Pavel Dutta

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.

SELECTIVE AND UPHILL TRANSPORT OF URANYL ION IN THE PRESENCE OF SOME BASE METALS AND THORIUM ACROSS BULK LIQUID MEMBRANE BY DI (2-ETHYLHEXYL) PHOSPHORIC ACID

Di (2-ethylhexyl) phosphoric acid has been successfully used as a selective carrier for the transport of uranyl ion across a bulk liquid membrane of chloroform. More than 98% uranium is selectively transported from a binary or a multi component mixture of cations in less than 3 hr. The pH of the feed solution was kept between 4.0 and 5.0 while 0.1 M hydrochloric acid served as a stripping agent in the receiving compartment. The co-transport of Zn2+ and Fe3+ was significantly small (<4%) but that of thorium was very high. However, the interference from Th(IV) and transition metal ions could be eliminated by the addition of EDTA (E. Merck (India) Ltd., Mumbai) to the feed solution. Different experimental parameters affecting the transport process were optimized and the membrane separation method was applied to the recovery and pre-concentration of uranium from synthetic seawater sample. It was concluded that approximately 98% of uranium could be recovered and a hundred fold pre-concentration of the metal could be accomplished in less than 3 hr.

FACILITATED TRANSPORT OF Th(IV) ACROSS BULK LIQUID MEMBRANE BY DI(2-ETHYLHEXYL)PHOSPHORIC ACID

Di(2-ethylhexyl) phosphoric acid (DEHPA) has been successfully used as a carrier for the selective and efficient transport of Th(IV) across a bulk liquid membrane (BLM) of chloroform. The feed comprised of a solution of pure Th(IV) or a binary mixture of Th(IV) and a cation, such as Na+, K+, Ca2+, Mg2+, Cu2+, Pb2+, La3+, Fe3+, or UO2+ 2, in water maintained at pH 2, while 0.1 mol/L hydrochloric acid served as a stripping agent in the receiving compartment of the permeation cell. Greater than 99% of Th4+ selectively permeated across the membrane in less than 3 h, while the transport of other cations present along with thorium was less than 3% during the same time. Selectivity of the transport of thorium was greatly affected by the presence of Fe3+ and UO2+ 2 in the feed solution. Fe3+ could be effectively masked by the addition of SCN− or citric acid to the feed solution but the cotransport of UO2+ 2 could only be partially reduced by the addition of carbonate or thiocyanate to the feed solution.

Solvent effect on the morphology of P3HT/PCBM films

This work reports the study of solvent effects on the morphology for P3HT/PCBM films using chlorobenzene, 1, 2-dichlorobenzene and 1, 3-dichlorobenzene. Although extensive research has been focused on investigating devices using chlorobenzene and 1, 2-dichlorobenzene and it was found that 1, 2-dichlorobenzene led to an improved device performance, little work has been conducted in the morphology by comparing films fabricated via chlorobenzene, 1, 2-dichlorobenzene and 1, 3-dichlorobenzene. Atomic force microscopy (AFM) was performed to study the film morphology using chlorobenzene, 1, 2-dichlorobenzene and 1, 3-dichlorobenzene as solvents. Initial studies showed that the size of nanocrystallites in 1, 2-dichlorobenzene based films is smaller than nanoclusters in the other two films. Kelvin probe force microscopy (KFM) images, which were used to figure out the electron transport pathway, together with AFM images, showed the solvent effect on the morphology of these films. In addition, obvious red shifts were observed in the UV-Vis absorption spectra for the P3HT/PCBM blend from 1, 3-dichlorobenzene and 1, 2-dichlorobenzene compared to the one from chlorobenzene.

Comparative study of nc-Si: H deposited by reactive sputtering using crystalline and polycrystalline silicon targets

Nanocrystalline silicon (nc-Si:H) is an important material for solar cell applications. Thin films of nc-Si:H can be deposited cost-effectively using chemical vapor deposition (CVD) and sputter deposition. While CVD method offer higher deposition rate and is commonly used in PV industry, microstructural properties (such as hydrogen incorporation, crystallinity, surface morphology, dopant incorporation) can be efficiently controlled using sputter deposition. While all the previous studies on sputter deposited nc-Si used crystalline Si (c-Si) sputter target, the current study compares the properties of nc:Si-H films using single and polycrystalline Si (p-Si) targets. P-Si is relatively inexpensive and easy to fabricate in bulk quantities. Several sets of nc-Si:H films were deposited using both c-Si and p-Si targets under similar conditions on glass substrates and characterized using TEM, XRD, AFM, UV-VIS spectroscopy and DC conductivity. Results indicate that surface morphology, size of nanocrystals, crystalline fraction, optical bandgap, coefficient of optical absorption and activation energy of samples were independent of sputter target used. Preferred orientation of nanocrystallites for all the films was along (111) direction with a crystallite size of 4–10 nm. All the samples have wide optical band gap ranging between 2 and 2.1 eV. Comparable activation energies at low (≪370K) and high temperature (≫370K) regions was obtained and was attributed to carrier transport through nanocrystalline and amorphous phase of the films. These results indicate that nc-Si:H thin films for solar cell applications can deposited using low-cost p-Si targets in-lieu of conventional c-Si targets.

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.

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.

Optimization of a single crystalline-like germanium thin film growth on inexpensive flexible substrates and fabrication of germanium bottom junction

III-V semiconductor multijunction solar cells utilizing a germanium (Ge) bottom junction show high efficiency, however, are limited to the expensive Ge single-crystal wafers. Our approach focuses on thin film-based multijunction photovoltaics to replace Ge wafers by depositing single-crystalline-like Ge thin film on flexible and inexpensive metal tapes. The deposition temperature and film thickness were optimized to achieve optimal surface roughness, carrier mobility and carrier concentration. It has been found that 850°C is an optimum deposition temperature to achieve the sharpest texture, the highest carrier mobility and film smoothness. The surface roughness decreases while hall mobility increases with increasing Ge layer thickness up to 1.25 μm. Finally, a p-Ge/n-Ge bottom junction was fabricated based on the optimized Ge thin film.