Layered semiconductor structures with different materials

Layered semiconductor structures with different materials, one grown on another form heterostructures with a step-discontinuous potential profile. Organic-inorganic heterostructures are an emerging and interesting field of research for optoelectronics. In this work an efficient organic/inorganic hybrid heterojunction between PEDOT: PSS and n-type Silicon has been fabricated for optoelectronic applications. Samples with varying thickness of PEDOT: PSS were prepared by spin coating technique and structural and morphological studies were performed. The electrical conductivity of organic layers was modified using DMSO as additive. Post fabrication, the hybrid heterostructures were treated with nitric acid vapor so as to enhance the conductivity of the organic layer. Surface treatment with HNO3 was found to lower the roughness of the organic layer and enhance the transparency of the layer. Current – Voltage (I–V) characteristics reveal optimized behavior of HNO3 treated PEDOT: PSS layer. A low Ideality factor (n~3.2) has been estimated for HNO3 treated sample, with a barrier height (?B) of 0.9 eV. The findings of the study provide a promising efficient method to enhance the electrical and device properties of PEDOT:PSS/n-Si heterostructures for optoelectronic applications.At present homojunctions/heterojunctions based optoelectronic devices such as solar cells, light-emitting diodes, lasers and touch panel displays have been in demand in the global market. One of the major disadvantages of a homogenous semiconductor or homojunctions is that the carrier transport cannot be controlled independently. Even the potential barriers in homo p-n junctions for electrons and holes cannot be controlled independently. As a result it is imperative to introduce heterojunctions to overcome some of these problems. In addition, the fabrication of optoelectronic materials, require a transparent layer which acts as a light harvesting layer. Materials such as ITO (Indium doped Tin Oxide) are mostly used as the transparent material in these applications. However, new transparent materials such as conducting polymer, graphene and carbon nanotubes have been introduced recently for replacing ITO material to overcome its drawbacks such as high mechanical brittleness, poor adhesion to organic and polymeric materials etc. 1-3. Organic/inorganic hybrid heterostructures have gained good popularity among materials for photovoltaic device fabrication, of which organic/Si hybrid solar cells (HSCs) have been subjected to extensive studies owing to low-cost of Si-based photovoltaic devices. Also, a heterojunction could be achieved between Si and organic polymers through simple solution processes at low temperature, making it advantageous over traditional p-n junction fabrication involving temperatures as high as 1000 oC and hence have attracted significant research interest in recent times 4,5. Crystalline silicon (Si) is the most popular material when it comes to fabrication of solar cell. Its unique properties, such as availability of large single crystal, high mobility and also cost- effectiveness make it an important material for application based studies. However, in most of the cases high temperature and clean room facilities are required to fabricate p-n junction 6. Thus, hybrid solar cell fabricated by using organic and inorganic semiconductors, especially, organic conducting polymer and Si wafer is a feasible and more attractive option 6-8.
Organic materials have many advantages such as controllable electrical conductivity, good environmental stability under ambient conditions and also low cost compared with inorganic materials. Among the organic materials, Poly(3,4-ethylenedioxythiophene) :poly(styrenesulfonate) (PEDOT:PSS) is a widely used polymer in optoelectronic devices such as photodedetectors, light emitting diodes and photovoltaics due to its numerous advantages such as good environmental stability, low band gap, high optical transparency, low redox potential, high work function, water solubility, hole conducting ability, ease of deposition by solution process and p-type semiconductor characteristics 5, 9-11. Though the superior characteristics of PEDOT:PSS ensures its role as a promising material for replacing inorganic semiconductors in plastic electronics application, PEDOT:PSS thin films possess a drawback due to their low conductivity compared to Indium-Tin oxide (ITO) and other conductive polymers. The molecular structure of PEDOT:PSS is shown in figure 1.

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