Hamda A. Al-Thani

Direct evidence of a buried homojunction in Cu(In,Ga)Se2 solar cells

The built-in electrical potential of Cu(In,Ga)Se2 (CIGS) solar cells was measured quantitatively and resolved spatially using scanning Kelvin probe microscopy. Profiles of the electrical potential along cross sections of the device demonstrate that the p–n junction is a buried homojunction, and the p/n boundary is located 30–80 nm from the CIGS/CdS interface in the CIGS film. The built-in electric field terminates at the CIGS/CdS interface, indicating that the CdS and ZnO layers of the device structure are inactive for the collection of photoexcited carriers.

Effect of Cu deficiency on the optical properties and electronic structure of CuInSe2 and CuIn0.8Ga0.2Se2 determined by spectroscopic ellipsometry

Spectroscopic ellipsometry measurements of CuInSe2 (CIS) and CuIn0.8Ga0.2Se2 (CIGS) reveal that there are important differences in electronic properties between stoichiometric CIS (CIGS) and Cu-poor CIS (CIGS). We find a reduction in the absorption strength in the spectral region of 1–3eV. This reduction can be explained in terms of the Cu 3d density of states. Cu-poor CIS (CIGS) materials show an increase in band gap due to the reduction in repulsion between Cu 3d and Se 4p states. The experimental results have important implications for the function of polycrystalline optoelectronic devices.

Effect of Cu deficiency on the defect levels of Cu0.86In1.09Se2.05 determined by spectroscopic ellipsometry

Room temperature spectroscopic ellipsometry measurement of Cu0.86In1.09Se2.05 reveals that shallow defect states are found in the sub-band-gap region. The energies of these defect levels are in quantitative agreement with those calculated and measured by photoluminescence, electrical conductivity, optical absorption, and deep level transient spectroscopy at low temperatures. The results can be explained in terms of the defect physics of this material and suggest that the defect levels are due to Cu deficiency. This work opens up the possibility of measuring defect levels of off-stoichiometric or heavily doped semiconductors by spectroscopic ellipsometry at room temperature.

Properties of Cd and Zn partial electrolyte treated CIGS solar cells

We study the influence of Cd partial baths on the photovoltaic properties of CuInGaSe/sub 2/ (CIGS) and CuInGaSSe/sub 2/ (CIGSS) thin film absorbers. We find that efficient solar cells can be fabricated by this treatment, and we compare their properties; with those containing CdS window layers grown by chemical bath deposition. The results suggest that Cd plays a dominant role in establishing efficient photovoltaic junctions in Cu InSe/sub 2/ alloys. Micron scale photoluminescence scans show non-uniformity along the length probed. Cd treatment quenches one of the luminescence transitions, which indicates a change in shallow acceptor level density. We present a model that helps to explain the evolution of photovoltaic action.

Microstructure characterization of Mo back contact used for CIGS based solar cell

Two separate series of Mo thin films were deposited on Si/SiO2, and soda lime glass (SLG) substrates using direct-current planar magnetron sputtering, with a sputtering power density of 1.2 W/cm2. The working gas (Ar) pressure was varied from 0.6 mT to 16 mT to gain a better understanding of the effect of sputtering pressure on the morphology and porosity of the Mo thin films and the subsequent effect on the Na out-diffusion from SLG glass substrates. The morphology of Mo-coated SLG substrates was examined using high-resolution scanning electron microscopy (HRSEM). The porosity of the Mo films as a function of sputtering pressure was studied by transmission electron microscopy (TEM) on Mo-coated Si/SiO2 substrates. Secondary-ion mass spectrometry (SIMS) was applied to depth profile the Na in as-deposited Mo/SLG substrates.

Investigation of the effect of I-ZnO window layer on the device performance of the Cd-free CIGS based solar cells

Cu(In,Ga)Se2 (CIGS) thin films were deposited on Molybdenum coated soda lime glass (SLG/Mo) substrates, using physical vapor deposition (PVD) 3-stage process. The CIGS films were treated by ammonium hydroxide solution before depositing the I-ZnO buffer layer. The I-ZnO layer was deposited using metallorganic chemical vapor deposition (MOCVD) at different substrate temperatures of 200°C, 250°C, and 300°C. The thickness of this buffer layer was varied according to the location of the substrates with respect to the gas flow direction. The CIGS devices were completed by depositing the Al-ZnO window layer by rf magnetron sputtering and applying the Ni/Al front contact grids. The thickness of the I-ZnO buffer layer was measured using a Dek-tak profilometer on soda lime glass substrates which were used as reference samples during the deposition process of I-ZnO buffer layer on CIGS films. Surface depth profiling survey for the elements and their chemical states, as well as their relative concentration was analyzed by X-Ray Photoelectron Spectroscopy (XPS) for as deposited and ammonium hydroxide treated CIGS films. In addition, the performance of the completed CIGS devices was evaluated under standard conditions of 1000 W/m2 and 25°C.

