Emerging/Novel concepts for high efficiency at low cost
In 1961, Shockley and Queisser (S&Q)  established a fundamental limit for the efficiency of single gap solar cells. Some of the premises for obtaining this limit were, for example: a) the cells operate in the radiative limit; b) one photon can create only one electron-hole pair; c) no photons could be absorbed below the bandgap. By lifting up some of these premises it is possible to device solar cells for which this limit can be exceeded. Although the phrasing “novel concepts” could be applied to almost any concept that is recent, in this section we restrict ourselves to those “novel concepts” that have been specifically proposed having in mind exceeding S&Q limit. In particular we refer to the intermediate band solar cell (IBSC), to the hot carrier solar cell (HCSC) and to the multiple exciton generation solar cell (MEGSC).
Intermediate band solar cell (IBSC). The IBSC aims to exceed S&Q by absorbing below bandgap energy photons . This is achieved by engineering a semiconductor-like material that, in addition to the conventional conduction band and valence bands, exhibits an intermediate band (IB) in-between both. It is this IB the one that makes the absorption of below bandgap energy photons possible. Many approaches have been proposed for engineering this IB. We list next some of the strategies being followed to identify these materials and implement solar cells with them:
- Quantum dots ,
- Insertion of impurities at high densities ,
- Highly mismatched alloys ,
- First principle calculations ,
- Dye sensitized + TT annihilation 
The absorption of below bandgap energy photons in IBSC has been experimentally demonstrated  as well as the fact that the presence of this IB does not prevent the achievement of high output voltage in the cell . A review of the experimental achievements related to IBSC research can be found in .
Hot carrier solar cell (HCSC). The HCSC pursues the extraction of photogenerated carriers before they thermalize . The cell demands the so-called energy selective contacts in order to isoentropically cool down the carriers to room temperature . It has also been postulated that electrons could be allowed to interact with optical phonons as long as a sufficiently large phononic bandgap exists between optical and acoustic phonons . Resonant tunneling configurations have been suggested as the means for implementing the energy selective contacts . Conibeer et al.  and Yao and Koning  have recently screened several absorber material candidates.
Multiple exciton generation solar cell (MEGSC). The MEGSC relies its potential in the possibility for a high energy photon generating more than one electron hole pair. Conceptually it has its roots in the “impact ionization solar cell”, a study that was motivated after observing quantum efficiencies larger than one in silicon solar cells . Nozik  suggested the use of quantum dots (QDs) as the means to avoid some physical restrictions when implemented in bulk semiconductors, such as the conservation of the crystal momentum, leading to the MEGSC concept. External quantum efficiencies exceeding unity have been experimentally demonstrated in solar cells implemented with QDs .
The interested reader can find in  a review of the operation of the three concepts described above from the perspective of the electro-chemical potentials (quasi-Fermi levels) of electrons, photons and phonons.
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