection and device stability. The layers of ITO / PEDOT : PSS inject holes efficiently to the emitting layer. For electron injection, a low work-function metal or salt of a low work-function metal (e.g. Ca, Ba, Mg, CsF, LiF) is deposited on the organic layer by evaporation or sputtering. However one of the problems of using a low work- function material is that by their very nature they are corrosive and react readily with oxygen and water. Any reaction of the cathode with harmful contaminants such as oxygen and water will clearly adversely affect the stability (or lifetime) of the device. In order to protect the reactive cathode from such degradations due to exposure to oxygen and moisture it is therefore necessary to utilize high quality encapsulation of the device

Instead of using a reactive cathode metal, it is preferable to remove the highly reactive material, and use a stable cathode metal such as high work-function metals (e g Al, Ag, Cu, and Au) However, high work-function metals cannot inject electrons efficiently, owing to a high energy barrier between the organic layer and cathode To facilitate electron injection, one can either design special polymers that have very high electron affinity or insert another layer between the organic layer and the cathode Both require careful design, synthesis, and extra steps in fabrication The present inventors have now devised a more general and simple way to improve the device performance using high work-function metals A small amount of an evaporable small molecule additive (ESMA) is added to the light-emitting material solution The resulting solution is then handled as a normal light-emitting matenal solution for the fabrication of the device The additive can improve electron injection from high work- function cathode metals such as Al In addition, it can increase the electroluminescence (EL) efficiency of PLEDs using low work-function metals as the cathode Furthermore, the excess hole current is decreased by the additive

Summary of the invention According to the present invention, there is provided a light-emitting material comprising an organic material, in which the material includes an evaporable small molecule additive in an amount 0 001 to 1000 wt% compared to the organic material

The evaporable small molecule additive may be a material that can be evaporated in vacuo, so would not necessarily be a volatile material at atmospheric pressure The evaporable small molecule additive preferably has a boiling point at 760 mm Hg in the range of 150-250 ⁰C, more preferably in the range of 160-220 ⁰C, and especially in the range of 200-215 ⁰C It may comprise a material having a high vapour pressure at room temperature

The evaporable small molecule additive preferably has a molecular weight in the range of 55-180, more preferably in the range of 70-160, and especially in the range of 75- 100

The evaporable small molecule additive can an aliphatic, alicyclic. aromatic or heteiocyclic material The structure of ESMA can contain but is not limited to the following functional groups O

O Ii O Ii O it — r ^-N N— O Ii I I

—OH ) — O ) C— ) -C-OH » — C-O- ' H ) -C-N- ) -NH₂ ) — NH- )

S I

I ^ — S — — Si-Cl

— N— -N= — C≡N -NO₂ -SH — S— -SO₃H — s— O I

_^ Cl

I _c, -t Cl c, 4 I,-0- X I o- -I O—- 4 I- — F -Cl —Br — I

Preferably it contains at least one heteroatom comprising oxygen and / or nitrogen, silicon and / or sulfur, with oxygen and / or nitrogen being the preferred heteroatoms More preferably two or more heteroatoms are present

Preferably the evaporable small molecule additive comprises N-methyl-2- -pyrrohdone (ΝMP), ethyl acetoacetate, 3,7-dimethyl-l-octanol, 1,3-propanediol and / or N,N-dimethylacetamide (DMAC) Especially preferred are 1 ,3-propanediol and, in particular, ΝMP

The evaporable small molecule additive is preferably present in an amount 10 to 500 wt%, more preferably 50 to 200 wt%, compared to the organic material The evaporable small molecule additive may function in a manner similar to a dopant, but without the adverse effects of a dopant such as phase separation of materials, or otherwise improve the performance of the light-emitting material when incorporated in an electronic device

The light-emitting matenal may be made from a solution of the basic hght- -emitting organic material in a mixture of two or more components, one component comprising a solvent and another component comprising the evaporable small molecule additive The evaporable small molecule additive may be added to the solution of the basic light-emitting organic material in an organic solvent The ESMA would usually have a higher boiling temperature and / or a lower vapour pressure than the solvent The quantity of the evaporable small molecule additive vanes from 0 001 to 1000 wt% compared to the basic light-emitting organic material quantity in the solvent

