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Silicon Solar Cells with Nanoporous Silicon LayerTayyar Dzhafarov1[1] Department of Solar and Hydrogen Cells, Institute of Physics, Azerbaijan National Academy of Sciences, Azerbaijan1. The series resistance of cell depends on concentration of carriers in n- and p-region, depth of p-n junction, resistance and construction of frontal and back ohmic contacts. 2- require the presence of F- ions (from HF solution) and positively charges holes (from the silicon wafer) at the silicon interface.
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Physical and Technological Aspects of Solar Cells Based on Metal Oxide-Silicon Contacts with Induced Surface Inversion LayerOleksandr Malik1 and F. Last time I started to talk about the solar cell business, and whether there was any realistic opportunity for new technologies given the state of the industry. The sun is a hot black body radiation source that is being converted to electricity by a room temperature band gap semiconductor.
The alternate materials, CdTe and CIGS, are direct bandgap semiconductors, and as a result have much high absorbtion for a given thickness than indirect bandgap silicon.
Single crystal GaAs has the best efficiency that is close to the theoretical maximum with polycrystalline silicon at 20%.
The lower efficiency of the non silicon material means that there must be a proportionally larger area of cells to generate the same energy. To date, none of the single junction alternatives have created cells with better internal efficiency than thick polySi cells, and certainly do not suggest any immediate prospect of exceeding mature poly-silicon technology.
My first take is that with an efficiency disadvantage, the cost structure and manufacturing scale of non-Si thin film solutions today are behind polySi, and which explains their competitive challengies. To understand the opportunity in module cost, let’s consider a test case of a new factory that Panasonic is building in Malaysia for $540 M, with a capacity of 300 MWatts per year, and will require 1,500 staff. The manufacturing opportunity for thin film with the same cell efficiency and raw material cost is to halve the total module cost. My take is that the alternatives to silicon have a significant leaning deficit to overcome to get closer to efficiency parity, then they need to invest in the most productive manufacturing capacity. GlobalFoundries’ top technologists open up on next-gen FD-SOI, the economics and challenges of 7nm, and what lies ahead.
Older equipment is now very much in demand due to shifts in end markets and new options for packaging. Synopsys’ chairman looks at the biggest changes in the industry and how they will affect technology for the foreseeable future. Advancing to the next process nodes will not produce the same performance improvements as in the past. Industry leaders examine enablers, implications, and perspectives for a changing technology ecosystem. FinFET formation, mask challenges and back-end-of-line issues will make this node difficult and expensive. General-purpose metrics no longer apply as semiconductor industry makes a fundamental shift toward application-specific solutions. There are a number of assumptions associated with the SQ Limit that restrict its general applicability to all types of solar cells.
If the theoretical limit for silicon cells is about 30%, what happens to the other 6% that is lost from the best production cell efficiency of 24%? For a semiconductor electron to move into an external load circuit, its energy level must be increased from its normal valence level (tightly bound to one atom) to its higher energy conduction level (free to move around). Basically the strategies to obtain better efficiencies than the SQ Limit predicts are to work-around one or more of the critical assumptions listed above (and shown again below). The example of a multi-junction cell on the left has a top cell of gallium indium phosphide, then a "tunnel diode junction", and a bottom cell of gallium arsenide. Performing a calculation using the SQ methodology; a two-layer cell can reach a maximum theoretical efficiency of 42% and three-layer cells 49%.
Instead of a typical 6 inch by 6 inch solar cell, a 7 inch by 7 inch square plastic Fresnel (pronounced Fray-NELL) lens incorporating circular facets, is used to focus the sunlight as shown on the left. The Stanford University Photon Enhanced Thermionic Emission (PETE) prototype uses concentrated sunlight as its source of energy and in a two step process uses both the sun's photon energy and its heat. In a regular solar cell, each photon collision generates a particle pair consisting of one free hole and one free electron. Another characteristic of a Quantum Dot is that different sizes capture different wavelengths of light.
