So far, none of the existing semiconductors have proven to be endowed with the singular features required for a proper operation of an IBSC. This implies that a new generation of IB materials has to be engineered. For this reason an extensive research has recently begun on the identification of appropriate IB candidate materials. So far, most part of the research has been focused on two different families: the quantum dot (QD) IB approach and the bulk approach, which will be reviewed in the following sections. Nevertheless, other interesting configurations have been proposed, as for example the molecular approach [Ekins-Daukes and Schmidt, 2008] which are out of the scope of this work.
At the beginning of the IB research, the detailed balance model [Shockley and Queisser, 1961] was used to calculate the optimum bandgap distribution of an IBSC. Under maxi-
mum concentration (46050 suns at the surface of the Earth) and the irradiance distribution of a black-body at approximately 6000 K, the following values attain the theoretical 63.2% efficiency: EH=1.24 eV, EL=0.71 eV and EG=1.95 eV [Luque and Mart´ı, 1997b].
During the first years after the concept was released, more IB features were mod- eled, either to analyze the suitability of the different IB materials or the impact of non- idealizations regarding the practical implementation of the IBSC. In an initial stage the operating principles of the IBSC have to be tested, which requires the implementation of dedicated experimental set-ups in order to solidly assure the verification of such evidences, i.e. the production of sub-bandgap photocurrent and the preservation of the voltage.
1.3.1 State of the art of IB materials
Up to now, most practical attempts to manufacture an IBSC have been based on the in- troduction of nanostructured stacked layers into a III-V host material. In practice, these nanostructures have to be 0-dimensional in order to provide a true zero DOS between the IB and the CB and VB, ultimately allowing the existence and split of the corresponding QFL. Although the InAs/GaAs QD system does not satisfy the characteristics for opti- mal IBSC performance in terms of room temperature bandgaps, it is a relatively mature technology. On the other hand it is based on GaAs, which is an excellent semiconduc- tor in terms of optical and electronic characteristics. These are two of the main reasons why this QD system has been selected as a prototype material to verify the fundamental principles of operation of the IBSC. Furthermore, MBE and metal organic chemical vapor deposition (MOCVD) technologies have proven to produce excellent crystalline results for this system [Bimberg et al., 1999]. The QD approach for the implementation of the IBSC will be extensively reviewed in section 5.
Besides QDs, the other large group of IB materials is the one based on bulk semicon- ductors. Several research lines have been proposed in this direction, which are, in most cases, based on the introduction of suitable impurity atoms. Different material systems containing impurities of the appropriate nature, located in the adequate locations in a pe- riodic crystal have been theoretically investigated [Luque and Mart´ı, 2010b, Luque et al., 2012b]. As we will review, a common condition for any of these impurities to form an IB is to be present in a sufficiently high concentration.
Deep-level (DL) impurities, i.e. atomic species that introduce an energy level inside the host material bandgap far from the band edges, traditionally referred to as traps or recombination centers, are considered “lifetime killers” in conventional semiconductors. Therefore, their use in high efficiency solar cells requires further justification. The answer to why impurities at high concentration inside the crystal might not increase the Shockley-
Read-Hall (SRH) recombination [Luque et al., 2006a] is related to the so-called lattice relaxation multiple phonon emission [Lang and Henry, 1975]. The latter is believed to be the main responsible mechanism for SRH recombination, since the simultaneous emission of several phonons is considered highly improbable in bulk semiconductors. The electron is de-excited from the CB to a DL trap, which implies a transition from a delocalized state to a localized one, causing an important swift movement of the charge, because it was initially distributed throughout many atoms and it is afterwards highly packed next to the impurity atom. Then, the atom vibrates violently (in real space) in a mode other than the usual lattice phonons, the so-called breathing mode. This violent vibration is then attenuated through the successive emission of phonons produced by conventional electron- phonon interaction. Therefore, if the impurities giving rise to the IB are present within the bulk material in a sufficiently high concentration as to produce the overlapping of their wavefunctions, the recombination through these intermediate states takes place between delocalized states, which can “share” or dilute the charge increase along the lattice. This distributed charge transfer process between states can inhibit the production of breathing modes as well as the corresponding lattice relaxation by multiple phonon emission. As a result, the SRH recombination is also inhibited.
