# Coupling and Trapping of Light in Thin-Film Solar Cells Using Modulated Interface Textures
## 1 Introduction  
Transparent conductive oxides (TCOs) play an important role as contact materials in solar cells [1; 2]. The front contacts simultaneously fulfill numerous requirements: high transparency, high conductivity, and in some cases exhibit a surface texture that facilitates a good optical performance of the solar cell. Many publications have already addressed the importance of balancing the transparency to reduce absorption losses and the electrical properties of TCO films in thin-film solar cells [3; 4; 5; 6]. An even larger number of publications address the importance of rough interfaces for optical performance [7; 8; 9; 10; 11; 12]. While most studies focus on silicon thin film solar cells, light management has been studied in copper indium gallium selenide (CIGS) and metal halide perovskite technology as well [13; 14; 15; 16]. The majority of these publications refer to the necessity of good light-trapping in the device to foster the absorption of the otherwise weakly absorbed red portion of the solar spectrum. Ideally, the light is trapped completely within the device due to total internal reflection or in other words: it is coupled to waveguide modes [17]. The pathway enhancement [18] is of special importance for the wavelength range thatfaces a low absorption coefficient (in silicon: 700 nm-1100 nm). Numerous optical models have been designed to describe the scattering effects at the front ZnO:Al/Si interface [19; 20; 21]. While the haze, which represents scattering of the rough interface transmitted or reflected into air, is easily measured, scattering of that interface towards the subsequent silicon layer with a higher refractive index reduces the scattering angle by refraction [19]. Recent studies also addressed the importance of the back contact texture rather than the front contact texture [7; 22; 23; 24; 25; 26]. In a simple picture, the light passes the front contact interface with small scattering angles, while a significant scattering takes place at the rough reflector. However, the coupling to waveguide modes is a complicated interplay between front and back surface texture. Hence, the back contact topography is of great importance for the light-trapping ability of the solar cell. Before light can be trapped it has to enter the absorber layer first. This is facilitated by an improvement of the light-coupling into the device and is very important for technologies using absorbers with high refractive index like silicon. Some groups applied special anti-reflection coatings with intermediate refractive index between the TCO and the silicon [27]. In addition, rough interfaces are well known to reduce reflection losses. An effective refractive index grading at rough interfaces is given once the two materials, e.g., Si and TCO, mesh in the sub-wavelength length scale. The incoming light experiences a gradual change from the pure refractive index of ZnO to the one of Si [28]. The extent of the improvement and the wavelengths that benefit most depend strongly on the texture properties [22]. Small features are required for good light-coupling, while larger features more effectively scatter light. Thus, double textures have been developed including chemical deposition of ZnO [29] or SnO\({}_{2}\)[30], etched TCOs [31; 32; 33], a combination of those [34; 35], and textured substrates [36; 37; 38; 39; 33]. A theoretical approach to double textured gratings was derived by [PERSON] et al. [40]. Some papers mention the anti-reflection effect [29]. However, a detailed distinction of the effects of light scattering and its anti-reflection effect is lacking. In this study, we present the analysis of the impact of different surface topographies on the aspects of light-coupling and light-trapping. We utilize sputter-deposited aluminum-doped zinc oxide (ZnO:Al) and subsequent wet-chemical etching to demonstrate various kinds of textures that exhibit different abilities for light-trapping and light-coupling.