Guanidinium-Pseudohalide Perovskite Interfaces Enable Surface Reconstruction of Colloidal Quantum Dots for Efficient and Stable Photovoltaics.

Complete surface passivation of colloidal quantum dots (CQDs) and their strong electronic coupling are key factors toward high-performance CQD-based photovoltaics (CQDPVs). Also, the CQD matrices must be protected from oxidative environments, such as ambient air and moisture, to guarantee air-stable operation of the CQDPVs. Herein, we devise a complementary and effective approach to reconstruct the oxidized CQD surface using guanidinium and pseudohalide. Unlike conventional halides, thiocyanate anions provide better surface passivation with effective replacement of surface oxygen species and additional filling of defective sites, whereas guanidinium cations promote the construction of epitaxial perovskite bridges within the CQD matrix and augment electronic coupling. Additionally, we replace a defective 1,2-ethanedithiol-treated CQD hole transport layer (HTL) with robust polymeric HTLs, based on a judicious consideration of the energy level alignment established at the CQD/HTL interface. These efforts collectively result in high-performance and stable CQDPVs with photocurrents over 30 mA cm-2, ∼80% quantum efficiency at excitonic peaks and stable operation under humid and ambient conditions. Elucidation of carrier dynamics further reveals that interfacial recombination associated with band alignment governs both the CQDPV performance and stability.

Solvent Engineering of Colloidal Quantum Dot Inks for Scalable Fabrication of Photovoltaics.

Development of colloidal quantum dot (CQD) inks enables single-step spin-coating of compact CQD films of appropriate thickness, enabling the promising performance of CQD photovoltaics (CQDPVs). Today's highest-performing CQD inks rely on volatile n-butylamine (BTA), but it is incompatible with scalable deposition methods since a rapid solvent evaporation results in irregular film thickness with an uneven surface. Here, we present a hybrid solvent system, consisting of BTA and N,N-dimethylformamide, which has a favorable acidity for colloidal stability as well as an appropriate vapor pressure, enabling a stable CQD ink that can be used to fabricate homogeneous, large-area CQD films via spray-coating. CQDPVs fabricated with the CQD ink exhibit suppressed charge recombination as well as fast charge extraction compared with conventional CQD ink-based PVs, achieving an improved power conversion efficiency (PCE) of 12.22% in spin-coated devices and the highest ever reported PCE of 8.84% among spray-coated CQDPVs.

Hybrid Surface Passivation for Retrieving Charge Collection Efficiency of Colloidal Quantum Dot Photovoltaics.

Efficient charge collection in photovoltaics is a key issue toward their high performance. Despite the promising performance of colloidal quantum dot (CQD)-based photovoltaics (CQDPVs), they suffer significant dissipation of photocurrent due to imperfect surface passivation of the CQD hole transport layer (HTL) by a single 1,2-ethaneditihol (EDT) ligand. To address the critical drawback of existing CQDPVs, we offer a hybrid passivation strategy, including both EDT and thiocyanate (SCN). The hybrid passivation leads to seamless surface passivation of CQDs, remarkably suppressing charge recombination. This strategy also augments the p-doping density of the CQD, resulting in a pronounced energy level bending at the active layer/HTL interface and facilitating efficient charge separation. Moreover, enhanced electronic coupling across the CQDs (originating from reduced inter-dot spacing) promotes rapid charge extraction. Consequently, the flawless charge collection by a hybrid-passivated HTL successfully retrieves the photocurrent, achieving an enhanced CQDPV power conversion efficiency of 12.70% compared with 11.49% for the control device.

Improving Charge Collection from Colloidal Quantum Dot Photovoltaics by Single-Walled Carbon Nanotube Incorporation.

