Jun Hong Noh

High-performance flexible perovskite solar cells exploiting Zn2SnO4 prepared in solution below 100 °C.

Fabricating inorganic–organic hybrid perovskite solar cells (PSCs) on plastic substrates broadens their scope for implementation in real systems by imparting portability, conformability and allowing high-throughput production, which is necessary for lowering costs. Here we report a new route to prepare highly dispersed Zn2SnO4 (ZSO) nanoparticles at low-temperature (<100u2009°C) for the development of high-performance flexible PSCs. The introduction of the ZSO film significantly improves transmittance of flexible polyethylene naphthalate/indium-doped tin oxide (PEN/ITO)-coated substrate from ∼75 to ∼90% over the entire range of wavelengths. The best performing flexible PSC, based on the ZSO and CH3NH3PbI3 layer, exhibits steady-state power conversion efficiency (PCE) of 14.85% under AM 1.5G 100u2009mW·cm−2 illumination. This renders ZSO a promising candidate as electron-conducting electrode for the highly efficient flexible PSC applications.

Fabrication of metal-oxide-free CH3NH3PbI3 perovskite solar cells processed at low temperature

Efficient metal-oxide-free perovskite solar cells were successfully developed by employing the N–I–P architecture. The modified solvent engineering process employing a diethylether drip as an orthogonal solvent enabled fabrication of a multi-layered device comprising FTO/PEI/PCBM/MAPbI3/PTAA/Au at low temperature (≤100 °C). Optimization of the thickness of the phenyl-C61-butyric acid methyl ester (PCBM) layer in the planar device yielded an overall power conversion efficiency (PCE) of 15.3% with a large hysteresis but a steady-state efficiency of 13.9% under AM 1.5G 100 mW cm−2 illumination. The use of the low-temperature processed dense-TiO2 layer in conjunction with the PCBM layer gave rise to performance comparable to that of the single electron transport layer (ETL) device and enabled fabrication of an efficient, flexible perovskite solar cell with a PCE of 11.1%.

Fabrication of CuInTe2 and CuInTe2–xSex Ternary Gradient Quantum Dots and Their Application to Solar Cells

We report the first synthesis of colloidal CuInTe2, CuInTe2-xSex gradient alloyed quantum dots (QDs) through a simple hot injection method. We confirmed the composition of synthesized QDs to cationic rich phase of CuIn1.5Te2.5 and Cu0.23In0.36Te0.19Se0.22 with XPS and ICP analysis, and we have also found that the gradient alloyed Cu0.23In0.36Te0.19Se0.22 QDs exhibit greatly improved stability over the CuIn1.5Te2.5 QDs. The solution-processed solar cell based on the gradient alloyed Cu0.23In0.36Te0.19Se0.22 QDs exhibited 17.4 mA/cm(2) of short circuit current density (Jsc), 0.40 V of open circuit voltage (Voc), 44.1% of fill factor (FF), and 3.1% of overall power conversion efficiency at 100 mW/cm(2) AM 1.5G illumination.

Synthesis of CdSe-TiO2 nanocomposites and their applications to TiO2 sensitized solar cells.

CdSe-TiO(2) nanocomposites were synthesized via aminolysis of Ti-oleate complexes in the presence of CdSe nanocrystals, and their application as sensitizers for TiO(2) solar cells was investigated. The formation of CdSe-TiO(2) nanocomposites was confirmed using transmission electron microscopy and Raman spectroscopy. The emission spectrum of CdSe-TiO(2) nanocomposites revealed photoinduced charge separation at the CdSe-TiO(2) interface of the composite. The photocurrent-voltage properties of CdSe-TiO(2)-sensitized TiO(2) particle films compared favorably with those of CdSe-sensitized TiO(2) films. Evidence was also found indicating that the TiO(2) component of the composite protects CdSe against degradation during film annealing.

Engineering interface structures between lead halide perovskite and copper phthalocyanine for efficient and stable perovskite solar cells

Successful commercialization of perovskite solar cells (PSCs) in the near future will require the fabrication of cells with high efficiency and long-term stability. Despite their good processability at low temperatures, the majority of organic conductors employed in the fabrication of high-efficiency PSCs [e.g., 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD) and poly(triaryl amine) (PTAA)] have low thermal stability. In order to fabricate PSCs with excellent thermal stability, both the constituent material itself and the interface between the constituents must be thermally stable. In this work, we focused on copper phthalocyanine (CuPC) as a model hole-transporting material (HTM) for thermally stable PSCs since CuPC is known to possess excellent thermal stability and interfacial bonding properties. The CuPC-based PSCs recorded a high power conversion efficiency (PCE) of ∼18% and maintained 97% of their initial efficiency for more than 1000 h of thermal annealing at 85 °C. Moreover, the device was stable under thermal cycling tests (50 cycles, −45 to 85 °C). The high PCE and high thermal stability observed in the CuPC-PSCs were found to arise as a result of the strong interfacial and conformal coating present on the surface of the perovskite facets, located between CuPC and the perovskite layer. These results will provide an important future direction for the development of highly efficient and thermally stable PSCs.