No solar cells produced prior to December, 2011 have quantum efficiencies greater than 100 percent. The schematic representation of multiple light reflection (on left) and optical microscopy image of textured silicon surface (on right).Table 1. Photocurrent density-voltage chracteristics of (n+-p) silicon solar cell (1) with and (2) without PS layer. IntroductionToday’s photovoltaic solar panels are widely used to supply the power and buildings. Increase of shunt resistance and decrease of series resistance result in to higher fill factor and thereby to larger efficiency.For crystalline silicon solar cells efficiency about 25% in laboratory and 14% commercially is reached. The decrease of surface recombination velocity is usually reached by deposition of thin passivation films on top surface of cell (for example SiO2 or Si3N4 films) by chemical vapor deposition (CVD), plasma enhance chemical vapor deposition (PECVD) or thermal oxidation technique). The excess energy formed at absorption of the solar photons that is larger than the band gap energy of semiconductor is realized in the form heat.
The screen-printed technique is often used for deposition metallic contacts to silicon solar cells[8].
They are carried out at high temperatures resulting in an increase in the cost of solar cells [16].
Concentration of holes in p-Si is sufficiently high (about 1014 - 1018 cm-3) and this case the nano-size pores were formed.
Calculated variation of the barrier height with metal work function for different values of the interfacial layer thickness [3].3. Calculated dependence of Qs on the surface potential at the silicon surface for different donor concentrations in the silicon substrate.4. Optical transmission of the ITO films for two thicknesses as a function of the wavelengthTable 3. Dependence of the collector current versus the collector-base voltage (the emitter current is used as a parameter). Formation of the inversion layer in the ITO-nSi structure when the ITO film is deposited on the silicon surface with surface barrier q?b. ITO-nSi portable sector-shaped solar modules fabricated by waste-free technology on 3 inch silicon wafers. Javier De la Hidalga-W1[1] Instituto Nacional de Astrofisica, Optica y Electronica (INAOE), Puebla, Mexico1. From Table1 it is clear that the best result regarding the minimum density of surface states is obtained by using hot water. Using NH4F as the final etching agent under clean-room conditions, the remaining amount of surface charges results from the electro-negativity difference between silicon and hydrogen.After the wet-chemical oxidation of initially H-terminated surfaces, characteristic values of the surface Fermi-level EFs, as shown in Table 2, were obtained from the interface-trapped charge and from different kinds of oxide charges.
Any photon with energy greater than the band gap creates a single electron hole pair, so low energy photons below the bandgap do not create current, and high energy photons do not generate any more current than band gap photons. Higher absorbtion means that the cells can be 20x thinner, enabling “Thin-film solar cells”. This has proved to be a major barrier to home installations, but not to large generation plants.
Although there are numerous programs underway to find ways around the SQ Limit, it is still applicable to 99.9% of the solar cells on the market today. Some sunlight is always reflected off the surface of the cell even though the surface is usually texturized and coated with an anti-reflective coating. Some infrared, all microwave, and all radio waves do not have enough energy and pass right through the solar cell. Absorption of electromagnetic radiation is the process by which the energy of a photon from the sun is transformed into other forms of energy for example electricity or heat. Since sunlight will only react strongly with band gaps roughly the same width as their wavelength, the top layers are made very thin so they are almost transparent to longer wavelengths. Most of today's research in multi-junction cells focuses on gallium arsenide as one of the component cells as it has a very desirable band gap. The record for a multi-junction cell is held by the University Of New South Wales (UNSW) in Sydney, Australia at 43% using a five cell tandem approach. While the tiny solar cells use less of the expensive semiconductor materials, cost is a factor as a two-axis sun tracking heliostat is necessary to accurately keep the focus point on the solar cell as the sun travels east to west each day and north and south each season.
A thermionic converter consists of two electrodes separated by a vacuum, see the figure to the left.

Quantum efficiency (not to be confused with solar cell efficiency) per the National Renewable Energy Laboratory (NREL) located in Boulder, Colorado, is the “ratio of collected charge carriers (electrons or electron holes) to incident photons”.
The reverse saturation current is determined by the leakage current of carriers across the p-n junction under reverse bias.
The standard technique for the reduction of the surface state density of Si is thermal oxidation at 800oC for 15 min in dry oxygen [7]. The temperature rise of solar cell results in increase in the reverse saturation current Io due to an increase concentration of the intrinsic carriers ni and diffusion length of minority carriers (Eq.
