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Hydrogenated amorphous silicon germanium by Hot Wire CVD as an alternative for microcrystalline silicon in tandem and triple junction solar cells L.W. Veldhuizen1, Y. Kuang2, N.J. Bakker3, C.H.M. van der Werf3, S.-J. Yun4, R.E.I. Schropp1,3 1

Eindhoven University of Technology (TU/e), Department of Applied Physics, Plasma & Materials Processing, P.O. Box 513, 5600 MB Eindhoven, The Netherlands 2 Physics of Devices, Debye Institute for Nanomaterials Science, Utrecht University, High Tech Campus 2, 5656 AE Eindhoven, The Netherlands 3 Energy research Center of the Netherlands (ECN), ECN-Solliance, High Tech Campus Building 2, 5656 AE Eindhoven, The Netherlands 4 Thin Film Solar Cell Technology Team, Convergence Components and Materials Research Laboratory, Electronics and Telecommunications Research Institute, 218 Gajeongno, Yuseong-gu, Daejeon 305-700, Republic of Korea ABSTRACT We study hydrogenated amorphous silicon germanium (a-SiGe:H) deposited by HWCVD for the use as low band gap absorber in multijunction junction solar cells. We deposited layers with Tauc optical band gaps of 1.21 to 1.56 eV and studied the hydrogen bonding with FTIR for layers that were deposited at several reaction pressures. For our reaction conditions, we found an optimal reaction pressure of 38 µbar. The material that is obtained under these conditions does not meet all device quality requirements for a-SiGe:H, which is, as we hypothesize, caused by the presence of He that is used to dilute the GeH4 source gas. We present an initial single junction n-i-p solar cell with a Tauc optical band gap of 1.45 eV and a short circuit current density of 18.7 mA/cm2. INTRODUCTION The manufacturing cost of thin film Si based tandem and triple junction cells and modules is at present too high for thin film Si modules to meet current module market prices. Conventionally, microcrystalline silicon is used as the low band gap absorber in ‘micromorph’ solar cells (a-Si/µc-Si tandem cells). However, due to the considerable thickness needed for the µc-Si:H absorber, it takes three to four times as many deposition reactors compared to single junction cells to produce tandem cells, leading to high cost of ownership. One of the approaches to reduce processing time of the low band gap layer(s) in multijunction siliconbased solar cells is the use of hydrogenated amorphous silicon germanium (a-SiGe:H). Until recently, United Solar Ovonic has been very successful in the development of triple junction solar cells with a-SiGe:H absorber layers [1]. In general however, a-SiGe:H has not been regarded as a viable option because of (i) the high defect density for PECVD a-SiGe:H, at band gaps < 1.4 eV, and [2] (ii) the high cost of germane (GeH4). On the other hand, because of its direct gap nature, the thickness of an a-SiGe:H absorber layer can be kept 10 times smaller than that of µc-Si:H. The use of GeH4 is a viable option if cells can be made ultra-thin, by the implementation of light scattering nanostructure. We are investigating whether a-SiGe:H can be (re-)consider