**6. Conclusion**

We have fabricated TCP/nitride semiconductor heterojunction solar cell structures by the spin-coating method using PEDOT:PSS or PANI as the TCP layer and Si-doped GaN as the semiconductor layer. The devices exhibited high quality rectifying properties and have an approximately 1 eV barrier height. Both the PANI/epi.-GaN and PEDOT:PSS/epi.-GaN heterojunction solar cells exhibited ultraviolet-sensitive photovoltaic action. The observed open-circuit voltage was superior to previously reported values for metal/GaN Schottky photo-detectors. A characteristic frequency-dependent behaviour of the interface capacitance was found for the TCP/epi.-GaN solar cells. The *C*-*f* characteristics were analyzed based on the dielectric dispersion theory and the intrinsic capacitance and resistance were obtained. The considerable reduction of the interface capacitance in the high frequency region allowed for highly-sensitive detection of deep levels in GaN by DLOS measurements.