Thus, in a further aspect of the present invention, there is provided a method of manufacturing a light-emitting material from a solution of a light-emitting material in a mixture of two or more components, one component compπsing a solvent and another component comprising the evaporable small molecule additive having a higher boiling temperature and / or a lower vapour pressure than the solvent, in an amount varying from 0 001 to 1000 wt% compared to the basic light-emitting organic material in the solvent The solvent is not limited to one kind of material, in some cases two or more solvents may be mixed and used as the solvent Possibly more than one evaporable small molecule additive can be added into the solution of the light-emitting material Solubility of the light-emitting mateπal in the evaporable small molecule additive is not required The solvent may comprise chlorobenzene, xylene, tetrahydrofuran (THF), chloroform, toluene, etc or a mixture of these If necessary, a secondary solvent such as THF may be included, to prevent phase separation For example, chlorobenzene THF may be used at a volume ratio 4 to 1, so that the solvent is a mixture of 80% chlorobenzene and 20% THF The light-emitting mateπal can be applied to all cathode matenals including low and high work function metals such as Al, Ca, Ba, Ag, Cu and Au, and it also works for thin dipole layers such as CsF and LiF Thus, in a further aspect of the present invention, there is provided an electronic device incorporating the light- emifting mateπal The evaporable small molecule additive improves the electron-injection properties of the light-emitting material, and may further reduce excess hole current and lower the turn-on voltage Thus, in a further aspect of the present invention, there is provided the use of an evaporable small molecule additive to increase the luminance, increase the quantum efficiency or improve the electron-injection properties of a light-emitting mateπal, or to reduce the excess hole current or to lower the turn-on voltage of an electronic device

The light-emitting material may be a polymer, a dendπmer, a phosphorescent organic mateπal or a blend or composite of such organic materials Any material that can be used in a polymer light-emitting device is possible Some examples are homo- polymers and copolymers of polyfluorene, polypyπdine, polythiophene, poly(phenyl- ene vinylene), poly(phenylene vinylene pyridyl vinylene), and poly(pyπdyl vinylene) One light-emitting polymer which we have found to be particularly suitable is a poly- (p-phenylencvinylene (PPV) based yellow electroluminescent-emitting polymer available under the trade name SUPER YELLOW™ from Covion Organic Semicon¬ ductors GmbH

The invention is not limited to polymers Electronic devices incorporating small-molecule light-emitting materials can also be improved by an evaporable small molecule additive The evaporable small molecule additive may be vaporized and the vapour used to expose the top organic layer just before electrode deposition

Preferably a baking procedure is used to improve the reaction between the light-emitting polymer and an electrode, and to remove residual solvents A suitable baking temperature is in the range 50-250 ⁰C However, it is not always required A device incorporating a light-emitting material according to the present invention may have an organic light-emitting device (OLED) structure This may be, for example, a conventional organic light-emitting diode, or a bipolar electrolumines¬ cent device, as shown in US patent No 5,858,561 (Epstein et al I The Ohio State University It may comprise an anode on a πgid (e g glass) or flexible (e g plastic) substrate, a hole transport layer (HTL) for efficient hole injection, an organic emitting layer, and a cathode According to the invention, a small amount of evaporable small molecule additive (ESMA) is added to the solution of the light-emitting material, and spin coated on to the HTL to form an organic emitting layer (EML) On the top of the EML, a metal electrode is evaporated as a cathode, which can be a relatively high work-function metal such as Al, Ag, and Au or low work- function metals such as Ca and Ba, or a stack of or a stack of one or more salts and metals

In order to prepare a light-emitting material solution, first, the light-emitting material should be dissolved in a solvent The solvent dissolves the light-emitting material, and can be deposited to and form a uniform film on the substrate or HTL In some cases, the solvent is made of two or more solvents in a mixture After completely dissolving the light-emitting material in the solvent, the ESMA is then added to the solution as a neat material or a pre-prepared solution The amount of ESMA is in the range of 0 001-1000 wt% compared to light-emitting material weight in a solvent Examples of ESMA are N-methyl-2-pyrrohdone (ΝMP), ethyl acetoacetate, 3,7-dimethyl-l-octanol, 1,3-propanediol, and N,N-dimethylacetamide (DMAC) Many others may be selected from those available, which have the following physical properties ESMA Boiling point Molecular weight (760 mm Hg)

