Research progress of efficient green perovskite light emitting diodes

Perovskite light emitting diodes exhibit the advantages of high color purity, tunable wavelength and low producing cost. Considering these superiorities, one regards perovskite light emitting diodes as very promising candidates for solid state lighting and panel displaying. Human eyes are very sensitive to green light, thus green perovskite light emitting diodes receive the most attention from researchers. Since the advent of the very first green perovskite light emitting diode, the external quantum efficiency has climbed from only 0.1% to over 20%. In this review, we mainly discuss the history of green perovskite light emitting diodes, the basic concepts of perovskite materials and green perovskite light emitting diodes, and the common methods to improve the efficiency of green perovskite light emitting diodes. The bandgap of bromide perovskite is about 2.3 eV, which is located just on a green light wavelength scale and thus becomes the suitable emitting layer material for green emission. There are mainly two types of device structures, i.e. regular format and inverted format. The whole working process of green perovskite light emitting diodes can be divided into two stages, i.e. the injection and recombination of charge carriers. One engineers the energy levels of different layers to improve the injection of charge carriers. They also raise up the strategy so-called surface passivation to reduce the defect density at the interface in order to avoid the quenching phenomenon. One usually inserts a buffering layer to realize the surface passivation. Besides, perovskites possess very small exciton binding energy, which is at the same order of magnitudes as the kinetic energy at room temperature. Charge carriers become free in this case, which will severely reduce the radiation recombination probability due to the non-radiation recombination process such as Shockley-Read-Hall effect and Auger recombination. To solve the problem, people fabricate three types of perovskites, namely quasi two-dimensional perovskite, perovskite quantum dot, and perovskite nanocrystal. In this way, the charge carriers can be confined into a limited space and the exciton binding energy will hence be improved. From the efficiency perspective, the green perovskite light emitting diodes promise to be commercialized. However, another critical issue impeding the development of green perovskite light emitting diodes is the stability problem. Comparing with the organic light emitting diodes and inorganic quantum dot light emitting diodes, the lifetime of perovskite light emitting diodes is too limited, which is only approximately one hundred hours under normal conditions. The temperature, moisture and light exposure are all factors that influence the stability of perovskite light emitting diodes.