Optimal values of refractive index and thickness of the single-layer ARCs on silicon.
1. Introduction
Today’s photovoltaic solar panels are widely used to supply the power and buildings. The account of total solar cell product in 2010 was about 20 GW. 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 main reason for dominant role of silicon solar cells in word market is high quality silicon that produced in large quantities for microelectronic industry. Additionally silicon solar cell processing does not burden the environment.
The main requirements for ideal solar cell material are (a) direct band structure, (b) band gap between 1.1 and 1.7 eV, (c) consisting of readily available and non-toxic materials, (d) good photovoltaic conversion efficiency, (e) long-term stability [1]. Silicon is the second most abundant element in the earth’s crust (35 %) after oxygen. 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. can convert photons with energy of the silicon band gap. The theoretical curve for conversion efficiency of solar cell materials versus band gap for single junction cell (Figure 1) shows that silicon (1.1 eV) is not at the maximum of the curve (about 1.4-1.5 eV) but relatively close to it [2]. The efficiency for ideal silicon solar cell can reach about 30% (for AM1.5 at 300K).
Photoelectron properties related with indirect band structure and high reflectance of crystalline silicon (about of 30-35%) are still a challenger for creation solar cells with high conversion efficiency. 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). 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.
The first observation of a visible photoluminescence at room temperature in nanostructured porous silicon opened the possibilities of wide range photonic and biologic applications due to tunable refractive index, large surface/volume ratio an biocompatibility of porous silicon [3]. 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]. Porous silicon can be formed by chemical etching, electrochemical etching and photo-electrochemical etching of silicon in HF-based solutions at room temperature. Therefore the chemical technology can be more adapted to industrial fabrication of solar cells due to its simplicity and lower cost. Porosity, thickness, refractive index of layer, pore size etc. depend on formation parameters (electrolytes contents, current density, temperature, crystal orientation, doping type and concentration, time etching etc.). Sizes of pore and pores walls can be varied from 5-10 nm to hundreds micrometers dependent on fabrication parameters. 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. Last years the porous silicon layers are widely used in silicon solar cell applications.
This chapter has focused on review of investigations concerning using of porous silicon layers in silicon solar cells and also characterization of structure and properties of porous layers.
2. Photovoltaic characteristics of solar cells
For the solar cell with the series resistance
Here
The reverse saturation current is given
Here
When the solar cell is operated at open circuit (
Very low values of
The series resistance of cell depends on concentration of carriers in
The conversion efficiency of the solar cell is defined as the percentage of incident of solar power, which the can convert in electrical power
Here
The fill factor (FF) defines the portion of electrical power produced in solar cell in load. The fill factor is the ratio of the maximum electrical power divided by the open-circuit voltage and the short-circuit current
Substituting
The series and shunt resistance of solar cell influence on the fill factor. 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. A theoretical limit of efficiency of crystalline solar cell is about 30%. 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. To improve the performance of solar cells, the power losses must be reduced. The maximum absorption (i.e. minimum reflectance), minimum recombination and series resistance are most conditions for reaching a high efficiency solar cell. 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. Losses in solar cells
The losses in silicon solar cells can be related with: (a) recombination losses, (b) series resistance losses, (c) thermal losses, (d) metal/semiconductor contact losses,(e) reflection losses [6].
The incomplete chemical bonds presenting on the surface of semiconductor play role of traps for photo-excited carriers and therefore recombination on traps can result in reduction on photocurrent. The surface recombination velocity
Where
Impurities and crystalline defects, presenting in bulk region of semiconductor can play role of traps for carriers. Reduction of concentration of rest impurities in bulk of semiconductor, according to Schockey and Read model, will decrease the trap-assisted recombination velocity. Using the bulk semiconductor material having lower concentration impurities and defects can increase the diffusion length of minority carriers and thereby can decrease the recombination losses in bulk of solar cells.
