in fabricating solar cell with compound semiconductor its necessary to have the same band gap range in p type and n type material if soo why and what mechanism behind that ?
Dear vimal
In order to minimize the injection of holes from the absorber into the
n-type semiconductor an energy barrier should be introduced in the valence
band, difference EV, between the n-type semiconductor and the absorber. Ideally,
this can be achieved by choosing an n-type semiconductor that has a larger
band gap than that of the absorber and the energy difference is located in
the valence band of the two materials. Similarly, the injection of electrons
from the absorber into the p-type semiconductor could be suppressed by using a p-type semiconductor with a larger band gap than that of the
absorber and having the band off-set in the conduction band, difference EC, between
the absorber and the p-type semiconductor. The requirement of having the
band off-set in the conduction band means that the electron affinity, 1e, of
the p-type semiconductor is smaller than the electron affinity of the
absorber. The additional advantage applying membrane materials with
large band gaps is to allow almost all photons to be transmitted and
absorbed into the absorber
- claro la fisica de los semiconductores es clara la existencia de una banda de conducion y otra de valencia lo interesante es que para los dispositivos fotovoltaicos lo optimo es que el maximo de la banda de valencia este en la misma dirreccion que el minimo de la banda de conducion eso se le llama EG EN DIRECTA
la existencia de la bandas en directas y en indirecta es lo que atenta o no contra la eficiencia de estos dispositivos por que la transiciones directa(gap en directa el electron viaja a la banda de conduccion con un minimo de energia el caso de las idirectas necesita colisinar con fonon, claro para evitar esto se dopa con portadores ya sean huecos o electrones
Dear colleagues,
what has been introduced by Mr.Slewa concerns the the organic or the perovskite solar cells. The compound metallic solar cells are composed of two layers only, forming a hetero junction. The widow layer and the absorber layer. The window layer has a wideband gap allowing most of the solar radiation passing through it to the absorber layer. The absorber layer itself has a relatively narrow band gap in the order of 1.35 eV
such as GaAs, Cd Te or silicon. for optimum absorption of the solar spectrum.The wide band gap material reduces the back injection of minority carriers from the absorber to the window layer and thereby decreases the revere saturation current of the solar cells
and so increases the open circuit voltage and the fill factor compared to homo pn junction solar cells made from the absorber material alone.
These structures also called wide gap emitter structures. They are also used in hereojucnction bipolar transistors HBT. to increase the injection efficiency of the transistor by suppressing the back injection from the base to the emitter.
However one has to minimize the interface states formed between two different materials because of the lattice mismatch.
Thank you
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