Chapter 3 Porous Silicon (PS)
3.6 Anti reflecting coating on electrochemical porous silicon base
For the first time the attention of the PV specialists was concentrated on PS in 80 years thanking its porous nature of a rough surface, low refractive index and blue shift of fundamental absorption edge (Fig. 3.8) [43]. These properties of PS assumed its availability as a material of an antireflecting coating for Si SC. The first experimental researches in this direction were carried out by Prasad et al. [44]. The authors of this paper used the por-Si layer for reflection reduction of polycrystalline SC with diffusion p+ - n junction by depth 0.6 µm. Conventional electrochemical anodization for PS layer formation was not used. It was a method of photostimulated chemical etching. Thus the PS growth in the HF- based electrolyte take due to a course of a photocurrent, induced in structure with p+ - n junction by electrical illumination. A
porous film thickness supervised on changing its interference colour. It might be typically 74 ± 2 nm.
Figure 3.8 Optical absorption coefficient versus wavelength for self-supporting 20 mm thick PS layer. Data for crystalline Si and amorphous Si:H is also shown Ellipsometric measurements determined the refractive index of the PS layer. It was in arange 1.95 ± 0.05. Such thin frontal layer of the PS in the silicon SC structure reduces its optical losses from 37 to 8% (Fig. 3.9) and increases a short-circuit current by 25% and open-circuit voltage by 20 mV, as experimental researches have shown. If the nonreflecting properties can explain such essential increase of a photocurrent, the gain of a SC output voltage is a result of a passivation by the PS layer of a Si surface and as a consequence a reduction of a saturation current. The PS antireflecting coating insignificantly influences on the form of the current- voltage characteristic of SC shown in fig 3.9. Thus such characteristics as the fill factor, series and shunting resistance essentially did not vary. It is important that the PS antireflecting coatings have demonstrated the weatherproof fact during several months.
Figure 3.9 Spectral reflectance and illuminated I-V characteristics of polycrystalline solar cells with and without PS coating [44]
The researching of PS based antireflecting coverings was continued after more than ten years interruption. At that PS layer was formed on a surface of c-Si wafers by an electrochemical anodization and its thickness about 10 mm. The integrated reflectance of such nonreflecting coating changes from 1.6 to 3.4% in a spectral range from 400 to 900 nm have shown measurements of optical losses. The received results are compared on efficiency with the best antireflecting coatings of the double MgF2/ZnS layer on the basis put on a texturized Si surface [1]. At the same time, if similar PS layer to use on a substrate surface of polycrystalline Si, the integrated reflective ability decreases only up to 10% for light waves with length from 400 to 900 nm [41].
The antireflecting coating in SC structure with dot contact has given effect of increase efficiency in consequence of optical losses minimization. But this effect was accompanied by a degradation of an open-circuit voltage and short circuit current in time [41]. Dependence of porous layer reflectance of its porosity is investigated as PS parameter optimization necessity for its using as silicon SC antireflecting coating. It was established that the lowest coefficient of PS reflection is reached with its porosity about 70% (Fig. 3.10). Thickness optimization of an antireflecting coating was carried out as porous layer surface optical reflection minimization.
The integrated reflectance of PS layer a little bit depends from a type and doping degree of an initial polycrystalline substrate was established eventually and it changes in a spectral range 350 - 1120 nm from 4.7 to 4.9% (Fig. 3.11).
The further researches of Si SC with PS frontal layer have confirmed its antireflecting properties, but essential increase of photoconversion efficiency could not demonstrate in consequence of the unoptimized structure of a SC. The results of subsequent papers [45-47] were more successful.
Figure 3.10 Effective reflectance coefficient as a function of E, the measured porosity of the PS layers [1].
The result of paper [47] was the SC with the 5 x 5 cm2 area. PS layer on its frontal surface was received by chemical etching in solutions with controllable composition. Efficiency of this SC under AM 1.5 illumination was 14.1%. However its spectral sensitivity, extended in infra-red range, and high quality of a surface passivation testify to a potential opportunity of the further increase of the output characteristics of this type of SC.
Figure 3.11 Integrated reflectance of a PS layer formed on a p doped, 1 Ωcm, as cut, polycrystalline wafer (Wacker, SILSO). The reflectance of bare substrate is also shown.
The result of paper [47] was the SC with the 5 x 5 cm2 area. PS layer on its frontal surface was received by chemical etching in solutions with controllable composition. Efficiency of this SC under AM 1.5 illumination was 14.1%. However its spectral sensitivity, extended in infra-red range, and high quality of a surface passivation testify to a potential opportunity of the further increase of the output characteristics of this type of SC.
Reference [48] has presented the most wide-ranging and productive researches of influence of a thin frontal PS layer on photoelectric properties of the multicrystalline SC. The parameter optimization of a porous layer as for optical losses minimization on a reflection, as for achievement of the maximum of the output characteristics SC was its purpose. These two directions are incompatible among themselves, but even in some cases are opposite on technological realization for the first time was shown. This conclusion is based on the fact that the increase of PS layer thickness which is necessary for optical losses decreasing simultaneously conducts to increase of series resistance of structure SC and lowers its fill factor.
This was illustrated most clearly in [48] by the dependence of output parameters of SC on the charge value passed through the device structure during process of the PS layer formation. (Fig.
3.12). As it follows from Fig. 3.12, there is certain value of the charge (and, consequently, thickness of the porous layer), when the output electrical characteristics begin to decrease
sharply at exceedance of it. This means, that to reach maximal positive effect from the use of antireflective PS layer in the structure of SC is possible only under detailed optimisation of its parameters.
According to the results of ref [49] the optimal parameters of the por-Si- based antireflecting coating for multicrystalline SC studied with shallow (0.4 mm) n+ - p junction are the following: thickness 80 nm, porosity 69%, refraction index 1.64. The use of such optimised PS layer in structure of SC has allowed to increase the short-circuit current of the SC more than by 20% and improve the effciency from 7.5 - 8.5 to 10 - 11%. The increase of photoresponse of the PS/ n+-multi-Si/p- multi-Si structures.
Figure 3.12 Multicrystalline solar cell parameters as a function of the electrical charge passed through the cell during anodization. The data are normalised to the values before anodization
In conclusion, PS formation is simple and is accomplished by either electrochemical or chemical etching but is still not completely understood or developed for PV. PS offers a low-cost and high through-put route to surface texturing and can also serve as an anti-reflection coating.
This provided the motivation to understand and develop low-cost manufacturable porous silicon etching for crystalline Si cells understood or developed for PV. Porous silicon offers a low-cost and high through-put route to surface texturing and can also serve as an anti-reflection coating.