Quo vadis?

Latin phrase meaning «Where are you going?» Peter asks the question of Jesus, after the latter announces he is going to where his followers cannot come.

The researches conducted by Greenpeace International, European Thermal Electricity Association and International Energy Agency found that by 2050 about 25% of all necessities of humanity in energy will be satisfied by means of concentrating solar technologies. Such prospects multiply the requirements to optical concentrators - key elements of sun power engineering.

Concentrators of the first and second generations, which are based on the use of laws of geometrical and diffraction optics (reflection, refraction, dispersion and diffraction of light at transition of radiation through interfaces of environments with the different indexes of refraction, or at meeting with the obstacles), today are imperfect taking into account their big mass and sizes, large cost of making and exploitation, that limits their application and narrows the prospects of development.

At creation of highly effective and inexpensive optical concentrators the specialists lay great expectations on the use of «unoptical» concentrating technologies, compatible with photovoltaic sun elements. The important place among them is taken by luminescent waveguide concentrators (LWC).

Constructions and physics of dye luminescent concentrators. Principle of work of luminescent waveguide concentrators is based on the phenomenon of photoluminescence - surplus above the thermal radiation of body, which arises up at the irradiation of certain matter by the optical radiation and exceeds in time the period of light oscillations. By these parameters the luminescence differs from the phenomenon of dispersion and reflection of light, and also from the nonlinear parametric phenomena.

The photon, which is disengaged by a luminescent particle, can spread in different directions. Rays 2 and 4 that fall on the surface of waveguide under a large angle leave a waveguide. In a case, when the angle of incidence of the ray is equal or less than the angle of total internal reflection (TIR), a ray is reflected from the interface of two environments (rays 3 and 5) and spread within a waveguide in direction of PV Cell. Thus, a luminescent waveguide retains considerable part of radiation in small spacious and sends it to the photovoltaic element. Luminescent concentrator has considerable advantages above an ordinary geometrical waveguide; in particular, it provides absorption of sunlight both direct and dissipated, thus, does not need the use of the complicated system of tracking after the Sun (Covalent Solar, USA).

The basic parameters of luminescent concentrator are duration of luminescence, spectrum of absorption of radiation, spectrum of luminescence, power output of luminescence and quantum output of luminescence. 

Duration of luminescence is determined by duration of the excited state and, accordingly, by the properties of matter and parameters of environment.

Photoluminescence is excited by the ultraviolet, visible and infra-red ranges of sun light-spectrum. The radiation of luminescence also lies in ultraviolet, visible and infra-red ranges of light-spectrum.

The relation of energy of luminescence to the value of absorbed irradiative energy in stationary conditions is called the power output of luminescence. The relation of amount of photons of luminescent radiation to the number of absorbed photons is called the quantum output of luminescence. In some crystal luminophores a quantum output of luminescence grows with growth of frequency of excitant radiation. The power output of luminescence grows in proportion with the value of wave-length of excitant radiation to the certain value of λ_max, and then sharply falls to zero. Sharp reduction of power output at the values of wave-length of excitant radiation, more than λ_max, is explained by that the energy of photons is not enough for excitation of luminescence.

The parameters of luminescent concentrator in a considerable measure are determined by the parameters of photoluminophores - amorphous or crystalline matters that are able to emit light under action of excitant radiation. Luminescence of the photoluminophore can be conditioned both by properties of his basic matter - matrix, and admixtures – activators. Activators form the centers of luminescence in a matrix. The names of activated luminophores consist of the names of matrix and activators. Mixed photoluminophores consist of matrix and a few activators. Luminophores more frequently are used for transformation of invisible radiation into the visible one. Depending on the conditions of the use the requirements to the parameters of luminophore are claimed: to the type of excitation, spectrum of excitation, spectrum of radiation, power output of radiation, time characteristics. The spectrums of excitation and radiation of photoluminophores can be found in an interval from short ultraviolet to near infra-red ranges. Organic luminophores can radiate both in solutions (rhodamine), and in the solid state (plastics, anthracene and stilben).