N,N- D imethy lformamide 152-154 ⁰C 73.09

Methacrylic acid 159-161 °C 86.09

2-Fluoro-4-methylpyridine 160-161 °C 111.12

2-oxetanone 162 ⁰C 72.06

Butyric acid 164 ⁰C 88.11

N,N-Dimethylacetamide (DMAC) 164.5-166 ⁰C 87.12

4-methyl-2-oxetanone 164-165 °C 86.09

Pyruvic acid 165 ⁰C 88.06

?rα«j-3-(Trimethylsilyl)allyl alcohol 166-168 ⁰C 130.26

2-(Methylthio)ethanol 169-171 ⁰C 92.16

3-(Methylthio)propionaldehyde 170-175 °C 104.17

3,5-Lutidine 171-172 ⁰C 107.15

Thiomorpholine 175-178 ⁰C 103.19

Methyl carbamate 176-177 °C 75.07

3,4-Lutidine 178-180 ⁰C 107.15

Crotonic acid 180-181 ⁰C 86.09

4,5-Dihydro-3(2H)-thiophenone 180-182 ⁰C 102.16

Ethyl acetoacetate 181 ⁰C 134.1 1

Cyclopropanecarboxylic acid 182-184 ⁰C 86.09

Dimethyl sulfoxide 189 ⁰C 78.13

4-Pyridinecarbonitrile 195.4 ⁰C 104.1 1

Ethylene glycol 195-197 ⁰C 62.07

N-Methylformamide 198-200 °C 59.07

2-(Methylamino)pyridine 200-201 ⁰C 108.14

3 -Pyridinecarbonitrile 201 ⁰C 104.1 1

N-Methyl-2-pyrrolidone (ΝMP) 202 ⁰C 99.13

N-Ethy lformamide 202-204 ⁰C 73.09

Thiolactic acid 203-208 ⁰C 106.14

N-Methylacetamide 204-206 ⁰C 73.09

2-Aminopyridine 204-210 °c 94.1 1

6-Amino-2-picoline 208-209 °c 108.14 ESMA Boiling point Molecular weight

(760 mm Hg)

N-Vinylformamide 21O ⁰C 71 08

2(5H)-furanone 212-214 ⁰C 84 07

2-Pyπdinecarbonitπle 212-215 ⁰C 104 1 1

Propionamide 213 ⁰C 73 09

3,7-Dimethyl-l-octanol 213-218 ⁰C 158 29

1,3-Propanediol 214 ⁰C 76 10

Isobutyramide 216-220 ⁰C 87 12

2-Pyπdinemethanol 219-224 ⁰C 109 13

2-Oxazohdinone 220 ⁰C 87 08

Acetamide 221 ⁰C 59 07

N-Vinylacetamide 229 ⁰C 85 10

2-Amino-4-picohne 230 °C 108 14

2-Pyπdylacetonitπle 248-250 ⁰C 118 14

Such materials have polar groups or strong coordination ability that can potentially form a strong interaction, such as a covalent bond or ionic bond with an electrode to improve charge injection Usually, ESMAs have a higher boiling temperature or lower vapour pressure than the solvent Even long after the film formation of the light-emitting material layer, they can stay in the film due to the higher boiling temperature In order to remove remaining ESMA, an additional baking process may be needed after the cathode deposition, but not necessaπly for all cases There is an optimal concentration of an ESMA to maximize the device performance, which is dependent on a number of variables, to include but not be limited to, the characteristics of light-emitting material, solvent, the additive, and device structure

The solution of the light-emitting material including ESMA can be coated by conventional spin coating, ink-jet pπnting, dip-coating, screen-printing or any other suitable deposition process such as spray coating, gravure pπnting etc The preferred means of adding the ESMA to the organic material are

(1) adding the ESMA diiectly into the organic material pπor to spin coating of the layer, (2) adding the ESMA as a vapour to the surface, and

(3) adding the ESMA to the surface of the spin-coated organic layer as a liquid by ink-jet pnnting, dip-coating or screen-printing

After all of the films are stacked, a cathode electrode is vacuum deposited, but cathodes may also be screen-pπnted or ink-jet printed Then, the device may be baked after the cathode deposition to enhance the reaction between additive and cathode, and to remove residual additive in the film The baking process is distinct from post- annealing processes reported in the pπor art, such as are described in US patent 6734623-B 1 (Aziz et al I Xerox Corporation), because, normally, the purpose of post-annealing is to improve lifetime of the devices However, prior art processes such as the above may also be used in the invention

