Phil Ahrenkiel

Connecting physical properties of spin-casting solvents with morphology, nanoscale charge transport, and device performance of poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester bulk heterojunction solar cells

The correlation between the physical properties of spin-casting solvents, film morphology, nanoscale charge transport, and device performance was studied in poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester (P3HT:PCBM) blends, spin cast with two halogenated aromatic solvents: chlorobenzene (CB) and ortho-dichlorobenzene (1,2-DCB). 1,2-DCB-based blends exhibited fine phase separation of ∼10 to 15 nm length scale with ordered self-assembly of P3HT whereas blends spin cast from CB showed coarse phase separation with large isolated clusters of ∼25 to 100 nm of donor- and acceptor-rich regions. Higher solubility of both P3HT and PCBM in 1,2-DCB and a slower drying rate of 1,2-DCB (because of higher boiling point) facilitated self-organization and ordering of P3HT and promoted finer phase separation. Higher local hole mobility in 1,2-DCB-based blend was attributed to efficient hole transport through the ordered network of P3HT chains. Moreover, higher local illuminated current (dark + photocurrent) in 1,2-DCB-based blend suggested efficient diffusion and dissociation of excitons due to finer phase separation. As a consequence, 1,2-DCB-based devices exhibited higher short circuit current density (Jsc), external quantum efficiency and power conversion efficiency in contrast to the CB-based device. It was also observed that the device performance was not limited by light absorption and exciton generation; rather morphology dependent processes subsequent to exciton generation, primarily charge transport to the electrodes, limited device performance.

Nitrogen-modified biomass-derived cheese-like porous carbon for electric double layer capacitors

Lignin, an abundant biomass constituent in nature, was modified by pyrrole to produce nitrogen-doped porous carbon. The porous carbon was efficiently activated through simultaneous chemical and physical reactions using potassium hydroxide as an activation agent during the heat treatment. Surface area analysis showed that the activated carbon possessed mesopores (∼15 nm) and a large specific surface area of 2661 m2 g−1, with a cheese-like morphology. Electrochemical double layer capacitors fabricated using the activated carbon as an electrode material showed a specific capacitance of 248 F g−1 at a low current density of 0.1 A g−1 and 211 F g−1 at a high current density of 10 A g−1 in 6 M KOH solution. Charge and discharge for 1000 cycles at different current densities ranging from 0.1 to 10 A g−1 confirmed excellent specific capacitance retention and good cycling stability. This work demonstrates that the nitrogen-doped cheese-like porous activated carbon is a promising electrode material for electric double layer capacitors.

High opto-electronic quality n-type single-crystalline-like GaAs thin films on flexible metal substrates

In this report, we present a detailed study of the microstructural and opto-electronic properties of single-crystalline-like n-type GaAs thin films directly grown on inexpensive flexible metal substrates by metal–organic chemical vapor deposition (MOCVD). The ion-beam-assisted deposition technique (IBAD) was used to obtain highly oriented single-crystalline-like buffer templates on polycrystalline metal substrates. Single-crystalline-like n-type GaAs films were achieved with sharp biaxial texture and strong (00l) crystallographic orientation. Controllable n-doping from 1016–1019 cm−3 was achieved using silane (SiH4) as the dopant precursor. The GaAs films were granular with a wide distribution of grain sizes ranging from 1 to 4 μm. Strong optical photoluminescence at room temperature was obtained and the peak intensity increased with increasing doping level. Electron mobility values as high as 1320 cm2 V−1 s−1 were obtained, one of the highest reported so far for GaAs films grown directly on unconventional substrates. Unlike GaAs on wafer substrates, the electron mobility of single-crystalline-like GaAs films significantly increased with doping which appears to be controlled by the electrical properties of the grain boundaries. High-mobility n-GaAs films on low-cost metal substrates are promising for flexible electronic and photonic device applications.

Synthesis and Characterization of Electrospun TiO 2 /Ag Composite Nanofibers for Photocatalysis Applications

Electrospinning is an electrical, jet-based method of fabricating nanofibers that involves the application of a very high electrostatic force on the capillary containing the polymer solution or polymer-melt. The fibers are created by an electrically charged jet of the polymer solution, which can be collected on the surface of a grounded template. The incorporation of metal nanoparticles produces functional nanofibers. Among the noble metal nanoparticles, silver nanoparticles are promising because they have electronic and catalytic properties [1].

