Negar Naghavi

CD-free Cu(In,Ga)Se2 thin-film solar modules with In2S3 buffer layer by ALCVD

The atomic layer chemical vapour deposition (ALCVD) technique allows the deposition of highly homogeneous thin-films with an excellent step coverage. This method has already shown promising results for the deposition of cadmium-free buffer layers in Cu(In,Ga)Se2 (CIGS) thin-film solar cells (13.5% efficiency with indium sulphide buffer). In this work, the process has been up-scaled to module areas of up to 30×30 cm2. The indium sulphide buffer layer was deposited at substrate temperatures between 160 and 220 °C using indium acetylacetonate and hydrogen sulphide precursors. An efficiency of η=10.8% (open-circuit voltage, VOC=592 mV; fill factor, FF=62%; current density, jSC=29.5 mA/cm2) for a module area of 30×30 cm2 has been achieved. For laboratory cells even an efficiency of 14.9% was realised. Damp heat stability testing of CIGS mini-modules indicates a similar behaviour of both devices with ALCVD indium sulphide and solution grown cadmium-sulphide buffer layer.

Electrodeposition of ZnO nanorod arrays on ZnO substrate with tunable orientation and optical properties

The electrodeposition of ZnO nanorods on ZnO:Al films with different orientations is reported. The influence of the total charge exchanged during electrodeposition on the nanorods geometry (length, diameter, aspect ratio and surface density) and the optical transmission properties of the nanorod arrays is studied on a [0001]-oriented ZnO:Al substrate. The nanorods are highly vertically oriented along the c axis, following the lattice matching with the substrate. The growth on a [1010] and [1120] ZnO:Al-oriented substrate with c axis parallel to the substrate leads to a systematic deviation angle of 55 degrees from the perpendicular direction. This finding has been explained by the occurrence of a minority orientation with the [1011] planes parallel to the surface, with a preferential growth on corresponding [0001] termination. Substrate crystalline orientation is thereby found to be a major parameter in finely tuning the orientation of the nanorod array. This new approach allows us to optimize the light scattering properties of the films.

Towards Better Understanding of High Efficiency Cd-free CIGS Solar Cells Using Atomic Layer Deposited Indium Sulfide Buffer Layers

ABSTRACT: This paper presents optimization studies on the formation of cadmium free buffer layers for high efficiency copper indium diselenide (CIGS) thin film solar cells using a vapor phase route. Indium sulfide layers have been deposited on CIGS substrates by Atomic Layer Deposition (ALD) at substrate temperatures between 140 and 260 °C using indium acetylacetonate and hydrogen sulfide precursors. The parametric study of the deposition temperature shows an optimal value at about 220°C, leading to an efficiency of 16.4 % which is a technological breakthrough. The analysis of the device shows that indium sulfide layers give an improvement of the blue response of the cells as compared a standard CdS processed cell, due to a high apparent band gap (2.7-2.8 eV), higher open circuit voltages (up to 665 mV) and fill factor (78 %) . This denotes high interface quality of the system. Atomic diffusion processes of sodium and copper in the buffer layer are evidenced. INTRODUCTION In high efficiency thin film solar cells based on Cu(In,Ga)Se

Broadband light-trapping in ultra-thin nano-structured solar cells

Conventional light trapping techniques are inefficient at the sub-wavelength scale. This is the main limitation for the thickness reduction of thin-film solar cells below 500nm. We propose a novel architecture for broadband light absorption in ultra-thin active layers based on plasmonic nano-cavities and multi-resonant mechanism. Strong light enhancement will be shown numerically for photovoltaic materials such as CIGSe and GaAs. First experiments on ultrathin nano-patterned CIGSe solar cells will be presented.

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.

A better understanding of reaction pathways of the formation of CZTSSe absorbers using co-sputtered Cu-Zn-Sn-S precursors

The aim of this work is to have a better understanding of the effect of annealing on the crystallization behavior and the formation of secondary phases in Cu2ZnSn(Sx,Se1-x)4 (CZTSSe) absorbers. Cu-Zn-Sn-S precursors were deposited on glass/Mo substrate by co-sputtering of Cu, ZnS, and SnS and then annealed under Se atmosphere. In order to have a better understanding of the mechanisms of formation of CZTSSe and secondary phases during annealing, the effect of the variation of the annealing temperature between 250 and 600°C on structural and compositional properties of the materials has been studied. For that, a detailed material characterization of the bulk and the front and back surface of the CZTSSe absorbers at different temperature, combining x-ray diffraction, Raman, and Glow Discharge Optical Spectroscopy (GD-OES) is performed. These studies revealed that during the selenization, the formation of CZTSSe occurs via two simultaneous and separated reaction paths. A Se-rich CZTSSe is formed for which the reaction rate depends on the Se diffusion through the films, while a S-rich CZTSSe rapidly forms at the CZTSSe/Mo interfaces. When increasing the temperature this leads to the formation of SnSe2 and Cu2Se secondary phases at the surface of the absorbers while ZnS secondary phases are formed in the bulk of the layers.

Search for new bath formulations Of Zn(S, O, OH) buffer layer to outperform record performances Of CdS-based CIGSe solar cells

Zn(S, O, OH) represents the most studied Cd-free material for replacing chemical bath deposited (CBD)-CdS buffer layers in Cu(In, Ga)Se2-based solar cells. However, the record performances remain lower than the CdS. The aim of this work is to study new bath compositions for CBD-Zn(S, O, OH), by introducing new complexing and non-complexing reactants for higher efficiencies, and to analyze their effects on growth mechanisms. Promising bath compositions based on the combined used of H2O2 and tri-sodium citrate have been developed, and could be a new avenue to outperform the CdS-based solar cells.

Lambertian back reflector in Cu(InGa)Se2 solar cell: optical modeling and characterization

In the past years, reducing the thickness of the absorber layer in CIGS-based solar cells has become a key issue to reduce the global Indium consumption and thus increased its competitiveness. As the absorber thickness is reduced, less photons are absorbed and consequently the efficiency decreases. It is well known that scattering light in the absorbing layer increases the effective optical length, which results in enhanced absorption. In this study, we have deposited a transparent conductive oxide as a back contact to the cell with a white paint on the rear surface to diffuse the light back to the cell. A proof of concept device is realized and optically characterized. Modeling scattering by rough surfaces can be done by brute force numerical simulations but does not provide a physical insight in the absorption mechanisms. In our approach, we regard the collimated solar light and its specular reection/transmission as coherent. On an irregular surface, part of the collimated light is scattered in other directions. To model this diffuse light, we adopt the formalism of the radiative transfer equation, which is an energy transport equation. Thus, interference effects are accounted for only in the coherent part. A special attention is dedicated to preserving reciprocity and energy conservation on the interface. It is seen that most of the absorption near the energy bandgap of CIGS is due to the diffuse light and that this approach can yield very significant photocurrent gains below 500nm absorber thickness.

Multi-resonant light trapping in ultrathin CIGS solar cells

We investigate ultrathin CIGS solar cells with a nanostructured back mirror. Numerical calculations are used to optimize the optical design based on multi-resonant absorption. The impact of the different materials (CdS/ZnS, metal of the back contact: Mo/Au/Ag) is studied, and short circuit current densities above 36 mA/cm2 are predicted for CIGS absorbers as thin as 200 nm. We have developed a fabrication process based on the transfer of the CIGS solar cells, and nanoimprint lithography for the nanostructured back mirror. Light-trapping effects and Jsc improvement are evidenced in our first experimental results.

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.