The development of organic LED (OLED) technology resulted from the need for thinner, brighter, faster, and more efficient display technologies. OLED technology is used mainly for large display screens. PMOLED, a type of OLED, was later developed to be used in smaller displays where most of the features of OLED are not needed.
What is PMOLED
OLED technology is based on a concept known as electroluminescence. Besides, OLED is also known as Organic Electroluminescence (OEL). Essentially, electroluminescence is a tested and proven concept where organic materials emit light when they are acted upon by an electric current.
PMOLED is based on the electroluminescence concept. It stands for passive matrix OLED in full. The ‘passive matrix’ part stands for the arrangement and addressing of the individual pixels that make up a PMOLED display.
There are a few core concepts that you need to grasp in order to understand the basis of PMOLED technology. They include;
- Pixel arrangement and addressing
- Light and color emission layers
- Use case requirements
- The passive matrix technology
For the definition, let us focus on pixel arrangement and addressing. We will discuss the other concepts when looking at how the PMOLED works.
Pixel Arrangement and Addressing
In PMOLED, the passive matrix defines the pixel arrangement and addressing. As a display technology, PMOLED has a specific scheme that controls the individual pixels on the display panel. Specifically, the pixels are arranged in a matrix pattern. This particular layout is what dictates the properties and applications of PMOLED technology.
PMOLED displays are made using organic LED semiconductors. These semiconductors are activated through the electroluminescence process we talked about earlier. Subsequently, the LEDs emit light which leads to the formation of an image on the display.
Pixel arrangement patterns affect the pixel density, picture quality, and encapsulation processes. In PMOLED, the pixels are arranged in a square grid matrix. This means that pixels are arranged in a simple grid adjacent to each other.
The pixel density of the square grid varies depending on the overall resolution of the display. Most PMOLED displays have low resolution since it is rarely a feature that affects the application and use cases of this display technology.
The other aspect that describes PMOLED displays is the addressing approach used. This defines the scheme used in pixel control. For PMOLED displays, the addressing method used is passive matrix addressing.
In these types of displays, pixels are activated individually rather than simultaneously. This happens by allowing electric current to flow in a specific row or column which contains the pixels that need to be activated. Due to the matrix pattern created by the pixel arrangement, the LED pixels light up to create patterns on the display.
This is why you will find most PMOLED-based displays have a pixelated pattern with high contrast between dark and bright regions.
Clearly, the pixel arrangement and addressing approaches used in PMOLED displays have the biggest impact on the display. Not only do they stipulate the type of content that can be displayed, but they also inform the display features.
How PMOLED Works
The mechanism of PMOLED technology is highly influenced by pixel arrangement and addressing. Apart from that, the color and light emission technology is another aspect that informs the mechanism.
In OLED technology, the electroluminescence process takes place in sequence. The display is made up of materials that are embedded between positively and negatively charged electrodes. In other words, these are electric current conductors that allow the voltage to pass through them.
Electrons from the electric current flow to the negative and positive cathodes to initiate the electroluminescence process. Through a process known as electron excitation, the electrons move to a more energetic state.
The pixels in the PMOLED display have emissive layers where the excited electrons land and produce light through the electroluminescence process. This light varies depending on the type of display, the manufacturer, and the color reproduction of the semiconductor organic materials making up the LED.
Generally, the emissive layer of the PMOLED display is made up of organic materials. They include small molecules like Perylene and Naphthalene derivatives and polymers. The organic material used is mostly responsible for the color of the light emitted.
The Passive Matrix Technology
This technology encompasses the approaches, methods, techniques, and components used in the arrangement and control of pixels in a PMOLED display.
Pixel control is what enables you to perceive light, in this case, images, from a display screen. As such, the matrix arrangement applied to PMOLED displays has an effect on the image frame construction on a display. This construction influences the refresh rate and pixelation of the displayed content.
The pixels of a PMOLED display do not have thin-film transistors (TFTs). These transistors are primarily found in active matrix OLED (AMOLED) displays. They are responsible for the addressing technique in those types of displays. You will learn more about this in another section where we will compare AMOLED and PMOLED technologies.
Features of PMOLED Display Technology
It is intriguing to think about what this type of technology archives. Since it is mostly used for niche applications, its features are sufficient and appropriate. Let us look at some of the major features of PMOLED.
High Contrast Displays
Knowingly or unknowingly, you might have interacted with a PMOLED display. Most wearable devices utilize this technology for their high contrast.