Improving charge collection is one of the key issues for high-performance PbS colloidal quantum dot photovoltaics (CQDPVs) due to the considerable charge loss resulting from the low mobility and large defect densities of the 1,2-ethanedithiol-treated PbS quantum dot hole-transporting layer (HTL). To overcome these limitations, single-walled carbon nanotubes (SWNTs) and C60-encapsulated SWNTs (C60@SWNTs) are incorporated into the HTL in CQDPVs. SWNT-incorporated CQDPV demonstrates a significantly improved short-circuit current density (JSC), and C60@SWNT-incorporated CQDPV exhibits an even higher JSC than that of pristine SWNT. Both result in improved power-conversion efficiencies. Hole-selective, photoinduced charge extraction with linearly increasing voltage measurements demonstrates that SWNT or C60@SWNT incorporation improves hole-transporting behavior, rendering suppressed charge recombination and enhanced mobility of the HTL. The enhanced p-type characteristics and the improved hole diffusion lengths of SWNT- or C60@SWNT-incorporated HTL bring improvement of the entire hole-transporting length and enable lossless hole collection, which results in the JSC enhancement of the CQDPVs.

Vibrational Spectrocopic Studies of Organic Molecules After Encapsulation Inside Single-Walled Carbon Nanotubes.

7,7,8,8-tetracyano-p-quinodimethane (TCNQ), tetrathiafulvalene (TTF), and dodecanethiol (DoSH) were encapsulated inside single-walled carbon nanotubes (SWNTs), (TCNQ@SWNT, TTF@SWNT, and DoSH@SWNT). We measured the Fourier transform infra-red (FTIR) spectra and X-ray diffraction (XRD) patterns to confirm the encapsulation of organic molecules. Slight shifts of the FTIR peaks and the disappearance of an XRD peak at ~6°, corresponding to the SWNT (10) reflection, were observed. From the measurements of the current-voltage curves, it was revealed that the current of TTF@ SWNT and DoSH@SWNT decreased, and the current of TCNQ@SWNT increased compared with that of pristine SWNTs.

Electric Property Change of Single-Walled Carbon Nanotube by the Encapsulation of Aliphatic Thiols.

The encapsulation of single-walled carbon nanotubes (SWNTs) with aliphatic thiol compounds with a relatively small amount of ionization energy achieves n-type doping of SWNTs. Thiol compounds encapsulated inside nanotubes in vacuum drastically change the electric properties of SWNTs by a charge transfer between the two species. The simplicity of the synthetic process offers a viable route for large-scale production of SWNTs with controlled doping states by using mat-type SWNTs. Optical characterization (Raman and near-infrared spectrum) and electric property (conductivity) reveals that a charge transfer between the SWNTs and compounds occurs through the difference in the ionization energy and electron affinity. We confirm an electron density change in SWNTs through optical spectroscopy and conductivity measurement in vacuum. X-ray photoelectron spectroscopy also reveals that the compounds are predominantly encapsulated inside SWNTs.

Improved Field Emission of Three-Dimensional Single-Walled Carbon Nanotube Networks Suspended Between ZnO Nanorods.

We report field emission (FE) properties of three dimensional single-walled carbon nanotube (3-D SWNT) networks synthesized between ZnO nanorods on textured Si wafer. The FE properties are measured for turn-on field and field enhancement factor, and are compared with other types of SWNT films such as synthesized SWNT films and spray SWNT films. 3-D SWNT has lower turn-on field and higher field enhancement factor than other SWNT films.

A Study of the Wettability of Single-Walled Carbon Nanotube Films on Flat and Textured Si Substrates.

Contact angle measurements are investigated on the surface of single-walled carbon nanotube (SWNT) films directly formed on flat and textured Si substrates using a thermal chemical vapor deposition method. The SWNT films on the textured Si consist of a multiscale structure composed of nanoscale SWNTs and a microscale textured Si. They show superhydrophobic properties in which the water contact angle was around 161°. A direct surface treatment to them increase the contact angle to 174°. The reversible wettability of the SWNT films formed on the textured Si substrates is confirmed through the oxidation process using an acid mixture of nitric and sulfuric acids and a successive reduction procedure via heating treatment in an NH3 environment.

Conductivity Properties of Single-Walled Carbon Nanotubes Upon the Encapsulation of TTF and TCNQ.

