Muhammad Monirul Islam

Na-induced variations in the structural, optical, and electrical properties of Cu(In,Ga)Se2 thin films

The systematic variations in the structural, optical, and electrical properties of polycrystalline Cu(In,Ga)Se2 (CIGS) thin films with Na doping level were investigated. Precise control of the Na concentration in CIGS films was demonstrated using alkali-silicate glass thin layers of various thicknesses deposited on substrates prior to CIGS growth. The CIGS grain size was observed to decrease with increasing Na concentration, although the surface morphology became smoother and exhibited a stronger (112) texture, which has been demonstrated consequence of larger grain size. The Ga composition gradient in the CIGS films was found to become large due to the presence of Na during growth, which in turn led to a decrease in the nominal band gap energy. Variations in the photoluminescence spectra and electrical properties suggested that the formation of an acceptor energy state, which may originate from OSe point defects, was enhanced in the presence of Na. This result suggests that not only Na, but also the pres...

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.

Time-Resolved Microphotoluminescence Study of Cu(In,Ga)Se2

The carrier recombination processes in Cu(In1-x,Gax)Se2 (CIGS) thin films were investigated by time-resolved microscopic-photoluminescence (µ-PL) measurement at room temperature. For films with x = 0.45, the spatial distribution of the donor–acceptor pair luminescence is much larger than the grain size. The PL decay lifetime is directly correlated with the µ-PL intensity, but the spectral shape is identical regardless of the sample position. These results suggest that the nonuniform distribution of nonradiative recombination centers mainly affects the carrier recombination in CIGS thin films. At relatively high Ga concentrations (x ≥0.7), the spatial inhomogeneity in µ-PL intensity is enhanced and decay accelerated, suggesting an increase in the density of nonradiative recombination centers. Thus, suppression of nonradiative recombination is critical to enhancing the performance of CIGS-based solar cells.

Impact of Cu/III ratio on the near-surface defects in polycrystalline CuGaSe2 thin films

Polycrystalline CuGaSe2 thin films grown with various Cu/III(=Cu/Ga) ratios were investigated by positron annihilation spectroscopy (PAS). The line-shape parameter S of the spectra was used to characterize defects in CuGaSe2 films. The S-parameter in positron annihilation spectra increased with decreasing bulk Cu/III ratio in the CuGaSe2 film. Experimental results combined with theoretical calculation show the formation of multiple vacancy-type defect complexes in the near-surface region of the CuGaSe2 film when Cu-content in the film is decreased. These point defects appear to cause the higher S-parameter in PAS measurement.

Impact of Se flux on the defect formation in polycrystalline Cu(In,Ga)Se2 thin films grown by three stage evaporation process

Cu(InxGa1−x)Se2 (CIGS) films, grown under various Se fluxes, have been investigated by the positron annihilation spectroscopy. The line-shape parameter (S) of the positron annihilation spectra was used to characterize the defects in the CIGS films. When Se flux was decreased, the S parameter at the surface and subsurface region of the films increased. This phenomenon was attributed to the increased concentration of the defect complex formed between Se and Cu vacancies. S parameter at the surface region was significantly higher than that in the bulk region of each film. It was explained with the model of compositional inhomogeneities along the depth of the film. Solar cell performance strongly correlated to the S parameter at the surface region of the CIGS films. Sufficient Se flux was found to be effective to reduce the S parameter, thereby, to suppress the defects in the films.

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.