Contrast is defined as the ratio of illumination levels of the brightest white and darkest black on a display screen. High contrast means that there is a clear distinction between these two colors. In other words, there is a huge contrast ratio.
There are many reasons for this occurrence. One, OLEDs inherently have the ability to control their own color, hence the contrast. Secondly and most importantly, the main reason for high contrast is the passive matrix addressing technique.
To reiterate, the individual pixel control in the passive addressing ensures that pixels are lit depending on the activation pattern. This pattern is based on the type of information retained in the content. This is why you will find some regions in a display are completely dark because the pixels in that region are unlit.
Ultra-thin Designs
OLED displays are known to be super thin. The lack of backlighting eliminates the need for a color filter hence resulting in fewer layers of the entire display. Additionally, PMOLED pixels are encapsulated on thin and flexible substrates.
Essentially, most of the layers that make up a PMOLED display are usually super thin and flexible. This is because this display technology is usually simple to manufacture. Also, the lack of heavy equipment in the construction leads to a lighter, hence a thinner design.
Low power consumption
There are many factors that influence the power consumption of a display. Luminous intensity and brightness are top of the list. PMOLED displays generally emit less light per square meter (Candela). Like the high contrast feature, low brightness occurs due to the passive matrix addressing mechanism.
Other factors like the size of the display also contribute to low power consumption. The PMOLED technology is mostly used in small-scale display manufacturing. Devices that utilize these displays do not require a lot of energy to light up.
Light scattering and spillage are limited in PMOLED displays. This is because of the lack of backlight and filter layers in the entire panel. As you can imagine this scattering and spillage would contribute to high power consumption.
Narrow viewing angles
When looked at head-on, the vibrance, brightness, and color accuracy of PMOLED displays are usually perfect. However, as the position of the eye relative to the direction of the display changes, these distortions start to emerge.
The distortions lead to reduced perceived brightness, color accuracy, and vibrance of the displayed content. In other words, the viewing angles start to narrow. Looking on the bright side, PMOLED technology is mainly used for one-person devices. Therefore, the issue of narrow viewing angles would not be a major deterrence.
Why PMOLED instead of AMOLED
Pixel arrangement and addressing is the primary basis that is used to compare PMOLED and AMOLED displays. We explained that PMOLED uses a passive matrix addressing an arrangement method.
AMOLED displays use an active matrix display addressing and layout technique. Each pixel making up the AMOLED display has thin-film transistors (TFTs) that control its addressing approach. The TFTs act like switches for the pixels to control their brightness and response rates. Therefore, AMOLEDs have high refresh rates and brightness.
Evidently, AMOLED displays seem much more capable than PMOLED. That begs the question, why not do away with PMOLED completely? The following points might attempt to answer these questions.
Specific Use Cases
The main reason you would consider PMOLED over AMOLED is if your application requires a passive matrix arrangement and addressing scheme. There are several devices in the market that would hugely benefit from this technology. Benefit in the sense that the use case requires this specific modulation to function.
Performance Specifications
PMOLED has rather subpar specifications. However, when you consider certain functionalities, you will notice that these specifications are sufficient. For instance, a wearable that is meant for sports tracking does not require a lot of features.
If anything, there are some use cases that strictly require PMOLED displays. Apart from being the perfect solution for these use cases, they also augment the performance of the device. Consider a screen in a medical device that is used to display basic information like numerical values and statuses.
Cost-effectiveness
The simplicity and application scenarios of PMOLED make it a cost-effective display technology. Moreover, most of the devices that use PMOLED displays are low-budget, a factor that influences the cost of the display.
Advantages of PMOLED Displays
How advantageous is this display technology? By looking at its features, you would think this technology has limited benefits for you. However, you might be shocked to learn otherwise. Here are some of the common advantages of PMOLED displays.
Flexibility
The thin nature of PMOLED displays translates to an ability to conform to bending. In other words, they are flexible. This is quite an advantage since it accommodates various display designs. When a display is flexible, it does not limit you to flat screens. Due to this, PMOLED can be used to make curved displays.
Low Production Costs
Display technologies that are less costly to manufacture are quite advantageous. PMOLED is particularly inexpensive to produce for various reasons. To start with, most of these displays are made in small sizes. Secondly, the technology does not require a lot of labor or special components. These features contribute to fast and budget production processes.