The frontal contact has form of fine grid lines and the back contact is a metal plate covering entire back surface of cell. Traditional antireflection coating and surface texturing may reduce reflection efficiently (up to 10-15%) at low wavelength in the visible spectrum (about 400-800 nm), but they are less efficient at harvesting light in the near infrared spectrum (more than 800 nm).
The mean size of the grains, 30-50 nm, was determined using the classical Debye-Scherrer formula from the half-wave of the (400) reflections of the XRD patterns.
IntroductionWith the current concerns about the worldwide environmental security, global warming, and climate change due to the emission of CO2 from the burning of fossil fuels, it is desirable to have a wide range of alternative energy technologies. Most of the oxidizing solutions SC-2, HCl, and HNO3 cause a strong depletion of holes on p-type silicon surfaces due to the positive fixed oxide charge, which is also known from CV measurements of thermally grown oxides. The same applies to thin film silicon cells which much less efficient because they absorb less of the light, as a result about twice the area is needed to generate the same power. The STC conditions approximate solar noon at the spring and autumn equinoxes in the continental United States with the surface of the solar cell aimed directly at the sun.
The red colored wavelenghts do not have enough energy and the yellow ones have too much energy. This allows the junctions to be stacked, with the layers capturing the shortest wavelengths on top, and the longer wavelength photons passing through them to the lower layers. When the cathode is heated to a high temperature, electrons become excited, jump across the thin vacuum to the relatively cold anode, and drive a current through an external circuit back to the cathode. In layman terms - its the ratio of the number of electrons produced in a solar cell to the number of the sun's photons hitting the cell. It should be emphasized that the research into Quantum Dots is at a very basic stage of demonstrating scientific principles.
Over 95% of all solar cells produced world wide are composed of the silicon (single crystal, polycrystalline, amorphous, ribbon etc.) and domination of silicon-based solar cell market probably will be do so in the immediate future.
The leakage current is a result of recombination of carriers on either side of the junction. Comparison efficiency of industrially produced silicon solar cells with theoretical efficiency shows that about 85% power losses occur in commercially cells.The present efficiency and cost of the silicon solar cell in comparison with conventional energy sources limit the wider using of silicon cells.
Ag and Ag-Al pastes are used in conventional silicon solar cells by deposition of frontal and back ohmic contacts by screen-printed technique. The significant of portion of solar radiation, penetrating through the atmosphere, lies at wavelength greater than 800 nm, therefore solar cells with traditional ARC and surface textures leave a huge amount of potential energy out of the using.Use the single layer ARCs is most simple, low cost and suitable for silicon solar cell technology as compared to expensive and impractical double- or multilayer ARCs. Fabrication of porous silicon layers on n-type silicon substrates is usually produced under illumination.
Parameters of the oxide layer formed with different chemical agents [4], Dit,min is the minimum density of surface states. Photovoltaic, or solar cells, have already proven themselves to be a viable option as a nonpolluting renewable energy source, as well as a visible business that will grow stronger in the global economy of present and future centuries. The challenge in thin film PV solar is how to create thin sheets of low defect crystals on either glass, plastic or metal films; the most difficult challenge in hetero-epitaxy.
Seeing as all these material have the same theoretical maximum the differences are in the internal efficiency, which is a typically dependent on the quality of the epitaxy and the semiconductor growth process.
The market place has shown that even with significantly lower prices, thin film silicon has not been able to compete. The modern SQ Limit calculation is a maximum efficiency of 33% for any type of single junction solar cell. Only photons with at least the band gap energy will be able to free electrons to create a current.
The yellow wavelengths are absorbed and generate electricity, but a lot of their energy is lost. In addition to the cost issue, there are other constraints that make the tandem cells complex.
