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Imamov E. Z., Jalalov T. A.,Muminov R. A., Rakhimov R. Kh.
8.2. UNIQUE OPPORTUNITY TO CREATE CHEAP BUT EFFECTIVE SILICON SOLAR CELLS
Imamov Erkin Zunnunovich, Dr., Professor of physics deportment TUIT, Professor. Tashkent University of Information Technologies, e-mail: [email protected]
Jalalov Temur Asfandiyarovich, senior lecturer Tashkent University of Information Technologies, e-mail: [email protected]
Muminov Ramizulla Abdullaevich, Academician Uzbekistan Academy of sciences. Institute of Physics and Technology, Scientific and Production Association «Physics-Sun» of the Academy of Sciences of the Republic of Uzbekistan
Rahimov Rustam Khakimovich, Doctor of Technical Sciences, Head of laboratory №1. Institute of Materials, Scientific and Production Association «Physics-Sun» of the Academy of Sciences of the Republic of Uzbekistan, e-mail: [email protected]
Abstract: In this paper, we propose one of the solutions to the problem of creating efficient silicon solar cells, which should:
- has a cheap technology of their production;
- using cheap materials;
- be durable, resilient and stable in functioning.
The approaches and methods, providing the ability to create efficient silicon solar cells.
Proposed technological factors selection of material of nanocluster as well as an analysis of the growth of physics, dislocation less and nanoheterostructures stability on the substrate based on the fundamental effect of self-organization of semiconductors systems. By using nanotechnology the negative properties of cheap and faulty silicon to transform into advantages.
Index terms: solar energy, solar cell, nanoinclusions, quantum dots, nanoscale contact structure, nanoscale «p-n junction», self-organization.
Introduction. Modern solar power requires the development of innovative approaches to improve production efficiency of solar energy, without which it can not compete with hydrocarbon energy.
The main component of the solar power device is a silicon solar cell, the efficiency of which is determined by geographical factors, its geometric forms, as well as the properties of the contact, the contacting materials, current collecting electrodes, and many other factors.
The main component of a solar cell is occurring in the area of contact between two materials embedded electrostatic field, the characteristics of which largely determine the efficiency of the solar cell.
These two factors traditionally dominated still in selecting methods of increasing efficiency in the processes of converting light into electricity. The main method is used as a solar cell substrate, if possible, and strictly pure crystalline silicon.
However, this direction of research reaches its saturation and thus practically not possible to exceed 30^25% of the theoretical maximum efficiency of silicon solar cells.
Under the efficiency of silicon solar cells is expected:
- cheap their production technology,
- cheap used when creating materials
- durable, stability, and the stability of their operation in the process of mass production of electricity.
Therefore, to create solar cells that satisfies fully the above quality requirements necessary to search for new and innovative approaches and methods. In this work, just and it offers one of the solutions to this problem.
Nontraditionals new solar cell. The main factor is not traditionally offered in the approach is the use of technical silicon as
a solar cell substrate, which is not characterized by a high degree of purity and crystalline structure of the ideal. The use of technical silicon could lead to a sharp reduction in price of the process of conversion of irradiative energy of the sun into electricity, as well as to an increase in the scope of application of solar energy capacity. This is the uniqueness of the proposed solar cells with much of the defective technical silicon substrate. There was nothing like it before, because it is this strong crystal defects was considered a negative factor.
However, such a simple, seemingly a solution to reduce the cost of solar energy is still not implemented. The reason for existing in the minds of many of the statements about the impossibility of classical means to create a high-performance separating the contact on the basis of highly defective silicon. This is, unfortunately, true.
But, the truth, in the ordinary macro world where the substance of which all the elements of the devices are extremely stable electrical properties.
Moving away from the usual cliches, in a complex of [1], and we were asked to address the problem of principally different, where the built-in electrostatic field is created by the NCS - a new contact structure. Its efficiency is precisely determined by the presence in the material of the substrate rich spectrum of different quality defects (and different in size, form and type of electronic energy states). Between these volumetric defects of micron size, there are narrow and long defect-free region with a high degree of order and cleanliness, along which are built-in electrostatic field.
New contact structure can affect the efficiency of the solar cell because of the substitution had some component macroscopic on the nanoscale counterparts. For example, one of the two contact areas of the space charge is replaced by nanocluster (or nanoinclu-sion) formed complex and heterogeneous nanogeterocontact
structure (or NHCS - "nanoheterocontactstructure ") on the basis of which during the establishment of thermodynamic equilibrium with the substrate. Her principal novelty that the material nanocluster from strongly electric capacity, other than the silicon semiconductor with a transverse dimension of about 4 4 35nm.