Brief introduction to the figures

The invention is illustrated by the following Examples and drawings in which —

Fig l(a) shows a basic organic light-emitting device (OLED) having the Substrate / ITO / HTL / EML / cathode stiucture, and Fig l(b) an enhanced OLED having the Substrate / ITO / HTL / EML / ETL / cathode structure,

Fig 2 shows the additive effects on current and electroluminescence (EL) intensity of indium tin oxide / poly(3,4-ethylenedioxythiophene) poly(styrene- sulfonate) aqueous dispersion / SUPER YELLOW™ incorporating N-methylpyrrohdone / aluminium (ITO / PEDOT PSS / SY_ΝMP / Al), as well as a corresponding struc¬ ture replacing the N-methylpyrrohdone with 1,3 -propanediol ("alcohol") (ITO / PEDOT PSS / SY_Alcohol / Al), Fig 3 shows the efficiencies of silver cathode devices (ITO / PEDOT PSS /

SY_ΝMP / Ag),

Fig 4 shows the additive effects on EL efficiency of ITO / SY_NMP / Ca / Al,

Fig 5 shows the additive effects on EL efficiency of ITO / PEDOT PSS / SY_NMP vapour / Al,

Fig 6 shows the additive effects on current and EL intensity of indium tin oxide / poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) aqueous dispersion / poly(/?-pyπdylvinylene p-phenylenevinylene) incorporating N-methylpyrτohdone / aluminium (ITO / PEDOT PSS / PPyVPV_ΝMP / Al), and Fig 7 shows the additive effects in phosphorescence devices of ITO / PEDOT PSS / phosphorescence doped layer_NMP / Ca /Al

General method A device incorporating a light-emitting polymer according to the present invention having a conventional organic light-emitting diode structure is shown in Fig l(a) It compnses an anode ( 100) on a rigid (e g glass) or flexible (e g plastic) substrate (10), a hole transport layer (HTL) (200) for efficient hole injection, an organic emitting layer (210), and a cathode (1 10) A small amount of evaporable small molecule additive (ESMA) is added to the polymer solution, and spin coated on to the HTL to form an organic emitting layer (EML) 210 On the top of the EML, a metal electrode 1 10 is evaporated as a cathode, which can be a relatively high work- -function metal such as Al, Ag, and Au or low work-function metals such as Ca and Ba, or a stack of or a stack of one or more salts and metals In the enhanced OLED structure of Fig l(b), an electron transport layer

(ETL) (220) is inserted between the organic emitting layer (210) and cathode (1 10)

The polymer is first dissolved in a single solvent or, in some cases, two or more solvents in a mixture After completely dissolving the polymer in the solvent, ESMA is then added to the polymer solution The polymer solution including ESMA can be coated by conventional spin coating After all of the films are stacked, a cathode electrode is vacuum deposited, but cathodes may also be screen-printed or ink-jet printed Then, the device is baked after the cathode deposition to enhance the reaction between additive and cathode, and to remove residual additive in the film Fig 2 shows the current density and brightness versus applied voltage To see the additive effects of the device, three different devices are tested and illustrated in Fig 2 The device structures are ITO / PEDOT / SY / Al, ITO / PEDOT / SY (with NMP) / Al, and ITO / PEDOT / SY (with Alcohol) / Al for Device A, Device B, and Device C, respectively Current density of the devices is reduced by the additive com- pared to additive-free devices, the current density decrease is evident at low operating voltage range Even with the low cuirent of the additive-added devices (Devices B and C), the brightness is increased The additive-free Device A has a maximum brightness about 500 cd m^~2 However, if an additive is added to the polymer solution, then the maximum brightness of the device is increased to about 50,000 cd m^~2, approximately 2 orders are increased Overall efficiency of the additive-added devices is 100 times improved

Example 1. ITO / PEDOT : PSS / SY NMP / Al device Additive-free polymer solution is prepared in a solvent such as chlorobenzene

(CB) To make an ESMA solution, a small amount of ESMA is added into the polymer solution already dissolved in the solvent The ESMA quantity is defined by weight percentage (wt%) compared to emitting polymer weight in the solvent In the example, 100 wt% N-methyl-2-pyrrohdone (ΝMP) is added in SUPER YELLOW™ (SY, from Covion) polymer solution dissolved in CB, where the final SY polymer concentration is 6 mg ml^"1 in CB and ΝMP

