While thin-film devices can usually be designed via simple analytical formulas, ray-based optics cannot be used to model surface-plasmon effects and so a more rigorous approach is required. A simulation tool such as RSoft's FullWAVE, which is based on Finite-Difference Time-Domain algorithm (FDTD), provides a full-vector solution to model the electro-magnetic fields within the structure and to compute the absorption spectrum of the device. Given this spectrum and the incident solar spectrum (AM 1.5), the total number of photons absorbed in the CuPc and PTCBI regions can be computed for a specific wavelength range.