Quantum dots luminescent concentrators. Degradation of luminescent dyes in course of time requires of correction of approaches to a creation of luminescent concentrators. One of such promising directions is the use of nanocrystalline quantum dots as an activator in place of dyes (Fig. 2), in that number in combination with a polymeric matrix (Roman Brenda, MC Cormack, Doran John, Norton Brian - Focas Institute, School Physics, Dublin Institute Technology, Kevin Street, Dublin, Irlande). Passivation of core quantum dots by the shells of material with the wide band gaps allows to promote longevity of concentrators.

Quantum dots allow also to extend the spectral range of sun radiation, which can be taken in by sun elements and, thus, to promote energy effectiveness of the concentrating photovoltaic system (Shrink Nanotechnologies, Inc.).

For the complicated luminescent matters (molecular compounds and condensed environments) a spectral composition of photoluminescence does not rely on a wave-length of light that excites luminescence, and submits to the Stocks rule: a value of wave-length of luminescent radiation is more than a value of wave-length of excitant radiation. Part of energy of photon is used onto «unoptical» processes, and goes to heating of body. Sometimes the anti-Stocks radiation appears. Origin of anti-Stocks radiation can be explicated by a process of addition of energy of thermal vibrations of molecules and atoms of crystalline grating to energy of excitant photon.

The sun radiation is mainly concentrated in the range of wavelengths of 0,28…3,0 µm. His spectrum consists of ultraviolet radiation, concentrated in the range of wavelengths of 0,28.0,38 µm (~ 2 %), visible radiation, concentrated in the range of wavelengths of 0,38…0,78 µm (~ 49 %) and infra-red radiation, concentrated in the range of wavelengths of 0,78…3,0 µm (~ 49 %). The spectrum of absorption of every separate luminophore usually is narrower and provides the use of only part of sun spectrum for activating of dyes centers or quantum dots. With the purpose to extend the spectrum of absorption of luminescent concentrator, as an activator a few components are used (Fig. 3). Each component is sensitized by separate admixtures, which allow to displace the spectrum of absorption into the side of short wavelengths of ultraviolet radiation (Fig. 3,a,b) or into the side of long wavelengths of infra-red radiation (Fig. 3, c) radiation.

At absorption of the photon with a wave-length λ_UV  the center of activation QD1, sensitized for absorption of ultraviolet radiation (Fig. 3,a), radiates into a waveguide the two «infra-red» photons with a wave-length λ_IR, energies of which in a sum are equal to the energy of absorbed «ultraviolet» photon.

At absorption of a photon with a wave-length λ_UV¹ the center of activation QD2, sensitized for absorption of this range of ultraviolet radiation (Fig. 3,b), radiates into a waveguide one «infra-red» photon with a wave-length λ_IR. It energy is less than energy of absorbed «ultraviolet» photon. Part of energy of absorbed photon is dispersed in a waveguide as a heat.

Similarly, the center of activation QD3, sensitized for absorption of infra-red range of radiation (Fig. 3,b), after absorption of two infra-red photons with a wave-length λ_IR¹ radiates in a waveguide one «infra-red» photon with a wave-length λ_IR, energy of which is equal to the sum of energies of two absorbed «infra-red» photons. Thus, multispectral sensitizing allows to extend the spectral range of sun radiation, which interact with a concentrator and after re-emitting by activators achieves a PV Cell and takes part in forming of electric current. 

The structure diagram of multispectral luminescent concentrator is shown on Fig. 4. The luminophore of a concentrator consists of three groups of luminescent activators. «Color» centers of activation – quantum dots QD1, QD2 and QD3 are sensitized for absorption of ultraviolet λ_UV, visible λ_R and infra-red λ_IR radiation of the Sun, and for emission in the range of that wave-length λ_LC, for which a sun photovoltaic receiver has the maximal spectral sensitiveness. 

Without regard to large prospects the power efficiency of the best samples of photovoltaic elements with luminescent concentrators does not exceed of 7,1 %. A reason of such low indexes values are primary losses of radiation in the processes of absorption-emission and wave guiding. Light is taken in by a luminophore, but material does not radiate as it has low quantum efficiency. Other reason of low energy effectiveness is the escaping of radiation from a waveguide in the case when it spreads in a waveguide under the angles, which exceed the angle of total internal reflection. The use of spectral filter that is located on the upper surface of waveguide is an effective method to prevent these losses. This filter returns optical rays inward of the waveguide. As such filter a Bragg reflective diffraction grating on cholesteric liquid crystals can serve (Dick K. G. de Boer, Philips Research Europe, Eindhoven, Netherlands).

