Researchers Unearth Intriguing Physics of Hybrid Organic-Inorganic Perovskites

Certain innovative materials that seem too good to be true end up being good and true. An emerging category of semiconductors with the ability to inexpensively light up our homes and other places in the future with modulated colors emitted from lamps, lasers, and also window glass could be the new example.

Such materials, known as hybrid organic-inorganic perovskites (HOIPs), are easy to synthesize from solution, highly radiant, and energy-efficient. The recurrent doubt of whether HOIPs can be advantageous has been cleared by an innovative international research headed by physical chemists from the Georgia Institute of Technology.

The scientists detected in a HOIP a sort of “richness” of semiconducting physics formed by electrons dancing on chemical foundations that sway similar to a funhouse floor in an earthquake. This is in contrast to our traditional knowledge because conventional semiconductors are dependent on rigidly stable chemical underpinnings, or more silent molecular underpinnings, to generate the desired quantum characteristics.

We don’t know yet how it works to have these stable quantum properties in this intense molecular motion. It defies physics models we have to try to explain it. It’s like we need some new physics.

Felix Thouin, First Author and Graduate Research Assistant - Georgia Tech

Quantum Properties Surprise

The gyrating disorderliness of HOIPs has rendered their investigation highly difficult. However, the team of scientists from five research institutes belonging to four countries have been successful in evaluating a prototypical HOIP and discovered its quantum characteristics to be equivalent to those of molecularly rigid, conventional semiconductors, a majority of which are graphene-based.

“The properties were at least as good as in those materials and may be even better,” stated Carlos Silva, a professor in Georgia Tech’s School of Chemistry and Biochemistry. All the semiconductors do not have the ability to efficiently absorb and emit light. However, HOIPs possess this ability, which makes them optoelectronic and hence prospectively useful in LEDs, lasers, other lighting applications, and even in photovoltaics.

As HOIPs are not molecularly rigid, they can be flexibly synthesized and used.

Silva co-headed the research with physicist Ajay Ram Srimath Kandada. Their group reported the outcomes of their research on two-dimensional HOIPs in the Physical Review Materials journal on March 8, 2018. EU Horizon 2020, the Natural Sciences and Engineering Research Council of Canada, the Fond Québécois pour la Recherche, the Research Council of Canada, and the National Research Foundation of Singapore funded the study.

The “Solution Solution”

In general, semiconducting characteristics originate from static crystalline lattices of tidily interconnected atoms. For instance, in the case of silicon, which is used in a majority of the commercial solar cells, they are interconnected silicon atoms. The same principle holds good for graphene-like semiconductors.

“These lattices are structurally not very complex,” stated Silva. “They’re only one atom thin, and they have strict two-dimensional properties, so they’re much more rigid.”

“You forcefully limit these systems to two dimensions,” stated Srimath Kandada, who is a Marie Curie International Fellow at Georgia Tech and the Italian Institute of Technology. “The atoms are arranged in infinitely expansive, flat sheets, and then these very interesting and desirable optoelectronic properties emerge.”

Although these proven materials are magnificent, there is a reason for pursuing HOIPs, apart from investigating their perplexing physics. It is because they might prove to be highly practical in many significant ways.

“One of the compelling advantages is that they’re all made using low-temperature processing from solutions,” stated Silva. “It takes much less energy to make them.”

In contrast, graphene-based materials are synthesized in small amounts at higher temperatures, which could be tough to process. “With this stuff (HOIPs), you can make big batches in solution and coat a whole window with it if you want to,” stated Silva.

Funhouse in an Earthquake

Apart from the fact HOIPs wobble, they also include a highly ordered lattice with an intrinsic rigidity, although it is less restrictive when compared to the conventional two-dimensional materials.

“It’s not just a single layer,” stated Srimath Kandada. “There is a very specific perovskite-like geometry.” Perovskite here indicates the shape of the crystal lattice of a HOIP, which is layered scaffolding.

“The lattice self-assembles,” stated Srimath Kandada, “and it does so in a three-dimensional stack made of layers of two-dimensional sheets. But HOIPs still preserve those desirable 2D quantum properties.”

The sheets are bound together by interspersed layers of a different molecular structure quite similar to a sheet of rubber bands. This makes the scaffolding to wobble like a funhouse floor.

“At room temperature, the molecules wiggle all over the place. That disrupts the lattice, which is where the electrons live. It’s really intense,” stated Silva. “But surprisingly, the quantum properties are still really stable.”

It is highly significant to have quantum characteristics that work at ambient temperature without the need for ultra-cooling for practical application as a semiconductor.

Revisiting the expansion for HOIP—hybrid organic-inorganic perovskites—this is the way the experimental material fit into the HOIP chemical class: It was made of a blend of inorganic lead iodide layers (the rigid part) separated from each other by phenylethylammonium organic layers (the rubber band-like parts)—with the chemical formula (PEA)₂PbI₄.

The lead in the prototypical material can be substituted by a metal safe for humans to handle before designing an applicable material.

Electron Choreography

HOIPs are exceptional semiconductors since their electrons perform an acrobatic square dance.

In general, electrons are located in orbits surrounding the nucleus of an atom, or they are shared by atoms in a chemical bond. However, HOIP chemical lattices, such as those in all semiconductors, are designed to share electrons more widely.

A system’s energy levels could liberate the electrons to move around and take part in affairs such as the flow of heat and electricity. The orbits, which in this case are rendered empty, are known as electron holes that need the electrons back.

“The hole is thought of as a positive charge, and of course, the electron has a negative charge,” stated Silva. “So, hole and electron attract each other.”

The holes and the electrons move around one another similar to dancing partners pairing up to form an “exciton” in physicists’ term. Although the excitons are not really particles, they look and act quite similar to particles.

Hopping Biexciton Light

In the case of semiconductors, millions of excitons are interrelated, or choreographed, with one another, resulting in desirable characteristics upon applying an energy source such as laser light or electricity. Moreover, excitons can pair up with each other to form biexcitons, thereby enhancing the energetic characteristics of the semiconductor.

“In this material, we found that the biexciton binding energies were high,” stated Silva. “That’s why we want to put this into lasers because the energy you input ends up to 80 or 90 percent as biexcitons.”

Biexcitons get excited energetically to ingest the input energy. Then, they energetically contract and emit light. Apart from lasers, this phenomenon would hold well even for LEDs or other surfaces formed of the optoelectronic material.

“You can adjust the chemistry (of HOIPs) to control the width between biexciton states, and that controls the wavelength of the light given off,” stated Silva. “And the adjustment can be very fine to give you any wavelength of light.”

That yields light of any color as desired by a person.