Different types of polymer with synthesis process, their morphology, and application.
Many changes have arisen in the world of display technologies as time has passed. In the vast area of display technology, Organic light-emitting diode is a recent and exciting discovery. Organic light-emitting diodes (OLEDs) have received a lot of curiosity among the researcher in recent years as the next generation of lighting and displays due to their numerous advantages, such as superior efficiency, mechanical flexibility and stability, chemical versatility, ease of fabrication, and so on. It works on the theory of electroluminescence, which is a mechanism in which electrical energy converts to light energy. Organic LEDs have a thickness of 100 to 500 nanometers or 200 times that of human hair. In OLEDs, organic material can be used in two or three layers. The emissive layer plays a key role in OLEDs. Polymers are used in the emissive layer to enhance the efficiency of OLEDs at the same time self-luminescence materials are used in OLEDs. In displays, this self-illuminating property removes the need for backlighting. Compared to LEDs and LCDs, OLED displays are smaller, lighter, and more portable.
- Conducting Polymer
- emissive layer
Today, using electronics is so much a part of our daily lives that we rarely imagine what life will be like without them. Electronics and mechanical assets are used in a variety of operations ranging from food to music. Because of the high demand for electrons, chemists, physicists, and other scientists and engineers are developing a plethora of novel organic materials that will change how society views technology . Organic printed electronics are being developed for a variety of applications such as organic light-emitting diodes (OLEDs), organic photovoltaic cells (OPVs), electronic journals, portable electronics, and various sensors . It is anticipated that compact, small, and lightweight devices with cost-effective usability can lead creativity into our lives, making research and development (R&D) in this field highly ambitious and universal .
OLEDs, as self-emitting devices, have high image efficiency, are ultra-thin, and light in weight . They have become popular with the general public due to their use in mobile phones such as Samsung’s Galaxy, iPhone, etc.. For the time being, OLEDs are mass-produced using an evaporation technique. While this method is feasible for mass manufacture of small-scale to medium-sized OLED displays, but it is still thought that certain technological and cost challenges remain in the manufacturing of large TV displays. The main feature and efficiency of the OLED device depend on the types of materials used for fabrication.
The research of nanomaterials has recently gained increased interest due to their optical, mechanical, electrical, and chemical properties. Metals, semiconductors, carbon, and polymers are among the materials used to create nanoparticles, nanowires, nanotubes, and nanocomposites. These materials’ uses cover optical and electronic instruments, as well as chemical and biomedical areas. Over the last century and a half, new groups of advanced materials known as polymers have been developed and researched, not only challenging the old classical materials but also enabling the creation of new goods that have led to the expansion of mankind’s range of activities. Polymers are the foundation of many significant industrial products. Aside from social influences, their exponential rise in production is driven by the need to substitute conventional products. G. J. Berzelius, a Swedish chemist, invented the expression “polymer.” He found benzene (C6H6) to be an ethyne polymer (C2H2). Later, this term underwent a minor change. The concept of polymers is one of the twentieth century’s great ideas. It first appeared in the 1920s after much debate, and its acceptance is closely linked with the name of H. Staudinger, who won the Nobel Prize in 1953.
Thus, this chapter intended to describe and discuss the polymers used in OLEDs. Types of conducting polymer used and their synthesis process. Moreover, OLEDs mechanism and their structure. How the emissive layer affects OLEDs and their efficiency are reported. Also, types of OLEDs are used in this era and their benefits in day-to-day life.
2. Organic light emitting diode
OLEDs (organic light-emitting diodes) are innovative developments in the field of optoelectrical systems for modeling next-generation versatile displays and devices that use organic thin film as an electroluminescent diode sensitive to current. There are different types of OLEDs classified below according to their light luminescence properties.
2.1 Types of OLED
There are several types of OLEDs which are used for a different purpose in day to day life. These are classified as, passive matrix, active matrix, transparent, top-emitting, bottom emitting, foldable, white, and phosphorescent OLEDs as shown in Figure 1 .
2.1.1 Passive-matrix OLED (PMOLED)
PMOLEDs include a cathode, organic layers, and anode. The anode and cathode are arranged in the way they are parallel to each other. Light is emitted by the pixel due to the intersections of the cathode and anode. External applied voltage helpful for the specific anode and cathode strips, for specifying which pixels light up and which stay dark. Once again, the light of each pixel is equal to the amount of current added. PMOLEDs are simple to produce, but they use more power than other forms of OLEDs, owing primarily to the power necessary for the external circuitry. PMOLEDs are ideally suited for small displays and are most useful for text and icons.
