The functionality of photovoltaic devices (such as, solar cells) is converting photons to electricity. Characterization of photovoltaic devices is essential at production level and during use. Here we put forward a new concept of functional imaging for photovoltaic devices. Based on the concept, we also propose an optical technique that can simultaneously acquire functionality and structure images of a single or multiple photovoltaic devices. The technique is simple, low-cost, effective, and efficient. It requires no mechanic motion and instead structured illumination is adopted for image acquisition. The complementary information provided by the functionality and structure images acquired allows one to better identify any existing and developing faults of photovoltaic devices. What's more, the technique is able to give reliable results with much fewer measurements than image pixels by exploiting the sparsity of functionality and structure images in the Fourier domain. The technique allows not only users to monitor the 'health condition' of the utilized devices, but also manufacturers to improve their fabrication conditions. The proposed technique may be applicable for rapid characterization for other semiconductor-based devices.