A power source (212) is disclosed for charging a battery (330) within a portable electronic device (310). An apparatus (422), such as a photovoltaic or thermoelectric cell, for charging the battery (330) is disposed contiguous to and within a transparent housing (412) of the portable electronic device (310). A fluorescent species (416), such as quantum dots or a fluorescent dye, is disposed on a side of the housing (412) opposed to the apparatus (422). Light (430) striking the fluorescent species (416) is converted into photons (432, 434) having a narrower spectrum that passes through the housing (412) to the apparatus (422). An optional layer (418) may be disposed on the fluorescent species (416) that reflects light from the fluorescent species (416) to the apparatus (422). Photonic crystals (415) may be combined with the fluorescent species (416) to increase reflectivity.