Organic Photodiodes : Optical, electric, and morphological aspects of polymer based devices

Abstract: The use of semiconducting organic materials as the active layer in optoelectronic devices offers some advantages in processing and new possibilities of device fabrication such as large-area devices. The mechanism of charge generation upon light absorption in organic materials differs from that in inorganic semiconductors. While in many inorganic materials the photon absorption produces free charges directly, in molecular materials the light absorption creates excited states, bound electron-hole pairs called excitons. These excitons must then be dissociated and the charges transported to the electrodes to produce an appreciable photocurrent. The exciton dissociation is not a bulk process, they are dissociated at strong electric fields normally found at the interfaces of materials with different electron affinities and ionization potentials. One of the limiting aspects in device physics is the short lifetime (≈nanoseconds) and diffusion length of excited states (≈10nm). In order to obtain maximum photoconversion several approaches can be used and some of them are presented in this thesis where the electrical and photovoltaic properties of polymer based devices is discussed.Basically, it is necessary to move the excited states to a site for charge separation within their life time in distance and in time. One approach to improve the exciton dissociation envolves distributing the sites for photoseparation by forming a composite of two phase segregated materials with different electron affinities proving the spatially distributed interfaces necessary for exciton ionization. Blending donor and acceptor materials in the active layer does exactly this. In sandwich type devices the active layers are found between two conducting electrodes, one transparent to the light and the other normally a mirror (aluminum). The stationary optical wave created inside the layers due to interference of the incoming wave with the wave reflected from the aluminum electrode strongly depends on the materials thicknesses. The photocurrent strongly depends on the resulting optical field distribution inside the device. In this approach we may consider that the optical electric field near to the dissociation region must be enhanced, i.e., increase the light absorption in the active parts of the device. In a heterojunction, bi-layer device where the exciton dissociation occurs at the donor/acceptor interface, the light distribution can be controlled and maximized. In a bi-layer device when the donor layer was formed by a polymer blend, an enhancement in photoconversion was achieved by means of energy transfer, where one polymer with high absorption coefficient transfers the excitons to another with better transport properties as well as ionization of excitons. In a single layer device an improvement of absorption was achieved by trapping light with a grating at the rear part of the device. The grating pattern was soft embossed onto the active organic layer before the metal evaporation using a soft lithography method. Transport of charges is quite important issue in organic photovoltaic devices. The work function values of the electrodes, as well as the bulk properties of the layer are relevant. Modeling the current density - voltage characteristics of polymer based diodes in dark have shown that the choice of electrodes may change the device from contact limited current to bulk limited current regimes depending on the injection barrier, the mobility depends on electric field under the space charge limitation.