2.1.2 Active-matrix OLED (AMOLED)
AMOLEDs include complete layers of a cathode, organic molecules, and anode, but the anode layer is overlaid by a matrix of thin-film transistors (TFTs). The circuitry that decides which pixels are switched on to form an image is the TFT array itself. Since the TFT array consumes less power than other alien electronics. AMOLEDs absorb less power than PMOLEDs, making them ideal for large displays. AMOLEDs also have higher refresh times, making them perfect for the film. Computer displays, large-screen TVs, and electronic signage or billboards are the perfect applications for AMOLEDs.
2.1.3 Transparent OLED (TOLED)
Transparent OLEDs contain translucent components (substrate, cathode, and anode) only, and can be up to 85% transparent when switched off from their substrate. The light will continue in both directions when a clear OLED monitor is enabled. Transparent OLED displays may be either active- or passive-matrix displays. This machine can be used for heads-up displays.
2.1.4 Top-emitting OLED (TEOLED)
The substrates of such OLEDs may be opaque or translucent. Since they are conveniently combined with a non-transparent transistor backplane, they are favored for active-matrix applications. Top-emitting OLED used in smart cards by manufacturers.
2.1.5 Bottom-emitting OLED (BEOLED)
In this type of OLED organic materials are used. BEOLED contains transparent glass, TFT, ITO, an emission layer, and a cathode. Light emits from the bottom of the devices. Manufacture use bottom emitting OLED in smaller as well as larger displays.
2.1.6 Foldable OLED (FOLED)
The substrates of FOLEDs are made of lightweight plastics or metallic foils. They have the benefits of being flexible, durable, and lightweight. Since the material is strong, corrosion and breakage are reduced, so it is used in GPS cameras, cellular phones, and big curved TVs. FOLEDs have many advantages, including higher image resolution and quicker response time. It has implementations in smartphones, GPS receivers, and OLED displays.
2.1.7 White OLED (WOLED)
White OLEDs provide the true color of incandescent lamps that produce brighter white light than fluorescent lights and bulbs that are standard and energization effective. They substitute fluorescent lamps which can reduce energy costs for lighting because they are manufactured in large sheets, are cost-effective, and use less electricity. It fits well for car lighting. White OLEDs are small and light, allowing automobiles to be more lightweight and powerful.
2.1.8 Phosphorescent OLED (PHOLED)
Heat production is minimized by using PHOLEDs. As a result, we will find it in a broad-sized OLED TV or lamps. PHOLEDs can significantly reduce temperature because they are energy-efficient. It also removes the amount of air conditioning needed to remove the heat produced and makes it a key component of a green or environment building strategy. PHOLEDs are used in computer displays, TV screens, and light tables.
2.2 Structure and mechanism of OLEDs
OLEDs operate similarly to traditional diodes and LEDs, but instead of using layers of n-type and p-type semiconductors to contain electrons and holes, they use organic molecules. Eight separate layers consist of an advanced OLED . There are protective glass or acrylic layers on the top and bottom. The upper layer is the seal and the underlying substratum layer. There are a negative terminal and a positive terminal within these layers (called cathode and anode). Finally, two layers between the anode and cathode, the emissive layer (where light is produced). In between the emissive layer and anode, there are two layers to control and ejection of the hole called the hole injection layer and hole transport layer. Similarly, there also two layers in between the emissive layer and cathode called the electron transport layer and blocking layer . Figure 2 describes the structure and mechanism of OLEDs.
We simply connect a voltage (potential difference) between the anode and cathode to make an OLED light up. The cathode absorbs electrons from the power supply while the energy flows, while the anode loses them (or it “receives holes,” if you prefer to look at it that way). An electron from the cathode moves towards the emissive layer through the electron transport layer. At the same time holes moves towards the emissive layer through-hole transport layer. The blocking layer and hole injection layer are used to control the electron and holes. At an emissive layer both electron and hole combined. As a hole (a lack of electron) collides with an electron, the two cancel each other out and emit a fleeting burst of energy in the form of a photon . This is known as recombination, and since it occurs hundreds of times per second, the OLED emits constant light as long as the current flows. Overall, OLED goes through four fundamental steps ,
Electron and hole injection at the electrodes.