In the Stanford prototype, the cathode emitter is a semiconductor material rather than a metal electrode. Researchers from the NREL have reported quantum efficiencies of 114 percent in solar cells “excited” from photons from the high-energy region of the solar spectrum. The main reason for dominant role of silicon solar cells in word market is high quality silicon that produced in large quantities for microelectronic industry. Reduction of concentration of rest impurities in bulk of semiconductor, according to Schockey and Read model, will decrease the trap-assisted recombination velocity. A single layer ARC allows a reduction of reflectance (up to 11%) only in a narrow wavelength range of solar spectrum. Such parameters are suitable for the fabrication of Schottky diodes based on metal-semiconductor structures.
Sunlight photons with less than the band gap energy will simply pass through the solar cell. That is because photons with excess band gap energy generate a free electron and a hole, but their extra energy gets dissipated as heat. For example, all the layers must be lattice compatible with one another in their crystalline structure and the currents from each individual cell must match the other cells.
That is from the near ultraviolet through the visible light spectrum - 350 to 700 nanometers. Production solar cells using Quantum Dots are thought to be about 10 years into the future.
Presence of acceptor type impurity (Al) in back contact (Ag-Al) results in decrease the resistance of near-back contact region of p-type silicon substrate due to diffusion penetration of aluminum during thermal treatment. A single-layer coating cannot reduce the reflection in a wide wavelength because of neighboring interference maxima.
Now, it is interesting to know the band bending of the silicon surface after different processes for obtaining the chemical grown oxide.
Silicon growth has been researched for as long as transistor have been made, so there is a huge learning advantage to silicon.
Multi-junction cells are commercially used in only special applications because their expense currently outweighs any efficiency improvement. The series resistance controlled by the top contact design and emitter resistance must be carefully designed for each type of solar cell in order to optimize efficiency of solar cell.
Increase in Io means the rising the “leakage” of carriers across the p-n junction under reverse bias due to recombination of carriers in the neutral regions on neither side of the junction.
A wider spectral range may be obtained either by increasing the number of layers or by using an inhomogeneous layer with gradient of refractive index. Herewith, pore formation begins on surface defects of Si and further growth of pores into silicon substrate proceeds due to the holes diffusion to Si-electrolyte interface.
It is the most studied material, and the lifetime of silicon solar cells and modules is 15-30 years. Chemical oxide after treatment in hydrogen-peroxide The chemical oxide can also be created on the silicon surface with the treatment of the wafer in an aqueous solution of hydrogen-peroxide. There is a particular wide range in efficiencies reported for CIGS, lower cost production techniques produce much lower efficiencies. At the moment they are used in space where weight is most important and in concentrated PV systems where the sunlight is focused on a very small cell area requiring only small amounts of semiconductors per cell. The tiny solar cells are mounted on a supporting plate at locations corresponding to the focus point of each Fresnel lens.
PETE converts about 25% of the sunlight's energy into electricity at 200°C and higher efficiencies at higher temperatures, i.e. It is the base material for photovoltaic conversion of solar spectrum radiation ranging from ultraviolet to the near infrared, however it can absorb the small portion of solar radiation, i.e. The reduction of different energy losses in crystalline silicon solar cells is the most problem of improvement of the conversion efficiency and thereby of reduction of cost.3.
Usage of such inhomogeneous layer allows to suppress the interference maxima narrowing the spectral range. The higher efficiency of silicon solar cells (up to 24%) that can be achieved using a complicated cell design, and applying new technological processes, lead to an undesirable increase of their total cost. High values of series resistance will produce a significant reduction in short-circuit current. Using the ARC with monotonous changeof the refractive index on the depth can raise the performance of silicon solar cells. It ensures the uniform etching of silicon surface and formation a smooth surface of substrate (the so-called the electro-polishing process).

From this point of view, solar cells based on more simple Schottky contacts and metal-insulator-semiconductor (MIS) structures are promising for solar energy conversion due to their relatively low production cost.Since 1978, a new class of photovoltaic devices, namely the semiconductor-insulator-semiconductor (SIS), has emerged, using a deposited conductive top layer made of a degenerated wide-bandgap oxide semiconductor. Losses caused by series resistance are given by P= VRsI = I2Rs and increase quadratic with photocurrent and therefore they most important at high illumination intensities.
