Several key issues concerning the toxicity of lead, lack of long-term stability and limitations in deposition area impede further progress in research. In EML, we focus on solving these critical issues. EML focuses on solving the key issues of current hybrid perovskite research. First, we aim to improve the stability of perovskite devices under ambient conditions by changing the composition of the perovskite absorber as well as replacing unstable organic layers with stable inorganic ones. Second, we are working on a unique large-area deposition method called solution-shearing to deposit the perovskite absorber layer. Lastly we aim to replace the toxic element present in high-efficiency perovskite solar cells.

Low-dimensional Perovskite

Perovskite has structure in which the octahedral of [BX6]4- shares the corner and the A cation occupies the 12-fold cuboctahedra site. When the halide metal octahedron network is separated by ‘A’ cationic lattices and is present in a plate, rod or cluster form, charge carriers become localized. This improves the exciton binding energy potentially leading to a highly efficient Light-Emitting Diodes.

Colloidal Perovskite NCs

Colloidal perovskite nanocrystals are emerging as a strong candidate for next-generation LED material. Perovskite nanocrystals exhibits high photoluminescence quantum yields (PLQY ~ 80 %, without applying core- shell structure) with high color purity (FWHM~20 nm), which are superior to Ⅱ-Ⅵ or Ⅲ-Ⅴ quantum dots. By modifying the compositions of A sites (MA+, FA+, Cs-) and X sites (I-, Br-, Cl-), they can produce wide color gamut (Covering 150% area of NTSC). Such excellent properties produce colloidal perovskite nanocrystals can be adapted in next generation light emitting applications. However, several issues concerning the toxicity of lead, and colloidal stabilities of nanocrystals after purification need to be resolved.

The two main topics that our lab currently interested in are post deposition treatment (PDT) and developing new buffer materials

Alkali post-deposition technique is needed highly-efficient CIGS solar cells prepared on a flexible substrate. Alkali elements, such as Na, K, Rb, Cs, has been known to passivate surface/interface. We are currently working on improving efficiencies with new alkali elements using a vacuum thermal co-evaporation technique and trying to understand the mechanism behind the beneficial effects of alkali treatments.

Second topic is development of new buffer layer. The most common buffer layer that forms a p-n junction with CIGS absorber is CdS because of high effiencies reported with the CdS buffer. However, for safety concerns, there is need for developing Cd-free buffer materials. We are working on ZnO-based buffer materials such as ZnSnO, ZnTiO as a potential candidate to replace CdS.

Cu2ZnSn(S,Se)4 (CZTSSe) has attracted massive attention as an alternative for CdTe and CIGS because it consists of earth-abundant and non-toxic elements. It also holds a promise for high efficiency due to its proper optoelectronic properties such as a high absorption coefficient (~104-105 cm-1) and a tunable band gap (1.0-1.5 eV depending on S/Se ratio). However, compared to other thin film solar cell technologies such as CdTe, CIGS, and perovskite solar cells, CZTSSe suffers from the high open-circuit voltage (Voc) deficit (Eg/q - Voc, where q is the elemental charge), which hinders further improvement in device performance. Our group is currently working on resolving Voc deficit issue in CZTSSe solar cells with various approaches concerned with passivating the absorber (increasing carrier concentration, grain boundary passivation, and band gap engineering) as well as the interfaces.

Sb2Se3 is an emerging material in earth-abundant chalcogenide solar cells. Since stable chemical potential area of CZTSSe is very narrow, a quaternary CZTSSe is vulnerable to secondary phases and defects. In this regard, a binary compound Sb2Se3 recently arises as a remarkable photovoltaic material with its benign ribbon-like structure as well as its optoelectronic properties (a band gap of 1.0 – 1.2 eV and a high absorption coefficient of ~105 cm-1). Application of Sb2Se3 into the field of inorganic thin film solar cells is relatively new, and our group recently yielded a power conversion efficiency of 4.03 % using thermal co-evaporation technique. (cf. a world-record Sb2Se3 solar cell has an efficiency of 5.90 % using rapid thermal evaporation technique.) Our group aims to enhance the device performance by focusing on the interfaces and the buffer layers in Sb2Se3 solar cells.

SnSe is promising thermoelectric materials because of its intrinsic low thermal conductivity originated from bonding anisotropy. Since 2014, when an article that achieved the highest thermoelectric efficiency of ZT = 2.6 with SnSe single crystals has been reported, SnSe thermoelectric device has been studied very widely. In our lab, we has been studied about the thermoelectric properties of thin-film SnSe. On this topic, we are working on collaborative research with KRISS.

Currently, we are working on the synthesis of both CIGS and BiVO4 and testing the suitability of them as a photoelectrode where water splitting reaction take place. Our ultimate goal is producing unassisted PEC tandem cell, which is solely driven by solar illumination, consisting of highly efficient and stable photoelectrodes with low manufacturing costs.

A promising and environmental-friendly way of producing hydrogen and oxygen gases is water photolysis, where solar energy generates electron-hole pairs in a photocatalytic material which, in turn, reduce water into hydrogen (at photocathode) and oxidize it into oxygen (at photoanode). A quaternary chalcogenide compound CIGS emerged as a promising candidate as an electrode on the cathode side. However, long-term stability of CIGS-based photoelectrodes in aqueous solution is a concern. We study the degradation mechanism of CIGS photoelectrode and how to prevent it. Additionally, we consider the application of CIGS-based photocathode for CO2 reduction.

Among various photoanode candidates, BiVO4 based photoanode is a promising photoelectrode for solar-driven water splitting applications in photoelectrochemical (PEC) devices because of its easy separation of the evolving gases (H2 an O2) with separated two-electrodes and high oxygen evolution reaction (OER) activity that achieving from applying with external bias and tandem cell configurations. However, PEC performances of the fabricated BiVO4 are varied and do not reproducible even though the BiVO4 is fabricated with the similar recipe using a metal-organic decomposition (MOD) process that one of widely used solution based techniques. Therefore, we focus on the fabrication of a highly efficient and reproducible BiVO4 based photoanode by using various strategies such as introduction of a buffer layer, decoration of co-catalysts and engineering the chemical compositions (secondary element doping) of BiVO4 during the fabrication of BVO film.