Barium disilicide (BaSi2) has been regarded as a promising absorber material for high-efficiency thin-film solar cells. However, it has confronted issues related to material synthesis and quality control. Here, we fabricate BaSi2 thin films via an industrially applicable sputtering process and uncovered the mechanism of structure transformation. Polycrystalline BaSi2 thin films are obtained through the sputtering process followed by a postannealing treatment. The crystalline quality and phase composition of sputtered BaSi2 are characterized by Raman spectroscopy and X-ray diffraction (XRD). A higher annealing temperature can promote crystallization of BaSi2, but also causes an intensive surface oxidation and BaSi2/SiO2 interfacial diffusion. As a consequence, an inhomogeneous and layered structure of BaSi2 is revealed by Auger electron spectroscopy (AES) and transmission electron microscopy (TEM). The thick oxide layer in such an inhomogeneous structure hinders further both optical and electrical characterizations of sputtered BaSi2. The structural transformation process of sputtered BaSi2 films then is studied by the Raman depth-profiling method, and all of the above observations come to an oxidation-induced structure transformation mechanism. It interprets interfacial phenomena including surface oxidation and BaSi2/SiO2 interdiffusion, which lead to the inhomogeneous and layered structure of sputtered BaSi2. The mechanism can also be extended to epitaxial and evaporated BaSi2 films. In addition, a glimpse toward future developments in both material and device levels is presented. Such fundamental knowledge on structural transformations and complex interfacial activities is significant for further quality control and interface engineering on BaSi2 films toward high-efficiency solar cells.@en

This contribution investigates the effect of seeding the growth of thin film microcrystalline silicon (µc-Si¿:¿H) deposited by radio frequency plasma-enhanced chemical vapor deposition on the material properties of µc-Si¿:¿H film and the device performance of p-i-n and n-i-p µc-Si¿:¿H solar cells. By means of Raman measurement, x-ray diffraction (XRD) and transmission electron microscopy (TEM), we investigate the structure of seeded µc-Si¿:¿H. In particular, the effect of seed layers on the crystallinity development is investigated. Measurements of the depth profile of the crystalline mass fraction using Raman spectroscopy show that seed layers lead to a more rapid and uniform crystallinity development in growth direction. The amorphous incubation layer is suppressed and crystallization begins directly from onset of film growth without evolving through the intermediate growth phases. From TEM analyses, we observe that crystal sizes are not affected by seed layers. Horizontal cracks are however observed to dominate the early growth of µc-Si¿:¿H in p-i-n solar cell and this is reduced upon seeding. For the n-i-p cells, these cracks are not affected by seeding. XRD results also indicate that the use of seed layers does not affect the crystal sizes but affects the direction of preferential orientation. Solar cell external parameters show that seeding of p-i-n solar cells leads mainly to increase in short-circuit current density, Jsc with a slight drop in open-circuit voltage, Voc. For the n-i-p cells, a reverse effect is observed. In this case, the Voc increases and the Jsc decreases.@en

In this work we report a simple approach by which we are able to probe crystallinity evolution in thin film microcrystalline silicon (¿c-Si:H) using depth-profile Raman measurement. Crystalline volume fraction f, is used as a measure of crystallinity indicator and varies with varying film growth phases. Each phase is identified by certain range of crystalline volume fraction extracted from the phase material. Our investigation shows that radio frequency plasma enhanced chemical vapour deposition (rf PECVD) growth of ¿c-Si:H evolves through various material phases from an initial amorphous incubation layer (ASIC) through other phases into the full microcrystalline regime. We found that f increases gradually in growth direction and saturates at the full microcrystalline regime. By means of seeding, we show that we are able to modify the growth process such that ¿c-Si:H evolution is more rapid, more uniform, less dependent on substrate and begins nearly directly from the crystalline phase hence omitting the amorphous and proto-crystalline phases. X-ray diffraction measurement is used to further investigate the structure of ¿c-Si:H grown on different substrates with and without seed layers and the results are also presented.@en

