Microwave annealing is applied for enhancing the efficiency of polymer organic photovoltaic (OPV) devices, where the unique selectivity and short annealing times might make this method suitable for the efficient industrial production of OPV devices. The unique selectivity and short annealing times might make this method suitable for efficiently using industrial production of OPV devices. One of the method for in situ monitoring of temperature in making the use of the fluorescent dye rhodamine B decomposes thermally as a temperature probe. Experiment result showed that microwave irradiation is a selective heating method that improves the efficiency of OPV devices. No heating effect of the microwaves occurred at the anode and the selectivity of the microwaves heated the active organic materials and the cathode. X-ray diffraction analysis indicated that microwave annealing facilitated the crystallization of P3HT, which in effect increased efficiency of the device.