Recent attempts have been made to increase the efficiency of solar cells by introducing an impurity level in the semiconductor band gap. We present an analysis of such a structure under ideal conditions. We prove that its efficiency can exceed not only the Shockley and Queisser efficiency for ideal solar cells but also that for ideal two-terminal tandem cells which use two semiconductors, as well as that predicted for ideal cells with quantum efficiency above one but less than two. [S0031-9007 (97)03454-6] PACS numbers: 84.60.Jt, 72.40. + wRecent attempts have been made to increase the efficiency of solar cells by introducing an impurity energy level in the semiconductor band gap that absorbs additional lower energy photons [1][2][3]. The question has been raised [4,5] whether these cases have their efficiency limited too by the Shockley and Queisser (SQ) model [6] for ideal cells, as it is the case for conventional solar cells. In this paper an ideal solar cell with an intermediate energy level or band within the semiconductor gap is analyzed presenting an efficiency well above the one permitted by the SQ model. What we precisely mean by ideal solar cell is developed along the text in seven conditions labeled IC1 to IC7.In Fig.

The intermediate-band solar cell is designed to provide a large photogenerated current while maintaining a high output voltage. To make this possible, these cells incorporate an energy band that is partially filled with electrons within the forbidden bandgap of a semiconductor. Photons with insufficient energy to pump electrons from the valence band to the conduction band can use this intermediate band as a stepping stone to generate an electron-hole pair. Nanostructured materials and certain alloys have been employed in the practical implementation of intermediate-band solar cells, although challenges still remain for realizing practical devices. Here we offer our present understanding of intermediate-band solar cells, as well as a review of the different approaches pursed for their practical implementation. We also discuss how best to resolve the remaining technical issues.T he intermediate-band (IB) solar cell consists of an IB material sandwiched between two ordinary n-and p-type semiconductors 1 , which act as selective contacts to the conduction band (CB) and valence band (VB), respectively (Fig. la). In an IB material, sub-bandgap energy photons are absorbed through transitions from the VB to the IB and from the IB to the CB, which together add up to the current of conventional photons absorbed through the VB-CB transition. In 1997 2 , researchers used hypotheses similar to those adopted by Shockley and Queisser in 1961 3 to derive a detailed balance-limiting efficiency of 63% for the IB solar cell, at isotropic sunlight illumination (concentration of 46,050 suns) and assuming Sun and Earth temperatures to be 6,000 K and 300 K, respectively (Fig. lb). This work also presented the Shockley-Queisser limiting efficiency of 41% for single-gap solar cells operating under the same conditions 2 . The limiting efficiency of the IB solar cell is similar to that of a triple-junction solar cell connected in series. However, the IB can be regarded 4 as a set of two cells connected in series (corresponding to the VB-IB and IB-CB transitions) and one in parallel (corresponding to the VB-CB transition). This provides the IB solar cell with additional tolerance to changes in the solar spectrum.An optimal IB solar cellhas a total bandgap of about 1.95 eV, which is split by the IB into two sub-bandgaps of approximately 0.71 eV and 1.24 eV The quasi-Fermi levels (QFLs) or electrochemical potentials of the electrons in the different bands are usually close to the edges of the bands. Because the voltage of any solar cell is the difference between the CB QFL at the electrode in contact with the n-type side and the VB QFL at the electrode in contact with the p-type side, the maximum photovoltage of the IB solar cell is limited to 1.95 eV, although it is still capable of absorbing photons of energy above 0.71 eV In contrast, single-gap solar cells cannot supply a voltage greater than the lowest photon energy they can absorb. IB solar cells can deliver a high photovoltage by absorbing two subbandgap photons to produce one high ...

We present intermediate-band solar cells manufactured using quantum dot technology that show for the first time the production of photocurrent when two sub-band-gap energy photons are absorbed simultaneously. One photon produces an optical transition from the intermediate-band to the conduction band while the second pumps an electron from the valence band to the intermediate-band. The detection of this two-photon absorption process is essential to verify the principles of operation of the intermediate-band solar cell. The phenomenon is the cornerstone physical principle that ultimately allows the production of photocurrent in a solar cell by below band gap photon absorption, without degradation of its output voltage.

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