Nazmul Ahsan

Intermediate band solar cells: Recent progress and future directions

Extensive literature and publications on intermediate band solar cells (IBSCs) are reviewed. A detailed discussion is given on the thermodynamics of solar energy conversion in IBSCs, the device physics, and the carrier dynamics processes with a particular emphasis on the two-step inter-subband absorption/recombination processes that are of paramount importance in a successful implementation high-efficiency IBSC. The experimental solar cell performance is further discussed, which has been recently demonstrated by using highly mismatched alloys and high-density quantum dot arrays and superlattice. IBSCs having widely different structures, materials, and spectral responses are also covered, as is the optimization of device parameters to achieve maximum performance.

Two-photon excitation in an intermediate band solar cell structure

We present evidence for the production of photocurrent due to two-photon excitation in an intermediate band solar cell structure. The structure consists of an n-GaNAs intermediate band layer sandwiched between a p-AlGaAs emitter and an n-AlGaAs barrier layer with suitable doping level to block electron escaping from the intermediate band to the bottom n-GaAs substrate. Multi-band transitions observed in two-photon excitation experiments are explained using photo-modulated reflectance spectrum, and further support for intermediate band solar cell operation of this structure is given by current-voltage measurements.

Effect of Sb on GaNAs Intermediate Band Solar Cells

We present a comparative study on the material properties and two-photon excitation (TPE) experiments that involve three bands between a GaNAs and a GaNAsSb absorber designed for intermediate band solar cells. The absorber layers were sandwiched between p-AlGaAs emitter layers and n-AlGaAs IB barrier layers. This permits production of above the bandgap electron-hole pairs by TPE involving two subband photons with the intermediate band as the stepping stone. A recovery in the carrier population in the intermediate band of the GaNAsSb absorber was realized due to an improved material quality. An enhancement in the photocurrent production due to TPE and an associated improvement in the open-circuit voltage were observed.

Heterologous expression in Pichia pastoris and single-step purification of a cysteine proteinase from northern shrimp.

A distinct cysteine proteinase (NsCys) of northern shrimp Pandalus borealis belonging to cathepsin L subgroup of the papain superfamily has been overexpressed as a precursor form (proNsCys) in Pichia pastoris. We adopted a simple and quick procedure to generate an expression cassette by constructing a donor vector harboring proNsCys followed by recombination with an acceptor vector in a way so that the proNsCys gene was placed downstream of the methanol-inducible AOX1 promoter and alpha-mating factor signal sequence gene. In addition, we used glycerol complex medium that supported high growth of yeast before induction while induction was carried out in minimal methanol medium thereby facilitating the secreted protein to be purified with a single size-exclusion chromatography. The recombinant enzyme was purified in two enzymatically active fractions: both corresponding to mature NsCys with, however, the major one comprising two molecular species of NsCys which had their severed prodomain non-covalently attached. The overall yield was about 100 mg of crude or 60 mg of purified recombinant enzyme comprising both mature and prodomain-attached forms of NsCys per liter of yeast culture. The recombinant NsCys was biologically active as observed by gelatin zymography and its ability to cleave Z-Phe-Arg-MCA, a synthetic substrate for cathepsin L. The development of the system reported here provides a cost-effective and easy to manipulate expression system to obtain large quantities of fully functional shrimp enzyme that will enable the functional characterization of this unique enzyme for both research and industrial purposes.

Defect study of molecular beam epitaxy grown undoped GaInNAsSb thin film using junction-capacitance spectroscopy

Defects in undoped GaInNAsSb thin film (i-GaInNAsSb) were investigated by junction-capacitance technique using admittance and transient photocapacitance (TPC) spectroscopy. An electron trap D2 was identified at 0.34 eV below the conduction band (EC) of i-GaInNAsSb using admittance spectroscopy. Optical transition of valance band (EV) electrons to a localized state OH1 (EV + 0.75 eV) was manifested in negative TPC signal. Combined activation energy of OH1 and D2 defect corresponds to the band-gap of i-GaInNAsSb, suggesting that OH1/D2 acts as an efficient recombination center. TPC signal at ∼1.59 eV above EV was attributed to the nitrogen-induced localized state in GaInNAsSb.

