Optical properties

Reflection without antireflection coating – HCl samples were prepared by optimising the temperature, the HCl concentration and time so that a rough surface was realised. The temperatures were varied between 900° and 1100°C.

HCl concentrations between 6% and 20% were investigated, as lower concentrations showed nearly no structuring of the surfaces. Finally, the etch times were kept short (1-5 minutes) in order to have a low silicon removal, which enables the applicability to the EpiWE. The investigations were firstly performed on Cz substrates. From the surface morphology, two promising processes were found which are shown in Figure 6-9. Both processes show no crater formation, but homogeneous surface structures covered with inverted

pyramids. Figure 6-9-A has smaller and deeper structures, whereas the pyramids of Figure 6-9-B are larger and smoother.

Figure 6-9: Silicon surfaces etched at 1000°C with 10% HCl concentration for 1 minute (A) and 6% HCl concentration for 3 minutes (B).

Figure 6-10 shows the wavelength-dependent reflection of an HCl-etched sample (1000°C, 10% HCl concentration, 1 minute) compared to an untreated surface. Both samples show a spectral reflection of 35% at 900 nm. However, the HCl-etched sample results in a far higher percentage of diffused reflection in contrast to the untreated surface. By variation of the etching parameters a reflectivity down to 10% for an etched surface can be achieved with 20% HCl concentration and 3 minutes at 1000°C. However, with these parameters deep spikes are created (Figure 6-5), which could lead to highly recombinative surfaces and shunts. This disables the application as a texture. Since antireflection coatings can decrease the spectral reflection, it makes also sense to investigate structures which increase the diffused fraction but do not lower the total reflectance. As shown in Figure 6-10, the reflectance between 600 and 1000 nm is quite similar and therefore only the reflectance at 900 nm is discussed. Especially, red light is weakly absorbed and would benefit more from an elongated light path.

Depending on the etch process, the direct fraction of the reflection can be reduced. In general, increasing both the HCl concentration and the etch time, more structures are formed and the direct fraction of reflection is lowered.

Figure 6-11 shows the reflectance measured at 900 nm for three HCl concentrations and depending on the etch time. The spectral reflectance is nearly unchanged for longer etching. However, the diffused fraction increases with etching time and it can be clearly noticed that etching for 1 minute is insufficient

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to reach a highly diffusive reflection. A 10% HCl concentration etches slower than with 20% and the maximum diffused percentage is reached after 5 minutes instead after 3 minutes for 20% HCl concentration. Longer etching does not improve the diffuse reflectance.

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HCl texture diffuse Ref spectral Ref diffuse

Reflection [%]

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Figure 6-10: Reflection depending on the wavelength for a 1 minute etched sample with 10% HCl concentration at 1000°C and an untreated reference.

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Figure 6-11: Specular and diffuse reflection depending on the etch time at 1050°C for 6%, 10% and 20% HCl concentrations.

Figure 6-12 shows the reflection for 3 minutes etched samples at 1100°C. The higher the HCl concentration, the lower the spectral reflectivity. The diffuse reflectance is nearly 100% for all concentrations. Therefore, higher HCl concentrations could seem beneficial, but the lower spectral reflectivity is induced by large craters and inhomogeneous etching, which would be detrimental for solar cells. All direct reflections are low for all HCl concentrations at 3 minutes etching. The temperature dependence is shown in Figure 6-13 for 10% HCl concentration and 3 minutes, but similar behaviour is found for the other HCl concentrations. The spectral reflection is lower at

1000°C, as craters like those shown in Figure 6-7 are created. Additionally, this is also the optimum temperature for a high diffuse reflection. Etching at 1000°C would be the optimum temperature if the reflection was the decisive parameter.

At higher temperatures of 1100°C, the diffuse reflectance is lowered because the etching is essentially horizontal and fewer structures are formed.

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Figure 6-12: Specular and diffuse reflection depending on the HCl concentration for samples etched at 1100°C for 3 minutes.

Figure 6-13: Specular and diffuse reflection depending on the temperature for samples etched with 10% HCl concentration for 3 minutes.

Reflection with antireflection coating – Some HCl textures reduce the direct fraction of the reflection, but the spectral reflection is nearly unchanged. As AR coatings lower the spectral reflection, several processes with direct reflection below 3% were investigated. Firstly, a double layer AR coating of 58 nm MgF_x and 105 nm TiO₂ was applied with a subsequent annealing at 350°C. This kind of AR coating is used for high-efficiency solar cells. The second AR coating was performed with 70 nm SiN_x and then fired at 880°C, which is usually applied in a screen-printed cell process. Figure 6-15 shows the reflection of samples before and after SiN_x and MgF_x/TiO₂ AR coating, depending on the HCl concentration. The reflection measured at 700 nm shows similar behaviour, but the reflectivity is reduced after AR coating compared to 900 nm. The AR coating is optimised for wavelengths in the visible light range, the maximum of the solar spectrum. It can be clearly seen that the AR coatings reduce the spectral reflection dramatically, especially the double layer coating. However, with increasing HCl concentration the spectral reflection increases with AR coating. For large structures, it is difficult to optimise the AR coating, as the incident light has to pass through a distance W that exceeds the thickness d of the AR coating (Figure 6-14). The larger the texture structures, the higher the

deviation from the optimum thickness. The quarter-wave principle is then not fulfilled and the interference will be lowered. The diffuse reflectance is very high for all HCl-etched samples and therefore a shallow HCl etching at 1000°C for 3 minutes seems to be sufficient. In comparison, a KOH/IPA texture shows a lower spectral reflection before and after the AR coating and a diffuse reflectance of nearly 100%.

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Figure 6-15: Specular and diffuse reflection measured at 900 nm depending on HCl concentration etched at 1000°C and 3 minutes. The values are shown before and after AR coatings with SiNx (left) and MgFx/TiO2 (right).