An IB solar cell is formed of an IB material situated between two ordinary semiconductors —n- and p-type respectively— that play the role of selective contacts to conduction band (CB) and valence band (VB) electrons respectively. The IB material has a band of states inside the band gap between the CB and the VB. In this way photons with less energy than the one necessary to pump an electron form the VB to the CB can be absorbed by transitions that pump an electron form the VB to the IB and form the IB to the CB. Thus a full VB->CB electron transition (or electron-hole pair generation) can be completed by means of two photons of energy below the band gap. This mechanism should increase the solar cell current.However, any increase of cell current is usually accompanied by a reduction of the voltage. To avoid this, it is necessary that three separate quasi-Fermi levels appear in the IB material, two of them associated to the VB and to the CB, as in ordinary solar cells, and the third one associated to the IB. The voltage extracted from the cell is precisely the difference of the CB and VB quasi-Fermi levels at the n- and p-contacts respectively (changed of sign and divided by the charge of the electron). Nevertheless, photons of lower energy than this voltage can contribute to the current thanks to the IB.Limit efficiency of this concept for maximum concentration (the one providing isotropic illumination on the cell with the radiance of the sun’s photosphere) is 63.2% to compare with the Shockley-Queisser limit of 40.7% for an ordinary cell in the same conditions.IB GaAs solar cells have been fabricated based on this concept using InAs quantum dots (QD) to form the IB. A small increase of the short circuit current has been measured by using two sources of photons of different energy and evidence of the electron-hole formation through the described two-photon mechanism has been produced. In addition, a separation of the quasi-Fermi levels has been experimentally found for cells forward biased. However the efficiency is not higher than the one of the cells without quantum dots mainly because of the low current enhancement due to the weak absorption in the QD, due to their inherent low density and low IB material thickness. The solution proposed for this is to use strain-balanced growth permitting the growth of more quantum dot layers and the improvement of the light on the quantum dots by using diffraction methods.While QD IB cells have revealed to be very useful for proving the basic principles, practical cells seem more promising based on alloys, with higher IB atoms density. Deep levels situated in the mid of the gap are know to be introduced by certain impurities and to act as effective centres of SRH recombination. But when their density exceeds the Mott transition the electrons in the level become delocalised, the deep level becomes an impurity band and the SRH recombination is thought to disappear.

Wei and Forrest have proposed using thin, high band gap, energy fence barrier layers to improve the efficiency of quantum dot intermediate band solar cells by reducing charge trapping and subsequent recombination in the quantum dots. Their calculations indicated GaAs-based quantum dot intermediate band cells, employing such energy fence barrier layers, could achieve power conversion efficiencies as high as 45%. To the authors’ knowledge, this proposal has not so far been experimentally tested. We report here results obtained on the epitaxial growth and characterization of InGaAs/GaAs quantum dot intermediate band cells containing thin GaAsP energy fence barrier layers. The quantum dot pin cell samples, containing 10 layers of undoped InGaAs quantum dots (6.1 ML, In concentration ~ 0.47) embedded between thin undoped GaAsP energy fence barrier layers and separated by ~ 40 nm undoped GaAs barrier layers, were grown by metal organic chemical vapor deposition on p+ (100) and (311)B GaAs substrates. The structural and optical properties of these quantum dot samples have been characterized by atomic force microscopy, transmission electron microscopy, high resolution scanning transmission electron microscopy, and photoluminescence. Processed cells were characterized by current-voltage and quantum efficiency measurements. The results obtained from these structures indicate that 4 nm thickness GaAsP energy fence barrier layers are adequate to completely bury the InGaAs quantum dots. Initial cell measurements, however, indicate slightly lower efficiencies for the quantum dot cells containing the thin GaAsP energy fence barrier layers in comparison to InGaAs/GaAs quantum dot cells of nearly identical structure except the GaAsP energy fence barrier layers were not included.

III-V dilute nitrides have attracted much interest over the past few years due to their captivating properties, which include a substantial reduction of the band gap and an increase of the electron effective mass upon incorporation of small amounts of N. These properties make this family of materials very attractive for a wide range of optoelectronic applications, including high-efficiency solar cells. The use of ~1.0 to 1.25 eV GaInNAs subcells, lattice matched to GaAs and Ge, has been suggested to enhance the efficiency of existing triple and quadruple junction solar cells. However unfavorable effects resulting from the addition of N in III-V semiconductors, such as poor minority carrier properties have thus far limited the success of this approach.(1)The use of III-V dilute nitride multi quantum wells (MQWs)inserted in the i-region of conventional III-V solar cells is a promising method to increase their efficiency, while alleviating many of the problems associated with bulk III-V based solar cells. Indeed, the insertion of multi quantum wells within the intrinsic region of conventional GaAs p-i-n solar cells has been predicted to yield practical efficiencies exceeding 35% at AM0. 2 III-V-N MQWs should extend the photon absorption range while alleviating minority carrier issues encountered in thick bulk-like GaInNAs. An added advantage of the use of III-V dilute nitride quantum wells is the stronger absorption coefficient, a result of the increase of the electron effective mass. In this work we discuss the development of 1.1 eV GaAsN/GaAs MQW solar cells grown by Chemical Beam Epitaxy (CBE). Device structures were fabricated on n-type GaAs (001) substrates by radio frequency (rf) nitrogen plasma-assisted CBE. 10 to 20 period MQWs of not intentionally doped strained GaAs1-xNx/GaAs quantum wells were inserted in the i-region of conventional GaAs solar cell structure at relatively low temperatures (440C