Research On High Performance Perovskite Optoelectronic Devices

Perovskite materials possess the same crystal structure as calcium titanate?CaTiO₃?.They are represented by a general chemical formula,ABX₃,where A and B are cations and X is an anion.The most important feature of this structure is that ions with widely different radii can coexist and form a stable structure.This peculiar crystal structure gives them unique physical and chemical properties,such as visible light absorption and electrocatalysis.They have also found applications in the chemical and physical fields.Particularly,the field of thin film photovoltaics has seen a rapid surge in the power conversion efficiency?PCE?of organic-inorganic hybrid perovskite solar cells from an initial value of 3.8%to the current certified value of 23.7%.Furthermore,characteristics such as;low cost,solubility,high fluorescence quantum efficiency,high color purity and adjustable color reveal the potential of applying perovskites in the fields of plane display and solid-state lighting.Inspite of these fascinating properties,perovskites still face many challenges.For example,perovskite materials are more commonly used as light-emitting layer or light-absorbing layer.Fewer reports exist for other uses of perovskite materials.Perovskite light emitting diodes?LEDs?show poor stability and low external quantum efficiency?EQE?,creating much room for improvement.Although perovskite solar cells?PSCs?have achieved high power conversion efficiency?PCE?,the intrinsic mechanisms and other basic issues require further study.Such mechanisms and issues include;crystallization kinetics of perovskite film,environmental stability of the device,and defects in the perovskite crystal lattice.To address these issues,this research work focuses on exploring the relationship between device performance and the active layer,and the preparation of high-performance light emitting diodes?LEDs?and solar cells by doping.At the same time,an in-depth study of the intrinsic mechanisms was conducted from the perspective of device physics.The research mainly includes the following:1.Since perovskite is a bipolar material,and the hole mobility of the all-inorganic perovskite CsPbBr₃ is 9.27 cm² V^-1 s^-1 higher than that of the conventional organic hole transporting material,we report for the first time,the use of all-inorganic perovskite?CsPbBr₃?thin film as hole transport layer?HTL?in a doped organic light-emitting diode?OLED?.This approach simplifies the preparation method and conditions.In addition,placing an electron blocking layer and an exciton blocking layer between HTL and the light-emitting layer,a high performance red OLED is obtained with a maximum brightness of 20,750 cd/m² and a maximum electroluminescence efficiency of 10.64 cd/A.2.Thermally activated delayed fluorescence?TADF?was brought in the perovskite LEDs.TADF materials have sufficiently small single and triplet energy level difference(?E_ST)which allows triplet excitons to be converted to singlet excitons through the reverse intersystem process at room temperature.The light is emitted via a delayed fluorescence process,thereby bringing the theoretical internal quantum efficiency upper limit of the device to 100%.As far as we know,it is the first time that the TADF material2CzPN was added to the light-emitting layer of the all-inorganic perovskite LEDs.By adjusting doping concentration,the effect of the TADF material on the light-emitting layer was studied,and the internal quantum efficiency of the doped devices was close to100%.A high-performance device with a brightness of 19,941 cd/m² and an electroluminescence efficiency of 8.74 cd/A was obtained with the TADF material as the dopant.3.Furthermore,the ionic radius of Ag⁺?129 pm?is like that of the Pb²⁺?119 pm?.Therefore,adding a small amount of AgI into the organic-inorganic hybrid perovskite?MAPbI₃?precursor solution does not affect the lattice structure of perovskite.However,the introduction of Ag⁺into MAPbI₃ affects the distribution of electron cloud density by altering the orientation of MA⁺.The AgI doping achieves n-type doping of perovskites by increasing electron mobility.AgI also acts as an additive to control the perovskite crystallization with improved crystallinity and film morphology.Consequently,a maximum power conversion efficiency>20%is achieved.This finding provides a direction to fabricate high performance PSCs by controlling the charge balance via an intensive doping technique.4.Dense and uniform perovskite films can also be formed on the substrate by TiI₄doping.This is critical for high-performance perovskite solar cells as defects in the films increase charge carrier recombination and decrease the open circuit voltage and short circuit current density,thereby reducing the performance of the cells.Therefore,it is necessary to reduce defects as much as possible in perovskite solar cells.By doping the TiI₄ into the organic-inorganic hybrid perovskite?MAPbI₃?precursor solution,the defects formed during the crystallization process of the perovskite were reduced and the I^-vacancies were filled.Finally,the PCE of the fabricated PSCs reached 20.55%.