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.

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.

Electron transfer properties of iodine-doped single-walled carbon nanotubes using field effect transistor.

Single-walled carbon nanotubes (SWNTs) are known to have a p-type charge transfer character in the atmosphere. The energy state of SWNTs can be modulated by doping with either an electron donor or an acceptor. In this study, iodine molecules are chosen for intercalation to SWNTs to predict the charge transfer tendency between them. Field-effect transistors (FETs) using iodine intercalated SWNTs (I-SWNTs) are fabricated and their electronic properties are investigated to better understand the charge transfer between iodine and SWNTs by changing gate voltages. Under vacuum, I-SWNT FETs exhibit weak n-type character, indicating that electrons are transferred slightly from the iodine to the SWNTs. After exposure to O2 gas, n-type characters are reduced; however, they still retain their original type.

Large scale synthesis and enhanced field emission properties from hybrid CuO/ZnO nanorod.

Upper-directionally grown nanorods were synthesized on a large scale by a simple method of direct heating of Cu foil in air. Hybrid CuO/ZnO nanorods were fabricated by ZnO thin film coating using magnetron sputtering. Field emission (FE) measurements of CuO and hybrid CuO/ZnO nanorod films show that they have turn-on field of 3.81 and 3.24 V/microm and a current density of 0.39 and 1.1 microA/cm2 under an applied field of about 6.6 V/microm, respectively. By comparing X-ray photoelectron spectroscopy analysis and the FE properties of two types of samples, we concluded that the narrowing of band gap due to the change of electron binding energy of hybrid CuO/ZnO nanorods effectively improved FE.

Photoluminescence study of encapsulated dye inside single-walled carbon nanotubes dispersed in solvents.

Photo-luminescent dye, 1-(2-amino-phenyl) naphthalene-2-ylamine (APNA) molecules were synthesized and encapsulated inside single-walled carbon nanotubes (APNA@SWNTs) in vacuum. Here we measured X-ray diffraction (XRD) pattern and attenuated total reflectance (ATR) spectrum to confirm the encapsulation of APNA molecules inside SWNTs. Strong photoluminescence (PL) spectrum was observed around 400 nm at an excitation of 326 nm. We employed the PL intensity of the dye to reveal suspension stability of SWNTs in solvents. The intensity of PL spectrum increased as a function of SWNTs suspension stability, i.e., the PL intensity was proportional to suspension stability of SWNTs in various solvents.