The characteristic equation for solar cell described by Eq. (3) shows that an increase in reverse saturation current
Reduction of emitter layer resistance is reached by optimization of the doping concentration of layer and the
Pyramids are usually formed by etching the surface with acid (H2SO4, HNO3:H2O etc.) or with alkaline etch (NaOH, KOH etc.). The light bouncing from pyramid to pyramid increases the optical path and increases absorption of visible light in silicon, thus increasing the efficiency of solar cell. Antireflection coating presents thin film of a transparency material with refractive index (
The optimization of parameters (the refractive index and thickness) of ARC was based on the stratified medium theory and Bruggeman effective medium approximation [12]. The zero-reflection for normal incidence of light on ARC/Si system is given [13]
Here
The optimal single-layer ARC (SLARC) thickness (
If conditions (8) and (9) for air/SLARC/Si system are to be satisfied (
For double-layer antireflection coating (DLARC) with refractive index of top and bottom layers n1 and n2, respectively, on silicon (air/ARC(top)/ARC(bottom)/Si system) the zero-reflectance at normal incident light is realized at conditions [15]
Here
Different types of SLARC (SiO2, ZnS, Al2O3, Ta2O3), DLARC (MgF2/ZnS, SiO2/TiO2, MgF2/TiO2, MgF2/CeO2, SiO2/SiH etc.) and multilayer ARCs are used for reduction for reflectance of silicon solar cells. Optimal values refractive index and thickness for single-layer ARCs on silicon surface (air/ARC/Si system, for λ = 650 nm) are presented in Table 1.
|
|
|
SiO2 | 1.4 | 116 |
Si3N4 | 2 | 81 |
ZnS | 2.25 | 72 |
ZnO | 2 | 81 |
MgF2 | 1.4 | 116 |
TiO2 | 2.5 | 65 |
SnO2 | 1.9 | 86 |
SiNx:H | 1.9-2.4 | 68-86 |
Por.Si | (1.2-2.2)* | 74-135 |
As stated above ARCs are generally fabricated by chemical vapor deposition, plasma-enhanced chemical deposition, thermal oxidation processes. They are carried out at high temperatures resulting in an increase in the cost of solar cells [16]. 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 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. A single layer ARC allows a reduction of reflectance (up to 11%) only in a narrow wavelength range of solar spectrum. A single-layer coating cannot reduce the reflection in a wide wavelength because of neighboring interference maxima. 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. Usage of such inhomogeneous layer allows to suppress the interference maxima narrowing the spectral range. Using the ARC with monotonous changeof the refractive index on the depth can raise the performance of silicon solar cells. ARCs with a graded refractive index constituted from silicon and titanium oxides mixtures were studied in [17]. 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. 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. Fabrication and Properties of Porous Silicon
Porous 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]. 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. Moreover, a wide range of porous layer thickness, porosities, surface areas and morphologies can be formed depending on the etching conditions. The bulk silicon was shown modifies during the etching to sponge-like structure with silicon columns and hydrogen covered pores.
The simplest electrochemical cell is shown in Figure 5. The Si wafer acts as the anode and the platinum is the cathode. The thickness of porous silicon layer on Si substrate is determined by duration of etching. The porosity, i.e. the void fraction in the porous layer is determined by the current density (about 10 - 100 mA/cm2), composition electrolyte, resistance and the doping density of Si substrate.
The anodic reaction on the Si substrate can be written during pore formation as [19]
Silicon atoms are dissolved as SiF6
2- require the presence of F- ions (from HF solution) and positively charges holes (from the silicon wafer) at the silicon interface. Concentration of holes in
The porous silicon layers are often prepared in composition of HF:H2O, HF:C2H5OH, HF:C2H5OH:H2O, HF:HNO3, HF:HNO3:H2O. Fabrication of porous silicon layers on
The structure and size of pores in porous silicon layer formed on
Pores, depending on the diameter, denoted as micropores (R < 2 nm), mesopores (2 nm < R < 50 nm) and macropores (R > 50 nm). Under illumination the pore size dependent on doping density and anodization conditions, with diameters in the range 100 nm - 20 μm (macropores).