The idea of using DLs to study the possibility of sub-bandgap photon absorption comes from the 60’s. Wolf introduced the concept of multitransition solar cell in 1960 with the purpose of improving the solar cell performance [Wolf, 1960] and a year later, based on Wolf proposals, Grimmeiss manufactured and tested a DL impurity-based solar cell with negative results [Grimmeiss and Koelmans, 1961]. This is not surprising because both the theoretical proposal and the experimental implementation paid no attention, for example, to the need for selective contacts, i.e. they did not identified the need for non-DL-doped portions of semiconductor separating the IB material from the metal contact. Also, in the proposals by Wolf, there was no explicit mention to the need for a high enough impurity concentration as to enable the formation of a band with the aforementioned NRR blockade. Thanks to the IBSC theory, we now know that because of the previous considerations were not taken into account, the voltage of the DL solar cell proposed by Wolf and implemented by Grimmeiss was fundamentally limited. The efficiency ceiling of such configuration was, in the best case, the one of a low bandgap conventional solar cell.
DLs introduced by impurities of any kind can be considered as an option for bulk IB materials if they are located at an appropriate energy and they can be half filled with electrons at room temperature. Actually, even DLs associated to a vacant or an interstitial atom might also lead to adequate configurations.
The first stage after the IBSC concept was first presented in 1997 dealt with theoretical analysis of possible IB candidate materials. In this respect, some bulk materials doped with transition metal impurities were proposed, as for example, Ti-substituted GaAs or GaP [Wahn´on and Tablero, 2002], as well as other substituting impurities such as Sc, V or Cr. An experimental research line has been proposed using an MBE reactor for the insertion of a high impurity concentration of Ti inside a GaAs matrix [Linares et al., 2013, Mart´ı et al., 2009].
Another bulk-based IB candidate material is a well-known semiconductor for PV ap- plications: silicon. In spite of its low limiting efficiency (according to the IBSC theory) because of its reduced bandgap, it can be used to test some of the principles of the IBSC theory that are related to the insertion of impurities. In this respect, high doses of Ti are implanted in the Si matrix by means of ion-implantation techniques combined with pulsed laser melting in order to recover the crystallinity of the implanted sample ( [Olea et al., 2008, Gonz´alez-D´ıaz et al., 2009]). Some important conclusions, such as the verification of the lifetime recovery in highly Ti-doped silicon samples have been extracted from this work [Antol´ın et al., 2009].
IB solar cells based on thin-film-based materials, such as CuInS2, CuGaSe2and CuGaS2,
which can also be doped with transition metals, such as Ti, V, Cr or Mn [Mart´ı et al., 2008c, Palacios et al., 2007, Tablero and Fuertes Marr´on, 2010] have also been proposed. This research line seems to be very encouraging since chalcopyrite structures can constitute an easy and versatile way to produce IB materials, i.e. the insertion of certain transition metal species seems realizable (in a sufficient concentration) from a thermodynamic point of view. An experimental line has also begun in this topic [Mart´ı et al., 2009].
Another research line also based on the thin-film technology is being developed at the University of Michigan for the ZnTe material doped with oxygen [Weiming et al., 2009]. So far, it has rendered promising results, including the measurement of the absorption of two below bandgap energy photons.
In-thiospinels such as In2S3 including a high vanadium concentration have also been
proposed. This last work allowed the recent implementation of the IB material by solvo- thermal synthesis, in which sub-bandgap absorption was measured [Lucena et al., 2008].
Recent works on bulk IB materials are based on the so-called highly mismatched mate- rials [Yu et al., 2006], e.g. the GaNAsP alloy, which has also shown IB properties through photo-reflectance measurements. This kind of nitrogen dilute alloys is believed to expe- rience the splitting of the CB because of the interaction between the nitrogen level and the CB of the non-nitrogen-doped host matrix (in this case, the GaAsP). In some cases, a real bandgap is created between the two split CBs as a result of this interaction and the
lower split CB is then regarded as the IB. The mechanism leading to this effect is known as band anticrossing [Shan et al., 1999].
InGaN-based alloys have also been presented as feasible IB material candidates [Mart´ı et al., 2008c], since they present exceptionally high solubility ranges for some metal tran- sition impurities (e.g. Mn) while the ternary alloy also covers a wide range of energy bandgaps.