Chemical and Physical Deposition
The synthesis of thin-film semiconductors can be obtained using a variety of techniques like solution based process or physical deposition. Among the solution based, chemical deposition (CBD) is widely used for deposition of smooth and homogeneous films (d <100 nm) used as buffer layers in solar cells.
In the picture we can observe the chemical deposition of over TEC/CdS substrate, which can be used a solar energy absorber. CBD is widely used as technique for prototyping materials due the low required infrastructure and cost of the supplies.
The physical deposition like thermal evaporation, sputtering, close-space sublimation is used thinking in a long term development of the technology (commercial use). This methods reduce considerably the amount of defects in the bulk and interface of the materials. However, the cost of research and development is huge compared to the solution processes.
Optical, Electrical & Structural Characterization
Structural characterization (XRD or AFM) is used to identify the structural composition and crystallinity of the developed thin films. These results can be complemented by morphology analysis using a scanning electron microscope (SEM) and semi-quantitative composition analysis using the energy dispersive X-ray spectroscopy (EDS) attachment.
Optical analysis like UV-VIS-NIR spectroscopy is necessary for determination of the optical bandgap (Eg) in thin film semiconductors. The calculation of Eg is obtained from Tauc-plot or multiple reflection theory derived from the optical transmittance (T) and reflectance (R) spectrum.
In the picture we can see the most common electrical probe for the measurement of resistivity, mobility, and carrier concentration (Hall Effect – Van der Pauw method). The electrical parameter are essential for the development of solar cells. With this techniques, we take care to design and fabricate semiconductors with the correct carrier concentration. For example, most of the solar absorber materials used in high efficiency solar cells use a p-type carrier concentration of . All this techniques help us to understand the basic properties of a semiconductor.
Numerical simulation of solar cells in SCAPS-1D
Numerical simulation is widely used to understand the behavior of a solar cell when it operates in dark and illumination conditions. The simulation in SCAPS-1D allow us to design photovoltaics structures with up to seven layers in a pn-junction configuration.
For a realistic simulation of a solar cell device we need to measure most of the optical and electrical properties of each component. Moreover, we need to determine the type and amount of carrier recombinations. These aspects are very important when the simulation pretends to match the real JV-curve of a solar cell.
In the analysis of thin-film solar cells we can vary most of the parameters of the device. For example, we can change thickness, carrier density, and recombination of each layer. However, we must take care of every change to obtained the expected results.
The results parameters we can find in the software after the simulations are:
- JV Curve: Rs, Rp, Voc, Jsc, FF, η
- Energy diagrams
- External quantum efficiency: EQE
- Capacitance voltage: CV.
- Among others.