Spectroscopic and Intensity Modulated Photocurrent Imaging of Polymer/Fullerene Solar Cells

Molecular spectroscopic and intensity modulated photocurrent spectroscopy (IMPS) imaging techniques are used to map morphology-dependent charge recombination in organic polymer/fullerene solar cells. IMPS uses a small (similar to 10%) sinusoidal modulation of an excitation light source and photocurrent responses are measured while modulation frequencies are swept over several decades (similar to 1 Hz-20 kHz). Solar cells consisting of either poly(3-hexylthiophene) (P3HT) and poly(2-methoxy-5-(3'-7'-dimethyloctyloxy)-1,4-phenylenevinylene) (MDMO-PPV) blended with a soluble fullerene derivative, [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) are used as targets. The morphologies of these polymer/fullerene systems are distinctly different due to PCBM miscibility in various polymer conformers. IMPS responses of both blend solar cells show unique morphology-dependent charge generation, transport and extraction signatures that can be spatially correlated to microscopic variations in local composition and packing by constructing IMPS images along with corresponding molecular spectroscopic imaging over the same scan area. We find that boundaries separating enriched polymer and fullerene domains promote nongeminate charge recombination appearing as positive phase shifts in the IMPS response. These zones are susceptible to degradation and we propose the approaches herein can be used to probe material and device degradation in situ under various conditions, such as oxygen content, temperature and ionizing radiation.