ARCs with a graded refractive index constituted from silicon and titanium oxides mixtures were studied in [17]. The raising the current density above the critical value at the end of anodization process results in a detachment of the porous silicon film from Si substrates. XPS analysis shows that the oxide thickness saturates on this level after 10 minutes of immersion in the solution. Very low values of shunt resistance Rsh will produce a significant reduction in open-circuit voltage. 3.7% average reflectance between wavelength from 300 to 1100 nm and 48% improvement of the photocurrent was reached on using silicon and titanium oxides mixtures as graded ARC on silicon. The behavior at high current densities turns out to be useful to produce porous silicon free-standing layers.
High refractive index of crystalline silicon (about 3.5) in solar spectrum region of 300-1100 nm creates large optical losses which can be reduced by using antireflection coating (ARC). It is believed that the porous silicon with tunable refractive index can be adapted production of silicon solar cells due to the simple and cheap technology.4. Of course, the optical and photoelectrical properties of SIS structures exceed the properties of MIS devices. Although highly efficient double and triple antireflection coatings are available, most manufactured crystalline silicon solar cells employ simple and inexpensive single-layer ARC with relatively poor antireflection properties. Both series and shunt resistance losses decrease the fill factor and efficiency of a solar cell.The characteristic equation for solar cell described by Eq. Fabrication and Properties of Porous SiliconPorous silicon layer on monocrystalline Si substrate and its manufacture by the technique of electrochemical etching of silicon substrate in HF solution or by chemical etching in HF-HNO3 mixture are known as early as from 1956 [3,18]. They show that the oxide thickness obtained in H2O2 solutions for different concentrations as function of time does not exceed 0.9 nm. Electrochemical etching of silicon is attractive because of the possibility to tune the pore size from a few nanometers to a few tens of micrometers, just by choosing wafer doping level and etching conditions. In order to explain this experimental fact they used the results of Stoneham and Tasker, where the effect of image charges and their influence on the grown oxide films are studied.
Today the porous silicon is quickly becoming very import and versatile material for solar cell technology.The crystalline structure, chemical, electrical, photoluminescence and optical properties of porous silicon have been extensively studied by various experimental techniques [4]. Increase of reverse saturation current means rising of leakage of carriers across the p-n junction under reverse bias due to recombination of carriers in depletion regions on either side of junctions.
Moreover, a wide range of porous layer thickness, porosities, surface areas and morphologies can be formed depending on the etching conditions. It was shown that such structures present a high barrier height that is not typical for Schottky diodes. These authors found that the polarization energies associated with localized charges near the interface between oxides and silicon provide a driving force, over short distances, which affect the transport of peroxide anions HO2- (principal oxidant) to the silicon surface. Porous silicon can be formed by chemical etching, electrochemical etching and photo-electrochemical etching of silicon in HF-based solutions at room temperature. Recombination proceeding in the depletion regions is less significant as compared to the surface recombination due to the electrical field of p-n junction that separates the photo-generated electrons and holes.Reduction of emitter layer resistance is reached by optimization of the doping concentration of layer and the p-n junction depth. The bulk silicon was shown modifies during the etching to sponge-like structure with silicon columns and hydrogen covered pores. The authors developed a physical model of the ITO-Si solar cells based on an inversion p-n junction similar to that reported by J. As the oxide thickness grows, the image charge reduces the transport of the ionic species, and the oxidation process is limited. Therefore the chemical technology can be more adapted to industrial fabrication of solar cells due to its simplicity and lower cost. For (n+-p) silicon solar cell the optimal values of junction thickness and doping concentration about of dpn? 0.8 ?m and n+? 2x1019 cm-3 respectively. According to Verhaverbeke, the model based on the charge transport (Figure 3) that predicts the frequently observed limitation of an oxide thickness of around 0.8-1 nm, may also be applied to the oxide grown in SC-1 solutions with a certain content of hydrogen-peroxide.
It is not possible to know in advance the sign of the charge in the oxide formed with ultrapure H2O2, but below we show that the presence of some impurities in the hydrogen-peroxide solution can change drastically the situation. The thickness of porous silicon layer on Si substrate is determined by duration of etching. According to this model, the I-V characteristics are dominated by a diffusion current flow in the bulk of the silicon substrate and show the usual behavior for a Shockley diode.The aim of this chapter is to discuss some new physical aspects of spray deposited ITO-Si solar cells which are tightly connected with the fabrication technology.
