Not in depth, just so I could understand. Just explain the key details.At what level do you need an answer? It's a result of the physical construction of the P/N junction.
https://en.wikipedia.org/wiki/Amorphous_siliconOnly photons with sufficient energy to create an electron–hole pair, that is, those with energy greater than the semiconductor bandgap will contribute to the energy conversion process.
When the minimum of the conduction band occurs at the same value of the crystal momentum as the maximum of the valence band, as it does in Figure 3.3, the semiconductor is a direct bandgap semiconductor. When they do not align, the semiconductor is said to be an indirect bandgap semiconductor. This is especially important when the absorption of light by a semiconductor is considered later in this chapter.
Even amorphous materials exhibit a similar band structure. Over short distances, the atoms are arranged in a periodic manner and an electron wavefunction can be defined. The wavefunctions from these small regions overlap in such a way that a mobility gap can be defined, with electrons above the mobility gap defining the conduction band and holes below the gap defining the valence band. Unlike crystalline materials, however, there are a large number of localized energy states within the mobility gap (band tails and dangling bonds) that complicate the analysis of devices fabricated from these materials.
Since only photons with hν > EG can create electron–hole pairs and contribute to the output power of the solar cell, it is clear that the bandgap determines how well the solar cell is coupled to the solar spectrum. A simple analysis can be performed to predict the ideal efficiency. This is plotted in Figure 3.17 for an AM1.5 global spectrum and shows a maximum efficiency of 48% at about EG = 1.1 eV, close to the bandgap of silicon, although bandgaps between 1.0 and 1.6 eV have comparable ideal efficiencies.
The source of the low efficiency of amorphous silicon photovoltaics is due largely to the low hole mobility of the material. This low hole mobility has been attributed to many physical aspects of the material, including the presence of dangling bonds (silicon with 3 bonds), floating bonds (silicon with 5 bonds), as well as bond reconfigurations. While much work has been done to control these sources of low mobility, evidence suggests that the multitude of interacting defects may lead to the mobility being inherently limited, as reducing one type of defect leads to formation others.
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