Study on the device structure of GaInNAs(Sb) based solar cells for use in 4-junction tandem solar cells

We have evaluated n-GaAs/i-GaInNAsSb/p-GaAs (nip) and p-GaAs/i-GaInNAsSb/n-GaAs (pin) solar cells grown by RF-molecular beam epitaxy. The absorption edges of these samples were at around 1180 nm. The currents generated in GaInNAs(Sb) layers, which were calculated from AM 1.5 spectrum and QE spectra at λ >; 870 nm region, were 4.6 (nip) and 5.1 (pin) mA/cm2, respectively. Illuminated current voltage curves show that the open circuit voltages (Voc) and FF values were 0.42 V and 0.71, respectively, for both structures. From these measurements, since almost the same PV properties are achieved for both samples, we found no distinct disadvantage to adopt nip structure substituted for pin GaInNAsSb solar cell, although there seems to be some difficulties in carrier collection near by the deep i-GaInNAsSb/p-GaAs interface in nip sample. We observed an enhancement in the filtered current density when GaInNAsSb absorber thickness is increased in nip structure without any degradation of other photovoltaic parameters. In order to further improve filtered current density, increase of GaInNAs(Sb) absorber thickness is necessary.

Identification of defect types in moderately Si-doped GaInNAsSb layer in p-GaAs/n- GaInNAsSb/n-GaAs solar cell structure using admittance spectroscopy

Bias dependence of the admittance spectroscopy of GaInNAsSb based solar cell structure has been performed to identify and characterize the type of defects, for example interface and/or bulk type defects in a moderately Si doped GaInNAsSb (n-GaInNAsSb) layer in the structure. From the zero bias admittance spectrum, three peaks namely E1, E2, and E3 corresponding to the localized level at 0.03 eV, 0.07 eV, and 0.16 eV below the conduction band edge (EC) of n-GaInNAsSb material, respectively, were found. Constant position of E2 and E3 peak in the admittance spectra in response to the various applied DC reverse bias suggests that E2 and E3 are related to the bulk type defects being spatially homogeneous throughout the bulk of the n- GaInNAsSb film. However, bias dependence admittance of the E1 peak along with the capacitance - voltage (C-V) measurement as well as characteristic feature in the temperature dependent junction capacitance value strongly suggests that E1 peak might be originated due to the free ca...

Effect of antimony on the deep-level traps in GaInNAsSb thin films

Admittance spectroscopy has been performed to investigate the effect of antimony (Sb) on GaInNAs material in relation to the deep-level defects in this material. Two electron traps, E1 and E2 at an energy level 0.12 and 0.41 eV below the conduction band (EC), respectively, were found in undoped GaInNAs. Bias-voltage dependent admittance confirmed that E1 is an interface-type defect being spatially localized at the GaInNAs/GaAs interface, while E2 is a bulk-type defect located around mid-gap of GaInNAs layer. Introduction of Sb improved the material quality which was evident from the reduction of both the interface and bulk-type defects.

High electron mobility in Ga(In)NAs films grown by molecular beam epitaxy

We report the highest mobility values above 2000 cm2/Vs in Si doped GaNAs film grown by molecular beam epitaxy. To understand the feature of the origin which limits the electron mobility in GaNAs, temperature dependences of mobility were measured for high mobility GaNAs and referential low mobility GaInNAs. Temperature dependent mobility for high mobility GaNAs is similar to the GaAs case, while that for low mobility GaInNAs shows large decrease in lower temperature region. The electron mobility of high quality GaNAs can be explained by intrinsic limiting factor of random alloy scattering and extrinsic factor of ionized impurity scattering.

Antimony enhanced homogeneous nitrogen incorporation into GaInNAs films grown by atomic hydrogen-assisted molecular beam epitaxy

In this work, we investigated the effect of Sb on N-induced localized states in GaInNAs films. From 77 K photoluminescence (PL) spectra for GaInNAs(Sb) films grown with different Sb fluxes, single band-to-band emissions observed in all samples. The emission wavelength redshifts with increasing Sb flux, which corresponds to an increase in Sb composition. The PL intensity and full-width at half maximum (FWHM) are both improved for a narrow range of Sb flux of 1 – 5×10−8 Torr. On the other, higher Sb flux deteriorates the PL characteristics. It can be thought that any higher Sb flux induces some kind of Sb-related defects such as SbGa antisites. Further in temperature dependent PL measurements, energy shifts, so-called “S-shaped” curves, were observed in all samples, which indicate a strong carrier localization. Although GaInNAs sample showed a large energy shift of 53 meV, irradiation of Sb decreases the localization energies to 13 – 22 meV. These results show that Sb can enhance the homogeneity of GaInNAs alloy, since the carrier localization is led by inhomogeneous N incorporation. The internal quantum efficiency characteristics for GaInNAsSb solar cell also improved by introducing an optimum amount of Sb and a redshift of fundamental absorption edge into 1 eV range was achieved.