Semitransparent OLEDs with Ultrathin Light Emitting Layer and Extraordinarily Thick Perovskite Hole Transport Layer

Organic light-emitting diodes (OLEDs) have had a huge gain in popularity by virtue of their simple fabrication process, thin structure and for the possibility to devise transparent devices. However, the production of stable, cheap and efficient devices remains a challenge. Ultrathin nondoped emitting layers have been investigated as a way to simplify fabrication process and lower material consumption, but the consequent reduction in thickness may decrease device stability. Overcoming this issue with thicker transport layers is alluring but tough, as organics have intrinsically low charge-carrier mobilities. On the other hand, metal halide perovskites (MHPs), promising materials in many fields of optoelectronics, have also been used as charge transport materials, where their transparency in the visible region and high hole conductivity make it possible to increase the thickness of the layer to the micrometer-scale without increasing the operation voltage or reducing luminous efficiency.<br/><br/>In light of this, we developed a semitransparent OLED having a thick cesium lead chloride all-inorganic perovskite hole transport layer, an ultrathin (&lt;0.1 nm) [Ir(ppy)₂acac] film as emitting layer and a semitransparent top indium-tin oxide contact deposited on top of a thin metallic layer. The resulting devices have a thickness exceeding 1 µm, a combined peak luminance of over 1000 cd m^-2, a current efficiency of 34 cd/A and a transparency in excess of 60% over the visible spectrum above 480 nm. The hole transporting CsPbCl₃ layer includes two thin (2 nm) CsCl layers at the interfaces, which have been demonstrated to be able to passivate interfacial halide vacancies, improving the photoluminescence and reducing exciton quenching in turn. The nature of the ultrathin layer was investigated by means of contact angle measurement, which confirmed that the molecules forming the LEL are not in the form of a neat layer. The metallic layer could be deposited with little damage to the underlying organic layers but despite its low thickness, essential to guarantee transparency, it still protected towards the final ITO sputtering deposition. The combination of transparent top electrode, thick perovskite HTL and ultrathin LEL, described herein for the first time, entails a reduction in material and fabrication costs and is promising towards future cheap and stable semitransparent OLEDs.