Stephen T. Connor

Solution-processed metal nanowire mesh transparent electrodes.

Transparent conductive electrodes are important components of thin-film solar cells, light-emitting diodes, and many display technologies. Doped metal oxides are commonly used, but their optical transparency is limited for films with a low sheet resistance. Furthermore, they are prone to cracking when deposited on flexible substrates, are costly, and require a high-temperature step for the best performance. We demonstrate solution-processed transparent electrodes consisting of random meshes of metal nanowires that exhibit an optical transparency equivalent to or better than that of metal-oxide thin films for the same sheet resistance. Organic solar cells deposited on these electrodes show a performance equivalent to that of devices based on a conventional metal-oxide transparent electrode.

Optical Absorption Enhancement in Amorphous Silicon Nanowire and Nanocone Arrays

Hydrogenated amorphous Si (a-Si:H) is an important solar cell material. Here we demonstrate the fabrication of a-Si:H nanowires (NWs) and nanocones (NCs), using an easily scalable and IC-compatible process. We also investigate the optical properties of these nanostructures. These a-Si:H nanostructures display greatly enhanced absorption over a large range of wavelengths and angles of incidence, due to suppressed reflection. The enhancement effect is particularly strong for a-Si:H NC arrays, which provide nearly perfect impedance matching between a-Si:H and air through a gradual reduction of the effective refractive index. More than 90% of light is absorbed at angles of incidence up to 60 degrees for a-Si:H NC arrays, which is significantly better than NW arrays (70%) and thin films (45%). In addition, the absorption of NC arrays is 88% at the band gap edge of a-Si:H, which is much higher than NW arrays (70%) and thin films (53%). Our experimental data agree very well with simulation. The a-Si:H nanocones function as both absorber and antireflection layers, which offer a promising approach to enhance the solar cell energy conversion efficiency.

Wafer-scale silicon nanopillars and nanocones by Langmuir-Blodgett assembly and etching

We have developed a method combining Langmuir–Blodgett assembly and reactive ion etching to fabricate nanopillars with uniform coverage over an entire 4 inch wafer. We demonstrated precise control over the diameter and separation between the nanopillars ranging from 60 to 600 nm. We can also change the shape of the pillars from having vertical to tapered sidewalls with sharp tips exhibiting a radius of curvature of 5 nm. This method opens up many possible opportunities in nanoimprinting, solar cells, batteries, and scanning probes.

Phase Transformation of Biphasic Cu2S−CuInS2 to Monophasic CuInS2 Nanorods

We synthesized wurtzite CuInS(2) nanorods (NRs) by colloidal solution-phase growth. We discovered that the growth process starts with nucleation of Cu(2)S nanodisks, followed by epitaxial overgrowth of CuInS(2) NRs onto only one face of Cu(2)S nanodisks, resulting in biphasic Cu(2)S-CISu heterostructured NRs. The phase transformation of biphasic Cu(2)S-CuInS(2) into monophasic CuInS(2) NRs occurred with growth progression. The observed epitaxial overgrowth and phase transformation is facile for three reasons. First, the sharing of the sulfur sublattice by the hexagonal chalcocite Cu(2)S and wurtzite CuInS(2) minimizes the lattice distortion. Second, Cu(2)S is in a superionic conducting state at the growth temperature of 250 degrees C wherein the copper ions move fluidly. Third, the size of the Cu(2)S nanodisks is small, resulting in fast phase transformation. Our results provide valuable insight into the controlled solution growth of ternary chalcogenide nanoparticles and will aid in the development of solar cells using ternary I-III-VI(2) semiconductors.

CuInS2 Solar Cells by Air-Stable Ink Rolling

Solution-based deposition techniques are widely considered to be a route to low-cost, high-throughput photovoltaic device fabrication. In this report, we establish a methodology for a highly scalable deposition process and report the synthesis of an air-stable, vulcanized ink from commercially available precursors. Using our air-stable ink rolling (AIR) process, we can make solar cells with an absorber layer that is flat, contaminant-free, and composed of large-grained CuInS(2). The current-voltage characteristics of the devices were measured in the dark and under 100 mW/cm(2) illumination intensity, and the devices were found to have J(sc) = 18.49 mA/cm(2), V(oc) = 320 mV, FF = 0.37, and eta = 2.15%. This process has the ability to produce flat, contaminant-free, large-grained films similar to those produced by vacuum deposition, and its versatility should make it capable of producing a variety of materials for electronic, optoelectronic, and memory devices.

