Study on the Effects of Active Layer Morphology on Photocurrent and Photovoltage Generation in Organic Solar Cells

Abstract

The morphology of the active layer of organic solar cells has significant impacts on the current and voltage generation of the organic solar cells. It is important to clarify the relationship between the active layer morphology and photovoltaic properties of the organic solar cell. Based on the understanding of the relationship, one can systematically approach the improved performance of future organic solar cells. Although previous studies have achieved significant progress, there are still many remaining issues and challenges in understanding the morphology-photovoltaic property relationship. Therefore, this thesis aims to unravel the missing connections between morphological factors and their effects on photovoltaic properties, in order to contribute to a deeper understanding of the morphology-performance relationship. Especially, the studies in this thesis investigate the effects of the structure of donor-acceptor heterojunctions on photocurrent and photovoltage generation in organic solar cells. In Chapter 2, effects of the molecular orientation at donor-acceptor interface on the photocurrent generation are investigated. The efficiency of photocurrent generation is compared for the cases that the conjugated plane of the donor polymer is oriented in parallel or perpendicular to the donor-acceptor interface. Previous studies hardly investigated this effect due to difficulties in characterizing the molecular orientation at donor-acceptor interfaces in complex bulk heterojunction structures. A few studies have reported on this orientation effect, but their studies had complex experimental systems which had limitations in observing the orientation effect independently. To avoid these problems, model devices are fabricated in this study which clearly shows the molecular orientation effect. The molecular orientation is effectively changed to be face-on or edge-on to the donor-acceptor interface by controlling the film drying kinetics during spin coating process. This orientation-controlled donor polymer film is then transferred onto the acceptor film by using a method which is non-destructive and does not require any application of heat or solvent. As a result, planar heterojunction devices with sharp and non-mixed donor-acceptor interface are obtained. Dependence of photocurrent generation on the orientation is observed in the devices, showing higher photocurrent in the device with the face-on interfacial orientation than in the device with the edge-on interfacial orientation. Optoelectrical and spectroscopic analyses reveal that the higher photocurrent generation in the face-on device than in the edge-on device is due to the longer exciton diffusion length of the face-on oriented polymer film as well as the higher dissociation efficiency of excitons into free charge carriers at the donor-acceptor interface with the face-on orientation. This study implies that the molecular orientation at the donor-acceptor interface is a critical factor that should be controlled elaborately to achieve efficient organic solar cells. In Chapter 3, effects of inhomogeneous spatial distribution of different donor-acceptor interfacial structures within the active layer are investigated. Especially, its effects on photovoltage generation are investigated, considering that the different interfacial structures give different molecular energy levels. Most existing expressions that describe open-circuit voltage (Voc) of an organic solar cell assume a homogeneous spatial distribution of donor-acceptor interfacial structure and thus homogeneous molecular energy levels within the entire active layer. Therefore, they cannot account for the case where the active layer has variation in interfacial structure and energy levels depending on position. To resolve the problem, model devices comprising two different donor-acceptor interfacial structures which are spatially separated within the active layer are fabricated. In this way, the inhomogeneous spatial distributions of molecular energy levels are realized. Large difference in Voc of the devices is observed depending on the spatial distribution of the energy levels. Optical and electrical analyses reveal that the Voc is not relevant to the energy levels of the spatial region where the photocurrent is generated, and that the Voc is not determined by the relative amount of charge-transfer states with different energies. Instead, it is suggested that the energy levels of the spatial region where the electrons and holes are dominantly populated are critical in determining the Voc of the devices.