We report a mathematical formulation that successfully describes the holographic photocurrent produced in photo-emf experiments, with large oscillation amplitudes, in strongly absorbing photorefractive materials. The large amplitude produces a sensible enhancement of the photocurrent signal and in this way facilitates measurements. Accounting for bulk light absorption of the sample is essential in order to adequately describe the experiment. We measure pure and doped photorefractive Bi12TiO20 (BTO) crystals and show that these data are in excellent agreemnt with theory. From data fitting we are able to determine some material's parameters.