IMPROVEMENT OF SHORT-CIRCUIT CURRENT IN MULTIJUNCTION A-SI BASED SOLAR CELLS USING TIO 2 ANTI-REFLECTION LAYER C. Das, M. Berginski, J. Huepkes, A. Gordijn, J. Kirchhoff, W. Reetz, A. Lambertz, F. Finger and W. Beyer Institute of Energy Research-Photovoltaics, Research Center Juelich, Juelich, D-52425, Germany Tel: (49) 2461-61-2615, Fax: (49) 2461-61-3735, E-mail: c.das@fz-juelich.de ABSTRACT: The development of TiO 2 thin films as anti-reflection layer (ARL) has been investigated and the application of TiO 2 in a-Si based multijunction superstrate thin film solar cells has been demonstrated. First, the TiO 2 thin films have been applied to the a-Si single junction solar cell and with optimized TiO 2 thin films, the FF of the a- Si solar cells does not seem to be affected due to the incorporation of ARL in the device configuration. A slight decrease in the blue response (<500nm) of the solar is observed in the quantum efficiency, which is counterbalanced by the increase in the same from 500-800nm wavelength region. The TiO 2 thin films have been applied to double (a- Si/µc-Si) and triple (a-Si/a-SiGe/µc-Si) junction solar cells and ~10% improvement in the short-circuit current has been obtained without deterioration of other solar parameters. Keywords: a-Si, TiO 2 , multijunction solar cells 1 INTRODUCTION In the recent years, improvement in efficiency of a-Si based solar cells has shown a clear direction to follow the multijunction structure in device fabrication. The most successful device configurations being double (a-Si/µc- Si) and triple (a-Si/a-SiGe/µc-Si) junction cells where a- Si, a-SiGe and µc-Si layers have been used as active layers. The record efficiency in any amorphous silicon based devices has been reported as 15.1% [1]. Thanks goes to high short-circuit current (J sc ) drawn from each of the spectrum splitting component junctions, that mainly boosted up the efficiency level. However, when the increase in the current drawn from each component junctions is only associated with increase in the thickness of the individual intrinsic layers, severe light induced degradation is expected with a-Si based layers. The initial and stabilized performance of the a-Si based multijunction solar cell differs huge when the thickness of amorphous layers is increased to gain short-circuit current. Therefore, a precise current management is needed for the thin film solar cells, which minimizes associated loss of illumination in the device due to inefficient device design. This could reduce the thickening of the active layers without affecting the short-circuit current. In case of a glass superstrate based device structure, such a loss of illumination occurs due to refractive index (n) mismatch between the transparent conducting oxide viz. ZnO (n~1.8) and Si (n~3.6) layers. A layer of TiO 2 (n~2.5) at ZnO and silicon interface has shown to have potential to act as an anti-reflection layer (ARL) for improvement of the current loss due to refractive index mismatch [2]. In this paper, application of TiO 2 as an anti-reflection layer has been investigated for a-Si based solar cells, to be used as components in a multijunction device structure. The effect of incorporation of TiO 2 has been studied for double and triple junction solar cells where a-Si, a-SiGe and µc-Si have been used as active layers. 2 EXPERIMENTAL 2.1 TiO 2 thin film The TiO 2 thin films have been developed by rf- magnetron sputtering of TiO 2 target using Ar and O 2 gas mixture environment. The doping concentration of the TiO 2 target is varied where undoped and Nb 2 O 5 doped ceramic targets have been used. In the preparation of TiO 2 films, the substrate temperature, rf-power, the O 2 concentration in gas phase and the layer thickness has been varied. The detail experimental process and consequent optimization of the TiO 2 film properties with electrical and optical characterizations is described elsewhere [Berginski et al, this conference]. The layer properties are also optimized with their application in single junction p-i-n solar cells. 2.2 Silicon thin films The silicon thin films have been deposited by either RF (13.56 MHz) or VHF (95 MHz) PECVD method using SiH 4 , H 2 and GeH 4 gases for the active layers and using PH 3 and TMB as dopant gases for n and p doped layers respectively. The a-Si, a-SiGe, µc-Si materials have been used as active layers for the multijunction solar cells. Both the 10x10 cm 2 and 30x30 cm 2 substrate area compatible PECVD systems have been used for the thin film deposition process where low (<1 Torr) and high (3-10 Torr) pressure regimes in the deposition process have been used. 2.3 Solar cells The glass substrate is used in superstrate p-i-n configuration for the fabrication of solar cells. The standard solar cells are deposited on ZnO coated glass, where the ZnO is texture-etched with HCl. In case of the solar cells with ARL, the TiO 2 layer has been deposited on the texture-etched ZnO and covered with a very thin film of ZnO (10nm) to protect it from H 2 plasma reduction process during exposure to PECVD silicon deposition. The rest of the fabrication process is same as that of a standard solar cell. The back contact of the solar