The effect of deposition temperature on the structural and optical properties of amorphous hydrogenated silicon (a-Si:H) thin films deposited by plasma-enhanced chemical vapour deposition (PECVD) from silane diluted with hydrogen was under study. The series of thin films deposited at the deposition temperatures of 50 ¿ 200°C were inspected by XRD, Raman spectroscopy and UV Vis spectrophotometry. All samples were found to be amorphous with no presence of the crystalline phase. Ordered silicon hydride regions were proved by XRD. Raman measurement analysis substantiated the results received from XRD showing that with increasing deposition temperature silicon-silicon bond-angle fluctuation decreases. The optical characterization based on transmittance spectra in the visible region presented that the refractive index exhibits upward trend with increasing deposition temperature, which can be caused by the densification of the amorphous network. We found out that the scale factor of the Tauc plot increases with the deposition temperature. This behaviour can be attributed to the increasing ordering of silicon hydride regions. The Tauc band gap energy, the iso-absorption value their difference were not particularly influenced by the deposition temperature. Improvements of the microstructure of the Si amorphous network have been deduced from the analysis.@en

A systematic and detailed analysis of the influence of hydrogen (H2) dilution of silane (SiH4) on the structural properties of thin hydrogenated silicon (Si:H) films was carried out. The Si:H films were prepared by plasma enhanced chemical vapour deposition method at different H2 to SiH4 flow ratios (R) ranging from R¿=¿0 to 40. A detailed structural analysis was carried out by X-ray diffraction (XRD). In order to suppress the XRD intensity from the substrate, Si:H films were deposited onto a [100]-oriented single-crystalline silicon substrate and XRD was carried out with a thin-film attachment. The XRD analysis demonstrated that the first diffraction peak in the Si:H films corresponded to the signal from silicon hydride (Si4H) ordered domains having tetragonal lattice structure. The full width of half maximum (FWHM) of the peak was decreasing with increasing H2 dilution, and remained nearly constant for R¿¿¿20. The FWHM values confirmed that the Si:H films grown at R¿¿¿20 are more ordered than films deposited at lower R. This trend was confirmed by the Raman spectroscopy from the analysis of the shift of the Raman amorphous peak.@en

The introduction of aluminium doped zinc oxide (ZnO:Al) as a back reflector is an important part of the light-trapping techniques in present thin-film silicon solar cells. However, the application of the sputtered ZnO:Al at the back contact often results in an S-shaped J¿V characteristic and a low fill factor. Using the advanced semiconductor analysis (ASA) simulation program we investigated the origin of the S-shape in the hydrogenated amorphous silicon (a-Si:H) solar cells. We carried out a sensitivity study of ZnO:Al properties and back-contact interface parameters on the performance of a-Si:H solar cells. The simulation results pointed out that the origin of the S-shape in the J¿V curve was due to a poor quality of the n-type a-Si:H/ZnO:Al interface and/or a Schottky barrier at the ZnO:Al/metal interface. The simulated S-shape that matched the experimental one resulted from the presence of a high concentration of defects at the n-type a-Si:H/ZnO:Al interface. These defects can be formed during the sputtering of the ZnO:Al film using deposition conditions that favour strong bombardment of the a-Si:H layer. Introduction of a soft deposition start by adapting the sputtering pressure of ZnO:Al resulted in the J¿V characteristic without the S-shape. The short-circuit current of the a-Si:H solar cells with ZnO:Al back reflector was enhanced by 1.3 mA/cm2 and the relative increase in the conversion efficiency was 10%.@en

Thin-film poly-crystalline silicon (poly c-Si) on glass obtained by crystallization of an amorphous silicon (a-Si) film is a promising material for low cost, high efficiency solar cells. Our approach to obtain this material is to crystallize a-Si films on glass by solid phase crystallization (SPC). As the grain size of SPC poly c-Si films will be smaller than that of multi-crystalline wafers, lower solar cell efficiencies are expected for this technology. Despite the smaller grain size, a 2-micron-thick polycrystalline silicon solar cell with light trapping was shown to have a conversion efficiency of more than 10% [1]. Obtainable efficiencies up to 15% are expected for solar cells made using SPC of a-Si:H films. Expanding thermal plasma chemical vapor deposition (ETP-CVD) was used to prepare hydrogenated a-Si films; this technique is chosen because the deposition rates are much higher than with plasma enhanced CVD. A-Si:H films with different hydrogen contents were annealed using temperatures ranging from 500 °C to 700 °C. The evaluation of the films after annealing treatments revealed that the hydrogen content and bonding configuration did not influence the structural properties of the crystallized films significantly. The average crystallite size in the fully crystallized films was between 100 and 150 nm. Full crystallization of 1 micrometer thick films was achieved within 20 minutes for annealing at 625 °C and 650 °C. During annealing at 600 °C crystallization is much slower, and no crystallization is observed at 500 °C. The relation between the annealing temperature and the rate with which the films are fully crystallized is of great importance to develop a solar cell technology, to limit the thermal budget and processing time.@en