Reversible change in electrical and optical properties in epitaxially grown Al-doped ZnO thin films

Aluminum-doped ZnO (AZO) films were epitaxially grown on sapphire (0001) substrates using pulsed laser deposition. As-deposited AZO films had a low resistivity of 8.01×10−4u2002Ωu2009cm. However, after annealing at 450u2009°C in air, the electrical resistivity of the AZO films increased to 1.97×10−1u2002Ωu2009cm because of a decrease in the carrier concentration. Subsequent annealing of the air-annealed AZO films in H2 recovered the electrical conductivity of the AZO films. In addition, the conductivity change was reversible upon repeated air and H2 annealing. A photoluminescence study showed that oxygen interstitial (Oi′) is a critical material parameter allowing for the reversible control of the electrical conducting properties of AZO films.

Quaternary semiconductor Cu2FeSnS4 nanoparticles as an alternative to Pt catalysts

We demonstrate an N719 dye sensitized solar cell based on Cu2FeSnS4 (CFTS) as a counter electrode. The elements for the material are all earth abundant and environmentally benign. The power conversion efficiency of a DSSC using CFTS was comparable to that of a DSSC using Pt under A.M. 1.5G (100 mW cm−2).

Mobility enhanced photoactivity in sol-gel grown epitaxial anatase TiO2 films.

Epitaxial anatase thin films were grown on single-crystal LaAlO3 substrates by a sol-gel process. The epitaxial relationship between TiO2 and LaAlO3 was found to be [100]TiO2||[100]LaAlO3 and (001)TiO2||(001)LaAlO3 based on X-ray diffraction and a high-resolution transmission electron microscopy. The epitaxial anatase films show significantly improved photocatalytic properties, compared with polycrystalline anatase film on fused silica substrate. The increase in the photocatalytic activity of epitaxial anatase films is explained by enhanced charge carrier mobility, which is traced to the decreased grain boundary density in the epitaxial anatase film.

SrNb2O6 nanotubes with enhanced photocatalytic activity

SrNb2O6 having a rhombic nanotubular structure with a diameter of 300 nm, average length of 3 μm and wall thickness of 50 nm was synthesized via a facile hydrothermal route. The prepared powders were characterized using X-ray diffraction, field-emission electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM) and Raman spectroscopy. Compared to the same material prepared by the solid-state reaction method, the rhombic nanotubes of SrNb2O6 exhibited enhanced photocatalytic activity for the production of H2 from pure water, which was mainly attributed to their higher surface area. Moreover, the increased optical absorption properties resulting from the variation of the crystal structure also contributed to their higher photocatalytic activity.

Synthesis of hierarchically organized nanostructured TiO 2 by pulsed laser deposition and its application to dye-sensitized solar cells

Hierarchically organized TiO2 nanostructures were synthesized on fluorine-doped SnO2(FTO)/glass substrate at room temperature under various oxygen pressures using pulsed laser deposition (PLD). The microstructures of the obtained TiO2 nanostructures were changed sensitively as oxygen pressure. A fully dense structure could be achieved below oxygen pressure of 50 mTorr. However, ultrafine nanoparticles (5∼15 nm) were produced over 50 mTorr and they assembled hierarchically in ordered forest consisted of characteristic tree-shaped structures. The hierarchical assemblies were gradually to be porous and finally the tree shape was disappeared and the assemblies were disordered with increasing oxygen pressure. The synthesized TiO2 nanostructures were changed from amorphous to anatase phase without morphological changes after air annealing at 450 °C for 1 hour. The anatase nanostructures were applied to dye-sensitized solar cells (DSSCs). Cell performance and electron diffusion characteristics of the hierarchical anatase assemble prepared by PLD as photoelectrode were investigated in DSSCs. Optimal TiO2 nanostructure synthesized by PLD for photoelectrode in DSSCs was discussed in terms of electron transport and dye absorption.