Sana Laribi

Ab initio calculation of intrinsic point defects in CuInSe2

Abstract Preliminary ab initio calculation of different point defects energy and electronic density of states have been performed on the prototype chalcopyrite semiconductor CuInSe 2 . The simulation method used is based on the density functional theory within the framework of pseudo-potentials and plane waves basis. The isolated neutral defects considered are: V Cu , V In , V Se , Cu i , In i , In Cu , Cu In and the complex defects are 2Cu i +Cu In , In Cu +Cu In and 2V Cu +In Cu , some of which being computed for the first time by advanced ab initio techniques. In agreement with previous results, we show that some point defects (such as V Cu ) and pair defects (2V Cu +In Cu ) have very low formation energies. Some energies of formation were found significantly lower than previous estimations. The comparison of the formation energies with the exchange correlation (LDA or GGA) is discussed. The perturbation induced by the presence of some of these ideal defects on the density of states is also presented.

Hot carrier solar cells: Controlling thermalization in ultra thin devices

From present day 40% conversion efficiencies to the thermodynamic limit (> 85%), there is still a lot of room for improvement. Hot carrier solar cells provide an attractive solution to fill this gap, by converting more efficiently the part of the incident power that is usually lost as heat. In those devices, the photogenerated carriers are not thermally equilibrated with the lattice. This occurs if the carrier thermalization pathways are saturated, either by reducing the electron-lattice interaction, or by increasing the carrier density. Antimony-based quantum well structures with 50 nm thick active material were synthesized for investigating their thermalization properties. The carrier temperature is determined as a function of the incident power density. Results indicate potential efficiency above 50% provided the incident power can be absorbed in a 50 nm thick absorber. Without specific care, reducing the absorber thickness would result in a reduced absorption and limited efficiency. Here, we propose a nanoscale structuration of the cell surface that enables strong absorption enhancement. 70 to 80 % of the incident power can be absorbed in a 50 nm thick GaSb layer. Going for high carrier density enables to lower the requirement on the cooling rate reduction. We show that using the structure described here and the thermalization rate measured on test samples, the potential efficiency is above 50%.

Monolithic Integration of Diluted-Nitride III–V-N Compounds on Silicon Substrates: Toward the III–V/Si Concentrated Photovoltaics

Abstract GaAsPN semiconductors are promising material for the development of high-efficiency tandem solar cells on silicon substrates. GaAsPN diluted-nitride alloy is studied as the top-junction material due to its perfect lattice matching with the Si substrate and its ideal bandgap energy allowing a perfect current matching with the Si bottom cell. The GaP/Si interface is also studied in order to obtain defect-free GaP/Si pseudo-substrates suitable for the subsequent GaAsPN top junctions growth. Result shows that a double-step growth procedure suppresses most of the microtwins and a bi-stepped Si buffer can be grown, suitable to reduce the anti-phase domains density. We also review our recent progress in materials development of the GaAsPN alloy and our recent studies of all the different building blocks toward the development of a PIN solar cell. GaAsPN alloy with energy bandgap around 1.8 eV, lattice matched with the Si substrate, has been achieved. This alloy displays efficient photoluminescence at room temperature and good light absorption. An early-stage GaAsPN PIN solar cell prototype has been grown on a GaP(001) substrate. The external quantum efficiency and the I–V curve show that carriers have been extracted from the GaAsPN alloy absorber, with an open-circuit voltage above 1 eV, however a low short-circuit current density obtained suggests that GaAsPN structural properties need further optimization. Considering all the pathways for improvement, the 2.25% efficiency and IQE around 35% obtained under AM1.5G is however promising, therefore validating our approach for obtaining a lattice-matched dual-junction solar cell on silicon substrate.

