Analyzing Outdoor Degradation of Flexible PEDOT-Free P3HT; PCBM Organic Solar Cells Using Impedance Spectroscopy

Flexible inverted organic solar cells (OSC) based on Poly(3-hexylthiophene-2, 5-diyl): [6], [6]-Phenyl C61 butyric acid methyl ester (P3HT: PCBM) have been manufactured and degraded using two different degradation protocols: six cells were kept in dark conditions, according to ISOS-D1 protocol, while other six were placed outdoor in a solar tracker, according to ISOS-O1 protocol. Pristine and degraded devices have been characterized in terms of I-V curves and Impedance Spectroscopy (IS). After two months, the efficiency drops from 2.83 % to 2.55 % for devices kept in dark conditions, and to 0.85 % for devices placed outdoor. Impedance spectra, measured before and after degradation have been fitted with an equivalent circuit that models recombination and diffusion processes. The evolution of the circuital parameters obtained from the IS fit show that devices degraded outside present i) a significant increase of the transport resistance, which hinders charge conduction, and ii) a decrease of majority carrier mobility, which is related to a loss of the blend crystallinity.