Furthermore, these nanoscale components interconnected na-noscale respective current collecting electrodes. These may be carbon nanotubes, or graphene coating having high conductivity with appropriate unusual properties.
Such replacement of some components of the macro to nano components in principle changed the properties of the solar cell. These changes are taking place very naturally, as in a state of many nanoscale electrical properties of the substances are principally different from the properties of macroscopic bodies (what is bad in the macroworld - well into the nanoworld, and vice versa!). The combination of the new solar cell macro- and nano-enabled component on the basis of highly defective silicon to create a sufficiently effective contact structure of a solar cell.
In particular, the difference arises from the fact that photoconversions functions of the proposed solar cell made no separate solid silicon p-n junction, as in a conventional solar cell, and so many parallel interconnected nanoheterocontact structures or "nanoscale p-n junctions" [1] .
The high electrical capacitance nanoinclusions at thermodynamic equilibrium is needed to increase the capacity of the limited space nanocluster a large number of carriers.
Accumulation of nanocluster, for example, a large number of electrons can consider it as a region of spatially localized negative charge or as a nanoscale "p-region".
The space depth of the substrate is regarded as an area of localized stationary, positively ionized residual impurities or a kind of "n-region" of the new contact structure, because the its free electrons moved in nanoinclusions.
In contrast to the "p-region", the length "n-region" is measured by several microns
Thus, the proposed nanoheterocontactstructure (or "nanoscale p-n junction'') can be regarded as a truly
- p-n junction (consists of two fixed parts or oppositely charged two conductivity types);
- hetero junction (contact of two different semiconductors);
- nanoscale structure (one of the elements - nanocluster).
Current collection with nanoheterostructures surface by carbon nanotubes or graphene coatings.
A lot of "nanoscale p-n junctions", connected in parallel to one another, forming an electrical circuit of a solar cell. This "solar cell with nanoscale contact structures " (SC NSCS).
Requirements of materials' of nanoheterostructures. Mono-crystalline ultrapure silicon condition characterized by a high degree of symmetry. But this state can not be maintained for long. Over time, the state will soften, stress will subside, entropy of the system will increase. All this is equivalent to slowly but surely move the structure into a state of thermodynamic equilibrium, that is, in a state with a much lower degree of symmetry. If the operating parameters of the unit are focused on concrete highly - state of monocrystalline silicon, the entropic processes will output will destroy the device, that is, the unit will begin to age over time.
This process of "thermodynamic aging" is almost imperceptible at the instruments with a lifetime of two or three years. But when it comes to solar cells that convert light energy into electricity in an open atmosphere for two or three decades years, it is clear that it makes sense to search for methods of tightening the "aging" process in the materials used.
In principle, the situation is different when the instrument is focused on the properties of highly entropic defective silicon:
- the velocity of the "aging" of the substrate material is reduced,
- improves strength and stability properties,
- slow with degradation processes.
All this is equivalent to increasing the life of the solar cell on the basis of highly defective silicon.
Thus, from thermodynamic considerations that the material used as a defective substrate of silicon with a uniformly distributed and defectiveness extremely high, not only ensures low cost solar cell, but also durability, and high stability.
Defectiveness of the substrate material of the categories lack the crystal goes into the category of advantages!
It should be borne in mind that the use of a defective silicon substrate material, can improve the efficiency of solar conversion only if you apply for this we offer solar cells with a "nanoscale p-n junctions."
Accordingly, with respect to new solar cells, in
- examine special requirements for material from its support and to the structure of the defects;
- province selection and classification of defects in silicon;
- defining the optimal parameters silicon defects.
Technically, it is not purified silicon band gap is literally crammed full [2] different electron energy levels of local states.
The silicon substrate randomly distributed defects are many, many varieties of them, their impact on the different physical processes. This chaotic arrangement of defects throughout the volume causes various local energy states of the electron, the location and distribution of which the entire band gap depends on the characteristics interdefect and interatomic interactions.
In turn, the presence of localized states in the band gap results in that the dependence of the energy E density of electronic states N(E), it is not only nonzero but also differently shaped nonmonotonic manifests itself within bandgap. At the same time, the most localized states - in the center, and less localized - near its edges.
Depending on the type and nature of possible defects:
- local state (ND or NA) donor or acceptor nature (impurity gives or receives electron) with a low ionization energy E=kT (shallow impurity states). This residual defect e s(RD);
- local state (NDD or NDA) or strictly donor or acceptor nature strictly (impurity gives or receives electron) with ionization energy E >> kT (deep impurity states). it e are structural defect s (SD). Here E - ionization energy, T is the temperature.
