Advanced textured monocrystalline silicon substrates with high optical scattering yields and low electrical recombination losses for supporting crack‐free nano‐ to poly‐crystalline film growth

Abstract

Crystalline silicon tandem devices with perovskites, CIGS, and nanocrystalline silicon, as well as the TOPCon design, are incompatible with the conventional pyramidal surface texture of silicon. This is a result of crack formation in nano to polycrystalline growth on large sharp surface features. In this work, three texturing approaches are investigated, using alkaline and/or acidic wet chemical etches, that can lead to the crack‐free growth of nano to polycrystalline materials on textured surfaces. In this work, we show that without acidic smoothening, the fraction of <111> pyramidal surface coverage has to remain relatively small to prevent crack formation during crystalline growth on these surfaces. Applying an acidic etch as a function of time continuously smoothens surface features. This shifts the reflection to wider scattering angles and results in higher total reflected intensity with respect to the conventional texture, making it an interesting option for a wide variety of tandem pv applications. Finally, we demonstrate crater‐like features on a <100> monocrystalline silicon surface using an etching process including a sacrificial layer. These craters increase light scattering into wider angles, but to a lesser extent than the former approach. In terms of passivation, we demonstrate the positive effect of a post deposition hydrogen treatment. Initial dilution of the silane plasma improves passivation on a <111> surface, but is detrimental to passivation on a <100> surface, likely because the hydrogen dilution results in epitaxial growth at the c‐Si/a‐Si:H hetero‐interface. A minority carrier lifetime of over 3 ms has been achieved for all texturing approaches, after deposition of a 15 nm a‐Si:H layer on both sides of the wafer, for different a‐Si:H deposition and annealing schemes.

The optical performance of random and periodic textured mono crystalline silicon surfaces for photovoltaic applications

“…in H 2 O) and 3 parts H 2 O. The influence of these etchings steps on the surface features, as well as the exact solutions and etching conditions, can be found in [12].…”

“…Details regarding the used scanning electron microscope (SEM) and atomic force microscope (AFM) are reported in [12]. The mean pore diameter D pore and other metrics for the surface features were extracted using the NanoScope Analysis software from Bruker.…”

“…In this section a sacrificial polycrystalline silicon film is used, where the porous grain boundaries in reference to the dense c-Si grains result in the desired anisotropic etching behaviour. The T sac texturing approach has been presented before in our earlier work [12], where it was concluded that the surface features were too small to yield the desired optical behaviour. In this section an optimisation of the sacrificial layer is performed to achieve larger surface features, which would result in lower reflectance and better wide-angle scattering.…”

“…For the first texturing approach, presented in Section 3.1 and referred to as T sac , a sacrificial poly-c-Si layer is used to develop a random surface texture. This approach is similar to the one presented in our earlier work [12], for which no optimization was performed on the poly-Si layer processing conditions. In Section 3.2, using a photolithographically developed SiO 2 mask, a periodic texturing approach is presented, resulting in a honeycomb structure (T honey ).…”

“…In Section 3.2, using a photolithographically developed SiO 2 mask, a periodic texturing approach is presented, resulting in a honeycomb structure (T honey ). In Section 3.3, the optical scattering yields of these two approaches are compared to a third texturing approach, referred to as T sp , which is the result of consecutive alkaline and acidic etching steps, producing random, smooth and concave structures [12]. Additionally, the performance of multijunction devices processed on the different textured substrates is optically simulated.…”