Interface Investigation of ZnO/CdS/CuIn 1−x Ga x Se 2 /Mo Solar Cells

Graded-band-gap CuIn 1−x Ga x Se 2 (CIGS) absorbers with Ga/Ga+In value in the 20%-30% range have a demonstrated efficiency of 18.8%. For CdS-containing devices, the shortcircuit current density (J sc ) has almost reached its expected maximum. However, the open-circuit voltage of CIGS solar cells is limited by the surface microstructure and chemistry. In this work, we examine the microstructural properties and chemistry of CIGS. We also attempted to correlate the above observations and device performance.

The influence of texturing bath conditions on the morphology and optical properties of crystalline silicon

Three different series of p-type (100) Crystalline Silicon (c-Si) FZ wafers of about 270 μm thickness with resistivity of 2–4 Ω-cm were textured by three different concentrations of KOH (2.5 wt.%, 3.5 wt.% and 4.5 wt.%). For every series of textured wafers, three samples were textured, each of 1×1.5 inch2 size. The texturing process was interrupted at different duration of 20 min, 40 min, and 60 min; after which the wafers were removed from the bath and rinsed by DI water, then blow dried with nitrogen. The total bath volume was 700 ml including the initial amount of 15 ml Isopropyl alcohol (IPA). Using an automated dispenser, IPA was also added during the texturing process with a dispensing rate in the range from 0.42 ml/min to 1.3 ml/min, also DI water was added with a fixed dispensing rate of 0.82 ml/min to maintain the concentration of KOH. Additionally, for each fixed KOH concentration, two different texturing bath temperatures were applied: 70°C and 75°C. The aim of this research work is to gain a better understanding on the influence of controlled etching rate (bath composition and temperature) on the silicon surface properties; morphological and optical. Scanning Electron Microscopy (FE-SEM) was applied to study the morphological structure of the textured surface. The surface topography, bearing area ratio, and root-mean-square (RMS) surface roughness of all textured wafers were investigated using Atomic Force Microscopy (AFM). Whereas, the reflectance measurements were carried out using UV-Vis-NIR Spectrometer.

XPS AND UPS investigation of NH/sub 4/OH-exposed Cu(In,Ga)Se/sub 2/ thin films

Photoelectron spectroscopy was used to determine the compositional and electronic changes occurring in Cu(In,Ga)Se/sub 2/ thin films as a result of immersion in aqueous ammonia solution. We find that NH/sub 4/OH-treated CIGS surfaces are preferentially etched of indium and gallium, resulting in the formation of a thin layer of a degenerate Cu-Se compound that we tentatively identify as Cu/sub 2/Se. The work function of ammonia-treated samples is found to increase by 0.6 eV relative to as-grown CIGS thin films. The uniformity of chemical bath effects (etching & deposition) was found to be improved by the addition to the bath of a non-ionic surfactant. Initial device results show that the new surfactant-based chemical bath deposition (CBD) method may lead to better and thinner CdS buffer layers.

Effect of Cu deficiency on the optical properties and electronic structure of CuIn1−xGaxSe2 (x = 0, 0.2) determined by spectroscopic ellipsometry

Spectroscopic ellipsometry measurements of CuInSe2 (CIS), CuIn0.8Ga0.2Se2 (CIGS), and (4CuInSe2)0.8(CuIn5Se8)0.2 reveal that there are important differences in electronic properties between α‐phase CIS (CIGS) and Cu‐poor CIS (CIGS). We find a reduction in the extinction coefficient k in the spectral region of 1–3 eV. This reduction is due to reduced Cu‐3d density of hole states near the valence band maximum. Cu‐poor CIS and CIGS materials show an increase in bandgap due to the reduction in repulsion between Cu‐3d and Se‐4p states. The experimental results have important implications for understanding why polycrystalline CIGS solar cells achieve higher efficiencies than their single‐crystal couterparts.