While the display specs are fine, what is raising concern in a lot of people’s minds is the fact that in a dark environment, when the brightness of the phone is set to below the 10-15% mark and there is a grey background on the screen, some areas of the display appear green rather than showing the actual color which is grey. This only happens at low brightness levels so if the brightness is increased or the background is of a different hue, this tinting effect disappears and colors look normal. In a practical scenario, the abovementioned conditions to replicate this green tint on the display occurs rarely and is not very obvious unless one actually goes looking for it. In about two weeks of using the OnePlus Nord, we did not encounter the tinting on the screen even while using the phone in a room with all the lights turned off. It’s just when we saw reports on social media that we tried to replicate it and were able to spot it upon close inspection. Now, while this should be a non-issue for most users, a valid argument is that everyone wants a perfect smartphone when they are paying a good amount of money for it. Nobody wants a phone with a defective display or one that has issues. But the question here is, is it even an issue? We tried to dig deeper into the manufacturing process of OLED displays and even further down to individual LEDs and we thought of documenting our findings to explain the tinting phenomenon. It is worth mentioning that a few concepts that we will be discussing hereon require some basic understanding of semiconductors and the way they work. We will try breaking it down to the basics for better understanding.
Working of Semiconductors
Let us begin by first understanding semiconductors and their basic properties. Semiconductors, as the name suggests are materials that are neither fully conductive nor are they complete insulators. Semiconducting materials like Silicon and Germanium behave like insulators under normal conditions but when subjected to thermal energy, which basically means when the temperature of the materials is increased, they start to exhibit conductive properties. The reason for the conductive nature of these materials at high temperatures is because of charged particles which are referred to as electrons and holes. Electrons carry a negative charge while holes are essentially voids that carry a positive charge. Now, if you still remember some high-school chemistry, every element in the periodic table has an atomic number. For an uncharged atom, the atomic number also signifies the number of electrons that the atom possesses. Silicon, for example, has an atomic number 14 which signifies that in one Silicon atom, there are 14 electrons. These electrons reside in circular orbits around the center (nucleus) of the atom. There are multiple orbits around the nucleus since every orbit (band) can only house a fixed number of electrons. The first band can house two, the following bands can house eight each. In the example that we considered, where Silicon has 14 electrons, two of them occupy the first band followed by the next eight that occupy the second band and the remaining four occupy the final band. We are interested only in the final band which is termed as the valence band and the electrons residing in the valence band are known as valence electrons. When heat is applied to a semiconductor, the electrons in the valence band get ‘excited’ which means they are free to move and no longer are bound by the force of the nucleus. Due to the heat energy and the fact that they are now free to move, the electrons in the valence band jump to something known as the conduction band. This movement of electrons from the valence band to the conduction band is what causes semiconductors to be conductive. Pure semiconductors, more commonly known as intrinsic semiconductors, however, are not as conductive by themselves and cannot be used for electronic purposes. Hence, they undergo a process called doping which turns them into extrinsic semiconductors. Doping essentially means adding impurities to the semiconductor in order to make it more conductive. The way to make a material more conductive is to add more charged particles i.e. by adding more free electrons or holes. This further gives rise to two types of semiconductors – n-type semiconductors where there are excess electrons and p-type semiconductors with excess holes. N-type semiconductors are doped using elements like Phosphorous, Arsenic, Antimony, etc. P-type semiconductors are doped with elements like Boron, Aluminum, Gallium, etc. These pre-requisites should be sufficient to understand further concepts that we will be discussing.
Diodes
A diode is a semiconductor device that is used to restrict the flow of current in one particular direction while allowing the flow of current in the opposite direction. The reason we are trying to understand the working of a diode is that LEDs are basically Light Emitting Diodes. A diode is made up of a p-type semiconductor fused with an n-type semiconductor. This gives rise to a depletion region where a process called recombination takes place when voltage is supplied across the ends of the diode. In simple terms, electrons combine with holes to release energy. This energy released due to recombination is in the form of light (photons) in LEDs. Usually, LEDs are not made of Silicon. Instead, they use Gallium Nitride which is also a semiconductor. OLEDs use an organic compound to produce light, but the basic working principle is the same.
