Highly Conductive Microcrystalline Silicon Layers for Tunnel Junctions in Stacked Amorphous Silicon based Solar Cells.
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HIGHLY CONDUCTIVE MICROCRYSTALLINE SILICON LAYERS FOR TUNNEL JUNCTIONS IN STACKED AMORPHOUS SILICON BASED SOLAR CELLS. K. PRASAD 1 , U. KROLL 1 , F. FINGER 1 , A. SHAH1, J-L. DORIER 2 , A. HOWLING 2 , J. BAUMANN 3 , M. SCHUBERT 4 1 Institut de Microtechnique, Universit6 de Neuchatel, CH-2000 Neuchatel, Switzerland 2 CRPP, Ecole Polytechnique Federale, CH- 1007 Lausannne, Switzerland 3 Fachbereich Physik, Universittit Konstanz, D-7750 Konstanz, F. R. Germany 4 Inst. fur Physikalisch Elektronik, Universitlit Stuttgart, 7000 Stuttgart 80, F. R. Germany ABSTRACT We have investigated the influence of substrate temperature on the optoelectronic and structural properties of heavily doped g.tc-Si:H, prepared with the Very High Frequency Glow Discharge process. At substrate temperatures as low as 160'C we obtain, for films with 0.5.tm thickness, maximum conductivities of 100 S/cm and 20 S/cm for and
material, respectively. Starting from these values the deposition parameters were optimised for ultrathin layers having thicknesses in the range of 100 to 500A . We observe.that boron doping plays a critical role in the crystallisation of ultrathin films. The thinnest layers investigated so far show conductivities of 0.2 S/cm at d=100A for , and 0.2 S/cm at d=250A for
material. These properties make gic-Si:H films attractive candidates to form tunnel junctions in tandem solar cells. INTRODUCTION The possibility of producing hydrogenated microcrystalline silicon (4tc-Si:H) films in the same reactor as that used for depositing a-Si:H, simply by tuning a few of the deposition parameters has stimulated considerable interest in this material in the past. The use of g±c-Si:H layers in thin film devices like solar cells and transistors requires that the preparation conditions of gic-Si:H are adapted to the corresponding device processes. The problems arising here are connected with the empirical preparation conditions for the formation of p.c-Si:H in PECVD processes: (1) high substrate temperature, (2) high discharge power and (3) strong dilution of the process gas (SiH4) with hydrogen. All of these conditions could unfavourably effect underlying layers or substrate coatings (like TCO) through strong ion bombardment, strong hydrogen etching or simply through thermal effects. Another general problem encountered is that, for solar cell applications, one requires ultra thin layers (between 50 to 250A) of highly conductive .tc-Si:H. Such very thin layers, deposited at 13.56 MHz, have been observed to be resistive and therefore not better suited as doped layers in p-i-n solar cells than their amorphous counterparts [1]. The reasons could be that (1) the initial film formation is in the amorphous phase or (2) the crystallites fail to form a percolation path. We have reported earlier that the Very High Frequency Glow Discharge (VHF-GD) favourises the formation of and gxc-Si:H at considerably lower levels of input power and temperature as compared with GD at 13.56 MHz [2]. It was therefore of interest to see in how far these favourable
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