L. Arzel

Atom probe study of Cu-poor to Cu-rich transition during Cu(In,Ga)Se2 growth

Atomic scale chemistry of polycrystalline Cu(In,Ga)Se2 (CIGSe) thin film has been characterized at key points of the 3-stage process using atom probe tomography. 3D atom distributions have been reconstructed when the layer is Cu-poor ([Cu]/([Ga] + [In])   1), and at the end of the process. Particular attention has been devoted to grain boundary composition and Na atomic distribution within the CIGSe layer. Significant variation of composition is highlighted during the growing process, providing fundamental information helping the understanding of high efficiency CIGSe formation.

KCN Chemical Etch for Interface Engineering in Cu2ZnSnSe4 Solar Cells

The removal of secondary phases from the surface of the kesterite crystals is one of the major challenges to improve the performances of Cu2ZnSn(S,Se)4 (CZTSSe) thin film solar cells. In this contribution, the KCN/KOH chemical etching approach, originally developed for the removal of CuxSe phases in Cu(In,Ga)(S,Se)2 thin films, is applied to CZTSe absorbers exhibiting various chemical compositions. Two distinct electrical behaviors were observed on CZTSe/CdS solar cells after treatment: (i) the improvement of the fill factor (FF) after 30 s of etching for the CZTSe absorbers showing initially a distortion of the electrical characteristic; (ii) the progressive degradation of the FF after long treatment time for all Cu-poor CZTSe solar cell samples. The first effect can be attributed to the action of KCN on the absorber, that is found to clean the absorber free surface from most of the secondary phases surrounding the kesterite grains (e.g., Se0, CuxSe, SnSex, SnO2, Cu2SnSe3 phases, excepting the ZnSe-based phases). The second observation was identified as a consequence of the preferential etching of Se, Sn, and Zn from the CZTSe surface by the KOH solution, combined with the modification of the alkali content of the absorber. The formation of a Cu-rich shell at the absorber/buffer layer interface, leading to the increase of the recombination rate at the interface, and the increase in the doping of the absorber layer after etching are found to be at the origin of the deterioration of the FF of the solar cells.

Influence of grain boundary modification on limited performance of wide bandgap Cu(In,Ga)Se2 solar cells

The reason why so-called wide-bandgap CuIn1−xGaxSe2 (CIGSe with x > 0.4) based solar cells show hindered performance compared with theoretical expectations is still a matter of debate. In the present Letter, atom probe tomography studies of CuIn1−xGaxSe2 polycrystalline thin films with x varying from 0 to 1 are reported. These investigations confirm that the grain boundaries (GBs) of low gallium containing (x   0.8) are Cu-enriched compared with GI. For intermediate gallium contents (0.4 < x < 0.8), both types of GBs are detected. This threshold value of 0.4 surprisingly coincides with solar cells output voltage deviation from theoretical expectations, which suggests modifications of GBs properties could participate in the loss of photovoltaic performance.

Impact of Mo density on Mo/CIGSe interfaces: An XPS study

We have investigated the impact of Mo density onto Mo/CIGSe interfaces. In this study, CIGSe layers have been simultaneously co-evaporated onto different Mo/soda-lime glass (SLG) substrates. Varying the Mo layer density allows to tune the amount of sodium available during CIGSe growth. The chemical composition of the Mo/CIGSe interfaces has been investigated by X-ray photoelectron spectroscopy (XPS) after lift the CIGSe layer off the molybdenum. Na and Ga amount on the Mo side are correlated to the Mo density. The CIGSe morphology is also impacted by Na amount. Two different chemical environments of sodium are distinguished for the fine-grained CIGSe.

Oxidation of In 2 Se 3 precursor films and its effects on preparation of CuInSe 2 based thin film solar cells

The present work deals with the air-annealing effects on In2Se3 precursor layers and the related CISe based heterojunction solar cell devices. CISe films were grown based on a modified 3-stage co-evaporation process that enabled the oxidation of In2Se3 precursor layer at the end of the first stage. To study the role of grain boundaries on oxidation, precursor layers were prepared at high and low temperatures. In2Se3 precursor thin film grown at high temperature shows a gamma-phase with (110) preferential orientation and grain size of 0.5-1 micrometer. Precursor layer prepared at low temperature showed amorphous structure with grains size around 300 nm. CISe films prepared with both precursor layers (high and low temperature) exhibit chalcopyrite structure with a (112) preferential orientation. Comparison between samples prepared with and without air-annealing do not exhibit clear morphological or structural changes. The effect of oxidation process on electrical properties of the solar cells was studied with current-voltage and external quantum efficiency measurements. These results showed that, as compared to devices with non-oxidized CISe, the device with 1h-oxidized CISe film exhibit a decrement in open circuit voltage of ~65mV. This could be related to passivation of interface states on the CdS/CISe interface. Comparing oxidized-CISe cells with different grain boundary density, more degradation of electrical parameters were observed on samples with high number of grain boundaries. Our result show that oxygen introduction to CISe films through the air-annealing of In2Se3 precursors is detrimental to the CISe based solar cell performance.

Interpretation of beneficial annealing of CIGSe cell buffured with co-evaporated In 2 S 3

The present contribution aims at better understanding the origin of the beneficial effect of the annealing of CIGSe-based solar cell buffered with co-evaporated (PVD)In2S3. For long, most of the works dealing with such alternatives to chemical bath deposited (CBD)CdS buffer layer have been focused on the CIGSe/buffer interface, where the pn junction is formed. Herein, we show that as already suggested by Nguyen et al. [1] in the case of (CBD)In2S3 buffer, the annealing post-treatment most probably improves the Voc and FF of the devices because it changes the nature and/or the density of defects at the In2S3/r-ZnO interface. Such a conclusion is motivated by our observation that rinsing the CIGSe/(PVD)In2S3 structures with water before the window deposition induces the same effect as the annealing, then such treated samples are less improved by annealing. In addition, the X-ray photoelectron spectroscopy analyses performed on the CIGSe/(PVD)In2S3 structure before and after the water treatment show that Na carbonates are removed from the surface by the water. This latter observation suggests that the main impact of the annealing of the cells buffered with co-evaporated In2S3 would be the destruction or the out migration of the Na-based compounds at the buffer/r-ZnO interface.

Depth resolved preferential orientation of Cu(In,Ga)Se2 thin films based on the 112 peak model

X-ray diffraction is a commonly used technique to characterize the Cu(In,Ga)Se2 thin films. It allows to derive information on the pureness of deposited layers, their preferential orientation and a degree of crystallization. In this work we present a detailed analysis of the shape of a very characteristic 112 peak and its dependence on the compositional profile of indium and gallium. Contrary to standard X-ray analysis we go one step further and from the comparison of calculated and measured 112 peaks we are able to determine the evolution of preferential orientation along the thickness of Cu(In,Ga)Se2 thin film.