Efficient Multiple Exciton Generation Observed in Colloidal PbSe Quantum Dots with Temporally and Spectrally Resolved Intraband Excitation

We have spectrally resolved the intraband transient absorption of photogenerated excitons to quantify the exciton population dynamics in colloidal PbSe quantum dots (QDs). These measurements demonstrate that the spectral distribution, as well as the amplitude, of the transient spectrum depends on the number of excitons excited in a QD. To accurately quantify the average number of excitons per QD, the transient spectrum must be spectrally integrated. With spectral integration, we observe efficient multiple exciton generation in colloidal PbSe QDs.

Probing carrier lifetimes in photovoltaic materials using subsurface two-photon microscopy.

Accurately measuring the bulk minority carrier lifetime is one of the greatest challenges in evaluating photoactive materials used in photovoltaic cells. One-photon time-resolved photoluminescence decay measurements are commonly used to measure lifetimes of direct bandgap materials. However, because the incident photons have energies higher than the bandgap of the semiconductor, most carriers are generated close to the surface, where surface defects cause inaccurate lifetime measurements. Here we show that two-photon absorption permits sub-surface optical excitation, which allows us to decouple surface and bulk recombination processes even in unpassivated samples. Thus with two-photon microscopy we probe the bulk minority carrier lifetime of photovoltaic semiconductors. We demonstrate how the traditional one-photon technique can underestimate the bulk lifetime in a CdTe crystal by 10× and show that two-photon excitation more accurately measures the bulk lifetime. Finally, we generate multi-dimensional spatial maps of optoelectronic properties in the bulk of these materials using two-photon excitation.

Three‐Dimensional Interconnected Silica Nanotubes Templated from Hyperbranched Nanowires

Figure 1. Flowchart of the fabrication process for interconnected silica nanotubes from hyperbranched PbSe nanowires. Inorganic nanofluidic devices, such as nanopores, nanochannels, and nanotubes (NTs) have been actively studied in bioseparation, bioanalysis, fluidic transistors, power generation, and fast mass transport. Compared to biological nanopores, inorganic nanofluidic devices have been demonstrated to be robust, to have easily tuned surfaces and to be integrable into arrays. One of the most powerful nanofluidic device fabrication methods is templating against a porous membrane or chemically synthesized or lithographically patterned nanowires (NWs). NTs or nanochannels made in this way have controllable dimensions, with diameters down to several nm and lengths up to tens of mm. Herein, we exploit hyperbranched PbSe NWs as templates to produce 3D interconnected hyperbranched silicon dioxide (silica) NTs by simple coating and etching steps. The obtained NTs with a thick enough shell retain the orientation of the original hyperbranched arrays and are either parallel or perpendicular to each other. These hyperbranched NTs afford interesting opportunities for constructing new 3D nanofludic devices. The fabrication process for silica hyperbranched NTs is shown in Figure 1. Hyperbranched PbSe NWs were grown on Si (100) substrates using vapor transport growth. Each hyperbranched PbSe NW exhibits 908 orientation between branches because of the epitaxial relationship. The details of hyperbranched NW growth can be found elsewhere. The samples with hyperbranched NWs were then coated by plasma enhanced chemical vapor deposition (PECVD) of silica. The deposition temperature was 350 8C. The growth rate for a silicon oxide layer based on thin-film deposition on silicon (100) substrate is around 6 nm min . Silica layers with different thickness (30 nm and 80 nm) were deposited on different samples of hyperbranchedNWs to evaluate the effect of silica thickness on the morphologies of the final silica NTs.

Nanowire batteries for next generation electronics

The scaling of electronic devices also requires the evolution of high energy density power sources. By using nanowires, high charge storage materials, which otherwise have mechanical breakage problems due to large structure transformations and volume changes, can be adopted as electrode materials. High power operation can also be possible due to the short lithium insertion distances in the nanowires. We have studied Si and Ge nanowires and demonstrated charge storage capacities several times higher than the graphite anodes used in existing battery technology. LiMn2O4 nanorod cathodes were found to show much higher power rates than commercial powders. Detailed morphology and structure characterization have shown that these improvements are attributed to facile strain relaxation, good electronic contact and conduction, and short Li insertion distances in the nanowire battery electrode. We also developed a Langmuir-Blodgett assembly technique to produce nanowire pillars as battery electrodes, which opens up the possibility for the fabrication of on-chip battery power sources.