Charge carriers are transported across the organic layers.
The shape of electron–hole bound pairs (excitons).
Radiative exciton decay and light emission.
3. OLED materials
OLEDs are constantly developing and emphasizing the existence of customized functions of organic materials that can be added to well-preserved thin films. As a result, the materials’ specifications are diverse, ranging from processability and film formulation to electrical transport, emission, and optical properties . The availability of effective and reliable light emitters over the entire visible spectral spectrum is more important. In this regard, the distinction between fluorescent and phosphorescent materials must be made. The arrival and further creation of emitters based on heavy-metal oriented metal–organic complexes, as seen in Figure 3, was a watershed moment. These materials are used in different layers of OLED to enhance properties and efficiency. Powerful spin-orbit coupling in these compounds combines singlet and triplet states even more than in pure hydrocarbons, allowing phosphorescence to become a permitted transfer. Meanwhile, promising efficiency data for OLEDs based on these materials have been published; however, the bottleneck remains the limited supply and stability of deep-blue phosphorescent emitters.
As there are different layers in OLEDs, we need different types of materials for enhancing the properties of OLEDs. The most important layer of the OLEDs is an emissive layer, where recombination takes place and light emits in a specific direction according to device manufacturing.
4. Emissive materials
As seen in Figure 4, OLED emissive materials are classified into two groups: small molecules and polymers . Subgroups of the polymer group are known as non-conjugated or conjugated. Small molecule and polymer groups may use dendritic compounds as intermediate materials. Organic material-based electroluminescent (EL) systems have excellent low-power driving voltage and bright emission properties . Tiny molecular organic compounds, conjugated oligomers, and polymers with precise chain lengths are examples of organic light-emitting materials that have distinct electrical and optical properties. The interpretation of the operating process in organic EL devices is dependent on the carrier mobility of organic materials. Conducting polymers must have high conductivity, strong solubility, and mechanical properties. They must also be resistant to acids and bases. Dendrimers are a new kind of polymer that can be used in OLEDs. Bernanose et al. made the first discoveries of electroluminescence of organic materials in the early 1950s .
4.1 Small molecules
These are materials that are crystalline or semi-crystalline and have high aqueous solubility. Tang et al. were the first to create efficient OLEDs using small molecules . Small molecules have advantages like simple synthesis and purification, and the techniques of vapor deposition allow for the production of complex, high-performance multi-layered layers.
4.2 Polymer used in OLED
The intrinsic versatility of the displays is determined by the material used. Polymers, thin metal tubes, and glass are examples of ductile materials that have been used. Conducting polymers, on the other hand, are more compact, lighter, and less costly. Polymers are widely used material forms for OLEDs because of these advantages .
In 1989, a team from Cambridge University discovered electroluminescence (EL) in a conjugated polymer . The system had a very short lifespan of a few minutes and only had a weak emission of 0.1 percent outward quantum efficiency (EQE). After this discovery, the vigorous development of polymer OLED materials and the optimization of system design has started with Sumitomo Chemical Co. Ltd., Covion in Germany, Dow Chemicals in the United States, and Cam-bridge Display Technology (CDT) in the UK. Strong EQE of 51% and a long operational lifespan of many tens of thousands of hours have been reached as the culmination of over 20 years of R&D .
Polymer OLEDs, like small-molecule OLEDs, have the following characteristics [15, 21]: (a) high contrast ratio (luminance-on/−off), (b) large viewing angle, (c) bright colors, (d) slim devices, (e) high-speed image switching, and (f) low power consumption. The applicability of a cost-effective fabrication method in mass manufacturing is a noteworthy characteristic of polymer OLEDs.
The color of light emitted by polymers is highly influenced by the form of polymer, its chemical structure, and the existence of the side groups. Thus, a series of soluble luminous polymers that emit from 400 nm to 800 nm across the entire VIS spectrum could be made available by chemical modifications to the polymer structure. A fascinating aspect that affects the colors of light-emitting polymers is the use of emissions additive, also known as dyes. If a small volume of an appropriate dye is applied to a polymer, the energy can be transmitted by light absorption from the dye from the polymer to the dye. Different dyes may be used to adjust the device’s color. A blue polymer containing a green dye will emit green light, while a blue polymer containing a red dye will emit red light. When choosing a material for a device, the glass transition temperature (Tg) of the polymer materials is critical. The study of several organic materials as active components is motivated by the need to refine the device’s characteristics. Burroughes et al. published a high-quality green light-emitting polymer-based system using poly(p-phenylene vinylene) in 1990, bringing polymer electroluminescence research to a close.
