Dear colleagues!
I have a question regarding the connection between absorption and emission of light in a poly-crystalline (solution processed inorganic) semiconductor.
from absorption data I extract the temperature dependence of the band-gap to follow Eg(T)=Eg(0)-2S /(exp(Eph/kT)-1) (ref 1)
and (the Huang Rhys constant) S=7.5 with an uncertainty of 0.5 (from the fit alone), Eph (the average phonon energy) results to roughly 50 meV.
When attempting to connect S and Eph with PL data (in terms of the Huang Rhys formula) however the data is clearly not comparable and the phonon energy seems close to 10 meV (or even less) and S also seems to be lower.
Generally, the PL peak is very narrow and from (Segall equation)
G(T)=G(0)+g*T+GLO/(ELO/kT)-1) (ref 2)
g~0 (acoustic scattering) and ELO~20 meV.
Since the first equation combines "average phonon energy" and (from literature reference) Segall's equation last term only the LO-phonons, I guess it can be said that generally speaking absorption and emission "sees" different phonon interaction but I still wonder about the large Huang-Rhys factor and very small interaction energy in that equation.
I hope someone can share his or her experience on this! Any help is greatly appreciated!
Yours,
Chris
ref 1: http://pubs.acs.org/doi/pdf/10.1021/jp800339m
ref 2: http://stacks.iop.org/cm/13/7053
do you want to keep you particlur semiconductor secret? Should we assume at least that it has a direct bandgap and is not doped?
Hi Christoph,
I was facing this issue investigating quantum dots (APL 99, 081901 (2011)). It is quite common that the Huang Rhys factors (HRFs) of absorption and emission do not coincide. Indeed, absorption and photoluminescence (PL) processes are different. The line shape of the emission is influenced by the lifetime of the carriers (however, there is no absorption lifetime) – and absorption is a unique process, whereas the recombining channels of a semiconductor are of multiple nature and might be “dark” or radiative. Additionally, most of the time the experiments are carried out with different light sources, i.e., I presume the absorption was measured with monochromatic light and the PL with laser excitation. So, during the PL experiments, you might have carrier temperatures above kT of the lattice.
Notwithstanding the former arguments, part of the answer to your question also underlies the fairly complex microscopic expression of the HRF, which can be found via the Fan parameter (see APL cited above).
Cheers!
Bruno
Dear Roman!
I am sorry I forgot to link the resources regarding the material: It is solution processed PbBr2 which seems to have a direct band-gap (according to Tauc plot) and is not intentionally doped (but I cannot exclude unintentional doping). ref: Physics Letters A Volume 100, Issue 7, 13 February 1984, Pages 375-378
Dear Bruno! Thank you for the excellent reference and your assumptions regarding the measurement are, indeed, correct. After studying the effects you mentioned further I might get back to you with any remaining question, otherwise I hope you can see another citation for the mentioned paper soon! ;)
Yours,
Chris
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