The determination of the dominant crystal orientation is of great importance in gaining insight into the growth mechanisms of hydrogenated microcrystalline silicon (¿c-Si:H). However, for this purpose most techniques are time-consuming, expensive and destructive. In this contribution, a new approach is investigated to determine crystal orientation in ¿c-Si:H films from Raman spectrum. Raman measurements have been applied under varying polarization angles to a series of samples of varying crystalline volume fraction (fc) covering the transition from amorphous to microcrystalline silicon regime. First, the dependence of the Raman intensity of the 520 cm¿1 TO peak on polarization angle is measured on [100], [110], and [111] single-crystalline silicon wafers. We found distinct patterns for these reference wafers and have used these patterns as a fingerprint for a specific crystal orientation in ¿c-Si:H. Subsequently, we measured the Raman intensity of the TO peak as a function of polarisation angle on the ¿c-Si:H films and compared the obtained patterns with that of the references. For most of the rf PECVD ¿c-Si:H samples we obtained a dominant crystal orientation of [111]. We found that the dominant crystal orientation is independent of the crystalline volume fraction of the ¿c-Si:H. The inferred crystal orientation will be compared with XRD results.@en

X-ray diffraction (XRD) analysis of thin silicon films was carried out using both the symmetric Bragg-Brentano and asymmetric thin-film attachment geometries. The asymmetric configuration allows quantitative phase analysis of the films and reveals that amorphous silicon films deposited from silane diluted with hydrogen have the strongest peak in the XRD patterns located around 27.5 degrees. This peak corresponds to the signal from ordered domains of tetragonal silicon hydride and not from cubic silicon crystallites. The full width at half maximum (FWHM) of this peak narrows from 5.1 to 4.8 degrees as the ratio of hydrogen to silane flow (R) increases to 20 and does not change significantly for higher hydrogen dilutions. The amorphous silicon films fabricated at different hydrogen dilution were applied as absorber layers in single-junction solar cells. Degradation experiments confirm a substantial reduction of the degradation when the dilution ratio is increased from R=0 to R=20. The light induced degradation of solar cells with absorber layers prepared at R > 20 is not further reduced by increasing R.@en

A systematic structural analysis was carried out on amorphous silicon films prepared from hydrogen-diluted silane using plasma-enhanced chemical vapor deposition. Hydrogen dilution of silane during the growth of a-Si:H absorber layers is used to suppress light-induced degradation of a-Si:H solar cells. Transmission electron microscopy (TEM) shows that for higher hydrogen dilution ratios the growth of films becomes strongly inhomogeneous and the transition from amorphous to microcrystalline phase occurs at a smaller thickness. A detailed X-ray diffraction (XRD) analysis of the amorphous films reveals that the strongest peak in the XRD patterns is located around 27.5° and corresponds to the signal from the ordered domains of tetragonal silicon hydride and not from cubic silicon crystallites. The full width at half maximum of this peak narrows from 5.1° to 4.8° as the ratio of hydrogen to silane flow (R) increases to 20 and does not change significantly for higher hydrogen dilutions. The presence of silicon hydride ordered domains in amorphous films is confirmed by the lattice imaging method applied to the high resolution TEM recordings. The amorphous silicon films fabricated at different hydrogen dilution were applied as absorber layers in single-junction solar cells. The degradation experiment confirms that the cells with absorber layers deposited using hydrogen dilution are more stable to light exposure. A clear reduction of the degradation is observed when the dilution ratio is increased from R = 0 to R = 20. The degradation of solar cells with absorber layers prepared at R > 20 is not reduced by further increasing R.@en