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The analysis of deep level defects of III-V nitrides (such as GaN or InxGa1-xN) is critical for the improvement of solar cell properties using these materials as an active layer. Schottky junctions are often used for the detection of level defects by deep level transient spectroscopy or deep level optical spectroscopy. Masatomo Sumiya and coworkers (pp. 470–473) have fabricated a transparent conducting polymer / GaN Schottky junction by using spin coating technique. The Schottky properties exhibited an ideal factor of 1.3, a Schttoky barrier height of 1.15 eV and a leakage current at -10 V of ∼10-6 mA/cm2, which was much better than that of a normal Schottky junction using a high workfunction metal like Au. Under irradiation of an AM1.5G solar simulator, the device performed an open circuit voltage of 0.72 V with a high fill factor of 0.71. Utilizing the features of transparency and good Schottky properties, the depletion layer in GaN at the interface beneath the conducting polymer was evaluated under light irradiation in order to detect the defects located in the GaN band-gap which must give an influence on the photovoltaic properties. This technique can be applied also to InGaN films with lower band-gap, even more suitable as photovoltaic material of III-V nitride film.