Optical developments for silicon thin film solar cells in the substrate configuration
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Optical developments for silicon thin film solar cells in the substrate configuration Thomas Soderstrom, Franz-Joseph Haug, Xavier Niquille, Oscar Cubero, Stéphanie Perregaux, and Christophe Ballif IMT/UNINE, Rue A.-L. Breguet 2, Neuchâtel, 2000, Switzerland ABSTRACT In the nip or substrate configuration thin film silicon solar cells, the choice of front TCO contact is critical because there is a trade off between its transparency which influences the current in the solar cell and its conductivity which influences the series resistance. Here, we investigate the optical behavior of two different TCO front contacts, either a 70 nm thick, nominally flat ITO or a 2 µm thick rough LPCVD ZnO. The back contact consists of LP-CVD ZnO with random texture. First we investigate the influence of the rough and flat front TCOs in µc-Si:H and a-Si:H solar cells. With the back contact geometries used in this work, the antireflection properties of ITO are effective at providing as much light trapping as the rough LP-CVD ZnO. In the second part, we demonstrate that total of 25 to 26 mA/cm2 is achievable in nip micromorph tandem cells and show short circuit current up to 11.7 mA/cm2 using an SIO based intermediate reflector. 1. INTRODUCTION In the thin film silicon solar market, the substrate or nip configuration can be a competitive advantage because flexible non transparent substrate can be used [1]. Furthermore, roll to roll deposition techniques [2] can be applied. Several companies develop solar cells with initial efficiencies over 15 % [3-5]. Here, we develop low temperature processes (below 200°C) compatible with plastic substrates such as PEN or PET. Our primary goal is to increase the stabilized efficiency of thin film silicon solar cell in the substrate configuration by using textured substrates, tandem micromorph structures [6]. In the nip configuration, substrate texture is compulsory to increase the current in the thin film silicon solar cell while reducing cell thicknesses. This leads to reduced light induced degradation effect [7] and effective cycle time in production. For efficient light trapping, the substrate textures should be in the dimension range of the incoming light wavelength. The common substrate used by the industries is hot silver which provide elevated light trapping in the cells. Here, we develop LP-CVD ZnO on glass substrate and it can also be applied on plastic. In addition, we compare two transparent conductive oxide (TCO) candidates as front contact; the first one is a flat thin ITO layer and the second one is a rough thick ZnO LP-CVD. First, we describe the difference between the TCOs by reflection and transmission measurements on glass substrate only. Then, we investigate the behavior of ITO and ZnO front contact on amorphous (a-Si:H) and microcrystalline (µc-Si:H) solar cells. Optically the situation between thin a-Si:H (200 nm) absorber and a thick (1 µm) µc-Si:H is different. First of all the optimum dimension of the substrate texture is different for the two types of cell because
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