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Considering both efficiency and stability in perovskite-based lighting

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Considering both efficiency and stability in perovskite-based lighting

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Ever since the successful demonstration of blue light-emitting diodes (LEDs) by Shuji Nakamura, Hiroshi Amano, and Isamu Akasaki (which, by the way, led to the 2014 Nobel Prize in Physics), our world has never been the same. Our room lighting, streetlights, computers, TVs, phones, and smartwatches benefit immensely from this discovery. Today, LED technologies are developed by both academic and industry researchers alike.

While the majority of commercial LEDs are based on semiconductors known as III-V (“three-five”) materials (i.e., materials that stem from groups III and V from the periodic table), these materials can be expensive and difficult to produce. That’s where perovskites come in!

Perovskites, like III-V materials, are great at emitting light! They can lead to bright and sharp colors such as red, green, and blue. Perovskites also have the advantage that they are cheap to synthesize and their LED fabrication is much easier as compared to III-V. However, perovskite’s main disadvantage is that the stability of their light emission is poor. Some of the best perovskite LEDs can only reliably emit light on the scale of a few hundred hours.

Thus, in order to commercialize perovskite-based lighting, we need to consider both the efficiency and stability of perovskite LEDs. Previously, our group showed that by introducing some manganese ions into the perovskite, we can improve the performance of perovskite LEDs. Additionally, other scientists have discovered a class of additive known as phosphine oxides that can be introduced into the formation of the perovskite in an LED configuration. These scientists found that these additives can repair some of the defects that occur during the perovskite’s deposition.

In an effort to improve the efficiency and stability of perovskite LEDs, we employed both manganese dopants and a phosphine oxide additive known as tris(4-fluorophenyl)phosphine oxide (i.e., TFPPO)! Initially, we saw that by increasing the concentration of TFPPO in our perovskite, we yielded brighter green films. Awesome! We then looked to see what this meant in an LED configuration.

By carefully engineering the layers around the perovskite, we were then able to produce bright green light! To quantify our perovskite LED’s efficiency, we use the external quantum efficiency (EQE). The EQE is defined as the number of photons emitted by our LED divided by the number of electrons flowed into it. This would mean that an EQE of 100% would emit one photon for every electron that passes through the LED. However, realistic EQEs are lower due to not all the light making it out of the device. Coming back to the manganese doped perovskite LEDs we fabricated, we see that similarly to the brightness of the green films, the green LEDs increased in EQE as we increased the concentration of TFPPO. In fact, our best LED reached an EQE of 14.0% which is the highest reported EQE for a manganese doped perovskite LED and among the highest for perovskite LEDs that consist of transition metal dopants at this time.

Now, we look at characterizing the stability of our LEDs. To do this, we measure how the brightness evolves in time as we supply a constant electrical current to the LED. To fairly compare the lifetimes of our LEDs, we define an “ adjusted T50’ ” time which is the time it takes for our LEDs to reach half of their maximum brightness adjusted for the time it reached its maximum brightness. To give an example, if an LED reached half of its maximum brightness after 30 minutes but it took 10 minutes for the device to reach its maximum brightness, the adjusted T50’ time is 30 – 10 = 20 minutes. Our stability curves above show an interesting trend. The higher TFPPO-treated perovskite LEDs result in decreased stability (i.e., adjusted T50’ times) lifetimes! If we then plot the peak EQEs (i.e., efficiency) and adjusted T50’ times (i.e., stability) of our manganese doped perovskite LEDs across TFPPO concentration, we have a clear trade-off. How equally exciting and perplexing!

This trade-off was quite puzzling to us and so we looked to try to understand it by probing the evolution of device-level parameters such as the turn-on voltage (i.e., the voltage at which our LED begins to emit light), maximum brightness, and device resistance by scanning our LEDs as they degraded. We also characterized photophysical processes (i.e., both the absorption and emission of light) within our LEDs before and after they were operated. Essentially, we found that all device-level parameters worsen in a greater fashion for higher TFPPO-treated perovskite LEDs as compared to lower TFPPO-treated devices at identical operating conditions (i.e., flowing the same amount of electrical current for the same amount of time). Additionally, we uncovered that the electrons flowing in our devices start to behave in ways that will not generate light to a greater extent in higher TFPPO-treated perovskite LEDs as compared to lower TFPPO-treated devices under identical conditions. Further details can be found in the paper!

To tell a long story short, we found that while TFPPO treatment does initially improve the performance of our manganese doped perovskite LEDs, these enhancements are not robust under identical operating conditions. This explains why EQEs are high but adjusted T50’ times are low for increased TFPPO treatment! Therefore, before we can commercialize manganese doped perovskite LEDs treated with TFPPO, we must mitigate their stability degradation tendencies while retaining their efficiency boosting properties. Only by considering both the efficiency and stability of perovskite LEDs will we be able to truly commercialize this amazing LED technology!