with holes in acceptor atoms, producing a layer of negatively-charged impurity atoms. The opposite action also takes place, holes from acceptor atoms in the p-region crossing into the n-region, combining with electrons and producing positively-charged impurity atoms [Book 4]. The net result of these movements is the disappearance of conduction electrons and holes from the vicinity of the junction and the establishment there of a reverse electric field, which is positive on the n-side and negative on the p-side. This reverse field plays a vital part in the functioning of the device. The area in which it is set up is called the "depletion area" or "barrier layer"[Book 4]. When light falls on the front surface, photons with energy in excess of the energy gap (1.1 eV in crystalline silicon) interact with valence electrons and lift them to the conduction band. This movement leaves behind holes, so each photon is said to generate an "electron-hole pair" [Book 2]. In the crystalline silicon, electron-hole generation takes place throughout the thickness of the cell, in concentrations depending on the irradiance and the spectral composition of the light. Photon energy is inversely proportional to wavelength. The highly energetic photons in the ultra-violet and blue part of the spectrum are absorbed very near the surface, while the less energetic longer wave photons in the red and infrared are absorbed deeper in the crystal and further from the junction [Book 4]. Most are absorbed within a thickness of 100 m. The electrons and holes diffuse through the crystal in an effort to produce an even distribution. Some recombine after a lifetime of the order of one millisecond, neutralizing their charges and giving up energy in the form of heat. Others reach the junction before their lifetime has expired. There they are separated by the reverse field, the electrons being accelerated towards the negative contact and the holes towards the positive [Book 5]...
