Microwave Annealing of Polymer Photovoltaic Devices

Organic photovoltaic (OPV) devices are receiving increasing attention because of their potential application for solar energy conversion. The advantages of using OPV devices over inorganic systems are mechanical flexibility, light weight, low cost, and fabrication at low temperature. Since the discovery of ultra-fast photoinduced charge separation between conjugated polymers and fullerenes, organic solar cells prepared from polymer semiconductors have been studied extensively. The so-called “bulk heterojunction” structure is commonly used on account of its simple device structure and thin-film processability. Recently, the external quantum efficiency (EQE) of OPV devices prepared from poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C-61-butyric acid methyl ester (PCBM) was improved through treatment thermally and/or under an electric field. In addition, Li et al. reported a solvent annealing method that achieved a remarkable efficiency (4.4 %). Meanwhile, Ma et al. improved the efficiency to 5 % through post-thermal annealing. Apparently, the annealing process is a key step toward obtaining high power conversion efficiency (PCE). Although thermal annealing is conventionally implemented through thermal conduction methods, such as the use of hotplates or thermal ovens, the energy loss and low efficiency of energy usage during such processes can be problematic. Because the degree of microwave absorption depends on the rotation of the dipoles of a material, microwave annealing, which can be used to heat materials selectively, is a potential approach toward enhancing the efficiency of energy usage. Moreover, microwave heating is a non-contact, rapid heating process; for example, the heating rate for amorphous carbon powders smaller than 1 lm can reach 1258 °C min at room temperature under microwave irradiation at 2.45 GHz. In addition, microwave-assisted annealing and sintering processes have been applied to improve the crystallization of amorphous silicon. Only a few studies, however, have focused on the behavior of conjugated polymers under microwave irradiation. In this paper, we describe the application of microwave annealing to enhance the efficiency of polymer OPV devices (Fig. 1). The unique selectivity and short annealing times might make this method suitable for the efficient industrial production of OPV devices.