Zakaria Djebbour

Distributed Bragg reflectors based on diluted boron-based BAlN alloys for deep ultraviolet optoelectronic applications

Highly reflective deep UV distributed Bragg reflectors (DBRs) based on the BAlN material system have been grown by metalorganic vapour phase epitaxy on AlN template substrates. These structures make use of the transparency of BAlN in the deep UV and the high refractive index contrast between BAlN and AlN, which has been demonstrated to exceed 0.27 at 280 nm. 18-pair BAlN/AlN DBRs showed experimental peak reflectivity of 82% at 311 nm and a stop-bandwidth of 20 nm. At 282 nm, a 24-pair BAlN/AlN DBR structure is demonstrated with experimental peak reflectivity of 60% and stop-bandwidth of 16 nm.

Studies of buried interfaces Cu(In,Ga)Se2/CdS XPS and electrical investigations

The formation of interface Cu(In,Ga)Se2/CdS in solar cells is not yet well understood but seems to be the key to further improvements of their performance. This interface depends on many parameters such as the initial chemical state of the CIGS surface or the chemical bath deposition conditions used to grow the CdS layer. In this paper, we focus our attention on the CIGS/CdS hetero-interface at different stages of its formation using mainly XPS studies of buried interfaces which were studied after gradual sputtering. We have investigated interfaces on CIGS submitted to various surface treatments, analogue to those involved in fabrication steps (NH3 dipping…). Kelvin probe and admittance spectroscopy measurements have been also performed on several interfaces prepared in the same conditions to correlate the chemical composition with the electrical response of the buried interfaces.

XPS and electrical studies of buried interfaces in Cu(In,Ga)Se2 solar cells

The active interface of Cu(In,Ga)Se2 solar cells is the key to further improvements of their performance. The formation of this interface is not yet well understood. It depends on the initial state of the CIGS layer, then on the evolution of the interfacial chemistry during the CdS deposition. We present a contribution to its understanding using XPS studies at different steps of the interface formation. Attention has been brought to the surface preparation and to the buried interface CIGS/CdS. Buried interfaces were studied after gradual sputtering. Well-resolved spectra have been obtained. We aimed at a clarification of the role of the various segregated/intermixed phases at the interface. To achieve this, admittance spectroscopy and Kelvin probe measurements have been performed on the same devices to correlate the chemical composition to the electrical responses associated with the buried interface.

Link between crystal quality and electrical properties of metalorganic vapour phase epitaxy InxGa1−xN thin films

We report on the crystal quality of metalorganic vapour phase epitaxy-grown InGaN with indium content ranging from 0% to 20%. Absorbance measurements are fit to a model including band tails and a defect represented as a Brendel oscillator (R. Brendel, Appl. Phys. A 50, 587, 1990). Band tail absorbance, corresponding to contorted bonds, increases with increased In content. Above 10% of In, the presence of another defect, the concentration of which increases with In content, has been correlated with x-ray diffraction and Raman. We suggest that this defect corresponds to nitrogen vacancies, in agreement with a reported model for GaN.

Subgap modulated photocurrent spectroscopy and its application to the study of the solar cell absorber defect distributions

In this paper, a theoretical background of subgap modulated photocurrent experiment is presented. It allows the investigation of the density of states DOS distribution, directly from the active region of a semiconductor heterojunction device. The junction is illuminated with a modulated subgap light excitation i.e., light with photon energy lower than the band gap of the active layer. Under specific considerations for the applied reverse bias voltage and the bias-light level, a simple theoretical relation of the imaginary part of the photocurrent versus the modulation angular frequency allows the determination of the energy profile of the gap states. This technique has been successfully applied to a Ga free CuIn,GaSe2 based solar cell to investigate the DOS distribution in the band gap of the absorber. Two distinct defect distributions have been exhibited in the absorber layer of the studied solar cell.

Ultrathin Cu(In, Ga)Se 2 solar cells

Within the UltraCIS project we have started to explore the possibility of reducing down the thickness of the Cu(In, Ga)Se2 (CIGSe) layer to the sub-micron level (0.1μm) while maintaining a high efficiency level of solar cells. The three main approaches we used are: — Reducing the CIGSe thickness by chemical etching combining the thickness reduction and smoothing effect of the absorber. Efficiency higher than 10 % on small area cells with an absorber thickness of 0.5 μm are obtained. Losses were attributed exclusively to the reduced photocurrent and the loss on texturation of the absorber — Optical management by front contact texturation or by replacement of the back contact by the “lift-off” of CIGSe layer from the Mo layer and deposition of a new reflective back contact. — Application of plasmonic structures to CIGSe solar cells enabling light confinement at the subwavelength scale