The average porosity (
Here
One can also get the porous silicon layer thickness
Here
The inhomogeneity in porosity and thickness of porous of the porous layers is often observed on fabrication with electrochemical anodization cell. They are most probably due to the bubbles that form and stick on silicon surface. The inhomogeneity in porous and thickness must be removed and the concentration of the HF has to be locally constant on the surface of the silicon substrate. Removal of the bubbles on the surface of the silicon and thereby preparation of homogeneous porous silicon layers is realized with using a stirrer. The distance between the silicon wafer and the platinum cathode also influences on the homogeneity, whereas the shape of platinum cathode almost does not influence on homogeneity. There is a certain distance for given cell when the porous silicon layers are homogeneous.
The thickness of the porous silicon layers mainly depends on duration of anodization process, whereas the porosity depends on the current density. It is be noted that the character of the thickness-etching time and porosity-current density relations depend on orientation, type and concentration level in silicon and conditions anodization process ( the electrolyte composition, distance between silicon wafer and platinum electrode, illumination etc.).
The thickness-etching time dependence for porous silicon layer fabricated on
The porosity of the porous silicon layer almost linearly increases with the current density once the other etching parameters are kept fixed (Figure 7).
Porous silicon is a particular form of crystalline silicon. The crystalline structure of porous silicon presents a network of silicon in nano (micro)-sized regions surrounded by void space with a very large surface-to-volume ration (up to 103 m2/cm3) [13]. The structure of porous silicon is like a sponge or columnar where quantum confinement effects play fundamental role [4]. The pore surfaces are covered by silicon hydrides (Si-H) and silicon oxides (Si-O).
Figure 8 shows Fourier transform infrared (FTIR) spectrum of free-standing PS film of thickness of 12 μm measured at room temperature [21]. The peaks related with absorption on vibration of Si-H (2100 cm-1) and Si-O bonds (1100 cm-1) located on pore surfaces were observed from Figure 8. These bonds play an important role in regulating optical, electrical and gas sensing properties of porous silicon.
The effect of isothermal annealing of free-standing PS films on changes of intensity of absorption coefficient of Si-H (2100 cm-1) and Si-O (1100 cm-1) peaks is used for estimation of diffusion coefficient [22]. Results of these measurements showed that in the range of 65-185°C the temperature dependence of hydrogen and oxygen diffusion coefficient along the porous surfaces are described as
The activation energy for diffusion of hydrogen along the porous surfaces estimated from response (or recovery)
The thermal oxidation of free-standing PS films in the range from 400 to 900°C is accompanied the structural phase transition [24]. The crystalline nanostructured silicon partly converts into amorphous and polycrystalline silicon, if temperature is about 500°C. At higher temperatures three Si structures (crystalline, polycrystalline and amorphous) produce SiO2 (combination of cristobolite and quartz) due to the oxygen diffusion and absorption in PS. An optimal oxygen-absorption temperature is about 700°C.
The characterization of the lattice deformation of porous silicon carried out by X-ray diffraction has been described in [25, 26]. Crystalline structure of porous silicon layers is equivalent to that of nearly perfect silicon. Porous silicon may be considered as an assembly of small silicon crystallites. These crystallites have two different dimensions, the bigger one being oriented perpendicular to the surface. Typical values seem to be about 1000 Å and 10-100Å. Lattice parameter of the porous silicon of 54% porosity prepared on
A linear increase in the lattice parameter expansion (the lattice mismatch parameter ∆
The origin expansion is attributed to the hydrogen-silicon bonds at the inner surface of the porous silicon. The hydrogen desorption results in a sharp contraction of the lattice parameter of porous silicon layer.
The pores on the surface of silicon increases absorption of light by increasing the effective thickness of emitter layer of solar cell (see Figure 4). Therefore the refractive index of the emitter layer, which strongly influences on efficiency of the solar cell depends on porosity of porous silicon. The current density applied during the formation process determines the porosity of the porous silicon layer and consequently its refractive index. We can say that the porosity-current density profile is transferred to the refractive index versus current density profile.
