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

ABSTRACT: Thin film microcrystalline silicon (¿c-Si:H) solar cell is an attractive material because of its extended response in the long wavelength region. It suffers less from degradation on prolonged exposure to light when compared with amorphous silicon. Based on these features, ¿c-Si:H solar cells are implemented as bottom cell the so-called micromorph tandem cells for enhanced light absorption resulting in a higher conversion efficiency. This contribution investigates the effect of seeding the growth of ¿c-Si:H deposited by radio frequency plasma enhanced chemical vapour deposition (rf PECVD) on the spectral response of ¿c-Si:H solar cells. We correlate the crystalline volume fraction of the absorber layer to the silane concentration used during the deposition and thickness of the seed layer. Our results show that with seeding of both the p- and i-layers, the overall solar-cell performance increases mainly due to extended spectral response of the solar cell in the long wavelength region of the solar spectrum. The Jsc is increased from 173 A/m2 to 239 A/m2. On the other hand, the Voc and fill factor drop by 11% and 2.9%, respectively. By means of bifacial Raman measurement, we demonstrate that the seed layers help to reduce the amorphous incubation layer and lead to a more rapid crystallinity development in growth direction. We show that the crystallinity profile in the growth direction is more uniform when compared with a ¿c-Si:H layer deposited without seeding. Keywords: Thin film microcrystalline silicon, seed layers, crystallinity, spectral response@en

Hydrogenated microcrystalline silicon (¿c-Si:H) has become attractive for use in thin-film silicon solar cells. The external quantum efficiency (EQE) of ¿c-Si:H solar cells extends up to 1100 nm, which is exploited in tandem solar cells. Properties of p¿i interface are critical for performance as it affects carrier collection, which is visible in the blue response. Here, we report how ¿c-Si:H p- and i-layer material properties influence the p¿i interface of ¿c-Si:H solar cells. The effect of RF PECVD parameters of these layers on the p¿i interface was investigated. We find that the blue response of the solar cell is sensitive to the crystallinity of both the p- and i-layers. We demonstrate that transient depletion during i-layer deposition affects the blue response of ¿c-Si:H solar cell. We obtained a narrow process window for optimal solar-cell performance. At the optimal deposition pressure of 9 mbar and using transient depletion, an EQE at 400 nm of 0.6 was obtained, achieving 16% higher short-circuit current density. Reducing the diborane flow during p-layer deposition yielded 13% relative increase in efficiency.@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