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Nano-Structured Solar Cells Laboratory of the Ioffe Physical-Technical Institute

This is a new laboratory in the Ioffe Physical-Technical Institute, established with the support of the Mega-Grant № 2012-220-03-121 under the scientific leadership of Academician Professor Antonio Luque as Principal Investigator (PI).

Its objective is to combine the considerable expertise of the Ioffe Institute in nanotechnology and photovoltaics for the research and development of new generation solar cells based on nanostructured materials.

In particular the Mega-Grant project supporting this Lab. aims at the research of Intermediate band solar cells based on quantum dots. This is the main activity at the NSSC Lab.

The intermediate band solar cell was proposed by the PI in 1997 [1]. An intermediate band (IB) of electrons located within the bandgap of a semiconductor shows three sub-bandgaps: the ordinary one between valence (VB) and conduction (CB) bands, the VB-IB sub-bandgap and the IB-CB sub-bandgap. Actually this material could yield efficiency very close to that of the triple junction solar cell and a stack of two IB solar cells might lead to a sextuple junction solar cell with efficiencies over 50% [2]. However the manufacturing of practical IB solar cells is a long term task that today engages thousands of scientists worldwide.

Quantum dots (QDs) of a low bandgap semiconductor in a wider bandgap semiconductor may form the IB with the confined states formed by the QD.

In this Lab research on the following topics are under way:

• The development of fast quantum calculations that allow for device modeling in order to understand the low light absorption obtained in the QDs and try to find methods to overcome this issue (e.g. increase of QD number and provide light confinement). In this respect the Empiric k.p Hamiltonian (EKPH) [3] is applied to the interpretation of several experiments in QD devices to verify its adequacy and also is being compared with the Luttinger-Kohn-Pikus-Bir Hamiltonian commonly used by solid state physicists, more accurate but slower in calculations.

• The development of QD solar cells by metal-organic chemical vapor deposition (MOCVD), faster and therefore more commercially oriented than the molecular beam epitaxy (MBE), masterly used at Ioffe for fundamental and laser research.

• The development of type II QD IB solar cells which, as a theoretical result of the first year of the project, should increase the open circuit voltage without a reduction of the short circuit current [4].

• General research of the behavior of the solar cells under concentrated sunlight: IB solar cells are complex and they will need of concentrator operation for cost effectiveness and also for technical reasons. In this respects advanced techniques used in Ioffe to make very good concentrator triple junction solar cells have been used [5] and research is made in the retrieval of modeling parameters from a bunch of different photovoltaic curves.

[1] A. Luque, and A. Martí, Physical Review Letters 78, 5014 (1997).

[2] A. Luque, Journal of Applied Physics 110, 031301 (2011).

[3] A. Luque et al., Solar Energy Materials & Solar Cells 95, 2095 (2011).

[4] A. Luque et al., Applied Physics Letters 103, 123901 (2013).

[5] N. A. Kalyuzhnyy et al., International Journal of Photoenergy 2014, 836284 (2014).