The devices are fabncated by a conventional spin coating method A hole transporting layer of PEDOT PSS (BAYTROΝ™ P 4083 EL₁ from H C Starck) is coated on the transparent anode of indium tin oxide (ITO) Subsequently, a polymer (either additive-free or additive-added) solution is spin-coated with a thickness 60- 100 nm Then, without any annealing process, the substrate is transported into a vacuum chamber to evaporate aluminium (Al) as a cathode After the cathode is evaporated, the devices are baked at a temperature 70 ⁰C with various baking times to enhance the reaction of additive with Al and to remove residual solvents Fig 2 shows current density and electroluminescence (EL) intensity versus applied voltage of the additive-free device (A) and 100 wt% ΝMP-added device (B) The current density is significantly reduced at low voltage range (2-8 V) by ΝMP additive On the contrary, EL intensity of the additive-added device (B) is highly increased in the entire operational voltage range, compared to additive-free device (A) At an operating voltage of 8 V, current density is 140 mA cm^~2 (1400 A nf²) for Device A and 95 mA cm^"2 (950 A m^~2) for Device B, and at the same voltage, EL intensity is 30 cd m^~" for Device A and 2400 cd m^~" for Device B Eventually, the efficiency of the additive device is approximately 100 times improved, and the corresponding efficiency is 0 02 cd A^"' for Device A and 2 5 cd A^"' for Device B Without any additive, Device A is not efficient because of the poor electron injection from high work function metal Al cathode Thus, the dominant current of Device A is hole current injected from ITO / PEDOT PSS It leads to excess hole current, which passes through the EML without any recombination with electrons Therefore, the reduced current of Device B is mainly due to the decreased hole current by the NMP additive This means that the additive functions as a blocking material for excess hole charge Although current is reduced, the EL intensity and efficiency are increased The reason for the higher brightness and higher efficiency of Device B is that electron injection is enhanced by the additive The NMP reacts with Al, and decreases the energy barrier between polymer layer and Al

Example 2. ITO / PEDOT : PSS / SY_Alcohol / Al device

Light-emitting devices are fabricated in accordance with Example 1 but, instead of the NMP additive, 50 wt% of 1,3-propanediol (herein referred to as "alcohol") is added to the polymer solution However, the alcohol is not compatible with CB, thus causing a phase separation from solvent CB To prevent phase separation, another solvent tetrahydrofuran (THF) is added to CB at a volume ratio 4 to 1 Thus, the solvent is a mixture of 80 % CB and 20 % THF

The current density and EL intensity of the alcohol-added device (C) is included in Fig 2 The transport and illumination properties of the alcohol added device are quite similar to the characteristics of NMP added device described in Example 1 Device C shows a lower turn-on voltage than additive-free Device A, in which the turn-on voltage is defined as the voltage at EL intensity 0 2 cd m^"2 The turn-on voltage is 2 7 V for Device C, and 3 9 V for Device A

Example 3. ITO / PEDOT : PSS / SY_NMP / Ag

Light-emitting devices are fabricated in accordance with Example 1 In this example, silver (Ag) is used as a cathode instead of Al cathode described in Example 1 The efficiency of the NMP added device with Ag cathode is increased more than 2 times compared to additive-free device However, the efficiency is not increased as much as the Al-cathode device The NMP may have less reactivity with noble metals such as Ag

Fig 3 shows the efficiencies of the silver cathode devices

Example 4. ITO / SY_NMP /Ca / Al devices

Light-emitting devices are fabricated in accordance with Example I In the example, the hole transport layer PEDOT PSS layer is not coated and a low work- - function metal, calcium (Ca), is used as a cathode, in which the Ca is capped by Al to protect from exposure to oxygen and moisture Two devices are tested for

- U - compaπson, the first device (D) is made of pure SY solution without any additive, and the other (Device E) is fabricated with 100 wt% NMP additive in SY solution The efficiency of the Device E clearly is improved by NMP in the entire operating voltage range (see Fig 4) The efficiency is 3 7 cd A^"1 for Device D, and 4 4 cd A^"1 for Device E at the operating voltage 6 V, the efficiency is about 20 % increased by the NMP