Localized energy states associated with structural defects usually located together with the Fermi level of the material - in the middle of the band gap, that is, the larger the disorder, the larger the localized states closer to the middle of the band gap.
Residual defect e s - a technical background is a silicon impurities in the likeness of doping impurities it's fine. This usually elements of groups 3 and 5 of the table of chemical elements. Desirably, the concentration (NA or ND) were as low (=10ls -1019 m-3) and alternately with the distance between them within 3 4 60 microns. About enough impurities - is shallow impurity energy states in silicon.
Structural defect e s - a job, alien tial and interstitial atoms, doubles, breaks chemical and intermolecular bonds, small imperfections and heterogeneity of structure, etc. They are randomly distributed along the crystal and homogeneous, create a band gap in the deep energy states acceptor and the donor whether nature. Their concentrations (NDA or NDD) should be as high (= 1024 4 1026 m-3) alternately with the distance between them within the 10 4 4 nm, the distribution of energy states the
Imamov E. Z., Jalalov T. A.,Muminov R. A., Rakhimov R. Kh.
on the band gap - quasieven and is quasihomogeneous. Therefore, the first silicon with such defects is nearing a state of thermodynamic equilibrium, in which the entropy change its properties (aging) occur slowly enough.
Structural defect e s - it is neutral defects. They are at enliven E >> kT. However, if their concentration are large enough, and approximately comparable, with a high probability of their mutual compensation can occur if NDD > NDA all the deep acceptors are ionized, if NDA > NDD all the deep donors are ionised. Due to the homogeneity of the silicon used positioning the district in the neighborhood of two deep acceptorsor two deep donors less likely than positioning the deep p th acceptor and donor is close to th.
Structural defects can be polyatomic systems that occupy much space large volumes. Therefore inevitably sc an individual on a list of defective blast it, and sufficiently puredefect-free areas it's between the two areas. They are essentially special zones of high purity crystal or single-crystal zone.
Requirements of the material of nanocluster. Define the parameters for choosing the material nanoinclusions. In [3] the possibility of spontaneous formation of equilibrium, periodically ordered governmental, coherently strained, three-dimensional nanoheter-ostructures on the surface of wide-gap semiconductors, if material of nanocluster is a narrow band gap (relative to the substrate) semiconductor. In addition, the nanocluster material should have high electric capacity to nanoinclusions could become the center of the electron concentration and the formation of the p-region na-noheterostructures. These requirements meet the family of lead chalcogenides: PbS, PbSe, PbTl. They are characterized by:
- a narrow band gap: in contrast to the "wide-band" silicon (ñEgSi~1,1eV) for lead chalcogenides ДЕ g PbS;Pbse;PbTi ~ 0,2-0,4 eV, that is relatively narrow;
- the high value of the dielectric constant: in contrast to silicon (sSi =12) for the lead chalcogenides: EPbS =175, EPbSe = 250, EPbTl = 450;
- non-small values of the effective mass of charge carriers: mn/mO=0,8=mp/mo for PbS, mn/mo = 0,04 and mp/mo=0,03 for PbSe [4].
On the illuminated surface of the silicon substrate by method of molecular beam epitaxy (MBE) formed nanostructures. Considering their carriers charge how limited space in three dimensions, we obtain for the energy spectrum of electrons nanostructures set of discrete levels, separated by regions of forbidden states, that is an exact copy of the ideal spectrum of the quantum dot.
Typically, the spectrum of the ideal quantum dot can have nanocluster, consisting of a thousand or a half thousand atoms [5], we have the perfect quantum dot is saved to the transverse size of 3-4 nm. However, viewed real nanostructures on surface of silicon consists of hundreds of thousands of atoms (with much greater transverse dimensions). In this case, the model of the quantum dot can be saved so long, until the operating temperature differences are not able to dilute its discrete energy levels.
Because of this, we define a minimum and a maximum transverse dimension of the nanocluster, in which are stored the applicability of the models the quantum dot.
Minimally possible size (Lmin) for real, spontaneously arising nanostructures is determined from the conditions of the emergence for the electron in its potential well at least one discrete energy level. The spherical quantum dot it is possible in the case of the conduction band discontinuity value of ДЕС between the silicon and the material nanoinclusions exceeds ДЕ1 - first electron energy level ( with an effective mass me) in the quantum well rectangular width of Lmin:
ДЕс > ДЕ1 = (fi2/2-meMп/ LmJ2
If the assessment of Lmin were rank ДЕС take in the range of 0.3-0.5eV, we find that the minimum value of the diameter of the quantum dot (or nanocluster) should not be less than 3-5nm.
