Untersuchung der Ladungsträgerkonzentration und -beweglichkeit in mikrokristallinen Siliziumlegierungen mit Hall-Effekt und Thermokraft

The electronic properties of amorphous and microcrystalline silicon layers in thin-film solar cells significantly affect the efficiency of solar cells. An important property of the individual layer is the electronic transport, which is described by the variables conductivity, photoconductivity, mobility, and carrier concentration. In the past, individual characterization methods were typically used to determine the electronic properties. Using the combination of Hall effect, conductivity, and thermoelectric power measurements additional variables can be derived, such as the effective density of states at the valence and conduction band edge, making a more detailed description of the material possible. To systematically study the electronic properties – in particular carrier mobility and carrier concentration – various series of silicon films are prepared for this work including microcrystalline silicon layers of different doping and crystallinity and a series of silicon films where the Fermi level is moved by irradiation with high energy electrons on one and the same sample. The results show that the transition from amorphous to microcrystalline transport is relatively abrupt. If the electron transport takes place in only amorphous regions, it is marked by the sign anomaly of the Hall effect. If a continuous crystalline path exists, the electronic properties are dominated by the crystalline volume fraction. The results of the measurements of silicon layers are compared with those of microcrystalline silicon carbide samples. Silicon carbide is especially interesting for future applications in thin-film solar cells due to high transparency and high conductivity. It is shown that the effective density of states at the valence and conduction band edge as a function of temperature in p- and n-type microcrystalline silicon and silicon carbide samples largely coincide with those of crystalline silicon or silicon carbide. A square root shaped profile of the density of states beyond the band edges is also assumed for microcrystalline silicon alloys. However, the mobility of charge carriers in silicon carbide samples is on average much smaller than those of silicon samples. To explain the variation of carrier mobility in microcrystalline silicon and silicon carbide samples two models were used: The model of differential mobility can justify the dependence of the Hall mobility on the position of the Fermi level and the influence of the band tail. The potential fluctuation model predicts a lower mobility of charge carriers in silicon carbide than in silicon samples due to the larger potential fluctuations in microcrystalline silicon carbide. This is confirmed by thermopower and conductivity measurements