Enhanced Light Absorption in a-Si:H Layers of Solar Cells by Applying Tco/Metal Back Contacts
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ENHANCED LIGHT ABSORPTION IN A-Si:H LAYERS OF SOLAR CELLS BY APPLYING TCO/METAL BACK CONTACTS G. TAO, B.S. GIRWAR, G.E.N. LANDWEER, M. ZEMAN, AND J.W. METSELAAR Delft University of Technology, P.O.Box 5031, 2600 GA Delft, the Netherlands
ABSTRACT The optimization of the back contact reflectivity for thin film a-Si:H solar cells has been performed. The results of optical calculations show that a-Si:H/TCO/Metal interfaces with a proper TCO thickness reflect much more than their a-Si:H/Metal counterparts. We compared solar cells which were deposited on a flat substrate with different back contacts. The back contacts consisted of a metal layer (aluminum, silver/aluminum) or combined TCO/metal layers (TCO/Al, TCO/Ag/Al). The same was done with solar cells which were deposited on a textured substrate. The solar cells with a TCO/metal back contact showed not only a significantly increased short-circuit current density but also an increase in the spectral response. The cells with TCO/Ag/Al back contact showed the best result.
INTRODUCTION a-Si:H solar cells with a thin intrinsic layer are considered to be more stable with respect to light degradation. However, thin layers usually absorb less light than relatively thick layers, resulting in a lower short-circuit current density. In order to maintain the short-circuit current density, the light absorption in the a-Si:H layers should be enhanced. To improve the light absorption, there is not much room left for improving the material (i.e. to improve the absorption coefficient of the material without degrading its electrical quality). However, light absorption improvement via solar cell structure design seems an effective way to achieve this goal. The new designs of the structure comprise: applying a textured TCO front contact [1, 2, 3] or using V-grooved glass substrate [4], applying a highly reflective backcontact [5], applying less absorbing p-type and n-type (micro-crystalline) layers, etc. In this paper, we present our experimental results to enhance the light absorption in the a-Si:H layers by applying different back contacts, including aluminium, silver, and the highly reflective TCO/metal back contacts.
THE LIGHT ABSORPTION The light absorption in the intrinsic layer of an a-Si:H solar cell is quite complicated. It depends not only on the optical parameters (refractive index, extinction coefficient, and the thickness) of the intrinsic layer, but also on the optical parameters of other related layers - the TCO layer, the back contact layer, the p-type and n-type layers. The light incident angle and the interface roughness also play important roles. To examine the light absorption improvement in the intrinsic a-Si:H layer due to enhanced back contact reflection, we first studied the back contact reflectance for samples with different back contacts while the other layers and interfaces remained the same. Because of our specific interest in solar cells, two categories of samples were used in our study, namely a-Si:H solar cells on flat TCO substrates and cells on textur
