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See detailElectrodeposition of Chalcopyrites from Deep Eutectic Based Ionic Liquids
Malaquias, Joao UL

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 ▲]

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See detailQuantification of surface ZnSe in Cu2ZnSnSe4-based solar cells by analysis of the spectral response
Colombara, Diego UL; Robert, Erika UL; Crossay, Alexandre UL et al

in Solar Energy Materials & 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 ▲]

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See detailSemiconductors for Photovoltaic Devices: Electrochemical Approaches using Ionic Liquids
Dale, Phillip UL; Malaquias, Joao UL; Steichen, Marc UL

in ECS Transactions (2014), 58(18), 1-12

Can electrodeposition be used to create high quality p-type inorganic compound semiconductors for photovoltaic applications? Thin film photovoltaic devices offer similar power conversion efficiencies to ... [more ▼]

Can electrodeposition be used to create high quality p-type inorganic compound semiconductors for photovoltaic applications? Thin film photovoltaic devices offer similar power conversion efficiencies to polycrystalline silicon devices and have the inherent advantages of consisting of less material and requiring less energy expenditure during processing. Thin film devices consist of a semiconductor pn heterojunction with front and back contacts to extract the excited charge carriers. The materials properties of the p-type layer are the most stringent, and determine the overall performance of the device. Common p-type semiconductors are CdTe, Cu(In,Ga)Se2, and Cu2ZnSn(S,Se)4. Typically the p-type semiconductor must form a continuous dense single phase layer two micron thick over metre squared areas. Most commercial producers of thin film photovoltaic modules choose evaporation or sputtering methods to deposit this layer. Of importance is the speed, cost, and quality of deposition. Electrodeposition offers the ability to deposit thin films over large areas with high materials usage, potentially at high speed. Can electrodeposition be used to create high quality p-type inorganic compound semiconductors? This talk will show that it is possible to directly deposit a working p-type semiconductor, but that a two step approach of depositing metals and then annealing them in a reactive atmosphere is a simpler, easier, and more robust approach. Both approaches can lead to semiconductors which provide working photovoltaic devices. However, improvements to the electrodepostion process are still required and the main challenges are outlined below. Challenges in directly electrodepositing a p-type semiconductor are (i) the inherent lack of electrons necessary for a reductive deposition process and (ii) the low thermal energy available at normal deposition temperatures to create micron sized well ordered crystals. Challenges for directly electrodepositing the metal alloys CuInGa or CuSnZn from aqueous solution are (iii) competition with hydrogen reduction leading to inefficient deposition, embrittlement, and dendritic growth (iv) control of the alloy composition over the micrometer and centimeter length scales due to the different reduction potentials, nucleation densities, and diffusion coefficients. In this talk it will be shown how these challenges can be met by using ionic liquids to replace aqueous solvents. Ionic liquids offer larger electrochemical windows, higher processing temperatures, and the choice of new forms of starting reagent. Furthermore, task specific ionic liquids or liquid metal salts, may even be employed to allow extremely high speed deposition. [less ▲]

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See detailCu-rich precursors improve kesterite solar cells
Mousel, Marina UL; Schwarz, Torsten; Djemour, Rabie UL et al

in Advanced Energy Materials (2013), 4

Detailed reference viewed: 137 (9 UL)