Synthesis and Modification of Conjugated Materials for Polymer Solar Cells

Doctoral thesis, 2016

Polymer solar cells have emerged as a promising alternative to silicon based solar cells. One of the advantages of polymer solar cells is the possibility to use roll-to-roll techniques for large-scale device production. However, in order to fully utilize this technique, several issues need to be resolved. The bottom electrodes of large-area devices are one critical aspect that typically requires modification with interlayers in order to achieve high performance. The main focus of this thesis has been on the design, synthesis and evaluation of such interfacial materials. In the first part of the thesis, a well performing DPP-based polymer was modified with alkoxy side chains to investigate the effect of various polymer properties. In addition to a redshift in the absorption, other polymer properties were altered, proposing increased flexibility in the polymer chain. The effect of adding an ultrathin layer of an amine-functionalized conjugated polymer between the cathode and active layer in polymer solar cells is studied using four different interlayer polymers. The introduction of these interlayer polymers to the device structure resulted in enhanced solar cell performance due to an improvement in surface and electrical properties of the substrate electrode. In addition, the polymers also improve the photo-stability of devices, mainly as an effect of a reduced decrease in open-circuit voltage and fill factor. Finally, two fullerene derivatives were used to simultaneously achieve both work function modification of the electrode and improved thermal stability of polymer solar cells. The use of fullerene interlayers resulted in higher photovoltaic performance. Moreover, the photovoltaic performance is retained in polymer solar cell blends that otherwise rapidly deteriorate at elevated temperatures.