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International audience; As the negative environmental effects of the current use of non-renewable energy sources have become apparent, hydrogenated amorphous silicon (a-Si:H) solar cell technology has advanced to provide a means of powering a future sustainable society. Over the last 25 years, a-Si:H solar cell technology has matured to a stage where there is currently a production of 30 MWpeak/year; and this production capacity continues to increase. The progress is due to the continuous advances made in new materials, cell designs, and large area deposition techniques for mass production. The absence of long-range order result in not only characteristics which make a-Si:H excellent for thin film solar cells, but also provide great flexibility in the design of different solar cell structures and in the manufacturing of large area monolithic modules. A review is presented here of the progress in the development of a-Si:H based materials as well as the evolution of solar cell structures which led to the continuous improvement in their performance and stability.

Using spectroscopic ellipsometry, the room temperature refractive index and absorption versus wavelength of the ferroelectric antiferromagnet bismuth ferrite, BiFeO3, are reported. The material has a direct band gap at 442 nm wavelength (2.81 eV). Using optical second harmonic generation, the nonlinear optical coefficients were determined to be d(15)/d(22)=0.20 +/- 0.01, d(31)/d(22) = 0.35 +/- 0.02, d(33)/d(22)=-11.4 +/- 0.20, and vertical bar d(22)vertical bar = 298.4 +/- 6.1 pm/V at a fundamental wavelength of 800 nm. (C) 2008 American Institute of Physics.

BiFeO3 thin films have been deposited on (001) SrTiO3 substrates by adsorption-controlled reactive molecular-beam epitaxy. For a given bismuth overpressure and oxygen activity, single-phase BiFeO3 films can be grown over a range of deposition temperatures in accordance with thermodynamic calculations. Four-circle x-ray diffraction reveals phase-pure, epitaxial films with omega rocking curve full width at half maximum values as narrow as 29 arc sec (0.008 degrees). Multiple-angle spectroscopic ellipsometry reveals a direct optical band gap at 2.74 eV for stoichiometric as well as 5% bismuth-deficient single-phase BiFeO3 films. (C) 2008 American Institute of Physics.

Deposition phase diagrams are convenient for categorizing the evolution of the surface microstructure and phase with accumulated thickness for hydrogenated silicon (Si:H) films during plasma-enhanced chemical vapor deposition (PECVD). They can also be used to assess the electronic quality and device suitability of Si:H, based on previous correlations. In this study, phase diagrams have been applied in a comparison of Si:H PECVD using two different plasma excitation frequencies (rf: 13.56 MHz; and vhf: 60 MHz). Smooth crystalline Si (c-Si) wafer substrates have been used to obtain the surface roughness evolution with maximum sensitivity in the amorphous silicon (a-Si:H) growth regime. This study has shown that under all explored conditions of plasma power, frequency, and gas pressure, yielding deposition rates of 0.5-20 Å/s, a-Si:H exhibits improved microstructural characteristics with increasing H2-to-SiH4 flow ratio R right up to the amorphous-to-(mixed-phase microcrystalline) [a→(a+μc)] boundary of the phase diagram. For depositions at R values much lower than the a→(a+μc) transition for a thick film, vhf PECVD can provide a significant improvement in microstructural evolution over rf PECVD, for a given deposition rate. For optimum R just below the a→(a+(ic) transition, however, vhf and rf a-Si:H films exhibit remarkably similar structural evolution for a given rate.

The inconsistencies associated with the common simple method of evaluating the quality and stability of hydrogenated amorphous silicon (a-Si:H) materials for solar cell applications from just the magnitude of subgap absorption (a(E)) are addressed in this study. The approach taken in this study is to characterize the entire a(E) spectrum by its derivatives, d[a(E)]/dE. Results are presented and discussed for two a-Si:H thin film materials differing in deposition rate by an order of magnitude, which illustrate the utility of this approach. The presence of two distinctly different light induced defect states at and below midgap are clearly identified and their evolution characterized. The results are not only consistent with the corresponding electron mobility-lifetime products but also illustrate the inadequacy of the simple approach, particularly when applied to a-Si:H materials which are deposited at fast rates.

