Increase in Photocurrent Density of WO3 Photoanode by Placing a Layer of an Ordered Array of Mesoporous WO3 Micropillars on Top of a WO3 Sheet Layer.

A facile stamping method was developed to assemble ordered arrays of mesoporous WO3 micropillars with uniform sizes, shapes, and lengths on F-doped tin oxide glass. Using this method, a series of WO3 heterostructural bilayer photoanodes consisting of an array of m-µm long ordered mesoporous WO3 micropillars at the top and the n-µm thick mesoporous WO3 plain sheet layer at the bottom (denoted as m/n) were prepared. Among them 2.5/7.5 displayed a steady state photocurrent density of 3.6 mA cm-2 at 1.23 V (vs RHE) under AM 1.5 (1 Sun), which is much higher than that of the plain 10-µm thick WO3 sheet (2.5 mA cm-2). This phenomenon occurs owing to the following six benefits: increases in charge carrier density, number of photogenerated electron, charge collection rate, thermodynamic feasibility for the vectorial charge transport from the outermost layer of the photoanode to the inner layer, the surface hydrophilicity, and the decrease in charge transfer resistance.

Silicotungstate, a Potential Electron Transporting Layer for Low-Temperature Perovskite Solar Cells.

Thin films of a heteropolytungstate, lithium silicotungstate (Li4SiW12O40, termed Li-ST), prepared by a solution process at low temperature, were successfully applied as electron transporting layer (ETL) of planar-type perovskite solar cells (PSCs). Dense and uniform Li-ST films were prepared on FTO glass by depositing a thin Li-ST buffer layer, followed by coating of a main Li-ST layer. The film thickness was controlled by varying the number of coating cycles, consisting of spin-coating and thermal treatment at 150 °C. In particular, by employing 60 nm-thick Li-ST layer obtained by two cycles of coating, the fabricated CH3NH3PbI3 PSC device demonstrates the photovoltaic conversion efficiency (PCE) of 14.26% with JSC of 22.16 mA cm-2, VOC of 0.993 mV and FF of 64.81%. The obtained PCE is significantly higher than that of the PSC employing a TiO2 layer processed at the same temperature (PCE = 12.27%). Spectroscopic analyses by time-resolved photoluminescence and pulsed light-induced transient measurement of photocurrent indicate that the Li-ST layer collects electrons from CH3NH3PbI3 more efficiently and also exhibits longer electron lifetime than the TiO2 layer thermally treated at 150 °C. Thus, Li-ST is considered to be a promising ETL material that can be applied for the fabrication of flexible PSC devices.

Novel Carbazole-Based Hole-Transporting Materials with Star-Shaped Chemical Structures for Perovskite-Sensitized Solar Cells.

Novel carbazole-based hole-transporting materials (HTMs), including extended π-conjugated central core units such as 1,4-phenyl, 4,4'-biphenyl, or 1,3,5-trisphenylbenzene for promoting effective π-π stacking as well as the hexyloxy flexible group for enhancing solubility in organic solvent, have been synthesized as HTM of perovskite-sensitized solar cells. A HTM with 1,3,5-trisphenylbenzene core, coded as SGT-411, exhibited the highest charge conductivity caused by its intrinsic property to form crystallized structure. The perovskite-sensitized solar cells with SGT-411 exhibited the highest PCE of 13.00%, which is 94% of that of the device derived from spiro-OMeTAD (13.76%). Time-resolved photoluminescence spectra indicate that SGT-411 shows the shortest decay time constant, which is in agreement with the trends of conductivity data, indicating it having fastest charge regeneration. In this regard, a carbazole-based HTM with star-shaped chemical structure is considered to be a promising candidate HTM.

50 nm sized spherical TiO2 nanocrystals for highly efficient mesoscopic perovskite solar cells.

Single crystalline TiO2 nanoparticles (NPs) with spherical morphology are successfully synthesized by a hydrothermal reaction under basic conditions. TiO2 NPs, selectively controlled to the sizes of 30, 40, 50, and 65 nm, are then applied to a mesoporous photoelectrode of CH3NH3PbI3 perovskite solar cells. In particular, a spherical TiO2 NP of 50 nm size (NP50) offers the highest photovoltaic conversion efficiency (PCE) of 17.19%, with JSC of 21.58 mA cm(-2), VOC of 1049 mV, and FF of 0.759 while the enhancement of PCE mainly arises from the increase of VOC and FF. Furthermore, the fabricated photovoltaic devices exhibit reproducible PCE values and very little hysteresis in their J-V curves. Time-resolved photoluminescence measurement and pulsed light-induced transient measurement of the photocurrent indicate that the device employing NP50 exhibits the longest electron lifetime although the electron injection from perovskite to TiO2 is less efficient than the devices with smaller TiO2 NPs. The extended electron lifetime is attributed to the suppression of electron recombination due to optimized mesopores generated by the spherical NP50.

14.8% perovskite solar cells employing carbazole derivatives as hole transporting materials.

Three novel carbazole-based molecules have been synthesized and successfully applied as hole-transporting materials (HTMs) of CH3NH3PbI3-based perovskite solar cells. In particular, the perovskite cell with SGT-405, having a three-arm-type structure, exhibited a remarkable photovoltaic conversion efficiency (PCE) of 14.79%.

Design and development of highly efficient PbS quantum dot-sensitized solar cells working in an aqueous polysulfide electrolyte.

PbS quantum dot-sensitized solar cells (QDSCs) with a photovoltaic conversion efficiency (η) of 5.73% have been fabricated by applying Au/CuS/FTO as a counter electrode (CE), post-annealing the deposited PbS QDs, and introducing the bilayered TiO2 nanostructure as a photoanode.