The reflectance can be calculated from refractive index. For film with parallel surfaces when light moves from a medium with refractive index
The reflectance spectrum of the porous silicon film is characterized by the multiple interference fringes caused by the air-porous silicon and porous silicon-silicon interfaces. A simple method for evaluating the refractive index on a thin film is to measure the interference fringes resulting from multiple reflections
Here λ1 and λ2 are the wavelength for two consecutive maxima,
Three type of porous silicon layer with different refractive index profile along the thickness are used in solar cells as antireflection coating:
As stated above the porous silicon consists of a network of nano-sized silicon walls and voids that formed when crystalline silicon wafer are etched electrochemically in HF-based electrolyte. Porous silicon presents the quantum system in which the charge carriers located in narrow crystalline silicon wall separating the pore walls. One of features of nanoporous silicon in comparison to the bulk silicon is shifting of fundamental absorption edge into the short wavelength of the visible region of the solar spectrum. It was confirmed by the measuring the optical absorption in the free-standing porous silicon layers [29].
PS layers with a thickness of 10-20 μm and an average porosity of 40 to 80% were prepared on
Figure 11 shows the effective energy gap in dependency on porosity of the free- standing PS films, calculated from extrapolation of the high energy part of {α2 (
Data on Figure 11 concerning increase of the energy gap in dependency on porosity of PS films can be explained by a model including the quantum confinement of carriers in the PS microcrystallites, causing the widening of the Si band gap.
The electrical measurements of the free-standing PS layers with 65% porosity (300 K, 45% RH) gave values of ρ = 1.8×106 Ω cm for resistance,
5. Porous silicon layers in silicon solar cells
As stated above the features of porous silicon (a quantum system, a sponge or columnar structure and an extremely large pore surfaces) provide many possible applications, such as light emitting diode, chemical and biological sensor, hydrogen fuel cell, photovoltaic cell, antireflection coating in solar cells etc.
Decrease of reflectance (between 30 and 3%) and increase of band gap of porous silicon layer (between 1.1 and 1.9eV ) with increase of porosity makes nanoporous silicon as a promising material for use in the solar cell technology. Therefore formation of nanoporous layer on frontal surface of PS/Si solar cell with lower reflectance and larger band gap, expanding the spectral range of photosensitivity, will contribute to increasing of conversion efficiency. Moreover,formation of Si-H and Si-O bonds on silicon surfaces followed by electrochemical etching in HF-based solution will provide passivation the pore surfaces.
Thus, the room temperature fabrication only nanoporous silicon layer on frontal surface of ready silicon solar cell, instead of three-step process (texturization, antireflection layer deposition and passivation), performed at high temperatures on standard technology can essentially improve the photovoltaic parameters and decrease the cost of silicon solar cells.
(1) Use as antireflection coating due to a lowering of the reflectance in the sensitivity range of silicon solar cell and possibility of formation PS layer with smooth change the refractive index between those Si and air
(2) Use as a wide-band optical window (the band gap shifts from 1.1 eV to 1.9 eV as the porosity is increased from 30 to 85% [29,31] that can broad the photosensitive region of the solar cell
(3) Use as front semiconductor layer with a variable band gap that can result in increase of photocurrent
(4) Possibility of the conversion of high energy ultra-violet and blue part of the solar spectrum into long wavelength radiations due to photoluminescence in nanocrystalline porous silicon
(5) Surface passivation and gettering role of porous silicon [32]
(6) Simplicity and lower cost fabrication technology of nanoporous silicon due to electrochemical modification of silicon
The theoretical requirements for the design of single- and double-layer porous silicon as antireflection coating on silicon solar cells are given in [33]. It is shown that the effective reflectance of
The significant reducing of the effective reflectance (up to 3%) was observed for monocrystalline silicon with porous silicon layer formed on previously texturized surface of sample [34]. Porous silicon layer was fabricated by electrochemical or chemical etch (stain etching) in HF:HNO3:H2O for 3-60 s on
Data of integral reflectance (for λ = 650 nm) of silicon samples without and with texturized layer, porous silicon layer or antireflection layer are presented in Table 2. The main conclusion which can be made from Table 2 is the effective reflectance for silicon samples with porous silicon layer is significantly smaller than that for samples without porous silicon layer. Moreover, the minimal effective reflectance (about of 3 %) is reached for porous silicon layer formed on previously texturized surface of silicon. The efficiency of PS/(
Polished surface | 33.9 |
mc-Si with ARC (SnOx) | 9.4 |
PS by stain etching (mc-Si) | 6.6 |
Texturisation by alkaline etching (Cz-Si) | 12.6 |
Texturisation by alkaline etching + PS by electrochemical etching (Cz-Si) | 3.4 |
Texturisation by alkaline etching + PS by stain etching (Cz-Si) | 3.1 |
PS by electrochemical etching (Cz-Si polish surface) | 9.7 |
The porous silicon layer formed on the textured surface of crystalline silicon by using silicon-dissolved tetramethylammonium hydroxides (TMAH) method results in significant decrease of reflectance [35]
Antireflection properties of nanoporous silicon layer on
The surface modification of silicon solar cells was used for improvement of photovoltaic characteristics of silicon solar cells in [37].