In the first samples of concentrators in quality of matrix the polymeric materials were used. But to polymeric matrices the considerable failing are inherent. In particular, at propagation of radiation within  plastic matrix a part of light is re-absorbed by dyes, and transformed into a heat, what decreases power efficiency of concentrator. The scientists of the Massachusetts Institute of Technology, which earlier used polymeric matrices in their devices, now came to the necessity of the use of glass as a matrix, which is simpler for making. In accordance with proposed technology a luminophore is posed on an upper surface of matrix. With the purpose of avoidance of the re-absorption of emitted photons, the researchers add to the dyes mixture an admixture with the inclusion of aluminum, which they named tris(8-hydroxyquinoline. The molecules of aluminum stimulate luminescence on frequencies, which do not allow re-absorbing of photons by the centers of activation.

In such system the amount of sunlight, which after falling on a concentrator is transformed into an electric current, grows almost on an order in comparison with traditional sun elements. The use of similar luminescent concentrators allows to promote energy effectiveness of photovoltaic sun elements by 50 %. Yet greater advantages the scientists plan to get from the use of glass matrices sensitized by dyes at production of window blocks. To this time the instability of parameters of luminescent concentrator interfered with rapid introduction of sun power windows. Today the group of researchers of the Massachusetts Institute of Technology works on problem of increase of longevity of the glass window modules.

Extraordinarily the wide prospects of increase of efficiency of luminescent concentrators are opened at the use of optical resonator for amplification of radiation (Fig. 5). Let dispose a matrix with activators between opaque Mirror 1 (100% of reflection) and semi-transparent Mirror 2. Then at illumination of centers of activation, the luminescent particles or quantum dots will begin spontaneously to radiate photons in different directions. Those rays that spread along an optical axis of resonator will excite the acts of luminescence of other centers of activation. Thus, intensity and as a result a quantum avalanche appears.

Every new act of radiation will be phase dependent from the radiation of primary photon. On the output of semi-transparent mirror the powerful beam of coherent laser radiation of certain wave-length will be formed. An optical resonator executes two functions in combination with luminescent concentrator: the first function consists in an additional activating of luminescence of active particles in a waveguide; the second function consists in reduction of angle of divergence of wave-guided radiation and, accordingly, in the increase of efficiency of concentration. 

In laboratories and industry a large experience of the use of luminescence and luminescent materials is accumulated. Especially, organic dyes luminophores are used for making of active materials of dye (cyanine) lasers. The wave-length of radiation of this laser can be changed depending on necessities by means of the use of diffraction gratings.

Polymeric luminescent concentrator on a cumarine dye is used for pumping of organic polymeric semiconductors lasers (Organic Semiconductor Centre, SUPA School Physics and Astronomy University St Andrews, North Haugh St Andrews KY16 9SS, UK). The edge of luminescent concentrator in the scheme contacts with a surface of MEH-PPV - laser with the distributed feed-back. The intensive green luminescent radiation, which goes out from the edge of concentrating film, «pumps» a laser.  

Part of hopes of specialists on the improvement of parameters of resonator waveguide concentrators is linked with the use of the well known luminescent glasses. Luminescent glass is made on the basis of glass matrices of a different chemical composition. At welding of glass into a charge the activators are added, more frequently, salts of rare heath elements or elements of actinide series. The output of luminescence is determined by the parameters of activator. Luminescent glass is characterized by good transparency and frequently used as active elements of solid state lasers.

Subject to the condition of intensive development of nanotechnologies, luminescent materials and scheme decisions the luminescent waveguide concentrators will occupy leading positions in the concentrating photovoltaic systems of solar power engineering.

Vasil Sidorov on May 28, 2010 from Technopark QUELTA

Queltanews Office e-mail: sidorovvasil@gmail.com