Invited ArticleSimulation of anti-reflecting dielectric films by the interference matrix method
Introduction
The processes of reflection of solar radiation and recombination of photo-electrons on the surface and in the active volume of the solar cell significantly reduce the efficiency of the solar cell. Today, there are two known ways to reduce the reflection coe-co-efficient of solar radiation, which have different physical mechanisms: the use of anti-reflection coating made of dielectric materials with low absorption and the formation of a profiled surface. At the same time, the problem of reducing the cost of technological processes and the materials used in them and preventing processes that lead to degradation of the properties of solar cells remain relevant. For example, for silicon solar cells, the losses associated with the reflection of solar radiation may be more than 35%, since the smallest reflection coefficient from the polished silicon surface for normally incident solar rays is approximately 30%.
In traditional solar cell technologies, anti-reflection surfaces were used, reducing the average reflection to 10–15%. For example, CdS films deposited on CdTe-based cells reduce the reflectivity by up to 14%. ZnO and In2S3 films for Cu(In,Ga)Se2 based solar cells provide reflections of 11.5–13.6% [1].
Modern optical technologies use films ranging in thickness from tens to hundreds of nanometres. The error of the thickness of the film deposited on a standard silicon wafers with a diameter of 200 nm can be only a few nanometres [2]. The deposition of films with thickness control, which is carried out with great accuracy, is achieved by the method of atomic layer deposition [3,4]. One of the most accurate methods for controlling the thickness of dielectric films is the method of laser ellipsometry [5] and X-ray methods in the scheme of small angle of incidence of rays [6]. Another advantage provided by the application of films with great accuracy of thickness control is the formation of multilayer and gradient films, which can provide the desired value of the reflection coefficient in a wide range of optical radiation.
Profiled surfaces with submicron relief have been introduced into the mass productions of solar cells. Such surfaces are formed by etching in acids (H2SO4, HNO3: H2O), and it is the most common etching technology in the industrial production of solar cells [7].
The development of technology has also led to the use of new materials as the base material of the solar cell: graphene, porous silicon, amorphous silicon or perovskite. This is also the reason for the search for new anti-reflective coatings.
As such coatings, along with the traditional single-layer and multilayer films, nanostructured surfaces having the property of variable-refractive films, also called gradient films, are used [8].
Technologies for the use of two-layer anti-reflective coatings when the outer layer is applied to a protective glass and the inner one with a higher refractive index on silicon are mentioned in Ref. [9].
Further increase in the number of layers can significantly reduce the reflection coefficient. However, a large number of layers can cause significant deformation and stress, which will greatly reduce the life of the device. This is due to the appearance of many defects at the junction of dielectric surfaces with different crystal structures or lattice constants.
This problem can be solved with gradient films. Among a wide variety of thin film technologies, the gradient film technology has been studied relatively less. It should be noted that practically all modern technologies of thin film deposition and control of their structure provide low values of reflection coefficient in a wide spectral range. In the process of choosing technologies for manufacturing solar cells, factors such as the possibility of technological realization of the production process, the cost of materials used and the resistance of manufactured devices to degradation begin to play. The justification for using gradient optical films is that they have better mechanical properties than layered structures, which is explained by the smooth change in the thickness of the lattice. Therefore, the gradient structures may have better properties against degradation under the condition of poor diffusion caused by the concentration gradient.
Thus, the purpose of the work is to analyze the materials and technologies of production of anti-reflective coatings, which provide a significant reduction in the reflection coefficient in the range of operation of silicon solar cells.
Section snippets
Reflection of polished silicon surface
The fraction of energy of reflected solar radiation for normal incidence on the polished surface of the silicon can be calculated by formula (1):where R is the fraction of reflected energy, n0 = 1 is the refractive index of the atmosphere, nSi = 3.5 is the refractive index of pure silicon.
Thus, for pure silicon, in the case of a normal incidence, the value of the reflected radiation fraction is almost 30%. In the general case, the incidence of solar radiation at an
Results of simulation of reflection coefficients for films with optimal parameters
The dependence of the reflection coefficient on the angle of incidence and the wavelength for a single-layer coating with a reflection coefficient equal to zero for a normal incident wave with a length of λ0 = 530 nm, whose thickness is d = λ0/(4 × 1.87)≈71 nm, is shown in Fig. 3.
The reflection coefficient was investigated for the wavelength range from 300 to 1000 nm, which corresponds to the characteristics of silicon solar cells and incidence angles from normal to low angle (5◦-10°).
The
Discussion
Despite the results achieved by reducing the reflection coefficient, various technologies continue to maintain their relevance. Each technology has both advantages and disadvantages. For example, multilayer films can perform the function of surface passivation. That is, the effect of narrowing the spectral range by the influence of the surface on the band structure of the base element in which the solar element effectively performs the photoelectric transformation can be further eliminated. For
Conclusion
Thus, the reflection coefficients of the most common dielectric films most commonly used in solar power were simulated: single-layer and double-layer films. Analysis of layer thickness optimality criteria that provide a zero reflection coefficient for a given wavelength is performed. The method of interference matrices showed that the films formed with these criteria provide the lowest average reflectance, taking into account the spectrum of direct sunlight for all possible angles of incidence
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
This project is supported by the European Union Project No 778156 (H2020- MSCA-RISE-2017 Program), Project Title “Innovative Optical/Quasioptical Technologies and Nano Engineering of Anisotropic Materials for Creating Active Cells with Substantially Improved Energy Efficiency” (IMAGE).
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