N-type and p-type single-walled carbon nanotubes (SWNTs) were formed via the encapsulation of tetrathiafulvalene (TTF) and 7,7,8,8-tetracyano-p-quinodimethane (TCNQ) inside SWNTs, respectively. Raman, near-infrared, and X-ray photoelectron spectrometer were used to confirm the encapsulation. From measurements of the current-voltage curves in a vacuum, it was revealed that current of TTF-encapsulated SWNTs decreased and TCNQ-encapsulated SWNTs increased comparing with that of pristine SWNTs. This was resulted from electron-donating (TTF) and withdrawing (TCNQ) character into SWNTs.

Suppressed Interfacial Charge Recombination of PbS Quantum Dot Photovoltaics by Graphene Incorporated into ZnO Nanoparticles.

Single-layer graphene (SLG) was incorporated into ZnO nanoparticles (NPs), and use of this material in photovoltaic devices generated significant changes. The Fermi level of ZnO NPs underwent a downshift, whereas the conduction and valence bands were maintained with increasing SLG concentrations. Furthermore, the effective defect densities were reduced and carrier mobility was enhanced. Colloidal quantum dot photovoltaics (CQDPVs) with the SLG-incorporated ZnO NP layer as an electron transporting layer achieved significant performance enhancement. Poor performing CQDPVs were also observed with incorporation of an excess amount of SLG. This trend paralleled the interfacial charge recombination trends of CQDPVs. Effective suppression of interfacial recombination was achieved for CQDPVs with an appropriate SLG concentration, whereas dramatically increased interfacial recombination was observed for CQDPVs with an excess of SLG. For CQDPVs with appropriate SLG incorporation, efficient defect passivation and enhanced electron mobility of ZnO NPs facilitated loss-less electron transfer and efficient electron extraction without compromising the favorable energy level alignment. Excess SLG incorporation led to an increase in recombination within the PbS QD layer due to the presence of an energy barrier. This simple and powerful strategy provides an effective method for modulating the interfacial properties of CQDPVs.

Field Emission and Electrical Properties of Perovskite.

We investigate characteristic field emission properties of methyl ammonium mixed-halide perovskite (CH3NH3PbI3-xClx) and their current change under one laser pulse. To analyze these properties, we fabricated inverted-type mixed-halide perovskite solar cells which exhibit a device efficiency of 9.31% under A.M 1.5 condition. Under one laser pulse varying from 420 nm to 580 nm, perovskite layer considerably reacted from 420 nm to 440 nm and then gradually decreased in current. A turnon field of 5.56 V and a field enhancement factor of 3183 were obtained from one spin-coating perovskite layer and in eight times of perovskite spin-coating cycles, a turn-on field of 6.70 V and a field enhancement factor of 5110 were observed.

Photocurrent Enhancement of CdSe Quantum-Dot Sensitized Solar Cells Incorporating Single-Walled Carbon Nanotubes.

We demonstrate quantum-dot sensitized solar cells (QDSSCs) which have colloidal CdSe quantum dots (TOPO-CdSe) as a sensitizer onto mesoporous TiO2 photoanodes. CdS quantum-dot (QD) layer plays a role of buffer layer for direct adsorption of TOPO-CdSe. We incorporate single-walled carbon nanotubes with TiO2 photoanode of our QDSSCs to facilitate efficient charge transfer. Shortcircuit current densities (Jsc) of our QDSSCs are enhanced while other parameters are maintained. Furthermore, we apply inert N2 pressure onto our sensitized photoanodes and observe 44% of Jsc enhancement with respect to pristine sample. Consequently, light-harvesting efficiency of our QDSSCs are increased. Significant series resistance reduction is observed from electrochemical impedance spectroscopy, indicating better interface contact between TiO2 photoanode and TOPOCdSe QD sensitizer are achieved.

Performance Enhancement of 3-Mercaptopropionic Acid-Capped CdSe Quantum-Dot Sensitized Solar Cells Incorporating Single-Walled Carbon Nanotubes.

