Chemical and Physical Deposition
Tin-film semiconductors can be fabricated using a variety of techniques like solution-based processes or physical deposition. Among the solution-based, chemical deposition (CBD) is widely used for the deposition of smooth and homogeneous films with thickness d <100 nm for solar cells.
In the picture, we can observe the chemical deposition of over a TEC/CdS substrate, which was used as the main absorber layer solar cell. CBD is widely used as a technique for prototyping materials due to the required infrastructure and low cost of the supplies.
The physical deposition like thermal evaporation, sputtering, close-space sublimation is used thinking in the long term development of the technology (commercial use). These methods reduce considerably the number 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) is used to identify the crystal structure 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 the 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.
The Hall Effect probe measures resistivity, mobility, and carrier concentration of semiconductors using the Van der Pauw method. Kowing the electrical parameters helps to design a correct junction of solar cells. For example, most of the p-type materials used in high efficiency solar cells like CIGS and CdTe have carrier concentration density of , while the n-type partner is ,
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 the 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.