In order to check the additive effect with a HTL, PEDOT PSS is placed as a HTL between ITO and SY polymer layer with Ca cathode The devices are tested at the same conditions, and the results are similar to the devices without HTL The overall efficiencies are increased for both additive-free and additive-added devices compared to the devices without PEDOT PSS For the devices with PEDOT PSS, the NMP-added device has 25% higher efficiency compared to the additive-free device

Fig 4 shows the effects of the additive on EL efficiency of such a device

Example 5. ITO / PEDOT:PSS / SY_NMP vapour / Al devices

Light-emitting devices are fabricated in accordance with Example 1, but no additive is mixed into polymer solution However, after the emitting polymer layer coating is deposited, the polymer layer is exposed to NMP vapour (50 ⁰C, 20 minutes) In this process, only the surface of the emitting polymer is exposed to NMP After the NMP vapour exposure, Al is evaporated as a cathode, and then the devices are baked at 70 ⁰C (2 hours) Fig 5 shows the efficiencies of the devices, in which the pure polymer device (F) is represented by the solid square symbol, and the 20 minute NMP exposed device (G) is symbolized by open-circle The efficiency of Device G is improved greater than 10 times by NMP exposure In this example, the reason for the efficiency improvement is mainly the increment of electron injection from the cathode

Fig 5 shows the effects of the additive on EL efficiency of such a device

Example 6. ITO / PEDOT : PSS / PPyVPV NMP / Al devices

Light-emitting devices are fabπcated in accordance with Example 1, with the 100 wt% NMP added into pyπdine-contaimng polymer PPyVPV solution The concentration of the light-emitting polymer is 10 mg mP¹ m solvent CB and THF (1 1 ratio) The pyndine-contained polymers have been shown to have bipolar properties See US patent No 5,858,561 (Epstein et al I The Ohio State University)

In the example, the effect of the NMP is tested for the pyndine-contained polymer device Fig 6 shows the current and EL intensity, those are increased by the NMP At the forward bias 10 V, the EL intensity of an additive free device (H) is only 0 3 cd πf², and that of an NMP-addtd device (I) is 1300 cd m^~2 The current density is increased from 7 to 135 mA cm^"" (70 to 1350 A m^~") by the NMP Overall

EL efficiency of Device I is improved approximately 250 times Although the reverse operation is not as effective, the NMP additive works for reverse bias with the same trend as forward bias

Fig 6 shows the effects of the additive on current and EL intensity of such a device

Example 7. Additive effects in phosphorescence devices ITO / PEDOT : PSS / Phosphorescence doped layer_NMP / Ca / Al

Light-emitting devices are fabricated m accordance with Example 1 In this example, the phosphorescence-doped solution is prepared, as described in "Polymer electrophosphorescence devices with high power conversion efficiencies", X H Yang and D Neher, Appl Phys Lett 84, 2476 (2004), the solution is made of 60 wt% of host polymer poly(N-vinylcarbazole) (PVK), 24 wt% of 2-(4-biphenylyl)-5,4- -(4-tert-burylphenyl)-l,3,4-oxiadiazole (PBD), 10 wt% of N,N'-diphenyl-N,N'-(bis(3- -methylphenyl)-[l , l-bφhenyl]-4,4'-diamine (TPD), and 6 wt% of the phosphorescent indium complex Ir(CioH₂iOPPy)₃ material in chlorobenzene Subsequently, as an ESMA, 50 wt% of ΝMP is added to the phosphorescence doped solution Fig 7 shows ΝMP effect in efficiency of the phosphoresced doped OLED

Without the ESMA, the maximum efficiency is only 5 3 cd/A at the operating voltage 1 1 4 V However, the maximum efficiency of the ESMA added device is increased to 10 6 cd/A at 9 3 V

Fig 7 shows the effects of the additive on phosphorescence in such a device

Claims

1 A light-emitting material comprising an organic material, in which the material includes an evaporable small molecule additive in an amount 0 001 to 1000 wt% compared to the organic matenal

2 A light-emitting material as claimed in claim 1, in which the evaporable small molecule additive has a boiling point at 760 mm Hg in the range of 150-250 °C

3 A light-emitting material as claimed in claim 2, in which the evaporable small molecule additive has a boiling point at 760 mm Hg in the range of 160-220 ⁰C

4 A light-emitting material as claimed in claim 3, in which the evaporable small molecule additive has a boiling point at 760 mm Hg in the range of 200-215 ⁰C

