Innovative Diodes based on Amorphous-Porous Silicon Heterojunction
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ABSTRACT In this paper we present an innovative diode based on the heterojunction between amorphous silicon and porous silicon grown on crystalline silicon. The device architecture gives several advantages. Deposition of amorphous silicon on porous material realises high performance junction at temperature less than 250'C and it passives the porous layer against the natural oxidation due to ageing in the environment. Porous technology allows to obtain a controlled textured silicon surface independently from crystalline silicon orientation just to give the opportunity to reduce surface reflectivity and the blue shift of the absorption spectra in solar cell application. Solar cells were characterised by I-V dark/light and quantum yield measurements. Under standard AM 1.5 light we obtained photovoltaic conversion efficiency greater than 10%. Change in photoluminescence in different gas environments showed for gas sensor applications give rise encouraging results. In dark condition we found the typical diode behaviour. INTRODUCTION Thin film materials and low temperature processes are currently widely investigated to produce a new generation of electronic devices. Since many problems still need a solution and further studies, heterojunctions represent a special way to introduce new family of devices involving different properties of various materials. In particular heterostructure with thin amorphous silicon film seems to be very appealing since large area and low temperature are involved to obtain a junction on base material. However amorphous silicon chemical and physical compatibility with crystalline wafer and thin film silicon does this material a good candidate to solve problems introduced when porous layer is produced on substrate; such as adhesion and electrical problems of metal contact on porous material. Porous silicon is usually obtained by an electrochemical etching of a silicon substrate. This material consists of a silicon nano-crystallites network (similar to a sponge) which shows interesting properties for both optical and electrical fields. Its strong room temperature visible photoluminescence has stimulated considerable interest in view of its possible use in silicon based optoelectronics. In this work we studied the compatibility of amorphous thin film technology with the porous silicon in case of large area applications like solar cells and sensors. For solar cells the problem of light confinement and collection is still open, since the proposed technology represents the way to enhance the photovoltaic conversion efficiency. Many solutions are showed for this aim: some implying steps derived from microelectronics technology [1], other involving easier chemical bath. These solutions need improvements, the first for high cost process, the latter for low effective light confinement. Recently a new approach to produce textured silicon surfaces has been shown. This method consists of p type silicon electrochemical etching in HF based solution obtaining the formation of a macroporous layer (with micromet
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