Limit maximum size of the nanocluster (or nanoinclusions) is determined from the condition comparable thermal second scatter energy (the value of~kT) with the energy spacing between levels at which dramatically increases are not desirable populated of high levels (which is the same -still working a model of the quantum dot). Therefore for quantum point condition of the disregard occupied of high lying levels can be represented as: кТ < (E2 - EJ/3,
where E1 and E2 -the energy of the first and second levels of the size quantization, respectively. The calculated value of the maximum possible size of the nanocluster depending on the value of the effective mass varies within the range 20-35 nm.
Thus, for real spontaneously occurring nanostructures conditions:
кТ < E1 and (3-5) nm < L < (20-35)nm fully ensures the applicability of the models and the quantum dot. There of L- the diameter of the quantum dot.
Physics growth, dislocation and sustainability nanoheterostruc-tures. According to [3] growth on the surface of the silicon substrate equilibrium periodically ordered governmental, coherently strained, three-dimensional nanoheterostructures became possible thanks to the fundamental effect of self-organization [6] semiconductors systems. As a result its manifestations are creation nano-heterostructures (NGS), each of which represents a complex system of two components. The first component - p-region in the form of nanoclusters with the properties of an ideal quantum points (with discrete energy spectrum, with high crystalline perfection and high uniformity in size).The second component - quite extended, positively charged n-region.
Self-organization nanoheterostructures - this is the effect of the spontaneous occurrence in the original homogeneous system mak-roskoscopic ordered nanoclusters, which are in the process of establishment of thermodynamic equilibrium with the silicon substrate are become nanoheterostructures.
Spontaneous occurrence ordered nanoclusters associated with long-range fields of elastic stresses
- two-contact surfaces (substrate and nanostructures), which have different constants of crystalline lattices and different structures of surfaces;
- the outer surfaces of the themselves nanostructures - stresses arising from the presence of their faces and edges borders.
The ratio between the energies of long-range fields of elastic stresses determines the conditions of three-dimensional growth mode nanoheterostructures that occur on the open top of the substrate and on top of nanoheterostructures. Intensive growth is possible if the amount of elastic strain energy of the two contact surfaces is greater than the energy of only one border surface of the substrate.
According to thermodynamic representations in possibility of spontaneous occurrence nanoheterostructures due to the small-ness of the free energy of a solid solution with a non-uniform profile as compared to the free energy of a solid solution with a constant composition. Therefore, the system tends spontaneously or spontaneously go into a state with a low free energy. So processes of spontaneous formation of coherent nanoheterostructures accompanied by a shift in the energy system sustainable equilibrium state with the minimum free energy.
According to the findings of [3] identifies the following conditions of formation of sparse array nanoheterostructures equilibrium:
- the average distance between nanoheterostructures far large compared with their size;
- nanoheterostructures interaction between them, ensuring their stable, determined elastic anisotropy of substrate and a dipole-dipole (it is proportional to г-3, has the character of repletion at any occurrence between them);
- with temperatures far from the melting point, equal equilibrium form contains only the face with the small in surface energy;
- equilibrium shape single nanoheterostructures does not depend on its volume, and coincides with the experimentally observed form - a pyramid with a square base;
- minimum energy per unit area falls on the nanocluster with a two-dimensional square lattice, where the profitability of energy compared to other forms is explained by the elastic anisotropy of the substrate and not form a separate nanoheterostructures;
- thermodynamically tendency to coalescence (merger) is not available, if the ratio of alteration of the surface energy of the system during the formation of one nanoheterostructures and contribution to ribs in the energy of elastic relaxation is less than one;
- emergence of nanoheterostructures connected by chemical bonds with the substrate and the strain dependence of the surface energy.
In the real case, the balance can be established in part, and then only on the surface, rather than in the bulk. Possible occurrence of various violations of spontaneous stability of equilibrium states in different nanoclusters. However, the transition to an ordered state of inhomogeneous systems (ie formation nanoheterostructures) is typical for any nanostructures. When the above conditions on the surface of the solid substrate formed strictly ordered, coherent nanostructures with identical dimensions and same distances between them.
Conclusion: We demonstrated the possibility of using a defective silicon as a substrate, to create efficient solar cells. The role of residues and contaminants deep in the formation of the built-in electrostatic field, as well as the role of electrically capacious quantum nanostructures. The mechanism of formation of coherent identical nanoclusters on the surface of the substrate. Purpose - to show the possibility of using the advances of nanotechnology to transform the negative properties of cheap and faulty silicon dignity.
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