Search

We critically examine how two processing steps commonly applied in the preparation of cadmium telluride (CdTe)-based solar cells, i.e., the cadmium chloride treatment and the subsequent stepwise bromine/methanol wet etching process, impact the structural and optical properties of polycrystalline CdTe thin-films. In particular, drawing upon a conjuncture of photothermal deflection spectroscopy and spectroscopic ellipsometry experimental results, we determine the spectral dependence of the optical absorption coefficient, α(E), over the photon energy range from 1.1 to 2.0 eV for samples of rf sputtered (RFS) and close space sublimation (CSS) CdTe. The impact of these processing steps on shaping the grazing incidence x-ray diffraction pattern is also examined. We extend the analysis to devices through interpretation of the spectral dependence of the external quantum efficiency associated with two cadmium chloride treated CdTe-based solar cells. The cells are comparably prepared with the exception of the absorber, one by RFS and the other by CSS. Through the use of our results for the thin-film CdTe optical functions and a model for the solar cell multilayer structure, we simulate the resultant external quantum efficiency spectrum. Through a critical contrast with the corresponding solar cell acquired experimental results, we glean insights into the carrier trapping and recombination processes that occur within the two types of CdTe absorbers. [ABSTRACT FROM AUTHOR]

Both the origins of the high open circuit voltages (VOC) in amorphous silicon solar cells having p layers prepared with very high hydrogen dilution and the physical structure of these optimum p layers remain poorly understood topics, with several studies offering conflicting views. This work attempts to overcome the limitations of previous studies by combining insights available from electronic measurements, real time spectroscopic ellipsometry, atomic force microscopy, and both high-resolution transmission electron microscopy (TEM) and dark field TEM of cross sections of entire solar cells. It is found that solar cells fabricated with p layers having a low volume fraction of nanocrystals embedded in a protocrystalline Si:H matrix possess lower recombination at the i/p interface than standard cells and deliver a higher VOC. The growth of the p layers follows a thickness evolution in which pure protocrystalline character is observed at the interface to the i layer. However, a low density of nanocrystallites nucleates with increasing thickness. The advantages offered by the protocrystalline character associated with the amorphous phase of the mixed-phase (amorphous+nanocrystalline) p layers prepared with excess H2 dilution account for the improved VOC of the optimum p layers. In this model, the appearance of a low volume fraction of nanocrystals near the top transparent conductor interface is proposed to be incidental to the high VOC. [ABSTRACT FROM AUTHOR]

We have developed a Kramers-Kronig consistent analytical expression to fit the measured optical functions of hydrogenated amorphous silicon (a-Si:H) based alloys, i.e., the real and imaginary parts of the dielectric function (∈[sub 1], ∈[sub 2]) (or the index of refraction n and absorption coefficient α) versus photon energy E for the alloys. The alloys of interest include amorphous silicon-germanium (a-Si[sub 1-χ]Ge[sub χ] :H) and silicon-carbon (a-Si[sub 1-χ]C[sub χ] :H), with band gaps ranging continuously from ∼1.30 to 1.95 eV. The analytical expression incorporates the minimum number of physically meaningful, E independent parameters required to fit (∈1, ∈[sub 2]) versus E. The fit is performed simultaneously throughout the following three regions: (i) the below-band gap (or Urbach tail) region where a increases exponentially with E, (ii) the near-band gap region where transitions are assumed to occur between parabolic bands with constant dipole matrix element, and (iii) the above-band gap region where (∈[sub 1], ∈[sub 2]) can be simulated assuming a single Lorentz oscillator. The expression developed here provides an improved description of ∈[sub 2] (or α) in the below-band gap and near-band gap regions compared with previous approaches. Although the expression is more complicated analytically, it has numerous applications in the analysis and simulation of thin film a-Si:H based p-i-n and n-i-p multilayer photovoltaic devices. First, we describe an approach whereby, from a single accessible measure of the optical band gap, the optical functions can be generated over the full solar spectrum for a sample set consisting of the highest quality intrinsic a-Si:H based alloys prepared by plasma-enhanced chemical vapor deposition using the principle of maximal H[sub 2] dilution. Second, we describe quantitatively how such an approach can be modified for sample sets consisting of lower quality... [ABSTRACT FROM AUTHOR]

Presents information on a study which measured the unnormalized Stokes vector by using a rotating-compensator multichannel ellipsometer. Polarization state of light reflected from specular and textured SnO2: F films; Experimental details; Results and discussions.

