ELABORATION OF COMPLEX OPTICAL COATING OF MODERN CASCADE SOLAR CELL FOR SOLAR PANELS OF SPACECRAFT

Multi-junction solar cells or, in other words, photoconverters (PC) based on AIIIBV compounds used as part of a solar battery (SB), the primary power source for spacecraft, are complex optoelectronic semiconductor devices. To minimize optical power losses, which occur, among other things, due to reflection from the semiconductor surface up to 30% of the incident light of the AM0 spectrum [1], a thin-layer ceramic anti-reflective (AR) coating (ARC), which is also protective, is formed on its surface, a massive metal contact create not solid, but in the form of a comb [2].

In order to achieve the highest conversion factor of solar energy into a PC, it is necessary to select materials that are optimal for ARC in terms of optical properties, manufacturability, durability, etc., as well as to determine the thickness of the layers and their number. Today, the world's leading manufacturers (Spectrolab Inc., etc. USA, Azur Space Solar Power Gmbh Germany, CESI SpA Italy) have developed and industrially manufactured multi-junction PCBs with such combinations of ARC materials as, for example, Al₂O₃/TiO_x, SiO₂/Ta₂O₅ and some others. An analysis of publications shows that this issue is not widely presented in the open press, and also that data on calculations are presented only for ARC/semiconductor structure designs [3], which do not take into account the fact that for operation under the conditions of complex effects of outer space factors, the PC protected by gluing plates of special radiation-resistant glass. This element, which has its own optical properties, also contributes to the optical losses of the PC and SC (therefore, its own ARC is often formed on its surface), respectively, the choice of ARC materials for the PC, the determination of the optimal thicknesses of the coating layers, should be adjusted taking into account the use of glass. with ARC and adhesive composition.

This paper presents the results of calculating a two-layer ARC implemented using the special OptiLayer software [4] for the most common InGaP/InGaAs/Ge structure of a three-junction PC for the bleaching range of 400–900 nm, corresponding to the absorption of the upper and middle stages. Combinations of ARC Al₂O₃/TiO_x and SiO₂/Ta₂O₅ materials are considered, calculations are performed both for uncoated PC and taking into account the presence of a protective coating (in this case, K-208 glass with an antireflection coating of MgF₂) and an adhesive compound layer, (in this case case SIEL 159-322).

Because the thickness of the layer of the wide-gap window of the upper cascade is comparable to the thickness of the ARC layers and the spectral reflectances are large compared to the selected oxides, the epitaxial layers will have a significant effect on the results of the ARC calculations. The following characteristics of the three-stage structure were used in the calculations: the base and emitter of the upper stage — a layer of composition In_0.49Ga_0.51P with a thickness of ≥ 0.5 μm and, above it, a wide-gap window of the upper stage — a layer of composition In_0.5Al_0.5P with a thickness varying within 25 — 35 nm. The calculations were carried out for the angle of incidence of light 0^о, that is, along the normal to the plane of the PC. The dispersion dependences of the refractive indices of antireflection materials n(λ) used in the calculations were taken from [5], except for n_Ta2O5(λ) and n_SiO2(λ), which were determined experimentally [6] (because it is known that n(λ) of films of the vast majority of antireflection materials depend on the method and conditions of their application).

It should be noted that in this work the refractive index of the adhesive-compound layer is taken as a single value of 1.41 for the entire AM0 spectrum. To obtain more accurate results, it is necessary to determine a set of values for the refractive index of the compound for different wavelengths, which is planned in further experimental work.

As a result of the calculations, the results were obtained, in the analysis of which it can be concluded that the sticker of glass with antireflection coating on the PC worsens the antireflection properties of the coating based on SiO₂/Ta₂O₅ oxides. If the average reflection before sticking glass on the PC was 2.4%, then after sticking it becomes 4.3%. It should be noted that an increase in reflection from the PC by 2% is approximately equivalent to a decrease in the short-circuit current by 5–7 mA. Optimization of the thicknesses of this pair of layers (60 nm for Ta₂O₅ and 97 nm for SiO₂) makes it possible to obtain an average reflection of 3.1% after glass bonding.

The coating, which was optimal without taking into account the SiO₂/TiO₂ protective glass (with an average reflectance of only 1.1%), after glass bonding shows an average reflectance of 4.3% and cannot be optimized.

The best properties after gluing glass with anti-reflective coating on the PC were shown by ARC based on Al₂O₃ (72 nm)/TiO₂ (36 nm), the average reflectance of which over the anti-reflection range remains at the level of 2.4%, i.e. in fact, the reflectance increased by 0.1% and the layer thicknesses optimized for full optical assembly are close to the values before Al₂O₃ (78 nm)/TiO₂ (44 nm) optimization.

It is shown that the absence of an antireflection MgF₂ coating on glass significantly increases the total reflection of the system: for example, for Al₂O₃/TiO₂, the average reflection coefficient increases to 4.6%.