An analysis of an ultra-thin crystalline silicon solar cell with 200 nm thick active layer fabricated in our laboratory shows that the behavior of quantum efficiency as a function of wavelength is primarily determined by the absorption of light in the active layer, showing a series of interference peaks. We have now modeled this solar cell in detail, perceiving the solar cell as a waveguide. Intriguingly we find that the peaks in the quantum efficiency lie close to the frequencies of the trapped (internal) modes of the waveguide. This paper provides a detailed explanation of how these modes, normally inaccessible to external observation, can be detected in external quantum response, in terms of the position of poles of the absorbance in the complex wave number plane.