Presents a study which reported the results of ellipsometry measurements performed during the growth of hydrogenated amorphous silicon thin films on crystalline silicon substrates. Formation of low-density microstructure in the initial stages of growth; Effect of sputtering on the glow-discharge decomposition of gases; Discussion on the pseudodielectric function trajectories.

Presents information on a study which extended in situ ellipsometry characterization of the nucleation and microstrucutral evolution of amorphous silicon (a-Si:H) to the effect of deposition parameters on these processes. Details about the structural evolution of a-Si:H; Important aspects of the ellipsometer/glow-discharge reactor system; General features of in situ trajectories.

Presents a study which examined the effect of substrate microstructure on the growth habit of high-quality hydrogenated amorphous silicon. Experimental details; Results and discussion; Conclusion.

Presents information on a study that examined the growth and microstructure of α-Si:H/α-SiN [subx] :H heterojunctions. Experimental details; Results and discussion on the study; Conclusions.

Presents information on a study which analyzed the in situ ellipsometry measurements taken during the deposition of hydrogenated amorphous silicon on crystalline silicon substrates in a convention rf glow-discharge apparatus. Nucleation behavior of hydrogenated amorphous silicon; Experimental details; Results and discussion; Conclusions.

Examines the optical properties of ion-beam-sputtered silicon and germanium using spectroscopic ellipsometry. Discrepancies between the structure of the p-silicon deduced from Raman and ellipsometry spectra; Significance of thin-film polycrystalline silicon; Psuedodielectric functions for ion-beam-deposited silicon prepared at different substrate temperatures.

Analyzes the structure of a hydrogenated amorphous silicon based n-1 two-layer system using spectroscopic ellipsometry measurements. Details on the experiment; Data analysis; Results of the study.

Presents information on a study which investigated the stress in diamond films prepared by microwave plasma chemical vapor deposition as a function of methane concentration and deposition temperature. Calculation of thermal stress; Dependence of the total and intrinsic stress on methane concentration; Stress dependence on deposition temperature; Origin of tensile intrinsic stress.

Investigates the growth rate and optical properties of thin nanocrystalline diamond films prepared over a range of the carbon monoxide(CO)/H[sub2] ratio, using real time spectroellipsometry (RTSE). Comparison between diamond growth and properties from CO/H[sub2] and CH[sub4]/H[sub2]/O[sub2] at similar phase-diagram points; Advantage of RTSE over Raman spectroscopy for characterizing and optimizing nanocrystalline diamond film; Significance of methyl radical for diamond film growth.

Discusses a study which applied a real time spectroellipsometry data analysis procedure to characterize depth profiles in the optical gap for semiconductor alloy thin films prepared by plasma enhanced chemical vapor deposition. Theoretical background; Description of the experimental setup; Findings.

Probes the evolution of surface roughness over a wide range of film thickness during the growth of glow discharge hydrogenated amorphous silicon (a-Si:H) using in situ ellipsometry. Differences in the resulting film properties; Factor to which the columnar morphology observed for SiH[sub4]:argon films is attributed; Description of the pseudodielectric function.

Reports on the application of real time spectroscopic ellipsometry to characterize the nucleation of diamond on c-silicon by tungsten filament-assisted chemical vapor deposition (CVD). Stages of diamond growth by filament-assisted CVD; Preparation of the diamond; Information on the structural model and parameters deduced in a linear regression analysis of the final spectra.