Conceptual view of an EDLT device, where the photoluminescence (shown in red) emitted by a semiconductor (here perovskite) under external photoexcitation (blue laser beam) is controlled with a gate voltage applied to an ionic liquid or gel.Researchers have discovered a new type of electric field effect that can control light emission from perovskite devices [Yi et al., Materials Today (2019), https://doi.org/10.1016/j.mattod.2019.01.003].
The electric field effect usually refers to the modulation of electrical conductivity in a semiconductor by means of an applied voltage to a gate electrode and forms the basis of modern digital electronics. In a conventional field effect transistor (FET), the conductivity of a semiconductor layer can be turned on or off or gradually ramped up or down. Now a team from Rutgers and the Universities of Minnesota and Texas at Dallas has found that the photoluminescence (PL) of a perovskite device can be modulated in a similar manner.
“Our work reports a novel type of field effect in which PL, rather than conductivity, is tuned by an ‘electric knob’ – the gate voltage,” explains Vitaly Podzorov, who led the research.
PL, which arises from the recombination of free electrons and holes generated in a semiconductor exposed to a light source such as a laser, is sensitive in some materials to external factors such as temperature, pressure, strain, or magnetic field. But the gradual, reversible control of PL by an applied voltage has not been observed before, say the researchers.
“We believe that our work is a significant breakthrough in optoelectronics based on emergent materials,” Podzorov told Materials Today.
The team had, in fact, been looking for the conventional electric field effect in lead-halide perovskites, which are promising materials for solar cells and other light-emitting or lasing applications. They fabricated electric-double-layer transistors (EDLTs) based on various lead-halide perovskites including CsPbBr3, MAPbBr3, and FAPbBr3 with an electrolyte gel replacing the insulating layer. Molecular ions within the electrolyte layer are mobile and can be polarized by applying a very small gate voltage. Anions accumulating near the surface of the semiconducting perovskite generate a strong electric field, which affects the rate of radiative recombination in the material and, therefore, the PL.
“The fields generated in EDLTs can typically be up to 100 times greater than fields generated in conventional FETs,” explains Podzorov, “which allows to ramp up the carrier density in the semiconductor much more drastically than one can using a conventional FET.”
The ability to tune the PL intensity of a perovskite EDLT reversibly over a wide range simply via the gate voltage could be useful in many optoelectronic applications.
“If perovskites, where we have observed our PL gating effect, are ultimately used in optoelectronic applications for light emission, one can enhance or control their performance with an additional gate electrode,” points out Podzorov.
It is also possible that the PL of other emergent materials might be controllable in the same way.