Monika Rathi

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.

A Standards-Based Method for Compositional Analysis by Energy Dispersive X-Ray Spectrometry Using Multivariate Statistical Analysis: Application to Multicomponent Alloys

Given an unknown multicomponent alloy, and a set of standard compounds or alloys of known composition, can one improve upon popular standards-based methods for energy dispersive X-ray (EDX) spectrometry to quantify the elemental composition of the unknown specimen? A method is presented here for determining elemental composition of alloys using transmission electron microscopy-based EDX with appropriate standards. The method begins with a discrete set of related reference standards of known composition, applies multivariate statistical analysis to those spectra, and evaluates the compositions with a linear matrix algebra method to relate the spectra to elemental composition. By using associated standards, only limited assumptions about the physical origins of the EDX spectra are needed. Spectral absorption corrections can be performed by providing an estimate of the foil thickness of one or more reference standards. The technique was applied to III-V multicomponent alloy thin films: composition and foil thickness were determined for various III-V alloys. The results were then validated by comparing with X-ray diffraction and photoluminescence analysis, demonstrating accuracy of approximately 1% in atomic fraction.

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.

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.

Comparative studies of single-crystalline-like Ge thin film on inexpensive flexible metal substrates

Single-crystalline-like germanium thin films on flexible metallic substrates are being developed for III-V compound semiconductors and silicon. The germanium films were deposited via reel-to-reel radio-frequency magnetron sputtering on multi-layered templates fabricated by ion beam assisted deposition (IBAD) and on Ni-5%W metal foils, prepared by Rolling Assisted Biaxially-Textured Substrates (RABiTS) process. In both cases, the germanium thin films were epitaxially grown on biaxially-textured CeO2 films. The Ge film on IBAD template was found to exhibit a better crystallographic texture quality compared to films fabricated on Ni-W substrate. Although a similar micro-scale roughness was observed in Ge on both types of substrates, the grain size of Ge thin film on Ni-W was found to be more than 30μm which is much larger than that on IBAD tape with grain size of hundreds of nanometers.

Growth and characterization of quaternary (GaInAsP-GaAs) graded heterostructures

The growth conditions of Ga_xIn_1-xAsyP_1-y lattice-matched to GaAs using MOCVD on (001) oriented substrates are analyzed. The epitaxial layer composition is varied from GaAs to lattice-matched Ga_0.52In_0.48P via quaternary grade. The layer quality is more sensitive to experimental variables as the composition approaches the Ga_0.52In_0.48P side than when growing quaternaries. TEM images show promising results even though the top layer Ga_0.52In_0.48P needs to be further optimized. The fundamental objective of this work is to improve understanding over the growth of LMM alloys on GaAs with special emphasis on quaternary grades. We believe that incorporation of LMM quaternaries with ternaries in a single-junction device will provide flexibility in design which may ultimately lead to record PV conversion efficiencies.

Compositional and structural characterization by TEM of lattice-mismatched III-V epilayers

We discuss compositional and structural transmission electron microscopy (TEM) characterization of lattice-mismatched (LMM) III-V epilayers grown on GaAs by metalorganic chemical vapor deposition (MOCVD), with possible applications in high-efficiency multijunction solar cells. In addition to the use of TEM imaging to survey layer thicknesses and defect morphology, our analysis emphasizes the particular methods of energy-dispersive X-ray spectrometry (EDX) and convergent-beam electron diffraction (CBED). Outlined here is a standards-based method for extracting compositions by EDX, which uses principal-component analysis (PCA) [1], combined with the zeta-factor approach of Watanabe and Williams [2]. A procedure is described that uses the coordinates of high-order Laue zone (HOLZ) lines, which are found in the bright-field disks of CBED patterns, to extract composition and strain parameters from embedded epilayers. The majority of the crystal growth for this work was performed at NREL, which has accommodated the development at SDSM&T of the characterization techniques described. However, epilayer deposition capability at SDSM&T has recently been achieved, using a home-built system, which is presently being used to examine new lattice-mismatched structures relevant to photovoltaic technology.

Structure and composition of lattice-mismatched III–V epilayers for high-efficiency photovoltaics

Lattice mis-matched (LMM) multilayered structures were grown using metalorganic chemical vapor deposition (MOCVD) at NREL. Energy dispersive x-ray (EDX) spectrometric mapping was conducted on transmission electron microscope (TEM) sample cross-sections of GaAs1−yPy grades on GaAs substrates, with known layer compositions. The compositions were determined by x-ray diffraction, which serve as standards for EDX. Data were acquired by both TEM spot analysis and 2-D maps in scanning TEM mode. The IP/IAs intensity ratio obtained from EDX, plotted against y / (1−y) obtained from XRD shows a linear slope. We seek to parameterize the variations from these standard samples for accurate composition determination of unknown samples [1]. The quantitative method we are developing is based on principle component analysis (PCA) [2, 8]. We anticipate our PCA algorithm will have applications in future growth designs, including heterostructures with multiple quaternary steps, or even continuous grades.