Customization
Most display technologies are not easy to customize. They are limited due to inherent features of the technologies. However, PMOLED technology allows for custom modifications. Still, this depends on the display technology manufacturer and the intended use cases.
Touchscreen Functions
Being able to manufacture a cheap display and incorporate touch sensitivity has several advantages. Touchscreen is a feature that is quite helpful, especially in wearables and embedded displays. Therefore, PMOLED is at the forefront of revolutionary display technologies as far as advantages are concerned.
Disadvantages of PMOLED Displays
This technology has its downsides. It has weaknesses that limit its potential to niche use cases. However, in most of the niche use cases, these setbacks do not affect the overall performance of the devices. Some of the general disadvantages include;
Limited to Small Displays
While larger displays can be made from PMOLED, it makes more sense to manufacture smaller, one-person displays. As with everything else, it all comes back to the features that this technology entails.
Small displays mean anything less than ten inches in diagonal measurements. Such small displays are just right for the limitations of PMOLED technology. However, it is worth noting that not all small devices are PMOLED. Some device manufacturers use more advanced display technologies such as mini LEDs for their displays.
Low-resolution Displays
The pixel density of PMOLED displays is not as high as its counterparts. This is because of the pixel arrangement which ensures there are visible pixels when the display is lit. Also, the addressing techniques limit the appearance of clear displays.
Slow response rates
This is utterly the main disadvantage of PMOLED displays. Slow response results from the lack of a dedicated on-and-off switching mechanism for the pixels. AMOLEDs tend to respond to inputs faster due to the TFTs in the display architecture.
Slow response leads to issues like poor motion handling. This is why PMOLED cannot be used for devices that need to display dynamic content continuously. However, this is not limiting since PMOLED is not primarily meant for such applications.
Limited color gamut
Color gamut is the range of colors within the visible light spectrum that can be reproduced by a device. When it is limited, it means that a specific device is unable to reproduce a wide range of colors. Such is the case for PMOLED displays.
The lack of color reproduction capabilities stems from pixel control limitations, among other factors. Another culprit is the low illumination of PMOLED displays. Color depends on the light intensity of a display. Therefore, the low intensity would limit the perception of colors, even if they are reproduced.
Low outdoor visibility
Most of the drawbacks we have discussed to this end, point to a limitation in brightness, especially in low-lit environments. During the day, the ambient lighting conditions in outdoor settings limit the visibility of PMOLED displays. This is a challenge since it renders the device bearing the display ineffective.
Comparison of PMOLED vs AMOLED
These display technologies have distinct features and specifications that make them stand out. As such, each has conditions where it fits best. However, making a comparison between them is a great way to visualize their differences. We can compare PMOLED and AMOLED based on a few aspects.
Some of the aspects that can help us distinguish them include; application, structure, lifespan, current consumption, color saturation, and resolution.
The applications of PMOLED displays are varied. Essentially, PMOLED will be used to manufacture the following;
- Consumer electronic displays
- Medical devices displays
- Automotive displays
- Industrial displays
The displays required in these devices are not only small but also do not require advanced display technologies for their functions.
AMOLED technology on the other hand is used in the manufacture of more premium and capable displays. These include TVs, smartphones, and other large devices.
We already saw that the basic structure of a PMOLED display is made of a passive matrix pattern of pixels. The structure of AMOLED has an active rather than passive arrangement pattern. These differences are seen in how the pixels are controlled.
PMOLED displays have shorter lifespans compared to AMOLEDs. For one, the pixel addressing seen in PMOLED can lead to pixel aging, which lowers the performance of the pixels over time. Also, the manufacturing processes of AMOLEDs are much more advanced to improve the lifespan of the display.
Electric current consumption is another worthy comparison point that is considered when comparing PMOLED and AMOLEDs. Again, this goes back to the addressing method used in both technologies. The addition of TFTs in AMOLEDs increases the voltage requirements for controlling individual pixels. Therefore, AMOLED needs more current than PMOLED.
Color saturation and resolution of the displays differ as well. We discussed color saturation when talking about the limitation of color gamut for PMOLED displays. AMOLEDs on the other hand have wide color gamuts. This enables them to display vibrant and life-like pictures consistently.
Conclusion
PMOLED remains one of the common types of OLED displays. As a display technology, it has its defining features, benefits, setbacks, and applications. From this article, we learn that PMOLED is as useful and relevant as more advanced counterparts such as AMOLED technologies. PMOLED will remain relevant until a better alternative replaces it.
