Sehyun Lee

Planar heterojunction perovskite solar cells with superior reproducibility

Perovskite solar cells (PeSCs) have been considered one of the competitive next generation power sources. To date, light-to-electric conversion efficiencies have rapidly increased to over 10%, and further improvements are expected. However, the poor device reproducibility of PeSCs ascribed to their inhomogeneously covered film morphology has hindered their practical application. Here, we demonstrate high-performance PeSCs with superior reproducibility by introducing small amounts of N-cyclohexyl-2-pyrrolidone (CHP) as a morphology controller into N,N-dimethylformamide (DMF). As a result, highly homogeneous film morphology, similar to that achieved by vacuum-deposition methods, as well as a high PCE of 10% and an extremely small performance deviation within 0.14% were achieved. This study represents a method for realizing efficient and reproducible planar heterojunction (PHJ) PeSCs through morphology control, taking a major step forward in the low-cost and rapid production of PeSCs by solving one of the biggest problems of PHJ perovskite photovoltaic technology through a facile method.

Efficient work-function engineering of solution-processed MoS2 thin-films for novel hole and electron transport layers leading to high-performance polymer solar cells

The work-function of MoS2 interfacial layers can be efficiently modulated by p- and n-doping treatments. As a result, the PCE of devices with a p-doped MoS2-based HTL is increased from ∼2.8 to ∼3.4%. Particularly, after n-doping the PCE was dramatically increased due to the change in work-function compared with un-doped MoS2 thin-films.

Annealing effects on the magnetic dead layer and saturation magnetization in unit structures relevant to a synthetic ferrimagnetic free structure

The changes in the magnetic dead layer (MDL) and saturation magnetization of the CoFeB layers are investigated as a function of the annealing temperature for four different unit structures, that are relevant to the synthetic ferrimagnetic free structure in MgO-based magnetic tunnel junctions. The MDL results for these unit structures are then converted into those for the constituent interfaces of the free structure. Most of the changes in the MDL thickness occur during annealing at a low temperature of 150 °C while those in the saturation magnetization occur at a high annealing temperature of 350 °C. These results for the MDL and saturation magnetization are critically tested by using the synthetic ferrimagnetic free structures with various thickness asymmetries. The observed switching properties of these tested structures are in good agreement with those expected from the results for the MDL and saturation magnetization, confirming the accuracy of the present results. The accuracy of the saturation magne...

Magnetic Dead Layer in Amorphous CoFeB Layers with Various Top and Bottom Structures

The magnetic dead layer (MDL) in amorphous CoFeB layers is investigated for four different unit structures. These structures are relevant to the synthetic ferrimagnetic (SyF) free layer structure in magnetic tunnel junctions used for high density magnetic random access memory (MRAM). The MDL results for these unit structures are then converted to those for the constituent interfaces of the SyF free layer structure. These MDL results are critically tested by fabricating the synthetic ferrimagnetic free layer structures with various thickness asymmetries. The observed switching properties of these tested structures are in good agreement with those expected from the effective thicknesses after the MDL correction, confirming the accuracy of the present results for the MDLs at the constituent interfaces.

Slot-Die Coated Perovskite Films Using Mixed Lead Precursors for Highly Reproducible and Large-Area Solar Cells

Recently, many kinds of printing processes have been studied to fabricate perovskite solar cells (PeSCs) for mass production. Among them, slot-die coating is a promising candidate for roll-to-roll processing because of high-throughput, easy module patterning, and a premetered coating system. In this work, we employed mixed lead precursors consisting of PbAc2 and PbCl2 to fabricate PeSCs via slot-die coating. We observed that slot-die-coated perovskite films based on the mixed lead precursors exhibited well-grown and uniform morphology, which was hard to achieve by using only a single lead source. Consequently, PeSCs made with this precursor system showed improved device performance and reproducibility over single PbAc2. Lastly, a large-area module with an active area of 10 cm2 was fabricated with a power conversion efficiency of 8.3%.

Small-Molecule Organic Photovoltaic Modules Fabricated via Halogen-Free Solvent System with Roll-to-Roll Compatible Scalable Printing Method

For the first time, the photovoltaic modules composed of small molecule were successfully fabricated by using roll-to-roll compatible printing techniques. In this study, blend films of small molecules, BTR and PC71BM were slot-die coated using a halogen-free solvent system. As a result, high efficiencies of 7.46% and 6.56% were achieved from time-consuming solvent vapor annealing (SVA) treatment and roll-to-roll compatible solvent additive approaches, respectively. After successful verification of our roll-to-roll compatible method on small-area devices, we further fabricated large-area photovoltaic modules with a total active area of 10 cm2, achieving a power conversion efficiency (PCE) of 4.83%. This demonstration of large-area photovoltaic modules through roll-to-roll compatible printing methods, even based on a halogen-free solvent, suggests the great potential for the industrial-scale production of organic solar cells (OSCs).

Water dispersion of reduced graphene oxide stabilized via fullerenol semiconductor for organic solar cells

To obtain solution-processable graphene in bulk-quantities, the chemical route was widely used to prepare graphene oxide (GO); however, the reduction process was inevitably needed to transform insulating GO into semi-metallic reduced graphene oxide (rGO). Therefore, to obtain stable rGO dispersions, many researchers introduced insulating surfactants that prevent irreversible aggregation. Although insulating surfactants were introduced to produce the stable dispersion of rGO, some properties of rGO, such as its dispersibility, processability, and electricity, are still problematic. In order to solve these problems, we synthesized novel rGO (F-OH-rGO) using the reduction process after blending GO and fullerenol (F-OH). By replacing insulating polymers with semiconducting and highly water-soluble F-OH, F-OH-rGO with not only higher conductivity but also a high dispersion concentration as well as film-forming properties could be achieved. We introduced various GO derivatives as hole-transporting layers (HTLs) to organic solar cells (OSCs) with discrepancies in important properties, so the fabricated solar cells manifested significant differences in their performances as we expected.

A systematic study on molecular planarity and D–A conformation in thiazolothiazole- and thienylenevinylene-based copolymers for organic field-effect transistors

A new donor–acceptor type conjugated polymer (P1) composed of thiazolothiazole (TzTz) and thienylenevinylene (TV) units was synthesized to confirm that structural characteristics, such as the presence of coplanar units and a D–A conformation in the polymer main chain, can have an effect on charge carrier transport in high performance organic field-effect transistors. P1 was compared with two polymers that had bithiophene (BiTh) units instead of TV units (P2) or thienothiophene (TT) units in the place of TzTz units (P3) to investigate the relationship between the molecular structure and polymer properties. P1 had a higher order of orientation than P2 due to the rigid and planar structure caused by the presence of the TV unit in P1 compared to the distorted structure of P2. The effect of the D–A type configuration in P1 was also confirmed by demonstrating its stronger intermolecular interactions and more effective hole and electron transport compared with that of the D–D type polymer P3. Thus, in contrast to P2 (with a hole mobility of 0.076 cm2 V−1 s−1 and an electron mobility of 0.0025 cm2 V−1 s−1) and P3 (with a hole mobility of 0.2 cm2 V−1 s−1 only), P1 exhibited higher ambipolar charge carrier mobilities (1.66 cm2 V−1 s−1 of holes and 0.05 cm2 V−1 s−1 of electrons).