Understanding intermediate-band solar cells

Abstract

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 ...

The effect of band offsets in quantum dots

“…For an IBSC, the efficiency value determined in a similar manner is 63% [1]. There are several different ways of manufacturing IBSCs [3], whose operation is described further in [4].…”

In situ measurement of exciton energy in hybrid singlet-fission solar cells

“…O f the options for renewable energy, photovoltaic techno logies [1][2][3] with the potential to exceed the ShockleyQueisser limit 4 on single bandgap devices are particularly attractive [5][6][7][8] . One strategy is to sensitize a low bandgap semicon ductor with a higher bandgap material that can generate additional photocurrent from absorbed high energy photons 6,9 .…”

Intermediate bands in type-II silicon clathrate with Cu and Ag guest atoms

“…18 With the existence of IBs, besides the usual transitions between conduction bands and valence bands, additional absorption due to transitions involving the IBs occur, leading to an optimal efficiency of 63% by splitting a total band gap of 1.95 eV into two sub-gaps of 0.71 eV and 1.24 eV between the IB and the valence band minimum (VBM) and the conduction band maximum (CBM), respectively. 19 The IBs have been experimentally realized in quantum dot layout 20 and by impurity doping. 21 The unoccupied states of InAs quantum dots grown in GaAs matrix are located inside the energy gap of GaAs, while oxygen doped ZnTe:O shows strong emission in the middle of ZnTe gap, which is caused by oxygen defects.…”