Operando‐analysis of organic photovoltaic devices via ESR and EDMR methods

Organic photovoltaic devices (OPVs, see Fig. 1(a)) are light, thin, and cost-effective. Furthermore, OPVs have many unique properties which conventional inorganic semiconductor cannot achieve. Therefore, they are attracting considerable attention as next-generation renewable energy devices^[1]. The most important issues that inhibit commercialization are durability and stability (see Fig. 1(b)). To explore the origin of degradation in OPVs, precise analysis is needed on the view points in molecular scope.

Fig. 1　(a) Schematic drawing of an OPV  (b) Conceptual picture of J-V curves revealing light induced degradation of an OPV

Fig. 2 (a) Targets and principle of ESR measurement and (b) of EDMR

Organic semiconductor devices are generally evaluated by analyzing physical properties associated with trapped radicals and current paths through recombination centers derived from detected polarons, which act as charge carriers in organic conductors. Such microscopic analysis can extract information about behaviors at the molecular level, while a macroscopic J-V measurement, as shown in Fig. 1(b), cannot access it. As shown in Fig. 2(a), Electron Spin Resonance (ESR) method is a useful technique which can detect polarons and trapped radicals as a change of magnetic moments, sensitively and selectively. In contrast, Electrically Detected Magnetic Resonance (EDMR, as shown in Fig. 2(b)) can selectively detect electron spins related to recombination currents, which are associated with device performance.