Influence of interface morphologies on amorphous silicon thin film solar cells prepared on randomly textured substrates V. Jovanov a , X. Xu b , S. Shrestha a , M. Schulte b , J. H¨ upkes b , M. Zeman c , D. Knipp a,n a Research Center for Functional Materials and Nanomolecular Science, Electronic Devices and Nanophotonics Laboratory, Jacobs University Bremen, 28759 Bremen, Germany b Institut f¨ ur Energie- und Klimaforschung, IEK5—Photovoltaik, Forschungszentrum J¨ ulich, 52425 J¨ ulich, Germany c Delft University of Technology, PVMD Laboratory, Dimes, 2600 GB Delft, The Netherlands article info Article history: Received 25 October 2012 Received in revised form 21 December 2012 Accepted 14 January 2013 Keywords: Amorphous silicon solar cells Randomly textured substrate Back contact morphology Surface prediction Light-trapping abstract The influence of the interface morphologies on amorphous silicon thin-film solar cells prepared on randomly textured substrates was studied. A simple three-dimensional geometrical model was developed to describe the surface morphology of amorphous silicon films and thin-film solar cells. The simulated surface morphologies are confirmed by experimental measurements. A detailed under- standing of the interface morphologies is required to gain insights in the light-trapping of silicon thin- film solar cells on randomly textured substrates and derive strategies to improve the light-trapping properties. The morphology of amorphous silicon solar cell layers is calculated by using atomic force microscope scans of the randomly textured substrates and the thickness of layers as input data. The influence of the interface morphologies on the surface roughness, average film thickness and size of surface textures is investigated. Finally the influence of these parameters on the light-trapping and optical losses is discussed. & 2013 Elsevier B.V. All rights reserved. 1. Introduction In order to maximize the conversion efficiency of thin-film solar cells the electrical and optical losses have to be minimized. In the case of low temperature amorphous silicon thin-film solar cells, the silicon material exhibits a low charge carrier diffusion length. Therefore, better electrical performances are achieved when the thickness of the solar cell is limited to a couple of hundreds of nanometers. In order to achieve high short circuit currents for such thin solar cells, light-trapping or photon management is necessary. The aim of light-trapping is to max- imize the absorption of light in the thin absorber layer of the solar cell. Randomly textured transparent conductive oxides (TCO) are commonly used in silicon thin-film solar cells as front contacts. The surface textures of the transparent conductive oxides reduce reflection losses and increase scattering/diffraction of the incident light [1–8]. Randomly textured TCO can be realized by wet etching of sputtered zinc oxide (ZnO) films, direct deposition of textured zinc oxide by low pressure chemical vapor deposition (LPCVD) or tin oxide films by atmospheric pressure chemical vapor deposition (APCVD) [1–8]. Optical simulations and experimental results reveal that enhanced light-trapping is achieved when both front and back contacts are textured [9,10]. Texturing of the back contact is achieved by propagation of front contact surface textures during solar cell deposition. It is commonly assumed in literature that the back contact surface textures are an exact replica of the front contact textures [9,11–15]. However, experimental measure- ments reveal significant differences between the front and back contact morphology of silicon thin-film solar cells [10,16–20]. Fig. 1 exhibits a cross section of an amorphous silicon thin-film solar cell in superstate configuration. The interface morphology of the front and back contact surface textures in Fig. 1 are based on experimentally measured surface profiles. Light-trapping in silicon thin-film solar cells is determined by the front and back contact morphology. The front contact surface textures determine the incoupling and scattering/diffraction of the incident light. The back contact textures determine the scattering/diffraction of longer wavelength light and the optical losses in the metal back contact. In order to gain insight in the light-trapping and optics of thin-film solar cells, a detailed knowledge of the front and back contact morphology is required. The front contact morphology is determined by the surface textures of the TCO. On the other hand, the back contact morphology is determined by the front contact textures and the formation of the amorphous silicon solar cell layers. In this manuscript we investigate the interface morphologies of the solar cell and their influence on light-trapping. The surface coverage algorithm used to determine the interface morphologies is intro- duced in Section 2. In Section 3 the properties of the solar cell Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/solmat Solar Energy Materials & Solar Cells 0927-0248/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.solmat.2013.01.017 n Corresponding author. E-mail address: d.knipp@jacobs-university.de (D. Knipp). Solar Energy Materials & Solar Cells 112 (2013) 182–189