Defect reduction by liquid phase epitaxy of germanium on single-crystalline-like germanium templates on flexible, low-cost metal substrates

Single-crystalline-like germanium (Ge) templates were demonstrated on low-cost, flexible metal substrates and have been used to fabricate III–V materials and devices for photovoltaics and flexible electronics applications. However, these Ge templates, fabricated by magnetron sputtering or plasma enhanced chemical vapor deposition, contain a high density of defects. In order to improve the performance of optoelectronic devices made using the Ge templates, a homo-epitaxial Ge layer has been additionally grown by a liquid phase epitaxy (LPE) method. The LPE Ge layer is composed of significantly larger grains (∼26 μm) compared to that of the Ge template (∼200 nm). This large size of the LPE Ge grains effectively reduces the volume fraction of grain boundaries. The LPE Ge shows an out-of-plane texture Δω of ∼0.63° and an in-plane texture Δϕ of ∼1.26°, which signify improvements of ∼39.3% and ∼76.6% compared to that of the vapor-deposited Ge template, respectively. Further, the LPE Ge is strain free, compared to the strained Ge template hetero-epitaxially grown on cerium oxide. Defects caused by low-angle grain boundaries, impurities and strain relaxation in the Ge template are found to be suppressed in the LPE Ge. The threading dislocation density is roughly estimated to be ∼5 × 106 cm−2 in the LPE Ge compared to ∼1 × 1010 cm−2 in the Ge template.

InP thin films with single-crystalline-like properties on flexible metal substrates for photovoltaic application

We demonstrate heteroepitaxial growth of single-crystalline-like InP thin films by metal organic chemical vapor deposition (MOCVD) on low-cost flexible metal foils. The epitaxy was enabled by a multilayer oxide buffer made using ion beam assisted deposition (IBAD). The InP films were biaxially textured with sharp in-plane texture and exhibited strong (002) preferential out-of-plane orientation. Strong room-temperature photoluminescence was also observed with band gap of ~ 1.27 eV. Electron mobility of > 700 cm2/V-s at a carrier concentration of 5 × 1017 cm-3 was obtained. High quality single crystalline-like InP films on low-cost metal substrates may potentially be used in the fabrication of inexpensive flexible InP solar cells.

III-V thin-film photovoltaic solar cells based on single-crystal-like GaAs grown on flexible metal tapes

This paper describes the demonstration of the flexible single-junction III-V solar cells based on high-quality epitaxial GaAs thin films on a low-cost flexible metal substrate. The single-crystal-like semiconductor material structure is fabricated to photovoltaic devices with front illumination geometry. We fabricate a proof-of-concept epitaxial GaAs thin film solar cell with an open-circuit voltage of 0.3 V and short-circuit current of 6 mA/cm2, resulting in conversion efficiency of ~1% in AM1.5G condition. Relatively low efficiency can be further increased by material crystalline quality improvement and device optimization. This development has the potential to open a new avenue for next-generation low-cost and high efficiency flexible PV devices.

AlGaAs/GaAs DH and InGaP/GaAs DH grown by MOCVD on flexible metal substrates

High quality, epitaxial, AlGaAs and InGaP thin films have been grown by metal organic chemical vapor deposition (MOCVD) on flexible metal substrates using buffered GaAs on ion-beam textured epitaxial templates. The grown AlGaAs and InGaP films exhibit strong (001) orientation and sharp in-plane texture. We also report preliminary developments on AlGaAs/GaAs and InGaP/GaAs double heterostructures (DH) to measure minority carrier life-time of GaAs thin films grown using MOCVD. Deposition of undoped AlGaAs was done on flexible GaAs/Ge template with a target Al concentration of 10-40 %, at different growth temperatures (650-800 °C) and 20 Torr process pressure. We have observed minority carrier lifetime of greater than 2 ns for GaAs films grown at 650 °C and sandwiched between Al0.2Ga0.8As DH grown at 750 °C. Deposition of lattice matched updoped In0.48Ga0.52P/GaAs is also in progress. Epitaxial AlGaAs and InGaP can be further utilized in the fabrication of flexible low-cost III-V solar cells on metal substrates.

Thin film III–V photovoltaics using single-cry stalline-like, flexible substrates

High quality, epitaxial undoped, Zn- and Se-doped GaAs films have been grown by metal organic chemical vapor deposition (MOCVD) on flexible metal substrates using buffered Ge epitaxial templates. The GaAs films exhibit strong (400) orientation, sharp in-plane texture, strong photoluminescence and a defect density of ~107 cm-2. Furthermore, the GaAs films exhibit hole and electron mobilities as high as 106 and 872 cm2/V-s respectively. Also, roll-to-roll MOCVD growth of epitaxial GaAs films has been demonstrated for the first time on flexible metal substrates. These high mobility single-crystalline-like GaAs thin films on inexpensive flexible metal substrates are being developed for high efficiency, low cost thin film III-V solar photovoltaics (PV).

Computational Method for Composition Determination of Multilayer Epitaxial Semiconductor Structures Using Standards-Based Energy-Dispersive X-Ray Spectrometry

1. University of Houston, Mechanical Engineering, Houston, TX, U.S.A. 2. South Dakota School of Mines & Technology, Nanoscience & Nanoengineering, Rapid City, SD, U.S.A. Accurate measurements of composition provide critical information in understanding and optimizing epitaxial growth of compound semiconductors and alloys particularly used in optoelectronic devices. Energy-dispersive X-ray spectrometry (EDX) is widely used technique in transmission electron microscopy (TEM) to rapidly identify compositions of multilayer structures. It is easy to derive qualitative conclusions instantly with this method, quantitative analysis of the generated data remains a challenge that requires specialized software tools.