Sizes of pore and pores walls can be varied from 5-10 nm to hundreds micrometers dependent on fabrication parameters. Below, we will show that suitable process schedules for chemical treatment of the silicon surface with the presence of acceptor-type surface states, is the reason for the inversion of the conductivity type at the silicon surface. Chemical oxide fabricated with a contaminated SC-1 solutionGenerally, silicon wafers always present some charge in either, chemically or thermally grown native oxide. Possibilities of minimization of reflectance (due to light trapping in pores), increase of band gap of porous silicon layer (due to quantum confinement of charges in the PS microcrystallites) by changing of porosity allow to use PS layer as both ARC and wide-band gap photosensitive layer. A sufficiently high potential barrier can be formed before the deposition of the ITO film if a minimum amount of fixed charge appears within the interfacial layer very close to the silicon surface. Then the role of the ITO electrode is the formation of an ohmic contact on the inversion layer.2. It is also known that a large positive fixed oxide charge appears in naturally oxidized Si wafers soon after they are dipped in an aqueous hydrofluoric acid (HF), and then the charge decreases as the native oxide grows in air. Barrier height of MIS solar cellsSolar cells based on contact metal-semiconductor with a Schottky barrier really represent MIS structures due to the existence of a thin insulator layer between the metal and the semiconductor. MIS solar cells are receiving increasing attention because they present several inherent advantages such as low cost, high yield, fabrication at low substrate temperature, etc. The charge was significantly smaller when the treatment was conducted using a quartz container; this is because the Al (and also Fe) concentration in quartz is generally more than one order of magnitude lower than that found in Pyrex glass.
This fact clearly suggests that some species in the SC-1 solution must be the cause of the negative charge, whose density should be much higher than that of the positive fixed oxide charge.
At this moment, the exact chemical mechanism of the negative charge formation is not clear. The positive sign of the charge is typical for several chemical methods used for the fabrication of thermal thin oxide on the silicon surface. Authors assumed that the negative charge arises from (AlSiO)- networks, when 3-valence Al substitutes 4-valence Si in the oxide.
The role of Al to form the negative oxide charge on the Si surface after rinsing with the SC-1 solution was verified with especially Al-contaminated SC-1 solutions. In this case the sign of the third term in equation (1) is positive, and a higher value of the barrier height is possible. Thermally grown oxide on the silicon surface usually presents a positive fixed charge, however it may be possible to introduce a negative charge in the oxide by developing suitable process schedules for the chemical treatment of the semiconductor surface.
Relation between the charge and surface potentialFor our discussion, we must find a relationship between the charge on the semiconductor surface Qsand the surface potential ?s.
Work function of tin-doped indium oxide (ITO) films In this section we discuss the work function (q?ITO) of the ITO film. This parameter presents a strong dependence on the fabrication method, structure and morphology of the film, and also on the carrier concentration. Reported results for films obtained by thermal evaporation give a work function of 5.0 eV [11].
Surface potential of the silicon wafers after a chemical cleaningFor any technical application, the silicon wafer must be subjected to a certain schedule of chemical treatment. These are very important technological steps necessary to prevent the contamination of the future device from non-controlled sources such as some metals that can introduce deep energy levels into the substrate.The RCA Standard Cleaning, developed by W. Puotinen in 1965, and disclosed in 1970, is extremely effective for removing contamination from silicon surfaces, and it is a current industry standard.
For this reason, the overall wafer thickness is slowly reduced, but certain thickness of SiO2 forms on the wafer surface. With a suitable thickness and physical parameters, these oxide layers can be used for the fabrication of MIS solar cells.
However, the question is what the thickness of the silicon oxide layer will be obtained and which surface potential on the wafer will be developed after applying this technological procedure? One more question is connected with the density of surface states after the wet chemical oxidation.Many researchers have tried to find the answer to these questions.
For instance, a complete investigation on this issue was conducted by Angermann [4]; some parameters of the oxide layers formed with different chemical agents are shown in Table 1.

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