Predicting the Interface Morphologies of Silicon Films on Arbitrary Substrates: Application in Solar Cells Vladislav Jovanov, † Xu Xu, ‡ Shailesh Shrestha, † Melanie Schulte, ‡ Jü rgen Hü pkes, ‡ and Dietmar Knipp* ,† † Research Center for Functional Materials and Nanomolecular Science, Electronic Devices and Nanophotonics Laboratory, Jacobs University Bremen, 28759 Bremen, Germany ‡ Institut fü r Energie- und Klimaforschung, IEK5-Photovoltaik, Forschungszentrum Jü lich, 52425 Jü lich, Germany ABSTRACT: A three-dimensional model that predicts the interface morphologies of silicon thin-film solar cells prepared on randomly textured substrates was developed and compared to experimental data. The surface morphologies of silicon solar cells were calculated by using atomic force microscope scans of the textured substrates and the film thickness as input data. Calculated surface morphologies of silicon solar cells are in good agreement with experimentally measured mor- phologies. A detailed description of the solar cell interface morphol- ogies is necessary to understand light-trapping in silicon single junc- tion and micromorph tandem thin-film solar cells and derive optimal light-trapping structures. KEYWORDS: light-trapping, surface morphology evolution, thin-film solar cells, interface prediction ■ INTRODUCTION Photon-management or light-trapping is crucial for achieving silicon thin-film solar cells with high energy conversion efficiency. Efficient light-trapping in silicon thin-film solar cells is realized by texturing the front or back contact. Randomly textured transparent conductive oxides (TCO) are widely used as front contacts to achieve light-trapping in superstrate (p-i-n) configuration solar cells. 1-10 Light-trapping in substrate (n-i-p) configuration solar cells is achieved by texturing the back contact. 11-17 When depositing amorphous or microcrystalline silicon on textured substrates, the surface textures propagate through the thin-film solar cell leading to a solar cell with textured front and back contact. Experimental results and optical simulations show that solar cells with both contacts textured exhibit high short circuit currents. 14,18 Most publications in the literature assume that surface textures propagate unchanged through the thin-film solar cell. 18-21 However, experimental measurements reveal that surface textures are significantly affected by the solar cell deposition process leading to different front and back contact textures. 9,22-27 Determining the interface morphologies of silicon solar cells deposited on textured substrates allows for deriving new strategies to optimize and improve light-trapping. The interface morphologies can be measured by atomic force microscopy (AFM). However, complex aligning procedures are required to measure the corresponding front and back contact texture. As an alternative, cross-sectional scanning electron microscopy (SEM) images can be used. 9,23-27 However, cross sectional SEM images can be used only for two-dimensional (2D) textured substrates, such as line-gratings or three-dimensional (3D) textured substrates with periodical surface textures and radial symmetry. This manuscript describes a simple approach to predict the interface morphologies of amorphous and microcrystalline silicon thin-film solar cells deposited on textured substrates using plasma enhanced chemical vapor deposition (PECVD). The approach is demonstrated for silicon solar cells deposited on randomly textured TCO. However, the same approach can be used to determine the interface morphologies of silicon single junction and micromorph tandem solar cells deposited on different textured substrates, including coaxial nanowire solar cells or textured glass substrates. 1,4,12,22,24,25 ■ METHODS Modeling Interface Morphology. Light-trapping in silicon thin- film solar cells is influenced by the front and back contact morphology. The front contact textures should increase scattering/diffraction of the incident light, while minimizing reflection losses. The front contact textures in superstrate configuration solar cells are determined by the fabrication process of the TCO layer. Sputtered and etched zinc oxide (ZnO) films exhibit craterlike features, while pyramidlike features are observed for ZnO films prepared by low pressure chem- ical vapor deposition and tin oxide films fabricated by atmospheric pressure chemical vapor deposition. 2-9 The interface and back contact morphology are determined by the surface textures of TCO and the silicon film formation. Consequently, significant differences between the front and back contact morphology are observed (Figure 1b). Figure 1 exhibits cross sections of typical amorphous silicon thin-film solar cells prepared on commercial Asahi U substrate. 3-6 The common approach to model thin film solar cells by using identical front and back contact morphology is shown in Figure 1a. Figure 1b depicts the cross section of an amorphous silicon solar cell based on experimental measurements. The back contact morphology determines the optical losses and scattering properties of the metal back reflector. Hence, an Received: April 19, 2013 Accepted: July 15, 2013 Published: July 29, 2013 Research Article www.acsami.org © 2013 American Chemical Society 7109 dx.doi.org/10.1021/am401434y | ACS Appl. Mater. Interfaces 2013, 5, 7109-7116