I doped Fe2O3 with direct band gap semiconductor material to improve its photo current, but there is NO improvement in its photo-current after doping.
What is the dopant amount? Sometimes if the doping level is too high, a reduction in photoactvity can be observed although a red shift in absorbance is observed. A place to start is 1~5 at.% doping. Keep in mind too that a reduction in bandgap or increased absorbance does not necessarily always translate to an increase in photocurrent. An issue with fe2o3, in particular, is high recombination due to poor hole diffusion.
Dear York., Actually I started with multilayers of Fe2O3 by PVD with SC material (very thin layers) and carried out thermal annealing further. You are right that it may be due to high concentration of SC materials in Fe2O3. (SC material was 5% to Fe2O3 in terms of film thickness)
Now from your answer, I have another question: Why slightly high concentration of dopant does not improve (may decrease) the photoactivity. Why this dopant concentration is/should be generally less than 5%. Is there any reason?
With doping you'll get an increase in conductivity of a semiconductor. For photoelectrochemical applications 1-5 or even up to 10 at.% doping is a 'sweet spot'- or rather a place to start. This all depends on the materials you're using and what dopant, and even synthesis method. How does 2 or 7% loading perform in comparison?
Doping can create low lying trap sites which can help to prolong electron lifetimes, which can lead to increased photocurrent given specific conditions. If the trap sites become too dense from doping, however, they can act as recombination centers thereby reducing photocurret.
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