Muriel Bouttemy

Insights on the influence of surface roughness on photovoltaic properties of state of the art copper indium gallium diselenide thin films solar cells

The influence of Cu(In,Ga)Se2 (CIGSe) surface roughness on the photovoltaic parameters of state of the art devices is reported, highlighting the importance of the roughness of the as-grown CIGSe absorbers on solar cell efficiencies. As-grown CIGSe surface is progressively smoothed using a chemical etch, and characterized by SEM, AFM, XPS, μ-Raman spectroscopy, x-ray diffraction (XRD), and reflectivity. The decrease of roughness has no marked influence on crystal structure and surface composition of the absorber. The main effect is that the total reflectivity of the CIGSe surface increases with decreasing roughness. The samples are processed into solar cells and characterized by current-voltage measurements. While the open circuit voltage (Voc) and fill factor remain constant, the short circuit current (Jsc) decreases markedly with decreasing roughness, resulting in a reduction of the solar cell efficiency from 14% down to 11%, which exceeds the expected decrease from increased reflectivity. Quantum effici...

Deposition of ultra thin CuInS2 absorber layers by ALD for thin film solar cells at low temperature (down to 150 ?C)

Two new processes for the atomic layer deposition of copper indium sulfide (CuInS₂) based on the use of two different sets of precursors are reported. Metal chloride precursors (CuCl, InCl₃) in combination with H2S imply relatively high deposition temperature (Tdep = 380 °C), and due to exchange reactions, CuInS₂ stoechiometry was only achieved by depositing In₂S3 layers on a CuxS film. However, the use of acac- metal precursors (Cu(acac)₂, In(acac)₃) allows the direct deposition of CuInS₂ at temperature as low as 150 °C, involving in situ copper-reduction, exchange reaction and diffusion processes. The morphology, crystallographic structure, chemical composition and optical band gap of thin films were investigated using scanning electronic microscope, x-ray diffraction under grazing incidence conditions, x-ray fluorescence, energy dispersive spectrometry, secondary ion mass spectrometry, x-ray photoelectron spectroscopy and UV-vis spectroscopy. Films were implemented as ultra-thin absorbers in a typical CIS-solar cell architecture and allowed conversion efficiencies up to 2.8%.

Towards a better understanding of the use of additives in Zn(S,O,OH) deposition bath for high efficiency Cu(In,Ga)Se2-based solar cells

CBD-Zn(S,O,OH) remains one the most studied and promising Cd-free buffer layer for Cu(In,Ga)Se2-based solar cells, and has already demonstrated its potential to lead to high-efficiency solar cells. However Zn(S,O,OH) deposition time and metastable behavior of the final devices remain critical to outperform CdS-based devices. The aim of this work is to study and understand the influence of additives such as H2O2 on the deposition bath and on the surface of the absorber. These results will be related with final performances of the devices. A new promising additive, persulfate S2O82-, will be presented and could be the key to go beyond CdS-based solar cell records.

Cross strategy of surface and volume characterizations of chalcogenides thin films: Practical case of CIGS absorbers

Photovoltaic cells based on chalcogenides CIGS (Cu(In, Ga)Se2) thin films are a very promising technology. To improve cells performances, a fine optimization of the CIGS absorber properties is needed. Hence, we developed a cross strategy method combining the surface, volume and specific interfaces of the final device characterizations. These features deal with a large panel of physico-chemical techniques for the chemical composition (XPS, EDS, ICP-OES, GD-OES, AES), the morphology (SEM, AFM) and the optical parameters (spectroscopic ellipsometry) determination. This article demonstrates the crucial interest of this cross strategy on CIGS absorbers and focus on the accuracy and complementarities of each technique.

Impact of the deposition conditions of buffer and windows layers on lowering the metastability effects in Cu(In,Ga)Se2/Zn(S,O) based solar cell

The highest and most reproducible (Cu(In,Ga)Se2 (CIGSe) based solar-cell efficiencies are obtained by use of a very thin n-type CdS layer deposited by chemical bath deposition (CBD). However because of both Cadmium’s adverse environmental impact and the narrow bandgap of CdS (2.4–2.5 eV) one of the major objectives in the field of CIGSe technology remains the development and implementation in the production line of Cd-free buffer layers. The CBDZn( S,O) remains one the most studied buffer layer for replacing the CdS in Cu(In,Ga)Se2-based solar cells and has already demonstrated its potential to lead to high-efficiency solar cells up to 22.3%. However one of the key issue to implement a CBD-Zn(S,O) process in a CIGSe production line is the cells stability, which depends both on the deposition conditions of CBD-Zn(S,O) and on a good band alignment between CIGSe/Zn(S,O)/windows layers. The most common window layers applied in CIGSe solar cells consist of two layers : a thin (50–100 nm) and highly resistive i-ZnO layer deposited by magnetron sputtering and a transparent conducting 300–500 nm ZnO:Al layer. In the case of CBD-Zn(S,O) buffer layer, the nature and deposition conditions of both Zn(S,O) and the undoped window layer can strongly influence the performance and stability of cells. The present contribution will be specially focused on the effect of condition growth of CBD-Zn(S,O) buffer layers and the impact of the composition and deposition conditions of the undoped window layers such as ZnxMgyO or ZnxSnyO on the stability and performance of these solar cells.

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