Deliberate and Accidental Gas-Phase Alkali Doping of Chalcogenide Semiconductors: Cu(In,Ga)Se2

in Scientific Reports (2017), 7

Alkali metal doping is essential to achieve highly efficient energy conversion in Cu(In,Ga)Se2 (CIGSe) solar cells. Doping is normally achieved through solid state reactions, but recent observations of ... [more ▼]

Alkali metal doping is essential to achieve highly efficient energy conversion in Cu(In,Ga)Se2 (CIGSe) solar cells. Doping is normally achieved through solid state reactions, but recent observations of gas phase alkali transport in the kesterite sulfide (Cu2ZnSnS4) system (re)open the way to a novel gas-phase doping strategy. However, the current understanding of gas-phase alkali transport is very limited. This work (i) shows that CIGSe device efficiency can be improved from 2% to 8% by gas-phase sodium incorporation alone, (ii) identifies the most likely routes for gas-phase alkali transport based on mass spectrometric studies, (iii) provides thermochemical computations to rationalize the observations and (iv) critically discusses the subject literature with the aim to better understand the chemical basis of the phenomenon. These results suggest that accidental alkali metal doping occurs all the time, that a controlled vapor pressure of alkali metal could be applied during growth to dope the semiconductor, and that it may have to be accounted for during the currently used solid state doping routes. It is concluded that alkali gas-phase transport occurs through a plurality of routes and cannot be attributed to one single source. [less ▲]

High-speed Electrodeposition of Copper-Tin-Zinc Stacks from Liquid Metal Salts for Cu2ZnSnSe4 Solar Cells

in Chemmical Communications (2017)

Electrodeposition of Chalcopyrites from Deep Eutectic Based Ionic Liquids

Doctoral thesis (2015)

This thesis focuses on the electrodeposition of metal precursors from a deep eutectic based ionic liquid electrolyte and their annealing for Cu(In,Ga)(S,Se)2 thin film solar cells. Ionic liquids are ionic ... [more ▼]

This thesis focuses on the electrodeposition of metal precursors from a deep eutectic based ionic liquid electrolyte and their annealing for Cu(In,Ga)(S,Se)2 thin film solar cells. Ionic liquids are ionic compounds which are liquid at temperatures below 100 °C and contain no water. Therefore, by using an ionic liquid electroplating bath, the electrolysis of water is avoided and elements which are normally difficult to electrodeposit, such as gallium, can be easily deposited. In this sense, the electroplating current efficiency of the process (i.e. the energy efficiency of the process) is significantly improved by using an ionic liquid electrolyte instead of an aqueous electroplating bath. The objectives of this work are i) to electrodeposit Cu(In,Ga) metal precursors with high electroplating current efficiency, ii) to control the chemical composition of these precursors and iii) form Cu(In,Ga)(S,Se)2 absorber layers with adequate chemical composition and morphology for solar cell fabrication. In the frame of the third objective, it is intended to obtain absorber layers with a continuous gallium distribution as well. This last point is due to the fact that, during thermal annealing of metal precursors, gallium often segregates to the back of the absorber layer. Ultimately, this uneven gallium distribution can hinder the performance of solar cells. To meet these objectives, the co-electrodeposition of a) copper and indium, b) indium and gallium and c) copper, indium and gallium from an ionic liquid electrolyte is studied. The ionic liquid used in this work results from the 1:2 molar mixture of choline chloride and urea. From this work, the electrodeposition of Cu(In,Ga) metal precursors with an electroplating current efficiency above 75% was achieved. It was observed that the morphology of the electrodeposited precursors depended on the chemical composition of the electrolyte. In this frame, Cu(In,Ga) layers with dendritic or compact morphology were obtained. Precursors with dendritic morphology are not adequate, since this morphology persists after thermal annealing and ultimately results in devices with no efficiency. The chemical composition of the metal precursors can be controlled as well. Specifically, the gallium content of the metal precursor, which influences the optoelectronic properties of the absorber layer, was accurately tuned. The gallium content is usually expressed as the concentration ratio [Ga]/([Ga]+[In]) and could be tuned between 0.1 and 0.9. Therefore, the electrodeposition of Cu(In,Ga) metal precursors with high gallium content was achieved for the first time and would not have been possible without the use of an ionic liquid electrolyte. After subjecting the metal precursors to a thermal annealing, absorber layers with adequate chemical composition and morphology were obtained. Additionally, by employing a specific annealing routine developed at the Institute of Energy Conversion in the University of Delaware, absorbers with a continuous gallium profile and different gallium contents were obtained. It was observed that the performance of the solar cells was limited by the thermal annealing step and a maximum solar cell efficiency of 9.8% was achieved. In general, it can be concluded that different precursors require different thermal annealing routines in order to form quality absorber layers. [less ▲]

Quantification of surface ZnSe in Cu2ZnSnSe4-based solar cells by analysis of the spectral response

in Solar Energy Materials and Solar Cells (2014), 123

Absorber layers consisting of Cu2ZnSnSe4 (CZTSe) and surface ZnSe in variable ratios were prepared by selenization of electroplated Cu/Sn/Zn precursors and completed into full devices with up to 5.6 ... [more ▼]

Absorber layers consisting of Cu2ZnSnSe4 (CZTSe) and surface ZnSe in variable ratios were prepared by selenization of electroplated Cu/Sn/Zn precursors and completed into full devices with up to 5.6 % power conversion efficiency. The loss of short circuit current density for samples with increasing ZnSe content is consistent with an overall reduction of spectral response, pointing to a ZnSe current blocking behavior. A feature in the spectral response centered around 3 eV was identified and attributed to light absorption by ZnSe. A model is proposed to account for additional collection of the carriers generated underneath ZnSe capable of diffusing across to the space charge region. The model satisfactorily reproduces the shape of the spectral response and the estimated ZnSe surface coverage is in good qualitative agreement with analysis of the Raman spectral mapping. The model emphasizes the importance of the ZnSe morphology on the spectral response, and its consequences on the solar cell device performance. [less ▲]

Tuning the gallium content of metal precursors for Cu(In,Ga)Se2 thin film solar cells by electrodeposition from a deep eutectic solvent

in Physical Chemistry Chemical Physics (2014), 16

Electrodeposition of Cu–In alloys from a choline chloride based deepeutectic solvent for photovoltaic applications

in Electrochimica Acta (2013), 103

Cu-Rich Precursors Improve Kesterite Solar Cells

in Advanced Energy Materials (2013), 4