Monolayer transition-metal dichalcogenides (TMDs) are considered to be fantastic building blocks for a wide variety of optical and optoelectronic devices such as sensors, photodetectors, and quantum emitters, owing to their direct band gap, transparency, and mechanical flexibility. The core element of many conventional electronic and optoelectronic devices is the p–n junction, in which the p-and n-types of the semiconductor are formed by chemical doping in different regions. Here, we report a series of optoelectronic studies on a monolayer WSe2 in-plane p–n photodetector, demonstrating a low-power dissipation by showing an ambipolar behavior with a reduced threshold voltage by a factor of two compared with the previous results on a lateral electrostatically doped WSe2 p–n junction. The fabrication of the device is based on a polycarbonates (PC) transfer technique and hence no electron-beam exposure induced damage to the monolayer WSe2 is expected. Upon optical excitation, the photodetector demonstrates a photoresponsivity of 0.12 mA·W?1 and a maximum exteal quantum efficiency of 0.03％. Our study provides an alteative platform for a flexible and transparent two-dimensional photodetector, from which we expect to further promote the development of next-generation optoelectronic devices.