Hydrogenated Silicon Layers and Solar Cells Deposited at Very Low Substrate Temperature

Abstract

For direct production of solar cells on cheap plastics, the quality of VHF-PECVD deposited intrinsic and doped silicon layers made at substrate temperatures ≤ 100 °C was optimized. The investigation showed that at lower substrate temperatures, higher hydrogen dilution of the source gas silane was required to achieve the best, but lower quality optoelectronic layer properties, compared to state-of-the-art films made at 200 oC. At the lowest tested substrate temperature (40 °C), the layer with the best properties was a mixed amorphous-microcrystalline phase ("mixed-phase") silicon layer. X-TEM images showed that the amorphous part of this mixed-phase layer is organized in ~50 nm wide columns starting at a layer thickness of ~120 nm, in which cone-shaped "microcrystallites" are embedded. Similar ~50 nm wide amorphous columns were found in a mixed-phase layer with similar crystal fraction made at 70°C. Protrusions with a width of ~50 nm are still visible at the surface of the amorphous part in the mixed-phase layer made at 100°C. The presence of the columns was attributed to the amorphous-amorphous roughness transition of which the resulting structure is maintained up to the surface due to a temperature dependent effectiveness of the valley filling mechanism. The cone shape and apex angles of the microcrystallites, were found to be approximately the same for all observed microcrystallites in our layers, and similar to those found in literature. The shape resembles in detail that of a ballistic aggregate on a seed; a well explored mathematical growth model. It was postulated that an extended version of the “chemical annealing model” for microcrystalline growth leads effectively to the same geometrical features as described by this model. Knowing the typical dimensions of microcrystallites, a crystalline surface fraction of a mixed phase silicon layer can be converted into a crystalline volume fraction. A new analysis method was introduced to determine the crystalline volume fraction of a mixed-phase Si layer in this way. The method is based on combining information from different types of microscopes and was validated by comparison of the results to values determined by Raman spectroscopy. Knowing the shape of the microcrystallites and assuming a random distribution, the crystalline surface fraction can be converted into a 3-D model of a mixed-phase silicon layer. Which, in its turn, can be used for the interpretation of measurement results. For example, for low substrate temperature samples, the dark conductivity increases monotonously with increasing crystallinity up to full crystalline surface coverage. The commonly observed jump in conductivity at the phase transition, is therefore most likely not related to percolation through contacting microcrystallites. Possibly, the jump occurs when a sufficiently deep network of straight boundaries (X-TEM) between adjacent microcrystallites is formed. Interaction of these boundaries with ambient gases can influence the conductivity. H-, Ne-, and He-evolution measurements confirmed that the structure of mixed-phase low temperature layers is very open and accessible to ambient atoms. This work resulted in a p-i-n Si solar cell made at 100°C with a record conversion efficiency of 7.3% on textured SnO2:F coated glass.