Phonon lifetime in SiSn and its suitability for hot-carrier solar cells

We present a phononic and electronic study of SiSn in the zinc-blende phase. A detailed description of the longitudinal optical (LO) phonon decay in a three-phonon process is presented together with the corresponding lifetime. The necessity to go beyond the zone center phonon approximation in this case is highlighted as it reveals a steep dependence of the lifetime on the initial phonon wavenumber, which differs from usual semiconductors. The electronic band structure is calculated within the GW formalism and shows a small direct band gap. It is shown that the LO-phonon resulting from electron cooling has a lifetime four to eight orders of magnitude above all the known value in semiconductors for this process. We finally show the suitability of SiSn for hot-carrier solar cells, as it is endowed with ultra-slow cooling of hot carriers.

Theoretical study of optical properties of anti phase domains in GaP

III-V/Si heterostructures are currently investigated for silicon photonics and solar energy conversion. In particular, dilute nitride alloy GaAsPN grown on a GaP/Si platform exhibits lattice match with Si and an optimal band gap configuration for tandem solar cell devices. However, monolithic “coherent” growth of the GaP thin layer on Si suffers from the nucleation of extended structural defects, which can hamper device operation as well as the GaP/Si interface level and through their propagation inside the overall heterostructure. However, the effect of such structural defects on optical and transport properties is actually not well understood in details. In this letter, we investigate the anti phase domains defect (also called inversion domains) by means of ab initio calculations giving insights into the alteration of optical and transport properties of GaP due to the defective GaP/Si interface.

Optical Phonon Decay In Cubic Semiconductors: A Hot Carrier Solar Cell Picture

In the framework of hot-carrier solar cell absorber material design, we revisit the LO-phonon decay processes in a wide variety of III-V and group IV binary semiconductors. We present a detailed description of the two-phonon final states, from the exact dispersion relation calculated within the Density Functional Perturbation Theory formalism. We focus on the relation between Klemens surfaces features and atomic mass differences, and the importance of the Ridley channels in some group IV binaries.

Hot-carrier solar cells

As the importance of renewable energy sources grows, the development of highly-efficient solar cells is increasingly gaining relevance. Today, the most efficient laboratory prototypes for solar cells are based on thin film multi-layered structures. These cells have 40% solar-to-electricity conversion efficiencies, which are well below the theoretical limit of 87% and leave considerable room for improvement. However, the design of multilayered solar cells can be complicated, and their workings might fail to tolerate changes to their operational conditions, such as the cell temperature or the power of the incident sunlight. Hotcarrier solar cells (HCSC), with their simplicity of design and ability to approach limiting conversion efficiencies, provide an attractive alternative to the multi-layer approach.1 Heat dissipation occurs when a material absorbs photons with energies larger than its bandgap. To circumvent this problem, the photo-generated charge carriers have to be collected through specially designed contacts that are energy-selective. In this way, heat production can be minimized: carriers with large kinetic energies—‘hot-carriers’—reach these contacts before losing most of their energy as heat. In principle, efficiencies as high as 86% could be achieved.1 However, since hot-carriers normally transfer their kinetic energy to the material in sub-picosecond times, the collection through contacts should be fast. This could be achieved at high-injection conditions, under which the interaction between the absorbent material and the hot-carriers becomes inefficient.2 As for any solar cell design, conversion efficiency is expected to grow with the concentration of incoming light, as this increases the output voltage of the solar cell by increasing the extracted work per absorbed photon. However, an optimal coupling between the incident radiation and the solar cells will lead to the high-injection regime, where drops in the cell’s output Figure 1. Energy band diagram of a hot-carrier solar cell with bandgap Eg and voltage qV. Electron-hole pairs are photo-generated in the absorber and kept hot at a temperature of TH (TH > TC , where TC is the ambient temperature). They are subsequently extracted using energyselective contacts with a transmission range iE and an extraction energy Eext. The Fermi levels in the electrodes are n and p , and the electron and hole chemical potentials in the absorber are e and h. The difference e h D H is known as the quasi-Fermi level splitting.