In organic photovoltaic devices, outer interface structures play a significant role in establishing optimal contact conditions for efficient extraction (or blocking) of charge carriers. Buffer layers of different nature are currently employed to enhance both power conversion efficiency (PCE) and cell stability by improving contact performance. Several materials have been explored to enhance the electron selectivity of the cathode contact: alkali metal compounds (LiF, etc.), metal oxides (TiO x , ZnO, etc.), and low molecular weight organic compounds have been reported to contribute to the overall PCE and solar cell lifetime, as reviewed in recent reports.1À3 Among those approaches, the effect of the dipole moment associated with self-assembled monolayers (SAM) attached to the interface, which alter the energy level alignment between the cathode metal and the bulk of the blend, 4 is particularly interesting, as well as the inclusion of conjugated polyelectrolyte interlayers. 5 In all of these cases, the energy shift induced by the charge dipole built up at interface layers enables the use of air-stable high work function metals. It is then inferred that electrostatic mechanisms occurring at the nanometer scale, both in the active layer bulk and at interfaces, have a great influence on the overall device operation. 6,7 Interface dipole layers are regarded as a determining ingredient of the metal/organic contact equilibration. 8À12 Several models have been proposed to account for the energy level alignment at interfaces, depending on the degree of interaction between the metal contact and the deposited organic layer. When the chemical interaction between the metal and contacting conjugated molecules or polymers is not negligible, it is expected that molecules attached to the metal surface undergo both a shift and a broadening of their molecular energy levels. Energy distribution of the attached molecules should be modeled by a specific interfacial density of states (IDOS) which differs from that encountered in the bulk of the organic layer. The situation is

A connection is established between recombination and series resistances extracted from impedance spectroscopy and current‐voltage curves of polythiophene:fullerene organic solar cells. Recombination is shown to depend exclusively on the (Fermi level) voltage, which allows construction of the current‐voltage characteristics in any required conditions based on a restricted set of measurements. The analysis highlights carrier recombination current as the determining mechanism of organic solar cell performance.

In this paper, the electrochemical degradation of polypyrrole film was studied by means of overpotential application. The overpotential was 0.58 V versus SCE, and after every 5 min of application of 0.58 V, a cyclic voltammogram was recorded in the range of -0.7 to 0.5 V as well as an electrochemical impedance spectroscopy and electrochemical quartz crystal microbalance (EIS and EQCM). The main characteristic is the huge increase in the charge transfer resistance (r(ct)), which indicates that the insertion process of ions in the polymer matrix is hindered by the electrochemical degradation. Once the process of insertion is damaged, the number of intercalated ions in the matrix should decrease, which is expressed by the low-frequency capacitance, which is proportional to the number of intercalated ions in the polymeric matrix. The decrease of intercalated ions has an influence in the mass variation of the polymer film, which is confirmed by EQCM measurements.

This work investigates the loss in performance induced by molecular oxygen in bulk heterojunction solar cells. We observe that upon exposure to molecular oxygen both formation of P3HT