(a) Chemical structures of regio-regular poly(3-hexylthiophene-2,5-diyl) (rr-P3HT) and phenyl-C61-butyric-acid-methyl ester (PCBM). (b) Scheme of the hybrid solid–liquid cell for modulated photocurrent spectroscopy. (c) Spectral response of the systems ITO/rr-P3HT:PCBM/NaCl/Gold (blue line and triangles) and ITO/rr-P3HT:PCBM/Ringer/Gold (red line and squares), with the saline solutions working as ionic cathodes, recorded after 12 days of immersion in minimum essential medium. Typical spectral response (in arbitrary units for direct comparison with present results) of a conventional, solid-state solar cell (ITO/rr-P3HT:PCBM (1:1)/Al) is reported as a reference (dashed, grey line). Minor changes in the spectrum shape have to be ascribed to slightly different sample thicknesses and processing conditions20; however, one should note that main spectral features are well reproduced. (d) Photocurrent maxima (at 530 nm) recorded from eight distinct devices (represented in different colours by diamond symbols) immediately after fabrication (day 0) and up to 28 days later. Samples were kept completely immersed in the electrolyte for the whole period. Box plot limits represent the 25th and 75th percentile and whiskers represent the outliers (coefficient 1.5) of the distribution. The mean (square), maximum and minimum values of the distribution are also shown. (e) Stability of the photocurrent of the system ITO/rr-P3HT:PCBM/NaCl/Gold on continuous illumination. The data show the photocurrent recorded at 530 nm at various times up to 28 h.