L. Jiao

Performance and stability of Si:H p–i–n solar cells with i layers prepared at the thickness-dependent amorphous-to-microcrystalline phase boundary

Systematic studies have been carried out on the transition from the amorphous to the microcrystalline phase in intrinsic Si:H as a function of the accumulated film thickness and the effect of this transition on p–i–n solar cell performance [J. Koh, Y. Lee, H. Fujiwara, C. R. Wronski, and R. W. Collins, Appl. Phys. Lett. 73, 1526 (1998)]. Guided by a deposition phase diagram obtained from real-time spectroscopic ellipsometry, cell structures having i layers deposited with different H2-dilution levels and thicknesses were investigated. For these structures, the fill factors are controlled by the bulk i layers. From the systematic changes in the fill factors, specifically their initial and degraded steady-state values and their degradation kinetics, the effects of the transition from the amorphous to the microcrystalline phase within the Si:H layers are identified, and insights are obtained into the properties of these structurally graded materials.

An improved analysis for band edge optical absorption spectra in hydrogenated amorphous silicon from optical and photoconductivity measurements

The uncertainties inherent in the normalization of subgap photoconductivity spectra to the optical absorption spectra α(hv) in a-Si:H based films have been addressed. An analysis is presented which is based on optical transitions of constant dipole matrix element between parabolic distributions of extended states and exponential distributions of localized tail states. This analysis has been used to normalize the two sets of results accurately, as verified by photothermal deflection spectroscopy measurements, and is shown to be useful in the commonly encountered cases, in which the two spectra do not overlap over an extended region. Improved quantitative fits of α(hv), for photon energy from ∼1.5 to 2.4 eV, obtained on different a-Si:H based films indicate that the localized exponential band tail regions extend ∼60–70 meV above the optical gap.

Kinetics of light-induced changes in p-i-n cells with protocrystalline Si:H

Studies have been carried out on the thickness dependent transition between the amorphous and microcrystalline phases in intrinsic Si:H materials (i-layers) and its effect on p-i-n solar cell performance. P(a-SiC:H)-i(a-Si:H)-n({micro}cSi:H) cell structures were deposited with the intrinsic Si:H layer thickness and the flow ratio of hydrogen to silane, R=[H{sub 2}]/[SiH{sub 4}], guided by an evolutionary phase diagram obtained from real-time spectroscopic ellipsometry. The thickness range over which the fill factors are controlled by the bulk was established and their characteristics investigated with different protocrystalline i-layer materials (i.e., materials prepared near the amorphous to microcrystalline boundary but on the amorphous side). Insights into the properties of these materials and the effects of the transition to the microcrystalline phase were obtained from the systematic changes in the initial fill factors, their light-induced changes, and their degraded steady states for cells with i-layers of different thickness and H{sub 2} dilution.

Light-induced changes in hydrogen-diluted a-Si : H materials and solar cells: A new perspective on self-consistent analysis

Abstract We report a study on a-Si: H materials and p(a-SiC : H)/i(a-Si : H)/n(μc-Si) solar cells prepared without and with hydrogen dilution (10 : 1) at substrate temperatures between 240°C and 130°C. In contrast to previously reported studies, the cell characteristics in the annealed state of these ∼4000 A thick cells could be directly correlated with the properties of their corresponding i-layer materials. Also, despite the importance of the p/i interface regions, very similar kinetics of light-induced changes are observed in the cells and the corresponding films. In particular, both cells and films fabricated with hydrogen dilution reach a degraded steady state in less than 100 h of AM 1 illumination, which offers a well-defined “marker” for the direct correlation of their respective light-induced changes. Advantage is also taken of the differences in degradation kinetics between diluted and undiluted materials in fabricating custom-designed cells in which these well-characterized intrinsic materials are incorporated into either the bulk or the p/i interface regions.

Stability of a-Si:H solar cells and corresponding intrinsic materials fabricated using hydrogen diluted silane

We report on a study in which properties of p(a-SiC:H)/i(a-Si:H)/n(/spl mu/c-Si) a-Si:H solar cells and their i-materials prepared with hydrogen dilution are investigated and compared with films and cells prepared without hydrogen dilution. The cells and the corresponding intrinsic films were fabricated in a multi-chamber PECVD system with pure silane (SiH/sub 4/) and silane diluted with hydrogen in the ratio [H/sub 2/]/[SiH/sub 4/]=10. The initial performance of both types of cells (/spl sim/4000 /spl Aring/ thick) fabricated without optical enhancement are quite similar but the diluted cells are significantly more stable. Despite the reported importance of the interface regions in determining their solar cell characteristics, a direct correlation between the degradation of the diluted solar cells and their intrinsic films is observed in this study. Both diluted cells and films reach a steady state of degradation under AM1 illumination within 100 hours. Distinctly different kinetics from the undiluted materials and cells and the ability to reach steady state degradation in less than 100 hours offer a new probe for improving our understanding of the mechanisms limiting cell performance.

Impact of hydrogen dilution on the properties and light induced changes of a-Si:H based materials and solar cells

Studies have been carried out on a-Si:H materials and corresponding solar cells fabricated with and without hydrogen dilution of silane by rf PECVD. The effect of hydrogen dilution on the growth kinetics and microstructures and their dependence on the substrate temperature have been studied. Hydrogen diluted a-Si:H materials and solar cells exhibit improved properties and higher stability to light induced changes. Distinct differences are found in the electron mobility lifetime ({mu}{tau}) products and subgap absorption over a wide range of generation rates. Striking differences are also found in the kinetics of light induced degradation in both the materials and their corresponding solar cells. Direct correlations are presented between the degradation kinetics of p(a-SiC:H)/i(a-Si:H)/n({micro}c-Si) solar cells and those of thin film materials constituting the i-layers.

A new perspective on the characterization of materials for a-Si:H solar cells

A study has been carried out on a-Si:H solar cell materials fabricated under a wide range of deposition conditions in different laboratories. The results on both thin films and corresponding Schottky barrier cell structures demonstrate that analysis and characterization based solely on the neutral dangling bonds are clearly inadequate. Contributions of charged defects to the properties of a-Si:H, their effect on light-induced changes are identified together with the limitations of methods commonly used to characterize the solar cell properties and stability of a-Si:H materials. Self-consistent fitting of a wide range of results on films and Schottky barrier cell structures is obtained with a gap state distribution in which charged defects are included.

Advances in the characterization of compositionally-graded layers in amorphous semiconductor solar cells by real time spectroellipsometry

The authors have developed a real time spectroellipsometry data analysis procedure that allows us to characterize compositionally-graded amorphous semiconductor alloy thin films prepared by plasma-enhanced chemical vapor deposition (PECVD). As an example, the authors have applied the analysis to obtain the depth-profile of the optical gap and alloy composition with {le} {angstrom} resolution for a hydrogenated amorphous silicon-carbon alloy (a-Si{sub 1{minus}x}C{sub x}:H) film prepared by continuously varying the gas flow ratio z = [CH{sub 4}]/{l_brace}[CH{sub 4}] + [SiH{sub 4}]{r_brace} In the PECVD process. The graded layer has been incorporated at the p/i interface of widegap a-Si{sub 1{minus}x}C{sub x}:H (x {approximately} 0.05) p-i-n solar cells, and consistent improvements in open-circuit voltage have been demonstrated. The importance of the graded-layer characterization is the ability to relate improvements in device performance directly to the physical properties of the interface layer, rather to the deposition parameters with which they were prepared.