We fabricated a series of linker-assisted quantum-dot-sensitized solar cells based on the ex situ self-assembly of CdSe quantum dots (QDs) onto TiO2 electrode using sulfide/polysulfide (S(2-)/Sn(2-)) as an electrolyte and Au cathode. Our cell were combined with single-walled carbon nanotubes (SWNTs) by two techniques; One was mixing SWNTs with TiO2 electrode and the other was spraying SWNTs onto Au electrode. Absorption spectra were used to confirm the adsorption of QDs onto TiO2 electrode. Cell performance was measured on samples containing and not-containing SWNTs. Samples mixing SWNTs with TiO2 showed higher cell efficiency, on the while sample spraying SWNTs onto Au electrode showed lower efficiency compared with pristine sample (not-containing SWNTs). Electrochemical impedance spectroscopy analysis suggested that SWNTs can act as either barriers or excellent carrier transfers according their position and mixing method.

Selective Growth of Metallic and Semiconducting Single Walled Carbon Nanotubes on Textured Silicon.

We fabricated the etched Si substrate having the pyramidal pattern size from 0.5 to 4.2 µm by changing the texturing process parameters, i.e., KOH concentration, etching time, and temperature. Single walled carbon nanotubes (SWNTs) were then synthesized on the etched Si substrates with different pyramidal pattern by chemical vapor deposition. We investigated the optical and electronic properties of SWNT film grown on the etched Si substrates of different morphology by using scanning electron microscopy, Raman spectroscopy and conducting probe atomic force microscopy. We confirmed that the morphology of substrate strongly affected the selective growth of the SWNT film. Semiconducting SWNTs were formed on larger pyramidal sized Si wafer with higher ratio compared with SWNTs on smaller pyramidal sized Si.

Enhancement of Photo-Current Conversion Efficiency in a CdS/CdSe Quantum-Dot-Sensitized Solar Cell Incorporated with Single-Walled Carbon Nanotubes.

Cadmium sulfide (CdS) and cadmium selenide (CdSe) are sequentially assembled onto a nanocrystalline TiO2 film to create a quantum-dot (QD)-sensitized solar cell application by a successive ionic layer adsorption and reaction (SILAR) method. The results show that CdS and CdSe QDs have a complementary effect in the performance of light harvest of solar cell. Single-walled carbon nanotubes (SWNTs) are incorporated with a CdS/CdSe QDs solar cell by mixing them with TiC2 film to enhance electron transfer. SWNTs are also sprayed onto CdSe QDs (SWNTs onto CdSe) to apply p+ type properties of SWNTs. Absorbance is increased in a wide wavelength range. In particular, cells having the sprayed SWNTs onto the QDs show a clear increase in absorbance at a low wavelength region. The fill factor of CdS/CdSe QDs solar cell with SWNTs is higher than that without SWNTs, indicating the decrease in loss of electron from TiO2 to QDs. Short-circuit current in a QD-sensitized solar cell having SWNTs on CdSe shows maximum value. Photo-current conversion efficiency of cells is increased in both cell types containing SWNTs at 10~17% compared with pristine cells. We expect that solar cells using SWNTs will affect future energy technology and devices.

Structure and Hydrogen Adsorption Properties of SBA-15 Doped with Pd Nanoparticles.

Hydrogen adsorption properties of Pd-doped Santa Barbara amorphous No. 15 (Pd-SBA-15) were investigated and the results were compared with pure SBA-15 ones in terms of change of its structure and Pd concentration. Pd-SBA-15 samples were prepared by a hydrothermal reaction, using mixture of PEO20PPO70PEO20 (P123) and tetraethyl orthosilicate (TEOS). For the doping of Pd on SBA-15, PdC2 solution was added into the mixture of P123 and TEOS, and the solution was annealed at 80 degrees C for 2 hours under 800 Torr of hydrogen atmosphere. According to the X-ray diffraction and transmission electron microscope data, Pd-doped SBA-15 samples form a hexagonal array of mesoporous structure with 20-30 nm size of Pd particles. Values of specific surface area decreased from 630 to 414 m2/g as increasing the Pd doping level due to the increasing of the volume density. In fact, the volume density increased from 0.103 to 0.276 g/cc as increasing the mass ratio of PdCl2 to TEOS from 0 to 0.5. For the Pd-doped SBA-15, the amount of adsorbed hydrogen significantly increased from 0.49 to 0.99 wt% as increasing the Pd doping level from 0 to 0.5 demonstrating that Pd doping is an effect method for SBA-1 5 as a potential use of hydrogen storage application.

