Photophysical characterization and photovoltaic performance analysis of organic solar cells produced by using different nanostructures

Although electronic properties of organic molecules have been studied in 20th century (Brütting, 2006), they have been widely used in organic solar cells in the middle of the 1980s (Tang, 1986). The advantages of the organic materials is that they can be used in low cost production such as inkjet printing and spin coating due to ability of operating in solution-based (Ã et al., 2013). Furthermore, they are more flexible in solid state phase compared to inorganic films due to week Van der Waals interaction between molecules (Shi et al., 2008) And also photo absorption in desired spectrum range is possible by modification of molecules/polymers. In organic solar cell studies, regioregular Poly (3-hexylthiophene-2, 5-diyl) (P3HT) is the preferred donor material because of its stability and high mobility. However its short exciton diffusion length that limits the photovoltaic performance is the important disadvantage. Thus 1-D organic nanowires, which are formed by π-π stacking of adjacent polymers, are used instead of hacimsel structure to avoid exciton recombination. In this way, sufficient exciton dissociation can be achieved by using organic nanowires. Also free charges can reach to the electrodes through ordered polymers that contains less trap states. It is possible to produce P3HT nanostructures with different dimensions by different methods and ambient conditions. However, when the consideration of solar cell efficiency requirements, efficient donor/acceptor interface area is desired. Moreover, excitons should reach the donor/acceptor interface before recombination and free charges should transport through the interconnected domains of the hacimsel. Due to these requirements, ordered P3HT nanostructures should be a few micrometers lenght and a few nanometers thick. Furthermore, P3HT nanostructures should form a homogeneous blend with acceptor material. Thus, morphological optimization of ordered structures plays a crucial role for device fabrication. For this purpose, in this thesis, controlled P3HT nanowire formation is investigated by atomic force microscopy (AFM), and then device performances of nanowires are studied. In the chapter 3.1.1 of the thesis, nanowire formation and evolution are investigated by slow cooling technique. The reason of the preference of slow cooling technique is that nanowire formation and evolution are slow. Thus, the nanowire formation in stock solution may be imaged by AFM in different incubation time. Another method for nanowire formation is to add poor solvent on P3HT solution. Nanowire formation occurs in micro second range at room temperature. Thus, just nanowire evolution can be observed by using this technique. In chapter 3.1.2, nanowires are formed by changing the ratio of poor solvent:good solvent, and their evolutions in different incubation time are investigated by AFM. By this way, the ideal conditions are determined morphologically for organic solar cell fabrication. In chapter 3.5.1, free charge mobility and solar cell performance of nanowires are investigated. Hole mobility is calculated by using space charge limited current (SCLC) method, and and effect of donor:acceptor morphology on organic solar cell efficiency is clarified. One of the requirements for the high performance of the device, as mentioned before, is that the donor / acceptor interface area should be large enough. Thus, donor and acceptor should blend homogeneously. In this regard, in chapter 3.5.3, device optimization is investigated by using 1,8-diiodooctane (DIO) additive for the film morphology. Thus, the density of trap states of morphological defects is decreased by DIO additive while density of trap states, caused by DIO additive, are also aimed to be minimum concentration. The variety of trap states and effects of the traps on device efficiency are defined by impedance spectroscopy.