Real time spectroscopic ellipsometry (RTSE) has been applied to develop deposition phase diagrams that can guide the fabrication of hydrogenated silicon (Si:H) thin films at low temperatures (

For the first time direct correlations are obtained between the light induced changes under 1 sun illumination in the properties of a-Si:H and those in the characteristics of p-i-n cells incorporating identically-prepared i-layers. These correlations were obtained after account was taken of the effects that the location of the electron and hole quasi-Fermi levels have on the carrier recombination that occurs through the different gap states. The changes in midgap state density, as measured on the films and reflected in the subgap absorption at 1.2 eV, are directly correlated with changes in the dark I-V characteristics under low forward bias. In this case small quasi-Fermi level splitting is present so the recombination of the injected carriers is determined by the midgap states in the bulk of the i-layer. In addition, the changes in the electron mobility-lifetime products as measured on the films are correlated with changes in the fill factor measured on cells under the same conditions as long as large quasi-Fermi level splitting is present and recombination occurs through states spanning a wide region of the gap, such as occurs under 1 sun illumination. The results explain (i) the failure of numerous attempts to correlate the degradation of solar cells reliably with the creation of dangling bond defects and (ii) the inadequacy of the large number of modeling results that assume such a correlation.

A study was carried out with the goal of obtaining high open circuit voltages (Voc) in a-Si:H n-i-p solar cells, taking into account the evolutionary nature of the microstructure of the p-layers during growth. It is found that cells with p-layers in the protocrystalline Si:H growth regime give the highest values of Voc not those with microcrystalline Si:H p-layers. Evidence for this conclusion is presented whereby Voc is related directly to the microstructure of the p-layers, as characterized using spectroscopic ellipsometry, atomic force microscopy, and electrical measurements. The results clarify the origins of (i) inconsistencies associated with attributing high Voc in n-i-p cells to the microcrystallinity of the p-layers, as well as (ii) the inability to obtain similarly high values in p-i-n superstate cells. Because the microstructure of p-type protocrystalline Si:H depends on that of the underlying i-layer, it is not possible to optimize the cell parameters based on an understanding of the process unless detailed characterization of the p-layer in the actual cell configuration is performed.

Real time spectroscopic ellipsometry (RTSE) has been applied to develop deposition phase diagrams that describe the thickness evolution of the phase of hydrogenated silicon (Si:H) thin films. Such diagrams can be applied to establish optimization principles for the intrinsic (i-) layers incorporated into high performance solar cells based on amorphous Si:H (a-Si:H). The phase diagrams for the growth of Si:H on a-Si:H film substrates incorporate two transitions versus accumulated thickness, the first from the amorphous to the mixed-phase (amorphous + microcrystalline) growth regime [the a→(a+μc) transition] and the second from the mixed-phase to single-phase microcrystalline growth regime [the (a+μc)→μc transition]. Methods have been developed to extract the evolution of the volume fraction of microcrystalline Si:H (μc-Si:H) within the mixed-phase growth regime. Similar deposition phase diagrams have also been developed to optimize p-type Si:H layers for a-Si:H-based n-i-p and p-i-n cells.

We have investigated the low-temperature epitaxial growth of thin silicon films by hot-wire chemical vapor deposition (HWCVD). Using reflection high energy electron diffraction (RHEED) and transmission electron microscopy (TEM), we have found conditions for epitaxial growth at low temperatures achieving twinned epitaxial growth up to 6.8 µm on Si(100) substrates at a substrate temperature of 230°C. This opens the possibility of growing high quality films on low cost substrates. The H_2:SiH_4 dilution ratio was set to 50:1 for all growths. Consistent with previous results, the epitaxial thickness is found to decrease with an increase in the substrate temperature.