Yoo-Yong Lee

Highly efficient and bending durable perovskite solar cells: toward a wearable power source

Perovskite solar cells are promising candidates for realizing an efficient, flexible, and lightweight energy supply system for wearable electronic devices. For flexible perovskite solar cells, achieving high power conversion efficiency (PCE) while using a low-temperature technology for the fabrication of a compact charge collection layer is a critical issue. Herein, we report on a flexible perovskite solar cell exhibiting 12.2% PCE as a result of the employment of an annealing-free, 20 nm thick, amorphous, compact TiOx layer deposited by atomic layer deposition. The excellent performance of the cell was attributed to fast electron transport, verified by time-resolved photoluminescence and impedance studies. The PCE remained the same down to 0.4 sun illumination, as well as to a 45° tilt to incident light. Mechanical bending of the devices worsened device performance by only 7% when a bending radius of 1 mm was used. The devices maintained 95% of the initial PCE after 1000 bending cycles for a bending radius of 10 mm. Degradation of the device performance by the bending was the result of crack formation from the transparent conducting oxide layer, demonstrating the potential of the low-temperature-processed TiOx layer to achieve more efficient and bendable perovskite solar cells, which becomes closer to a practical wearable power source.

A Strain-Insensitive Stretchable Electronic Conductor: PEDOT:PSS/Acrylamide Organogels.

UNLABELLED Organogel-based stretchable electronic conductors exhibit electrical conduction even under a large stretching deformation of 300% without electrochemical reactions at DC voltages. The resistance change with stretching is almost strain-insensitive up to 50% strain and it remains at each deformation up to 1000 fatigue cycle. The polymeric conductive paths of PEDOT PSS are well preserved during the mechanical deformation.

Designing thermal and electrochemical oxidation processes for δ-MnO2 nanofibers for high-performance electrochemical capacitors

To date, the phase of electrospun MnOx nanofibers (NFs) after thermal calcination has been limited to the low oxidation state of Mn (x < 2), which has resulted in insufficient specific capacitance. The organic contents in the as-spun MnOx NFs, which are essential for forming the NF structure, make it difficult to obtain the optimum phase (MnO2) to achieve high electrochemical performance. Herein, δ-MnO2 NFs, which were obtained by galvanostatic oxidation of thermally calcined MnOx NFs, were successfully fabricated while maintaining the 1-D nanoscale structure and inhibiting loss of the active materials. The galvanostatically oxidized Mn3O4 exhibited an outstanding performance of 380 F g−1 under a mass loading of 1.2 mg cm−2. The effect of galvanostatic oxidation was strongly dependent on the concentration and energetic stability of the Mn2+/3+ ions in the MnOx phases.

Enhanced conductivity of solution-processed indium tin oxide nanoparticle films by oxygen partial pressure controlled annealing

A highly conductive and transparent indium tin oxide (ITO) film was developed using a nanoparticle-based solution process through the control of oxygen partial pressure during annealing. At an oxygen partial pressure of 2.1 × 10−3 Torr, a maximum conductivity of 313 Ω−1 cm−1 was obtained: a great improvement over the conductivity of conventional ITO nanoparticle films (at this conductivity, the sheet resistance decreased to 30 Ω sq−1, and the transmittance reached 90%). By analyzing the electron concentration and mobility using Hall measurements, we determined that the main factor contributing to the enhanced conductivity is the increase in electron concentration that occurs due to the formation of oxygen vacancies under low oxygen partial pressures. However, if the oxygen partial pressure is too low, the removal of the organic ligands covering the ITO nanoparticles is incomplete, and the electron mobility is reduced. Microstructure control is also necessary for further improvement of the mobility.

Blends of polyarylate with poly(styrene-co-acrylonitrile)

Abstract Blending of polyarylate and various types of styrene-acrylo-nitrile copolymers (SAN) was carried out to observe the phase behavior as a function of the AN content in SAN. SAN of about 25% AN content showed the most enhanced miscibility in blends with PAR. SAN had a greater tendency to diffuse into PAR than the reverse, which was confirmed by glass transition temperature and phase morphology.

Growth Mechanism of Strain-Dependent Morphological Change in PEDOT:PSS Films

Understanding the mechanism of the strain-dependent conductivity change in polymers in stretched conditions is important. We observed a strain-induced growth of the conductive regions of PEDOT:PSS films, induced by a coalescence of conductive PEDOT-rich cores. This growth due to coalescence leads to a gradual decrease in the electrical resistivity up to 95%, independent of the thickness of the PEDOT:PSS films. The primary mechanism for the evolution of the PEDOT-rich cores proceeds by the cores growing larger as they consuming relatively smaller cores. This process is caused by a strain-induced local rearrangement of PEDOT segments in the vicinity of PSS shells around the cores and also changes the chemical environment in PEDOT, induced by the electron-withdrawing effects around the PEDOT chains. The strain-induced growth mechanism is beneficial to understanding the phenomenon of polymeric chain rearrangement in mechanical deformation and to modulating the electrical conductivity for practical applications.