4.2.1 Conducting polymer
In 1976, conductive polymers were discovered. Shirakawa inadvertently produced the first conducting polymer polyacetylene capable of conducting electricity in the mid-1970s. While it was not stable in air, the fact that it could become conductive due to doping has prompted further studies into other recognized conjugated polymers. Many experiments have been conducted on conductive polymers such as Polypyrrole, polythiophene, and polyaniline since 1976 . The Nobel chemistry prize was awarded to MacDiarmid, Shirakawa, and Heeger in 2000 for the discovery and advancement of conducting polymers, demonstrating the significance of driving polymers .
Conductive polymers have a backbone that is π-conjugated (alternating single and double bonds), allowing overlapping of bound electrons in the polymeric chain . Through incorporating various electron releasing/withdrawing functional groups into the polymeric backbone and managing the electron–hole injecting/transporting ability of the synthesized conducting polymer, and the conductivity of the polymers can be effectively tuned to achieve emission in the desired luminance range . The creation of charge carriers induces an increase in conductivity. The mechanism of conduction in these polymers is described by a quasi-one-dimensional system and bandgap model. The basic self-localized nonlinear excitation, i.e. a quasi-one-dimensional structure is defined by solitons, polarons, and bipolarons [19, 26]. There are different types of conducting polymer used in OLED shown in Figure 5.
4.2.2 Synthesis of conducting polymer
Many researchers have reported different ways for the synthesis of conducting polymer. The structure and properties of the polymer depend on the synthesis process. There are many processes used to increase efficiency, few of them are tabulated in Table 1.
|Polymer||Synthesis process||Polymer morphology||Application||Refs.|
|Polyaniline||1. Chemical oxidation|
2. Nanotubes and nanowires
|ETL in OLED, FED, OLED||[24, 28]|
|Polypyrrole||1. Electrochemical||1. Nanotubes and nanowires||FED, OLED, solar cell|||
|Polyacetylene||RH catalyst||—||Sensor, diode, catalyst.|||
|Polycarbazole||1. Chemical synthesis,|
2. In-situ polymerization
|PH sensor, OLEDs||[31, 32, 33]|
|Polythiophene||1. Solvothermal||—||OLED, Supercapacitor|||
|Polyfurane||1. Suzuki coupling reaction||OLED, HTL,|||
|Polyazulene||1. Oxidizing||1. spherical nanoparticle||OLED, electrochemical,|||
|PEDOT||1. Electrochemical||1. Nanotubes and nanowires||FED, HTL, LED, Diode||[24, 37, 38, 39]|
|2. In-situ polymerization||2. Nanoflowers|
|3. emulsion droplet electochemisty||3.Nanoparticles|
|4. template-free solution method||4.Nanofibers|
|1. via witting reaction|
2. soluble precursor route
3. electrochemical routes
|LEDs, laser, optocouples, triodes, photodiodes, photodetectors.|||
A light-emitting center is typically connected to one or more branched dendrons in light-emitting dendrimers. Surface groups are bound to the distal end of the dendrons to provide the solubilities needed for solution processing. The center (light emission), branching groups (charge transport), and surface groups can all be modified independently to the dendritic structure (processing properties) tabulated in Table 2.
|Structural control||high||low||Very high|
|Synthesis||Solution technique||polymerization||Stepwise growth|
|Crystallinity||Semi crystalline/crystalline||Semi crystalline/crystalline||Non crystalline/ amorphous|
|Glass transition temperature||high||low|
|Viscosity||Linear relation with a molecular weight||Linear relation with a molecular weight||A nonlinear relationship with a molecular weight|
5. Efficiency of OLED
The effectiveness of an OLED depends on a variety of variables including the energy efficiency, the voltage it works with, the recombining efficiency, the number of photons that are released in the photons consumed, and the degree of charge carrier equilibrium injection, etc. Device efficiency can be enhanced by using highly filtered organic complexes to monitor the thickness of the substrate, proper HIL, HTL, ETL, and device structure selection (the process for the co-deposition of the suitable host into organic emission material) . In organic thin-film light-emitting systems, energy transfer from a conductive host to a luminescent dopant can result in high external quantum efficiencies.
