It was shown that ESR spectroscopy is a very useful technique for monitoring the photochemical and thermal degradation of conjugated polymers commonly used in organic solar cells. The relative stability of materials can be quantified by comparing the rates of trap accumulation (dC(R)/dt) estimated from their ESR profiles.
Herein, we reveal for the first time a comprehensive
mechanism
of poorly investigated electrochemical decomposition of CH3NH3PbI3 using a set of microscopy techniques
(optical, AFM, PL) and ToF-SIMS. We demonstrate that applied electric
bias induces the oxidation of I– to I2, which remains trapped in the film in the form of polyiodides, and
hence, the process can be conceivably reversed by reduction. On the
contrary, reduction of organic methylammonium cation produces volatile
products, which leave the film and thus make the degradation irreversible.
Our results lead to a paradigm change when considering design principles
for improving the stability of complex lead halide materials as those
featuring organic cations rather than halide anions as the most electric
field-sensitive components. Suppressing the electrochemical degradation
of complex lead halides represents a crucial challenge, which should
be addressed in order to bring the operational stability of perovskite
photovoltaics to commercially interesting benchmarks.
The problem of batch‐to‐batch variation of electronic properties and purity of conjugated polymers used as electron donor and photon harvesting materials in organic solar cells is addressed. A simple method is developed for rapid analysis of electronic quality of polymer‐based materials. It is shown that appearance of impurities capable of charge trapping changes electrophysical properties of conjugated polymers. In particular, a clear correlation between the effective relaxation time τeff and relative photovoltaic performance (η/ηmax) is revealed for samples of poly(3‐hexylthiophene) intentionally polluted with a palladium catalyst. This dependence is also valid for all other investigated samples of conjugated polymers. Therefore, fast impedance measurements at three different frequencies allow one to draw conclusions about the purity of the analyzed polymer sample and even estimate its photovoltaic performance. The developed method might find extensive applications as a simple tool for product quality control in the laboratory and industrial‐scale production of conjugated polymers for electronic applications.
Here we report the application of the Electron Spin Resonance (ESR) spectroscopy as a highly sensitive analytical technique for assessment of the electronic quality of organic semiconductor materials, particularly conjugated polymers. It has been shown that different batches of the same conjugated polymer might contain substantially different amounts of radical species which were attributed to structural defects and/or impurities behaving as traps for mobile charge carriers. Good correlations between the concentrations of radicals in various batches of conjugated polymers and their performances in organic solar cells have been revealed.
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