Vol.2, No.1, 63-66 (2010) Natural Science
Copyright © 2010 SciRes. OPEN ACCESS
Photocurrents induced by stimulated absorption of light
Alexey Kavokin1, Giuseppe Eramo2
1Physics and Astronomy School, University of Southampton, Highfield, Southampton, United Kingdom
2Dipartimento di Fisica, Universita di Roma II, via della Ricerca Scientifica, Rome, Italy
Received 23 October 2009; revised 13 November 2009; accepted 17 November 2009.
In organic materials absorption of light results
in creation of the Frenkel excitons, Coulomb
-bound electron-hole pairs having an integer
spin. Being composite bosons, Frenkel excitons
may accumulate in significant quantities in a
single quantum state. The probability of photon
absorption by such a state increases as (N+1)
where N is the occupation number of the state.
This enhancement is due to the stimulated ab-
sorption of light, which is a final-state stimu-
lated process analogous to the well-known
stimulated emission of light. We propose using
the stimulated absorption for creation of solar
cells of a record quantum efficiency. We are
going to use the organic microspheres to ac-
cumulate the Frenkel excitons at the discrete
frequencies corresponding to the photonic
whispering gallery modes of the sphere. The
dissociation of accumulated Frenkel excitons
will be effectuated periodically using the trans-
parent carbon contacts on a piezoelectric
mechnical support.
Keywords: SolarCell; Photovoltaics; Exciton;
Development of the renewable sources of energy is a
strategic economic priority nowadays. The solar energy
is especially important for the tropical countries due to
their favorable climate conditions. At present, the com-
mercially available solar cells are based on silicon
monocrystals which are relatively expensive and can
hardly compete with the conventional energy sources
therefore. Organic materials are widely discussed now as
a valuable alternative to silicon but several obstacles still
block the full-scale development of organic photovol-
taics. First, unlike silicon, absorption of light in organics
does not produce the free electron-hole pairs, but bound
electrically neutral states known as Frenkel excitons. A
significant external voltage needs to be applied to help
dissociating the Frenkel excitons with formation of the
unbound electrons and holes which contribute to the
photocurrent [1]. Second, the metallic contacts used at
present absorb light themselves which reduces signifi-
cantly the quantum efficiency of photocells. The revolu-
tionary improvement of the performance of the organic
photocells due to the use of new materials and new
physical effects is in the focus of this Letter. The starting
idea is to take advantage of the bosonic properties of
Frenkel excitons and use them to achieve the stimulated
absorption of light. The photocells based on stimulated
absorption are expected to posses a dramatically en-
hanced quantum efficiency with respect to traditional
photocells based on the spontaneous absorption of light.
The expected difference is as huge as the difference be-
tween a light emitting diode (producing light by sponta-
neous emission) and a laser source (producing light by
stimulated emission). Furthermore, we take advantage of
the experience accumulated in the recent years in pro-
duction of the optically transparent electric contacts
based on the diamond-like-carbon (DLC) with embed-
ded carbon nanotubes and to operate with the organic
microspheres characterized by the photonic whispering
gallery modes of extremely high quality factors. We
emphasize the important role of photonic confinement in
our photocells, which allows for resonant generation of
macroscopic number of Frenkel excitons in the same
quantum state during the life-time of the corresponding
photonic mode. In turn, once being created, the macro-
scopic population of Frenkel excitons reproduces itself
by stimulated absorption of new solar photons. We pro-
pose a new technology based on a new physics, new
structures and new materials. The targeted outcome is a
new generation of photocells with a drastically improved
quantum efficiency.
In 1917 Einstein has proposed the theory of radiation
of atoms which accounted for three essential processes
of light-matter interaction: the spontaneous emission of
light, the spontaneous absorption of light by atoms and
the stimulated emission of light. This latter process has
found its use in the new coherent sources of light (lasers,
64 A. Kavokin et al. / Natural Science 2 (2010) 63-66
Copyright © 2010 SciRes. OPEN ACCESS
from Light Amplification by Stimulated Emission of
Radiation, which we frequently meet in the everyday life
now). In lasers, generation of new photons by matter is
stimulated by existing population of the photon mode in
a laser cavity. The fourth essential process of light-mat-
ter coupling is the stimulated absorption of light. It may
only take place in the systems where the absorbed pho-
tons generate bosonic quasiparticles, e.g. excitons. Ex-
citons are Coulomb correlated electron-hole pairs, a di-
rect analogy of hydrogen atoms, created inside inorganic
semiconductor crystals due to resonant absorption of
light at the energies slightly below the bandgap (Wan-
nier-Mott excitons [2]) or in organic crystals at the en-
ergy below the HOMO-LUMO transition (Frenkel exci-
tons [3]). While electrons and holes are fermions, the
excitons possess integer spins and are composite bosons
therefore. As such, the excitons can be accumulated in
large quantities in a single quantum state, forming the
Bose-Einstein condensate (BEC). Recently, multiple
experimental evidences for the BEC of excitons coupled
to light modes in semiconductor microcavities have been
reported [4-6]. An exciton coherently coupled to a con-
fined photon in a microcavity forms a so-called exci-
ton-polariton. A new generation of semiconductor light
sources called “polariton lasers” and based on the
Bose-Einstein condensation of exciton-polaritons has
been suggested and experimentally realized very re-
cently [7,8].