5.1 Emission efficiency
The efficiency of OLED emissions can be computed with,
Where is electroluminescence quantum efficiency, is carrier balance of electrons and holes, is recombination rate, is photoluminescence efficiency and Q is quenching factor by the cathode. Higher photoluminescence and recombination rate, well-controlled electrons and holes injections, and cathode calming removal help to increase OLED efficiency according to the equation.
5.2 Light extraction efficiency
In glass or plastic substratum, the majority of light produced by the OLED will be caught and directed by the waves to the sides; normal, this is about 80% of the total. Current research activities are aimed at different surface treatments that can improve the extraction performance significantly.
5.3 Power efficiency
Power efficiency is one of the amounts that decide how long a mobile unit lives. The efficiency of power is calculated per watt in lumens. The reliability of the power is not influenced only by the device’s quantum efficiency, but also by its voltage . It is therefore critical that low voltages of 3–6 V compared to the load injection barrier be obtained. To provide battery compatibility, this prevents costly voltage up converters.
5.4 Recombination efficiency
Recombination performance normally occurs at or close to unity; i.e., as two charges come close to each other, they are guaranteed to destroy each other. However, if the destruction mechanism does not have energetic obstacles, then quantum spin figures show that, if the state has a single multiplicity, only 25% of the resulting excited states are useful. Though spin emissions in organic molecules are prohibited, progress was made in the development of three-state emitters which contain at least one atom with higher atomic weight. The excited state still will decay not radioactively, even though spin is permitted.
5.5 Luminous efficiency
Three parameters reflect OLED’s luminous performance: quantum efficiency, lighting quality and efficiency. Quantum performance can be split into internal quantum performance and external quantum performance . The external quantum efficiency ηext corresponds to the proportion of photons (NP) released by the OLED in a given direction to the amount of injected (NC) electrons. OLED is a multilayer structure and the waveguide effect is responsible for absorbs or loses light. The inner quantum efficiency ηint corresponds to the real device’s luminous effectiveness. The ratio of both reflects the ηC coefficient of optical coupling outside the apparatus in a certain direction.
5.6 Way to increase the efficiency
There are several ways to enhance the efficiency of OLED described below,
Efficiency of OLEDs is reduced since the light emission of undoped systems is only accountable in single states. Recent advances in the collection of the triplets using phosphorescent materials have resulted in increased performance and color selectivity. To produce the primary colors required for display applications, the electric phosphorescence achieved by doping organic metallic phosphorus in a host was successfully used .
The doping of the emissive layer of an OLED has been widely used to improve performance, durability, and color. Tang et al., used fluorescent dyes, 3-(2-benzothiazolyl)-7-diethylaminocoumarine (coumarin 540), and DCMs were first developed dopant in Alq3 in 1989 to increase system effectiveness and color purity .
Endothermic energy transfer from a molecular organic host (donor) to organometallic phosphors (trap) can lead to high-efficiency electron lighting.
One of the most important factors restricting the external quantic power of devices is the low extraction of light and hence improved coupling methods for improved efficiency. In wave directed modes, almost 80% of the light provided by the OLED is lost in the radiation optics because of glass substrates and ITO/organism content, i.e. the majority of light produced is either stuck in or out of the edges of an OLED in the glass substratum or the system . Various light refracting and dispersal approaches to reducing TIR at the interface have been identified to remove the trapped and wave-driven light into external modes, like using the shaped substitute, micro-lenses used on the backside of a substratum are used as a spreading medium and high-refractive-index substrate form the silica micro-sheet . The other method is to inject a very low refractive index of silica aerogels between the glass substrates and the translucent ITO electrode.
