Tae-Woo Lee - Organic-Inorganic Perovskite Optoelectronics: Solar Cells and Displays

Organic-Inorganic Perovskite Optoelectronics: Solar Cells and Displays

Bio: Tae-Woo Lee is an associate professor in the department of the materials science and engineering at Pohang University of Science and Technology (POSTECH), Korea. He received his Ph.D in chemical engineering from KAIST, Korea in February 2002. Then, he joined Bell Laboratories, USA as a postdoctoral researcher in 2002. From September 2003 to August 2008, he worked in Samsung Advanced Institute of Technology, Samsung Electronics as a member of research staff. He received a prestigious Korea Young Scientist Award from the President of Korea in 2008 and The Scientist of the Month Award from the ministry of science, ICT and future planning in 2013. He is author and co-author of 135 papers including Nature Photonics, Nature Communications, PNAS, Angewandte Chemie, Advanced Materials, Nano Letters, and Advanced Functional Materials, as well as inventor and co-inventor of 322 patents (143 Korean patents and 179 international patents). His research focuses on printed and organic/inorganic/hybrid nano-materials and devices based on organic, organic-inorganic hybrid, and carbon materials for flexible  electronics, printed electronics, displays, solid-state lightings, solar energy conversion devices, and neuromorphic devices.

Abstract: Methylammonium lead halide perovskites have been intensively studied as promising photo absorption in solar cells and an alternative emitters to conventional organics or inorganic quantum dots (QDs) light emitting diode, due to excellent semiconducting properties, a broad range of light absorption, and a high extinction coefficient in solar cells and its high color purity-light arising from exciton confinement between organic and inorganic 2-dimensional alternating layers in light emitting diode. Although PCE of perovskite solar cells has been increased remarkably, a few reports have considered  solution-processed planar heterojunction (SP-PHJ) structure solar cells without using a mesoporous or compact semiconducting metal oxide (e.g, TiO2) layer processed by high-temperature sintering, and the SP-PJH solar cells to date have shown lower PCE than those with a mesoporous or compact TiO2 layer. Commercialization of perovskite solar cells requires easy, scalable and low-temperature methods to fabricate them efficiently by a solution process without sintering. PEDOT:PSS can be considered as a good HEL because of simple solution processibility, planarization effect on the underlying ITO layer, and a low-temperature annealing process. However the work function WF of PEDOT:PSS (4.9 to 5.2 eV depending on the ratio of PEDOT to PSS) is lower than  the ionization potential IP of perovskite (e.g. 5.4 eV for methylammonium lead iodide (CH3NH3PbI3)), so the potential energy loss at PEDOT:PSS/Perovskite interface decreased built-in potential in perovskite solar cells. In addition, the large hole-injection barrier and luminescence quenching due to the conventional PEDOT:PSS are serious problems to use the perovskite as an emitting layer (EML).

            The WF in HELs can be tuned by using molecular surface engineering to control the surface composition in HEL films, which depends on the surface-enriched molecules and their concentration relative to the conducting polymer. Thus we used a self-organized HEL and HIL (SOHEL and SOHIL) which is composed of a conducting polymer composition (e.g., PEDOT:PSS) and a perfluorinated ionomer (PFI), i.e., tetrafluoroethylene-perfluoro-3,6-dioxa-4-methyl-7-octene-sulfonic acid copolymer. Here, we present solution-processed methylammonium lead iodide CH3NH3PbI3-based perovskite solar cells and the bright organic/inorganic hybrid perovskite light-emitting diodes (PrLEDs) with a high-WF SOHEL and SOHIL for good energy level alignment with the IP level of CH3NH3PbI3. The SOHEL and SOHIL interface can increase the built-in potential, the photocurrent, and thus the PCE of perovskite solar cells, in addition, can facilitate the hole injection to the EML (CH3NH3PbBr3) by reducing the hole injection barrier between PEDOT:PSS and IP of CH3NH3PbBr3 and reduce the luminescence quenching due to the large quantity of PFI on top of the surface. We obtained high PCE of 11.7% in SP-PHJ perovskite solar cells under 100-mW/cm2 illumination. We also demonstrated flexible perovskite solar cells on a poly(ethylene terephthalate) (PET) substrate; they had PCE as high as 8.0%. In terms of the PrLED with SOHIL, we demonstrated high luminance, current efficiency and EQE (417 cd/m2, 0.577 cd/A, 0.125%), and multicolored PrLEDs by substituting the Br^- ions with I^- ions and Cl^- ions. We also demonstrated flexible PrLEDs on a plastic substrate for the first time.