If conditions (8) and (9) for air/ARC/Si system are to be satisfied (
The growth rate of porous silicon on Si substrate, measured in this run for a current density of 60 mA/cm2, was about 8 nm/s. Therefore, the time of electrochemical etching under a constant current of 40, 50 or 60 mA/cm2 was 8-15 seconds. As a result, a blue colored PS layer between the grid fingers on the surface of the
A SEM micrograph of the front PS surface obtained by using scanning electron microscopy (SEM) (JSM-5410LV) is shown in Figure 12. Cross cut representation of silicon layer showed that the pores have a conical form.
Figure 13 shows the photoluminescence spectrum of the PS layer (60% porosity) on Si substrate, where the spectrum illustrates the peak at λ=580 nm (the orange region of the solar spectrum). Measurements of distribution of photoluminescence intensity along the thickness of the PS layer (of thickness 10 μm) showed that the intensity approximately linearly decreasesfrom the surface deep down. Similar results were also obtained by investigations of samples with PS layers of different thicknesses. Observation of photoluminescence in PS at visible region of the spectrum can be interpreted by quantum confinement effect causing the confinement of the charge carriers in nanocrystalline silicon wall separating the pore [39].
The integrated reflectance spectra of the polished silicon surface before and after porous silicon layer (of 60% porosity) formation showed the significant lowering of the reflectance (to 4%) as compared to polished silicon (about 38-45%). These data show that PS on (
The current-voltage characteristics of n+-p silicon solar cells without and with porous silicon layer of 60% porosity ((
The photosensitivity spectra of the solar cells with and without PS layer were presented in Figure 15. The value of photosensitivity for the PS/(
|
|
|
|
|
||
(n+-p) Si PS/(n+-p) Si |
23.1 34.2 |
500 520 |
0.74 0.75 |
12.1 14.5 |
28 4 |
[37] |
(n+-p) Si PS/(n +-p) Si |
21.5 28.4 |
580 585 |
0.55 0.74 |
7.5 12.5 |
12 3 |
[40] |
(n+-p) Si PS/(n +-p) Si |
95 mA 137 mA |
580 570 |
10.3 13.5 |
[41] | ||
(n+-p) Si PS/(n +-p) Si Text. (n+-p) Si PS/Text. (n+-p) Si |
17.2 20.1 23.3 25.5 |
598 606 592 595 |
0.74 0.75 0.70 0.74 |
7.6 9.5 9.6 11.2 |
7 12 3 |
[42] |
(n+-p) Si PS/(n +-p) Si |
18.5 27.2 |
580 601 |
0.73 0.77 |
7.85 12.54 |
7 | [43] |
PS/ (n+-p) Si SiOx/( n+-p) Si |
33.4 34.8 |
460 530 |
9 3.8 |
[44] | ||
(p+-n)PS/Si (100) (p+-n)PS/Si (111) |
15.9 mA 12.4 mA |
480 440 |
0.81 0.82 |
15.4 11.2 |
7 16 |
[45] |
(n+-p)PS/Si (111) SiO2/(n+-p)Si (111) (ZnO-TiO2)/(n+-p)Si |
12.4 mA 6.04 mA 10.2 mA |
440 370 410 |
0.82 0.79 0.81 |
11.2 4.4 8.4 |
[47] | |
(p+-n)PS/Si PS on one side (p+-n)PS/Si/PS PS on both sides |
8.8 mA 12.4 mA |
430 490 |
0.78 0.84 |
7.4 12.75 |
16 6 |
[48] |
(n+-p)PS/Si | 28.9 | 627 | 0.76 | 13.8 | 9 | [49] |
PS/(n+-p) Si SiN/(n+-p) Si |
26.3 28.4 |
602 606 |
0.76 0.75 |
12 13 |
10 | [50] |
(n+-p) mc-Si PS/ (n+-p) mc-Si |
26.6 28.9 |
572 582 |
0.75 0.76 |
11.3 12.7 |
15 5 |
[51] |
(n+-p) mc-Si (n+-p)PS/mc-Si |
29.8 30.2 |
577 587 |
0.5 0.76 |
12.9 13.5 |
8 | [7] |
Change the porosity deep down to PS layer can also stimulates the improvement of the photovoltaic parameters of solar cells. Experimentally observed decrease of intensity of the photoluminescence peak at 580 nm deep down to PS layer, which consists of pores of conical form, can be circumstantial evidence for decrease of porosity deep down. Taking into account that the band gap energy of nanoporous silicon increases with increment of porosity due to quantum confinement of carrier charges (see Figure 11), one can assume that the porous silicon layer on the (
The original formation technique of porous silicon layer on silicon solar cells was used in [40].