Creation of a new exciton in a quantum state already
occupied by N excitons has a probability (N+1) times
higher than creation of an exciton in an empty state. This
is why the stimulated absorption of light by excitonic
condensates is expected to be extremely efficient. Sev-
eral conditions need to be met to ensure stimulated ab-
sorption: 1) the seed exciton population exceeding 1
must be created; 2) the life-time of the exciton state (ac-
counting for its radiative and non-radiative life-times)
must be longer than the generation rate of excitons. The
first condition is readily satisfied due to the intensity
fluctuations in the solar light: statistically, simultaneous
excitation of two or more excitons is always possible
within a sufficiently long time interval. The second con-
dition can be satisfied if the intensity of incident light
exceeds some critical intensity dependent on the exci-
ton-life time. In this case the amplification of the seed
population takes place, the exciton condensates stabi-
lizes itself and the rate of stimulated absorption keeps
steadily growing up to the saturation value dependent on
the spectral density of incident light and the Mott transi-
tion threshold for the exciton system. Making use of the
stimulated absorption of light would be a fundamental
breakthrough in photovoltaics which may be compared
with invention of lasers in optical lighting. The research
in this direction is timely and relevant given the overall
high interest to organics materials for photovoltaic ap-
plications in the world.
The qualitative analysis shows that the most important
condition to be satisfied to ensure the stimulated absorp-
tion going on is a sufficiently long life-time of an exci-
ton state of interest. Sufficiently long means about 100
ps according to our estimations based on the average
intensity of the day-time light in the Mediterranian re-
gion. Another important practical issue is linked with a
necessity to have a significant absorption within the
large frequency range of the visible and near ultraviolet
light, in order to avoid losses of the solar energy. This is
partly resolved because of the natural inhomogeneous
broadening of Frenkel exciton resonances in the most
part of organic materials, which is of the order or several
hundreds meV, typically. In order to further extend the
frequency range of efficient stimulated absorption and to
control the life-time of the concerned exciton states we
propose using the organic microspheres as a main ele-
ment of the solar cell. In is well known that dielectric or
semiconductor microspheres are characterized by dis-
crete optical spectra composed by a multitude of sharp
resonances. These resonances are associated with so-
called whispering gallery modes, i.e. the light modes
confined near the surface of the microsphere (Figure 1).
The whispering gallery modes of high orbital and mag-
netic indices may have a quality factor (ratio of the mode
frequency to its linewidth) of the order of 105-106 which
ensures the photonic life-time of 100 ps or more. In the
same time, the finesse of these modes (ratio of the fre-
quency splitting between different modes to the
linewidth of the mode) remains quite low (of the order of
several units). This is why a microsphere filled by an
absorbing aggregate would absorb light at the huge mul-
titude of discrete frequencies corresponding to the whis-
pering gallery modes. The life-time of excitons coupled
to each of these modes would be long enough to ensure
stimulated absorption. Between these selected frequen-
cies the absorption of light would be still spontaneous.
In order to cover the most part of the visible spectrum of
light we propose to operate with the microspheres of
different radii ranging from several microns to several
tens of microns. Furthermore, in order to ensure efficient
collection of the charge carriers accumulated in the mi-
crosphere due to stimulated absorption, we propose us-
ing the transparent contacts made of a diamond-like-
carbon (DLC). Fabrication of such contacts which may
serve as Fresnel lenses as well is now a registered
know-how in Europe. The DSC cover layer would pro-
A. Kavokin et al. / Natural Science 2 (2010) 63-66
Copyright © 2010 SciRes. OPEN ACCESS
Figure 1. A dielectric sphere and the path of light in the whispering gallery mode (left); localization of light in a sphere due to
multiple internal reflections (right).
tect the organic solar cell from chemical and mechanical
damage, ensure focusing of light on the microsphere and
play a role of a non-absorbing upper contact (see the
scheme in Figure 2). In order to allow for efficient ac-
cumulation of Frenkel excitons inside the organic mi-
crosphere we plan discharging it periodically which
could be achieved by placing the upper contact on a
piezoelectric support which would extend and contract
Figure 2. Scheme of the proposed photovoltaic device struc-
ture composed by an organic microsphere, where the
Bose-Einstein condensate of exciton polaritons will be created
by resonant optical pumping, a lower metallic contact and an
upper transparent carbon contact supported by two piezoelec-
tric pillars operating in a THz modulating regime: when the
voltage between two contacts drops due to the current passing
through the sphere the piezoelectric supports extend which
leads to the break of current. As soon as the current is broken,
the supports contract again, which allows for the new discharge
of the microsphere etc.
with a Tera-Hertz frequency.
The proof-of-concept demonstration device would be
a standard porphirine microsphere of the radius from 10
to 100 microns connected to two platinum microcontacts.
One would measure a photocurrent generated by an ac-
cordable laser as a function of the frequency and the
intensity of the laser light. A threshold-like superlinear
dependence of the photocurrent as a function of the laser
intensity at the discrete frequencies corresponding to the
whispering gallery modes of the microsphere would be a
“smoking gun” for the stimulated absorption. In the next
phase the optimized microspheres would be assembled
with the metallic and carbon contacts on the piezoelec-
tric support. To improve the collection of solar light, the
organic Fresnel lenses covered by a DSC protective film
would be assembled on the top of the structure. The
main risk at this stage is linked with the proposed dis-
charge mechanism for the microspheres which has not
yet been tested experimentally.
In conclusion, the recent advances in the Bose-Einstein
condensation of excitons and exciton-polaritons open
new horizons for the organic photovoltaics. A revolu-
tionary concept of stimulated absorption of light may
allow for a dramatic increase of the quantum efficiency
of organic solar cells. In this Letter a device concept
suitable for demonstration of photocurrents generated by
stimulated absorption of light is proposed.
We acknowledge support from the EU IRSES grants
66 A. Kavokin et al. / Natural Science 2 (2010) 63-66
Copyright © 2010 SciRes. OPEN ACCESS
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