Impedance spectrometry of optimized standard and inverted P3HT-PCBM organic solar cells

A study of the optical and impedance behavior of optimized standard and inverted photovoltaic solar cells based on P3HT:PCBM active nano composites is presented. The standard cells sequence is ITO/HTL1/P3HT:PCBM/Ca/Al and the inverted cells one is ITO/ZnO/P3HT:PCBM/HTL2/Ag where HTL1 and HTL2 are Hole Transport Layers. Absorption and action spectra, together with I–V characteristics, are shown to be quite similar and lead to 4.05% and 3.90% energy conversions, for standard and inverted cells, respectively. Built-in potentials of 0.82–0.89 V and acceptor impurities concentrations of 1.6–2.4 1015 cm−3 are found through capacitance measurements. Impedance spectrometry shows the classical two-circle complex plan curves, one being related to the effective lifetime of charge carriers before recombination at low frequency, and the other one to the diffusion time of these carriers at high frequency. The shape of the curves is identical, showing the ohmic role of the ZnO layer. It is shown that overall resistances in the dark are higher for inverted cells as compared to standard ones, and that this feature is inverted under illumination, with a thousand-time decrease. Global mobilities are in the range 3.8–4.6 10−3 cm2 V−1 s−1, which is slightly higher as compared to the literature. Four different equivalent circuit models are tested on experimental results, and it is concluded that the classical RCPE model (or its Garcia–Belmonte variant) is suitable for this kind of cells.