Somewhat worse degradation was found on nip a-Si cells by NASA Langley [42]; they also found recovery with a 2-hr, 200°C anneal. 5. Thin-film Cascades Introduction An important technology for the production of high-efficiency thin-film arrays is the ability of thin films to be produced in multibandgap ‘cascade' structure [43]. In a solar cell made of a single semiconductor material, a fundamental limitation to the maximum possible efficiency is imposed by the photon utilization. Photons of energy less than the bandgap of the material Eg are not used, while the fraction of photon energy greater than Eg is wasted. This limitation can be avoided by using more than one semiconductor material in a ‘cascade' or ‘tandem' cell structure. In the cascade structure, short wavelength (high energy) photons are absorbed in a high bandgap material on the top of the solar cell. The high bandgap material is transparent to longer wavelength (low energy) photons, which pass through and are absorbed by a second layer consisting of a photovoltaic material with lower bandgap. This increases the utilization of the solar spectrum, since the excess energy of the high energy photons is not wasted. This is shown graphically in Fig. 4, where the Air Mass Zero (AMO) solar spectrum is graphed in the form of total number of photons with energy greater than E
RkJQdWJsaXNoZXIy MTU5NjU0Mg==