What could be the possible reason(s) if it is observed that optical absorption of a polycrystalline thin film is increased after annealing, but the photonsensitivity of the material diminished compared to the as-grown sample? Enhancement of absorption co-efficient (measured by UV-VIS spectroscopy in transmission mode) signifies increase in photon generated carriers, which eventually should contribute in photosensitivity of the material. But if it noticed photosensitivity of the material is decreased even its absorption co-efficient is increased, how could one interpret?
Bazul, in common they are not proportional. Photosensitivity by conduction (photoconductivity) means that photoexcited carriers give additional macroscopic current in the illuminated sample. But if the e-h recombination is fast, the photoconductivity will be small. Herewith optical absorption could be rather high but photocuurent conversion could be not effectivt. Material related aspects involvinh carrier mobility, deep centers and defect traps, uvwanted fast recombination mechanisms should be involved in the analyzis
Dear Vasil'evskii,
Thanks very much for your answer. Yes, you rightly pointed out that different parameters like carrier mobility, carrier lifetime (majority or minority), trap and/or recombination centers play crucial role in controling the photosensitivity or photoconducivity of the material. But existing reports and studies on polycrystalline solid say that after annealing the crystalline quality of the material improved via fusion of grains resulting decrease in gain boundary states and other associated defects like deep level etc. As a matter of fact, the optical absorption as well as photoresponse of the material gets better. So is it recombination mechanism determining the ultimate result???? Factors like dark current actively participates in manipulating the ultimate photoserponse of the material also. But these are all QUALITATIVE assumption/explanation. What do you mean by unwanted fast recombination? What is the origin of it and does it have any dependency upon annealing???
LASTLY I WANT TO ASK YOU DO YOU HAVE ANY REFERENCE(S) WHERE I CAN GET THE QUANTITATIVE MEASURE OF THE FACTORS YOU MENTIONED ABOVE, WHICH EFFECT THE PHOTOSENSITIVITY OF POLYCRYSTALLINE SEMICONDUCTOR SOLIDS.
Dear Bazlul Karim
The Enhancement of absorption co-efficient, that due to increase in surface roughness after annealing. Furthermore, the effect of the annealing temperature on the film formation, and morphology.
and the photosensitivity of the material is decreased as result of exciton recombination process.
Dear Alfahed,
Thanks for your response. But is excitonic transition possible for material with high dielectric constant like 8-9 and having low excitonic energy around 20 meV and especially when it is in polycrystalline form?
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