Color Reproduction in an LED
If you are wondering why we explained so much in detail about the working of a semiconductor, you will need it to understand how LEDs produce different colors. Now, there are two ways in which this is done. Displays consist of pixels that produce light and hence multiple pixels contribute to producing a complete picture. A pixel also has sub-pixels that individually produce different colors. These sub-pixels can be organized in different patterns with the most common one being RGGB. A Red LED, two green LEDs, and a Blue LED. First, let us look at how these individual LEDs in a pixel produce color.
There are two variables to consider here – The dopant being used in order to dope the semiconductor and also the bandgap of the semiconductor which is the distance between the valence band and the conduction band. These two factors decide the color of an LED. For example, if the band gap is small, the resultant LED may glow red. If the bandgap is large, the resultant LED may glow green. Basically, different bandgaps release different energies.
Varying Voltage – First Method
In order for these LEDs to emit different colored lights, they need to be supplied with some voltage. This voltage is provided by the battery on a phone which would be regulated through a dedicated circuit. It’s also important to note that the intensity of every single LED is directly proportional to the voltage supplied to it. If the voltage supplied is high, the LED will emit higher intensity of light, and that is how the brightness slider on your phone works. Coming back to the green tint on the OnePlus Nord, it is possible that when the brightness slider is got to the minimum value, the voltage that is being supplied to some green sub-pixels (LEDs) is not being reduced proportionally in some areas which may lead to higher intensity of green light in those specific areas of the display. However, it does not stop at just this.
Colour Masking/Shadow Mask Patterning – Second Method
There is another method of allowing OLEDs to display color and this is by using a process known as shadow mask patterning. This method involves depositing RGB emitting layers on each white pixel. The white light produced by the pixel is then filtered by the RGB deposit based on whichever color is supposed to be displayed on the screen. The way this is done is by arranging Red, Green, and Blue layers that emit light in each pixel of the OLED display. Like how we mentioned earlier about LEDs being arranged as sub-pixels inside a pixel in a pattern, similarly these light-emitting layers are also arranged in a particular pattern, for example, RBG. Which means each sub-pixel has an individual color.
Why does the display tint occur?
During this process is when the fault occurs which leads to the green tint on the OnePlus Nord’s display. These colored layers are deposited on the LEDs using a stencil referred to as a color mask. If the mask is disturbed or not placed accurately during deposition, there can be an error in the spacing of the color deposits which causes a non-uniform color output on the display as you can see from the image.
This need not be just green. There are cases where some phones, namely the ROG phone 2 from last year had a pinkish tint on the display. Moreover, there are cases where tinting is observed even on OLED TVs.
Is it really an issue?
Going back to the original question, is this really an issue? Smartphone manufacturers source their display panels from different vendors. Since these vendors manufacture displays on a very large scale, these faults that we talked about are regular and not easy to avoid. Manufacturing of OLED displays is a complex process and requires a lot of precision. If you ask why devices from Samsung or Apple or others do not have display tints, it’s probably because the manufacturing process used in those OLED panels are either different (there are other ways of manufacturing OLED displays too like Color Filtering or using Electron Beams) or the method being used is more precise which cancels out any human error. Since the display tint occurs during manufacturing itself, it essentially becomes a characteristic of the panel. With millions of displays being manufactured by a single vendor, it’s just not feasible to discard panels with such minor faults that otherwise perform normally. Hence, these displays also pass the QC test as one would hardly notice the tint in regular scenarios.
Should you get the OnePlus Nord in spite of the display tint?
If your OCD is going to get triggered upon spotting the green tint once in a while, while using the OnePlus Nord, this might seem like an issue to you. For everyone else, the green tint is not visible while using the phone regularly on a day to basis or while consuming content on the display so this should not be a deal-breaker. If you are lucky, your unit of the OnePlus Nord may not even have a tint if the display was manufactured with precision. Either way, we hope the entire green tint scenario is now clearer to you and you know the actual reason why it occurs. It’s not a problem per se, it’s just a bi-product of the complex manufacturing process.
