In today's era, a lot of interest is gained by solar cell formed by combination of organic and inorganic nano-particle semiconductors mainly because of its major features such as scalable solar power conversion and cost effectiveness, which makes the cell a desirable photovoltaic device. This piece of work is an attempt to make a solar cells by the combination of zinc oxide (ZnO) and graphite. ZnO is a good n-type material for the application in photovoltaic (PV) devices due to its better optical, electrical, structural, and environmentally friendly properties, and on the other hand, graphite, an organic semiconductor, enhances the rate of charge transfer in the device. These materials are so designed to help bring in more understanding in a wider range of the solar spectrum. This work focuses on developing solid-state polymer and hybrid solar cells.
TopMaterial Used
Zinc Oxide (ZnO) is an inorganic compound. As ZnO is insoluble in water, it is preferably used as an additive in numerous materials and products like rubbers, plastics, ceramics, glass, cement, lubricants. ZnO is a wide-band gap semiconductor. The native doping of the semiconductor is due to oxygen vacancies or zinc interstitials is n- type (Chiang et al., 1977). Hence the semiconductor has many desirable properties like high electron mobility, good transparency, strong room-temperature luminescence and wide band gap. Those propertiesare valuable in emerging applications for: transparent electrodes in liquid crystal displays, energy-saving windows, and electronics as thin-film transistors and light-emitting diodes. ZnO has a relatively large direct band gap of approximately 3.3 eV at room temperature (Dayal et al., 2009). The band gap of ZnO can further be tuned to approximately 3-4 eV by its alloying with grapheneoxide.
Even in the absence of intentional doping, most ZnO has n-type character. An alternative explanation has been proposed, based on theoretical calculations, that unintentional substitutional hydrogen impurities are responsible for the same (Dutta et al., 2012). Substitution of Zn with group- III elements such as Al, Ga, in is done to achieve controlled n-type doping. In past few decades, considerable attention has been paid to the research in the production of conductive polymers and organic molecules (Dutta et al., 2012). It has been on top interests of scientists to produce a new solar cell, they decided to construct the device using organic materials as shown in figure 1. This structure is known as bilayer solar cell. Layer by layer fabrication is done to fabricate a bilayer solar cell. As a basic solar cell, the device consists of four layers such as anode layer, two contact layers of ZnO and Graphite, the cathode (Halls et al., 1995). At the interface, excitons are dissociated into electrons and holes and successively there occurs diffusion of electrons at the cathode following which the electrons enter the external to generate electrical flow and complete the cell. To generate electric current, conversion of photon to electron is necessary. Light is absorbed by electron donor material to bind the excitons and the energy used in binding excitons is called Frenkel exciton (Hou et al., 2009).
Figure 1. Device architecture of Hybrid Cell
Methodology
Usually, construction of polymer solar cells is accomplished by sandwiching an active layer between two coplanar electrodes, along with a layer of Zinc Oxide and Graphite. The bottom electrode (anode) and the upper electrode (cathode) is made of a transparent substrate (glass) coated with a conducting film such as indium tin oxide (ITO). Advantage of incorporating conductive polymer as electrode is its simple film-forming techniques. The conductive polymer in solution can be moulded into different geometric drawings, and after baking it becomes conductive film (Kannan et al., 2003). Polymer electrode successfully replaces metal film making flexible plastic large-scale photovoltaic cells a future possibility.