Organic Solar Cell Fabrication and Study on the Influence of Spin-dependent Processes on the Photocurrent using Spin-sensitive Techniques

Sammanfattning: Research in recent years on novel materials in organic solar cells (OSCs) have contributed to a rapid advancement in OSC efficiency. Here, OSCs with the well-studied organic semiconductors poly(3-hexylthiophene):[6,6]-phenyl C60-butyric acid methylester (P3HT:PCBM) in a bulk heterojunction structure were prepared to establish a baseline procedure for fabrication and spin-sensitive spectroscopy. Spin-sensitive spectroscopy can be used to probe the spin-dependent processes and loss mechanism in OSCs. Understanding the microscopic processes enables research targeting loss mechanisms directly, which opens up for higher efficiency OSCs. Measurements with continuous wave electrically detected magnetic resonance (cwEDMR), continuous wave electron paramagnetic resonance (cwEPR) and transient (tr)EDMR were set up and followed by an initial study on the spin-dependent processes and their influence on the photocurrent. Signals from spin-dependent processes in the samples were seen for the three experiment series carried out, and well-functioning OSCs with consistent results were prepared. This thesis and the presented baseline fabrication and experimental setup procedures provide with valuable learnings for future research in the group AG Behrends enabling fabrication and spin-sensitive studies on novel materials in OSCs. It was found that post-processing annealing at 120 C for 5 min reduces S-shape behaviour in current-voltage curves, and over all improves poor current-voltage characteristics of the OSC samples fabricated. Further, influence from PEDOT:PSS was seen in cwEPR spectroscopy, indicating the presence of traps or free radicals in the PEDOT:PSS. In cwEDMR spectroscopy of the OSCs it was shown that the spin processes of bias-induced and photo-induced charge carriers influence the photocurrent in the same way. Finally, the authors suggest that results from spin-sensitive spectroscopy of degraded OSCs cannot be applied to non-degraded OSCs.