The porous silicon as ARC in (
The photovoltaic parameters of (
A significant improvement in efficiency of (
Double layers SiOx/PS structure as antireflection coating in (
The orientation of porous silicon influences of performance of silicon solar cells [45]. (
It is to be noted that for thick porous silicon layer when it thickness is larger than that of
The comparative investigations of (
Above we considered the results of studies on silicon solar cells with porous layer formed only on front side of cell. Interesting data for silicon solar cells with porous layers prepared
The formation of PS layer during the normal cleaning sequence at the beginning of the solar cell process was carried out in [49]. Porous silicon layers on p-type silicon wafer prepared by stain etching in HF:HNO3 showed reflectance as low as 9 %. Photovoltaic characteristics of the best porous silicon cells fabricated by screen-printed technique are given in Table 3. The high values of photovoltaic parameters of (
Thin porous silicon layer formed on dendritic web and string ribbon silicon by chemical etching in HF:HNO3:H2O solution for a few seconds increases the lifetime of the minority carriers by a factor 3.3 [50]. The additional heat treatment (860°C, 2 min) or simultaneous phosphorus and aluminum diffusion after PS layer formation results in lifetime enhancement by a factor 8.3 or 5.8 respectively. In opinion of authors [50] porous silicon induced lifetime enhancement can be caused by the gettering properties of PS layer. Photovoltaic characteristics of (
The improvement of photovoltaic parameters of
Gettering behavior of PS layer on silicon leading to enhancement of the lifetime of the non-equilibrium minority carriers and improvement of silicon solar cell characteristics have been considered in [52-54]. The porous silicon containing a large number of small pores with diameter between 4-50 nm shows very large the surface area-to-volume ratio (about of 500 cm2/m3). The extremely large pore surfaces and their very chemical activity ensure possibilityof application related with gettering of impurities and defects. The high-temperature annealing of the chemical etched porous silicon surface can enhance the impurity diffusion into the porous silicon network and thereby acting as an efficient external gettering site. The oxidation rate of porous silicon due to its mesapore structure is significantly larger (by 10-20 times) than that for single-crystalline silicon surface [52]. Gettering by porous silicon consists of oxidizing the porous silicon layer in wet oxygen ambient condition followed by the removal oxide in a dilute HF solution. The lattice mismatch between porous layer and silicon substrate can stimulate the gettering of impurities and defects.
The gettering-induced enhancement in the minority carrier diffusion length in multicrystalline silicon (mc-Si) with porous silicon layer on its surface was studied in [53]. The PS/Si/Al structures prepared by formation of porous layer in front surface of silicon and by vacuum evaporation of aluminum layer (2 µm) on the other side of sample were exposed to the wet oxidation of the porous silicon (950°C). Measurements showed that the minority carrier diffusion length for Si in such structure (about 190 nm) is larger than that for reference Si sample (about 100 nm). Co-gettering effect for PS/Si/Al consists in out-diffusion of impurities throughout the porous silicon layer and the aluminum. For {phosphorus layer/silicon/aluminum layer} structures (without porous layer) usually used for fabrication of silicon solar cell influence of co-gettering on the diffusion length of minority carriers is less (about 100 nm).
