Researchers have observed, in metals for the first time, transient excitons – the primary response of free electrons to light. Here, the researchers discovered that the surface electrons of silver crystals can maintain the excitonic state more than 100 times longer than for the bulk metal, enabling the excitons to be experimentally visualized by a newly developed multidimensional coherent spectroscopic technique (, "Transient excitons at metal surfaces"). 
An interferogram showing the photoelectron energy vs. delay time between identical femtosecond pump and probe pulses, which excite coherent three-photon photoemission at a single crystal silver surface. The interferogram is taken from a movie of photoelectron energy vs. momentum with one frame corresponding to a 50-attosecond delay. The oscillations in the intensity of photoelectron signal for emission normal to the surface show how long light is trapped in the form of excitonic polarization during the coherent nonlinear interaction with the silver surface. The Impact Detecting excitons in metals could provide clues on how light is converted into electrical and chemical energy in solar cells and plants. This research may also provide ways to alter the function of metals in order to develop active elements for technologies such as optical communications by controlling how light is reflected from a metal. Summary The act of looking in a mirror is an everyday experience, but the quantum mechanical description behind this familiar phenomenon is still unknown. When light reflects from a mirror, the light “shakes” the metal’s free electrons and the resulting acceleration of electrons creates a nearly perfect replica of the incident light – providing a reflection. Excitons, or particles of the light-matter interaction where light photons become temporarily entangled with electrons in molecules and semiconductors, are known to be important to this process and others such as photosynthesis and optical communications. Unfortunately, studying and proving how excitons function in metals is difficult because they are extremely short-lived, lasting for approximately 100 attoseconds, or less than a 0.1 quadrillionth of a second. For the first time researchers have observed excitons at metallic surfaces that maintain the excitonic state 100 times longer than in the bulk metal, enabling the excitons to be experimentally captured by a newly developed multidimensional multiphoton photoemission spectroscopic technique. This discovery sheds light on the primary excitonic response of solids which could allow quantum control of electrons in metals, semiconductors, and organic materials. It also potentially allows for the generation of intense femotosecond electron pulses that could increase resolution for time-resolved electron microscopes that follow the motion of individual atoms and molecules as they rearrange themselves during structural transitions or chemical reactions.
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An interferogram showing the photoelectron energy vs. delay time between identical femtosecond pump and probe pulses, which excite coherent three-photon photoemission at a single crystal silver surface. The interferogram is taken from a movie of photoelectron energy vs. momentum with one frame corresponding to a 50-attosecond delay. The oscillations in the intensity of photoelectron signal for emission normal to the surface show how long light is trapped in the form of excitonic polarization during the coherent nonlinear interaction with the silver surface. The Impact Detecting excitons in metals could provide clues on how light is converted into electrical and chemical energy in solar cells and plants. This research may also provide ways to alter the function of metals in order to develop active elements for technologies such as optical communications by controlling how light is reflected from a metal. Summary The act of looking in a mirror is an everyday experience, but the quantum mechanical description behind this familiar phenomenon is still unknown. When light reflects from a mirror, the light “shakes” the metal’s free electrons and the resulting acceleration of electrons creates a nearly perfect replica of the incident light – providing a reflection. Excitons, or particles of the light-matter interaction where light photons become temporarily entangled with electrons in molecules and semiconductors, are known to be important to this process and others such as photosynthesis and optical communications. Unfortunately, studying and proving how excitons function in metals is difficult because they are extremely short-lived, lasting for approximately 100 attoseconds, or less than a 0.1 quadrillionth of a second. For the first time researchers have observed excitons at metallic surfaces that maintain the excitonic state 100 times longer than in the bulk metal, enabling the excitons to be experimentally captured by a newly developed multidimensional multiphoton photoemission spectroscopic technique. This discovery sheds light on the primary excitonic response of solids which could allow quantum control of electrons in metals, semiconductors, and organic materials. It also potentially allows for the generation of intense femotosecond electron pulses that could increase resolution for time-resolved electron microscopes that follow the motion of individual atoms and molecules as they rearrange themselves during structural transitions or chemical reactions.
Atomic Force Microscopy (AFM) images show pinholes in the spiro-OMeTAD layer prepared by spin-coating (left) versus no pinholes when prepared by vacuum evaporation (right). The pinholes, identified by OIST’s Energy Materials and Surface Sciences Unit led by Prof. Yabing Qi, were described in the earlier this year ("Air-Exposure Induced Dopant Redistribution and Energy Level Shifts in Spin-Coated Spiro-MeOTAD Films"). The pinholes in the top layer of the solar cell, known as the hole transport layer, were identified as a key cause for the quick degradation of perovskite solar cells. Researchers around the world are investigating the potential of perovskite, a manmade organic-inorganic hybrid material, as an alternative to silicon-based solar cells. “Pinholes are a very critical problem because it's a pathway for moisture and oxygen to attack the perovskite material, which is the active layer converting sunlight to energy,” said Min-Cherl Jung, a staff scientist at OIST and first author of this work. “Without pinholes in the hole transport layer, the perovskite is protected and the lifetime improves.” The researchers eliminated the pinholes by using a different method to create the top layer of the solar cell, which is made of a material called spiro-OMeTAD. Instead of dissolving spiro-OMeTAD powder in a solution and then spin-coating it onto perovskite, they evaporated the powder in a vacuum chamber and the spiro-OMeTAD molecules deposited onto the solar cell. To create this layer, a solar cell is positioned upside down on the ceiling of a vacuum chamber. As the spiro-OMeTAD is heated up, it evaporates and the gas molecules that stick to the perovskite, creating an even layer -- much like when snow blankets the ground. Essentially, the spiro-OMeTAD molecules are snowing, but up rather than down. 
Many high-performance solar cells under development layer spiro-OMeTAD on top of perovskite, with other trace elements added to increase electrical conductivity. “Vacuum evaporation enables us to much more precisely control the deposition rate and thus the thickness of this layer,” Jung said. “We were able to reduce the thickness of the solar cell from over 200 nanometers to 70 nanometers.” This method also enabled the team to precisely control how and when they added other ingredients to the mix to make it more conductive. The result again was a significant improvement – they could finely tune the energy level of that layer to closely match the layer beneath it, which makes the movement of “holes” carrying positive charges around the solar cell circuit much easier. “A very small difference between the top layer and perovskite material means maybe we get greater energy efficiency,” Jung said. The evaporation method also resulted in a much longer-lasting solar cell. Before, the cells would lose the ability to efficiently convert sunlight into electricity after a couple of days. Now, their efficiency remains high for more than 35 days. While cheaper than conventional silicon-based solar cells, evaporation-based perovskite solar cells are more expensive than spin-coated cells. The team is now working to determine how to strike a balance between cost and efficiency, and hopefully find a way to use solution processing without creating pinholes.
These results open new opportunities, such as using the novel photolithography in a multiple patterning process. An example would be creating an OLED array that adds a fourth color to red, green and blue, as well as developing previously-unseen devices such as a new sensors that integrate OLED with the organic photodetector