Hybrid Structure: Use of Semiconducting Nanoparticles in Donor/Acceptor Type Organic Solar Cells

Organic solar cells are not intended to compare with silicon solar cells but they have several advantages regarding some specific applications, such as packaging flexible screens and recharging cell phones and laptops. Unlike crystalline silicon, organic solar cells could be manufactured at a lower cost. In organic or polymer solar cells, device operation involves (1) excitons generation, (2) charge separation and (3) carrier transport to the opposite electrodes. Since the three steps occur in sequence, efforts have been made to enhance efficiency or output of each of the steps. Generation of excitons expectedly depends on matching of electronic absorption spectrum of the active materials with that of the solar illumination. Hence active materials are chosen accordingly. Exciton dissociation or charge separation, which occurs at the interface between a donor and an acceptor layer, takes place due to the internal field that develops owing to the difference of energy levels at the interfaces. Carrier transport, the third step, is directed by the internal field generated by the difference in work functions of the two electrodes. Apart from the electric field, mobility of charge carriers plays a major role in determining short-circuits current of photovoltaic devices. Organic photovoltaic cells have a limitation considering poor charge carrier mobility and stability of the active materials. An effort has been made to fabricate solar cells based on organic and inorganic hybrid materials. In hybrid solar cells, inorganic nanocrystals are generally mixed in a host conjugated polymer to form a photoactive layer. Inorganic nanocrystals have several advantages in this direction. These nanoparticles have a better charge carrier mobility than organic thin films due to crystalline nature of the quantum dots. They are also very stable. The size control of II these nanoparticles allow for tuning of optoelectronic properties. Nanoparticles also have a large aspect ratio (surface area to volume ratio) which present more interface area with the host polymer matrix. These nanoparticles have however a limitation due to their insulating capping agent. An effort has been made to remove these capping agents by pyridine or by small chain amines (butyl amine, octaylamine, etc) which have a low vaporization point and could be removed through annealing after film formation. Besides using nanoparticles as photoactive materials, they could be used as charge transporting materials due to their superior charge carrier mobility. As a result, these nanoparticles have been used as channels for carrier transport in one or both photoactive layers of a photovoltaic cell. Carbon nanotubes (CNT) could also be used in this direction. Hybrid solar cells are a step forward for organic photovoltaic devices where we could tune the device performance to a large extent. We have carried the work of this thesis in this direction.The research was conducted under the supervision of Prof. A. J Pal of the Solid State Physics division under SPS [School of Physical Sciences]The research was carried out under CSIR & DST research gran