Schematic diagram of studied structures. Sweep voltage is applied to silver on top of silicon.
Metal-semiconductor contact for highly doped ZnO and a-Si:H not drawn to scale. All values are in eV. Horizontal arrows represent the different transport mechanisms present. (a) Thermionic emission over the barrier, (b) field emission or tunneling through the barrier, and (c) thermionic field emission.
J-V measurements of all ZnO/μc-Si:H and ZnO/a-Si:H heterojunctions. Current density levels are increased as doping concentration is increased for all samples.
Energy band diagrams for both types of heterojunctions. Low doping of a-Si:H (15 nm thick) makes its Fermi level to keep almost constant. Since the Fermi level of a-Si:H Fermi is almost at the same position than ZnO work function, a low charge transfer from ZnO to a-Si:H does not produce enough band bending. For ZnO/μc-Si:H samples, a high doping of μc-Si:H reduces the barrier height and makes the depletion region thinner, allowing and increased tunneling transport mechanism.
J-V-T forward bias measurements for ZnO/a-Si:H heterojunctions. Steeper slopes for lower doping concentrations and higher temperatures reveal the influence of thermionic emission transport.
Influence of thermionic emission transport mechanism is revealed as temperature increases.
Effective Schottky barrier height as a function of temperature for the ZnO/a-Si:H structures. The reduction of effective barrier as temperature decreases is attributed to contribution of transport mechanisms such as tunneling.
J-V characteristics for ZnO/μc-Si:H heterojunctions with medium and higher doping concentrations in linear scale. Rectifying characteristics are no longer present for the samples with higher TMB flows. Ohmic characteristics are observed instead.
External quantum efficiency measurements for thin film solar cells with different window layers. Despite the better electrical properties of ZnO/μc-Si:H contact, ZnO/a-Si:H window layer produces better cell performance due to allowing more light to reach the intrinsic region. The best result is obtained for a ZnO/μc-Si:H/a-Si:H window layer.
List of parameters used or TCAD simulations. 16–27
Solar cell parameters using different types of p-type window layers.
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