Seunghwan Bae

Two different mechanisms of CH3NH3PbI3 film formation in one-step deposition and its effect on photovoltaic properties of OPV-type perovskite solar cells

Although the one-step deposition method has intensively been studied because of the simple and easy fabrication of perovskite films, uncontrolled crystallization of perovskite during one-step deposition often results in films with small crystallites and low surface coverage, leading to low photovoltaic performance. In this study, we have proposed the optimum processing conditions to afford a favorable crystal morphology of perovskite films for achieving high power conversion efficiency of perovskite solar cells. Two different morphologies, tree-like and flower-like morphologies, are developed depending upon the spin-coating time and post-heat treatment temperature. When the perovskite is crystallized from the liquid film after a short spin-coating time, the flower-like morphology is developed, whereas the tree-like morphology is developed when the perovskite is crystallized for a long spin-coating time. When the morphology evolution is monitored using in situ optical microscopy and X-ray diffraction to investigate the origin of the difference between tree-like and flower-like morphologies, it reveals that the CH3NH3I–PbI2–solvent complex is formed to develop the tree-like morphology before CH3NH3PbI3 crystals are formed, whereas the flower-like morphology is developed when the CH3NH3PbI3 crystals are formed directly from the liquid film without the formation of the CH3NH3I–PbI2–solvent complex. The film with a flower-like morphology, as prepared from DMSO solution, has large-sized crystallites, and the crystallites are highly orientated along (112) and (200) directions, resulting in a high PCE of 13.85%, whereas the film with a tree-like morphology has small-sized crystallites with random crystal orientation, exhibiting very low PCEs.

Performance enhancement of planar heterojunction perovskite solar cells by n-doping of the electron transporting layer

Herein we report a simple n-doping method to enhance the performance of perovskite solar cells with a planar heterojunction structure. Devices with an n-doped PCBM electron transporting layer exhibit a power conversion efficiency of 13.8% with a remarkably enhanced short-circuit current of 22.0 mA cm(-2) as compared to the devices with an un-doped PCBM layer.

The effect of different chalcogenophenes in isoindigo-based conjugated copolymers on photovoltaic properties

New low bandgap conjugated copolymers based on isoindigo and three different chalcogenophenes (thiophene, selenophene and tellurophene) were synthesized (denoted as PIT, PISe and PITe, respectively) to investigate the effect of different chalcogenophenes on their photovoltaic properties. PISe and PITe show lower bandgaps and deeper HOMO energy levels as compared to PIT. The solar cell device based on PISe blended with PC61BM exhibits a promising PCE of 5.72% with a JSC of 10.21 mA cm−2, while PITe shows the lowest PCE of 1.16% with the lowest JSC of 2.51 mA cm−2 because of its coarse morphology although the field-effect hole mobility of PITe is the highest due to higher crystallinity than others.

A perylene diimide-based non-fullerene acceptor as an electron transporting material for inverted perovskite solar cells

Herein we introduce a new perylene diimide dimer (diPDI) as a non-fullerene electron transporting layer (ETL) material for inverted perovskite solar cells. The solar cell device with n-doped diPDI as ETL exhibits a notable power conversion efficiency of 10.0%.

Two-dimensional photonic crystal bandedge laser with hybrid perovskite thin film for optical gain

We report optically pumped room temperature single mode laser that contains a thin film of hybrid perovskite, an emerging photonic material, as gain medium. Two-dimensional square lattice photonic crystal (PhC) backbone structure enables single mode laser operation via a photonic bandedge mode, while a thin film of methyl-ammonium lead iodide (CH3NH3PbI3) spin-coated atop provides optical gain for lasing. Two kinds of bandedge modes, Γ and M, are employed, and both devices laser in single mode at similar laser thresholds of ∼200 μJ/cm2 in pulse energy density. Polarization dependence measurements reveal a clear difference between the two kinds of bandedge lasers: isotropic for the Γ-point laser and highly anisotropic for the M-point laser. These observations are consistent with expected modal properties, confirming that the lasing actions indeed originate from the corresponding PhC bandedge modes.

Synthesis of 6H-benzo[c]chromene as a new electron-rich building block of conjugated alternating copolymers and its application to polymer solar cells

For the purpose of reducing the bandgap of a fluorene-based semiconducting polymer, we introduced a strong electron-donating oxygen atom into fluorene (F) to synthesize benzochromene (BC), which was then polymerized with electron deficient dithienyl benzothiadiazole (DTBT) and diketopyrrolopyrrole (DPP2T) to afford two low-bandgap polymers, PBCDTBT and PBCDPP2T, respectively. The PBCDTBT-based solar cell exhibits a high power conversion efficiency (PCE) of 5.74%, which is much higher than that of the reference polymer (PFDTBT) (2.56%). The higher PCEs of BC-based polymers are mainly attributed to higher JSC, which arises from stronger and broader absorption, higher exciton generation rate, more effective charge separation and higher hole mobility as compared to fluorene-based polymers.