Organic materials, such as OLEDs, have a broad range of uses in electronics due to several advantages, including :
Incorporation of versatility by nature-organic synthetic processing has limitless precision in terms of molecule packing and macroscopic properties, (ii) solid-state system that is very small, (iii) lightweight- unlike glass displays in LCD applications, (iv) the substrates are shattered resistant, (v) luminous power efficiency is very high. Other than this there are more benefits described below,
An element of inactive OLED does not generate light or consume fuel to make true blacks. With a quick response time, you can create amusing animations. For the quickest color transfer, LCDs can achieve reaction times as low as 1 ms. Response times for OLEDs are 1000 times higher than 10 microsecond LCD response times. The viewing angle is wide. - OLEDs have a larger viewing range than LCDs because the pixels in OLEDs transmit light directly. The colors tend to be right. As it is self-emitting, there is no need for a backlight source. Color correction for full-color screens. Flexibility – OLED displays are made on flexible plastic substrates, which generate flexible organic LEDs. Cost benefits over inorganic systems- OLEDs are less expensive than LCD or plasma screens. Power use is minimal.
OLEDs use much less power than LCDs because they do not need backlighting. OLEDs are less expensive to make, and since they are made of plastic, they can be made into wide and thin sheets. Video files are more realistic and up-to-date as OLEDs reload quicker than LCDs.
6.1 Current status and future OLED applications
OLEDs are currently used in small-screen applications such as mobile phones, digital cameras, and PDAs. Sony Corporation revealed in September 2004 that it would begin mass production of OLED displays for its CLIE PEG-VZ90 brand of personal entertainment handhelds. In March 2003, Kodak was the first to introduce a compact camera with an OLED view, the Easy Share LS633 [source: Kodak press release]. Several firms have already developed prototype OLED computer displays and large-screen TVs. Samsung Electronics revealed in May 2005 that it had created a prototype 40-inch OLED-based ultra-slim TV, the first of its kind [source: Kanellos]. Also, Sony revealed in October 2007 that it would be the first to market for an OLED television. Customers in Japan will be able to buy the XEL-1 in December 2007. It is priced at 200,000 Yen (approximately $1,700 USD, nearly 1.4 lakh in INR) [source: Sony].
OLEDs have a wide range of applications that we see in our everyday lives. To create digital displays for televisions, mobile phones, PDAs, monitors, car radios, and digital cameras. OLEDs have a wide range of applications in lighting, such as the LUMIBLADE OLED samples developed by Philips. Similarly, in 2011, novaled AG, based in Germany, released the Victory OLED desk lamps. It’s used in watches. A fossil (JR-9465) and Diesel (DZ7086) were used for OLED displays. MCC of Mitsubishi Chemical Holdings in 2014 developed a 30,000-hour OLED panel which is twice the lifetime of conventional OLED panels. OLEDs have taken the place of CRTs (Cathode Ray Tubes) or LCDs (Liquid Crystal Display). Based on a white emitter, Samsung electronics produced full-color AMOLED displays. Top-emission systems provide advantages in the manufacture of TFT-OLED displays.
OLED technology and development are advancing steadily, which may lead to potential developments of heads-up displays, vehicle dashboards, billboard-type displays, home and office lighting, and modular displays . As the rate of OLEDs is almost 1,000 times higher than that of LCDs, an OLED device can almost change data in real-time. More detailed videos and daily revisions should be made. The newspaper of the future could be an OLED show that updates with breaking news (think “Minority Report”) – then, like a normal newspaper, you could fold it up and stuff it in your bag or briefcase when you are finished reading it.
Organic light-emitting diodes, which are more energy consuming, allow computer display more comfortable. OLED is so groundbreaking that it is being celebrated as the first invention since Edison in the field of illumination. Today, OLED technology is commonly considered to be the next-generation flat panel display component and will play an important role in the development of flexible displays. They are more compact and thinner than the crystalline layers in an LED or LCD. They have wide fields of view and they generate their light. We can improve efficiency by taking some precautions and by using appropriate polymers.
This chapter has seen the different types of OLEDs that have been implemented to date in order to increase their functionality and serve various purposes. The operation and mechanisms of oleds are discussed. The output of OLEDs is dependent on their layers, with the emissive layer playing a critical role. Various formulations are used for the emissive coating, depending on the material’s properties. A number of factors must influence lead performance, which can be improved with some tweaking. At the moment, OLEDs are gaining popularity for the future transforming age, and many more studies are being conducted to achieve remarkable performance.
Applause for all necessary assistance, grateful to IIT (ISM), Dhanbad. My friend and family are very grateful for helping me to write this chapter in this pandemic era.
Conflict of interest
“The authors declare no conflict of interest.”