Influence of the Grain Boundary Band Offset on Charge Transport Mechanism in Microcrystalline Silicon Analysed by Numeri
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INFLUENCE OF THE GRAIN BOUNDARY BAND OFFSET ON CHARGE TRANSPORT MECHANISM IN MICROCRYSTALLINE SILICON ANALYSED BY NUMERICAL SIMULATION
a,b)
Alessandro Fantoni, a)ManuelaVieira, a)Reinhard Schwarz a) ISEL, R. Conselheiro Emídio Navarro, 1900 Lisboa, Portugal b) FCT-UNL, Quinta da Torre, 2825 Monte de Caparica, Portugal ABSTRACT Microcrystalline silicon is a two-phase material composed of grains of crystalline silicon embedded in an amorphous silicon tissue. The boundary regions between the crystalline grains and the amorphous matrix are treated similarly to a heterojunction interface.The band offset at the grain boundaries causes the appearance of local electric field peaks, variables in intensity and direction. We present results obtained with two dimensional simulations of a µc-Si:H p-i-n junction in short circuit condition. Charge transport mechanism is described by the internal electric configuration assumed by the junction in thermodynamic equilibrium and illuminated with monochromatic radiations. Different configurations of the band offset at the grain boundaries are also considered and related to the transport properties in microcrystalline silicon
INTRODUCTION The first successful deposition of microcrystalline silicon (µc-Si:H) films has been realised in the late 60’s [1]. Since then, µc-Si has been a subject of interest because of its properties, like high electrical conductivity and low absorption coefficient, which made this material a good candidate for photovoltaic applications. The interest in producing microcrystalline silicon p-i-n solar cells is recently increasing, since they are effectively more stable then their a-Si:H counterpart [2]. Microcrystalline based solar cells have demonstrated a remarkable efficiency for entirely µc-Si:H p-i-n cells [3] and have been successfully incorporated in tandem structures [4]. However, the understanding of µc-Si:H based devices is still in an early stage and some questions about its properties are actually subject of discussion and investigation [5]. µc-Si:H can be considered as a two-phase material. Its composition can be interpreted as grains of crystalline silicon embedded in an a-Si:H tissue [6].The structural properties of µc-Si:H are therefore strongly dependent on the deposition and process conditions used [7]. Evidence of different film morphology have been found in samples deposited with different processes [8, 9, 10, 11, 12]. A proper description of the structural properties of such material can be quite a complicated task, as shape and size of the crystalline grains can be highly variable and their distribution within the amorphous matrix is probably different for each sample. Nevertheless,
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microcrystalline silicon has its own macroscopic characteristics, different from each of the single phase characteristics. MODELLING TRANSPORT MECHANISM IN TWO-PHASES MATERIALS The model used here to simulate µc-Si:H p-i-n device is similar to the one we have previously used for simulating a-Si:H p-i-n structures applying the ASCA program. Details abo
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