5 A light-emitting material as claimed in any preceding claim, in which the evaporable small molecule additive has a molecular weight in the range of 55-180

6 A light-emitting material as claimed in claim 5, in which the evaporable small molecule additive has a molecular weight in the range of 70-160

7 A light-emitting material as claimed in claim 6, in which the evaporable small molecule additive has a molecular weight in the range of 75-100

8 A light-emitting material as claimed in any preceding claim, in which the evaporable small molecule additive comprises an aliphatic, alicychc, aromatic or heterocyclic material containing at least one heteroatom comprising oxygen, nitrogen, silicon and / or sulfur

9 A light-emitting material as claimed in claim 8, in which the evaporable small molecule additive comprises N-methyl-2-pyrrolidone (ΝMP), ethyl acetoacetate, 3,7-

-dimethyl-1 -octanol, 1 ,3-propanediol and / or N,N-dimethylacetamide (DMAC)

10 A light-emitting material as claimed in any preceding claim, in which the evaporable small molecule additive is present in an amount 10 to 500 wt% compared to the organic material

1 1 A light-emitting material as claimed in claim 10, in which the evaporable small molecule additive is present in an amount 50 to 200 wt%, compared to the organic material

12 A method of manufacturing a light-emitting material as claimed in any preceding claim from a solution of the light-emitting material in a mixture of two or more components, one component compπsing a solvent and another component comprising the evaporable small molecule additive

13 A method of manufacturing a light-emitting material from a solution of a light-emitting material in a mixture of two or more components, one component compπsing a solvent and another component compπsing the evaporable small molecule additive having a higher boiling temperature and / or a lower vapour pressure than the solvent, in an amount varying from 0 001 to 1000 wt% compared to the basic light-emitting organic material m the solvent

14 A method as claimed in claim 13, in which the evaporable small molecule additive has a boiling point at 760 mm Hg in the range of 150-250 ⁰C

15 A method as claimed in claim 14, in which the evaporable small molecule additive has a boiling point at 760 mm Hg in the range of 160-220 ⁰C

16 A method as claimed in claim 15, m which the evaporable small molecule additive has a boiling point at 760 mm Hg in the range of 200-215 ⁰C

17 A method as claimed in any one of claims 13 to 16, in which the evaporable small molecule additive has a molecular weight in the range of 60-180

18 A method as claimed in claim 17, in which the evaporable small molecule additive has a molecular weight in the range of 70-160

19 A method as claimed in claim 18, in which the evaporable small molecule additive has a molecular weight in the range of 75-100

20 A method as claimed in any one of claims 13 to 19, in which the evaporable small molecule additive comprises an aliphatic, ahcyclic, aromatic or heterocyclic material containing at least one heteroatom comprising oxygen and / or nitrogen

21 A method as claimed claim 20, in which the evaporable small molecule additive comprises N-methyl-2-pyrrolidone (ΝMP), ethyl acetoacetate, 3,7-dimethyl-

- l-octanol, 1,3-propanediol and / or N,N-dimethylacetamide (DMAC)

22 A method as claimed in any one of claims 13 to 20, in which the evaporable small molecule additive is present in an amount 10 to 500 wt% compared to the organic material

23 A method as claimed in claim 22, in which the evaporable small molecule additive is present in an amount 50 to 200 wt%, compared to the organic material

24 A method as claimed in one of claims 12 to 23, in which a baking procedure is used to improve the reaction between the light-emitting material and an electrode, and to remove residual solvents

25 A method as claimed in claim 24, performed at a baking temperature in the range 50-250 ⁰C

26 An electronic device incorporating a light-emitting material as claimed in any one of claims 1 to 1 1

27 An electronic device as claimed in claim 26, in which the light-emitting mateπal is applied to a cathode materials including a low or high work function metal or a thin dipole layer

28. Use of an evaporable small molecule additive to increase the luminance, increase the quantum efficiency or improve the electron-injection properties of a light-emitting material; or to reduce the excess hole current or to lower the turn-on voltage of an electronic device.

29. Use of an evaporable small molecule additive as claimed in any one of claims 1 to 11 to increase the luminance, increase the quantum efficiency or improve the electron-injection properties of a light-emitting material; or to reduce the excess hole current or to lower the turn-on voltage of an electronic device.