Architectural Approaches for the Absorption Layer and their Impact on Organic Solar Cells

This study focuses on the architectural modification of pin-type small-molecule organic solar cells, in particular on the absorption layer and its influence on the key solar cell parameters, such as short circuit current density, fill factor and open circuit voltage. Three different approaches have been applied to improve the match between the solar spectrum and the spectral sensitivity of organic solar cells. In the first part, deposition parameters such as substrate temperature, gradient strength and (graded) absorption layer thickness are evaluated and compared to organic solar cells with homogeneously deposited absorption layers. Moreover, the gradient-like distribution of the absorption layer is characterized optically and morphological effects have been extensively studied. In order to isolate the origin of the efficiency improvement due to the graded architecture, voltage-dependent spectral response measurements have been performed and gave new insights. The second part concentrates on the efficient in-coupling of converted UV light, which is usually lost because of the cut off properties of organic light in-coupling layers. Via Förster resonance energy transfer, the absorbed UV light is re-emitted as red light and contributes significantly to higher short circuit current densities. The correlation between doping concentration, simple stack architecture modifications and the performance improvement is duly presented. In the third and last part, the impact of tri-component bulk heterojunction absorption layers is investigated, as these have potential to broaden the sensitivity spectrum of organic solar cells without chemical modification of designated absorber molecules. Along with the possibility to easily increase the photocurrent, an interesting behavior of the open circuit voltage has been observed. Knowledge about the impact of slight modifications within the solar stack architecture is important in order to be able to improve the device efficiency for the production of cheap and clean energy