The PS/Si/Si structures prepared by forming porous silicon layers on both sides of Si wafer and then coated with phosphorus dopant (POCl3) and heat treated at 900°C for 90 min discovered the improvement of the electrical and recombination characteristics of silicon [54]. Diffusion of phosphorus into PS layer is accompanied by gettering of eventual impurities towards the phosphorus doped PS layer. As a result of removal of impurities, increasing the mobility of the majority carriers and the diffusion length of minority carriers are observed. Removal of eventual impurities and defects away from the device active regions allowed to improve output characteristics of silicon solar cells.
Above, the results of influence of the porous silicon as an active element in thick crystalline silicon solar cells have been considered. It is shown the porous silicon as antireflection coating significantly improves the performance of silicon solar cells. In last year’s porous silicon layer were also used as sacrificial layer for fabrication of thin-film silicon solar cells. As known the monocrystalline silicon which intensively are used in commercial solar cell applications is high cost material. The reduction in the amount of high-quality expensive silicon material per solar cell is one of ways of lowering the cell coat. At present one of the technologies developed to fulfill the aim of reduction the cost of silicon solar cell is porous silicon (PSI)-transfer process [55]. The thickness of monocrystalline silicon solar cell prepared by PSI-process (about 5-50 µm) is significantly lower than that of crystalline silicon cell fabricated by standard technology (250-300 µm). The PSI-transfer process consists of four steps.
6. Conclusion
The review of investigations of the use of the nanoporous silicon in silicon solar cell showed that an increase in the conversion efficiency (about of 25-30%) is achieved for PS/Si solar cell compared to a cell without a PS layer. At the same time, the performance of silicon solar cells with PS layer is more than that of silicon solar cells with conventional ARC. The lower value of effective reflectance (up to 3%) for nanoporous silicon layer that significantly reduces the optical losses is one of main reasons of improvement of performance of PS/Si solar cell. A wide-band gap nanoporous silicon (up to 1.9 eV) resulting in widening of the spectral region of photosensitivity of the cell to the ultraviolet part of solar spectrum may promote the increase the efficiency of silicon solar cells with PS layer. The internal electric field of porous silicon layer with variable band gap (due to decrease of porosity deep down) can stimulate an increase in short-circuit current (Figure 16).
Additionally, the intensive photoluminescence in the red-orange region of the solar spectrum observed in porous silicon under blue-light excitation can increase the concentration of photo-exited carriers. It is necessary to take into account the passivation and gettering properties of Si-H and Si-O bonds on pore surfaces which can increase the lifetime of minority carriers.
Porous silicon, along with above advantages, in some cases e.g. due to its high resistivity can reduce the output parameters of PS/Si solar cells. Contribution of resistance (about 5x10-2 Ω) of thin porous silicon layer (about 100 nm) in total series resistance of the solar cell (about 0.5-1.0 Ω) is negligible and therefore it must not influence on parameters of cells. The resistance of thick PS layer (5-15 µm) can essentially increase the total series resistance and thereby it can reduce the photovoltaic parameters of silicon solar cell. However, as is seen from Table 3 reducing of parameters of PS/Si solar cells with thick PS layer was not observed. It tentatively can be explained that formation
Note should be taken that properties of PS layer and thereby photovoltaic characteristics of solar cell can change on running under illumination, heating etc. At present, as far as our knowledge goes, publications on temporal stability of PS-based solar cells are almost absent in literature.Works related with degradation phenomena in PS/Si solar cells is the matter of topical interest for further researches.
Judging by the results presented in this review and taking into account the simplicity of fabrication of porous silicon layer on silicon we can safely draw to the conclusion that the nanoporous silicon is a good candidate for use on preparation of low cost silicon solar cells with high efficiency. It gives hope for the industrial production of PS-based silicon solar cells.
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