A solar cell does not transform all of the sunlight that strikes it into electricity. In fact, two-thirds of sunlight’s energy is lost in solar cells. Half of the energy lost is attributable to a process called “hot carrier cooling,” in which high-energy photons lose their excess energy as heat before being transformed to electricity. AMOLF scientists have discovered a way to control the speed of this process in perovskites by exerting pressure on the material. This paves the way for perovskites to be used in a wider range of applications, including lasers and thermoelectric systems, in addition to solar cells.
Due to their outstanding photophysical properties, lead halide perovskites nanocrystals have recently emerged as a promising material not only for solar cells but also for lighting and display applications. Chemical engineering, such as cation exchange, will enhance the photophysical properties of these materials even more. One of the primary photophysical processes in lead halide perovskites is hot carrier (HC) cooling, which can have a major impact on the performance of lead halide perovskites nanocrystals-based devices.
A perovskite solar cell is a kind of solar cell that uses a perovskite-structured compound as the light-harvesting active layer, most commonly a tin halide-based material or hybrid organic-inorganic lead. Perovskite materials, such as all-inorganic caesium lead halides and methylammonium lead halides, are effective materials for future generation solar cells because they are inexpensive and simple to make, and their composition is easy to change to satisfy unique specifications, such as solar cells of any desired color. The Hybrid Solar Cells group at AMOLF is working to discover the fundamental properties of perovskites in order to improve the performance and lifetime of hybrid perovskite semiconductors. The rate at which hot carrier cooling occurs is one of these properties, which is also important if perovskites are used in other applications.
Hot carrier cooling
The term “hot electron” was firstly used to describe non-equilibrium electrons (or holes) in semiconductors. In a broader sense, the term refers to electron distributions that are describable by the Fermi function but has a higher effective temperature. The mobility of charge carriers is affected by this increased energy, and as a result, how their movement through a semiconductor system is affected.
Solar cells transform the energy of light that matches the semiconductor’s bandgap directly into electricity. Photons with higher energy cannot take this direct route. These photons produce “hot carriers,” which are high-energy electrons (and holes) that must cool before being harvested as electrical energy. The hot carriers lose their excess energy in the form of heat by scattering before they reach the semiconductor’s conduction energy level. Loreta Muscarella, a Ph.D. student, experiences a lot of challenges when trying to understand this method in perovskites, one of which is the timescale.
She claims that hot carrier cooling happens very quickly, on the order of femtoseconds to picoseconds, making it difficult to control or investigate the process. Our group is fortunate to have a one-of-a-kind setup with a Transient Absorption Spectrometer (TAS) in combination with pressure equipment. They can now calculate the electronic properties of perovskite under external stress just a few femtoseconds after shining light on it.
Manipulating with pressure
Perovskites are polar semiconductors, so their electronic properties should be strongly linked to lattice vibrations. External pressure has a direct effect on lattice dynamics, so it can be used to tune properties like electron-phonon coupling and phonon lifetimes that are highly dependent on lattice vibrations. The hot carrier cooling may be influenced by changes in one or both of these quantities. HCC should be faster if the electron-phonon coupling is increased.
It was already established that hot carrier cooling in perovskite semiconductors is much slower than in silicon semiconductors when exposed to bright light. This makes studying the process in perovskite rather than silicon much more feasible. Muscarella and her colleagues assumed that the rate of cooling was pressure-dependent. “Vibration and scattering allow the hot carriers to release their excess energy. Applying pressure increases vibrations within the material, which should speed up the cooling of the hot carrier.” She says that they wanted to put this theory to the test and discovered that they can control the cooling period by applying pressure. The method is two to three times faster at 3000 times ambient pressure.
Even though a solar cell will not be able to work at such high pressures, the internal strain may be used to achieve a similar effect. According to Mozzarella, “their experiments were done with external pressure, but it is possible to induce an internal strain in perovskites by chemically altering the material or its development, as they have previously shown in their group.”
Cooling speed for different applications
Perovskites can be used for a variety of applications other than solar cells because they can regulate the hot carrier cooling speed. They are interesting not only for colored solar cells but also for lasers and LED technology, according to Mozzarella, because they can be designed for specific colors. Fast cooling of hot carriers is critical in such applications, just as it is in conventional solar cells while slow cooling will make perovskites ideal for thermoelectric devices, which transform a temperature difference into electricity if they were allowed to cool slowly. As a result, the ability to tune the hot carrier cooling speed opens the door to a wide variety of perovskites-based applications. For a particular form of the solar cell, she also envisions adding negative pressure to the material to make the hot carrier cooling process even slower.
“Scientists are searching for ways to harvest hot carriers until they cool since heat dissipation accounts for nearly thirty percent of efficiency loss in solar cells. Even’slow’ cooling in perovskites at ambient pressure is currently too fast for hot-carrier solar cells. In picoseconds, the excess energy of these hot carriers is converted to heat. However, if a negative strain could be induced, the process could be made slow enough to be used in a working device.”