Measurement of contact resistance in CdSe-single-walled carbon nanotube hybrids.

The CdSe-single-walled carbon nanotube (SWNT) hybrids are synthesized for measuring contact resistance between CdSe quantum dots and SWNTs in two hybrid samples, i.e., spray-deposited CdSe on SWNTs, and pyrene-self assembled CdSe on SWNTs. Currents are measured through indium-tin oxide (ITO), CdSe-SWNT hybrids and the tip of conductive AFM (c-AFM) with and without light at 532 and 655 nm.

Field emission properties of a three-dimensional network of single-walled carbon nanotubes inside pores of porous silicon.

We report characteristic field emission (FE) properties of single-walled carbon nanotubes (SWCNTs) synthesized inside the pores as well as on the top surface of a porous silicon (PS) substrate. Turn-on fields and emission current densities were measured and compared with those of other types of SWCNTs in similar environments. Investigation of the FE properties of SWCNTs synthesized inside the pores of a PS substrate revealed a low turn-on field of approximately 2.25 V/µm at 10 µA/cm2 and a high field-enhancement factor (6182) compared with other samples. A life-time stability test was performed by monitoring the current density before and after repeated exposure to O2, suggesting that the pore channel can effectively prevent O2(+) ion etching from destroying SWCNTs within the pores of the PS layer.

Current flow of single-walled carbon nanotubes upon the encapsulation of beta-carotene by using conducting probe atomic force microscopy.

Beta-carotene was inserted into single-walled carbon nanotubes (SWCNTs) by using the encapsulation method in a solution phase, and the energy transfer process was studied under irradiation of visible light. The encapsulation of beta-carotene inside SWCNTs was confirmed by ultraviolet (UV)/visible (Vis) and near-IR (N-IR) spectroscopy, and the stability of encapsulated beta-carotene was also confirmed by a UV irradiation experiment. The N-IR absorption spectrum revealed that the beta-carotene donated electrons to the SWCNTs upon encapsulation. We measured current flow through SWCNT bundles by using conducting probe atomic force microscopy (CP-AFM) while the samples were irradiated by green light (532 nm) and red light (650 nm). The current changed with the irradiation of 532 nm light, where the beta-carotene has its own absorption, but not with the irradiation of 650 nm light. From these results, we concluded that the encapsulated beta-carotene inside SWCNTs efficiently absorbed 532 nm light and excited electrons of beta-carotene might be transferred to the SWCNTs like an energy transfer process. Our conclusion was consistent with a previously suggested energy transfer theory between beta-carotene and SWCNTs.

Formation of ZnO nanostructures grown on Si and SiO2 substrates.

ZnO nanorods are grown on Si-based substrate by chemical bath deposition method in aqueous solution using zinc nitrate hexahydrate. Various substrates having different surface morphology are used to evaluate their effect on growing ZnO nanorods, such as flat Si(100) wafer, small and large textured-Si wafer, porous silicon, flat SiO2 wafer, small and large textured-SiO2 wafer. The length, diameter, geometry, and coverage density of ZnO nanorods are investigated by field-emission scanning electron microscopy and summarized. SiO2 is a preferred substrate for the growth of ZnO nanorods to Si if the surface morphology of substrate is same, and the textured surface has much higher coverage density (> 95%) than the flat surface. Each nanorod is vertically grown along the c-axis on the top of each pyramid face for textured substrate, and forms the 3D sea sponge-like ZnO structure. The characteristics of ZnO nanorods grown on various substrates are analyzed by grazing-angle X-ray diffraction (XRD) and photoluminescence (PL) measurements.