Fabrication Of Highly Efficient Copper Indium Gallium Selenide Thin Film Solar Cells From Aqueous Solution

Copper indium gallium selenide?CIGS?is one of the most promising absorber materials for commercializable thin film photovoltaics due to its high efficiency and excellent stability.However,the vacuum-based deposition methods,which rely on expensive vacuum technologies and have a high capital cost,limit the market share of CIGS.Solution methods have emerged as promising alternatives for fabricating CIGS because of their low cost and availability for high throughput.Among all available solvents,water is the most desirable one for cost effective and green production,concerning the cost of the solvent itself and the environment and health issues caused by the solvents.This thesis focuses on the fabrication of highly efficinent CIGS thin film solar cells from aqueous solution.The stability issue of the metal ions in water and impurities originating from the uncoordinated metal ions and the oxidation products formed during film annealing in air account for the low efficiency of aqueous solution processed CIGS solar cells.To address these problems,copper thiourea complex?Cu TU₃Cl?and indium thiourea complex?In TU₃Cl₃?with preformed metal-S bonds are used as precursors,which keep their metal-S bonds in aquous solution and stabilize the metal ions.Copper indium sulfur selenide?CISSe?thin film and solar cells are fabricated from the simple aqueous complexes solution.Further,gallium alloying and potassium doping are applied to optimize the properties of the absorber.The contents and conclusions of this thesis are listed as follow:?1?Study on solution chemistry and the reaction pathway from the precursors to the thin film absorbers.The results show that all the metal ions coordinate with thiourea in the aqueous solution,which stabilize In³⁺and Cu⁺effectively.The direct bonding of all metal ions with sulphur leads to the formation of sulfide precursor film upon thermal annealing,avoiding impurities originating from the uncoordinated metal ions and related by-products that are detrimental to the growth of CIGS.High quality CISSe absorber film with high crystallinity,densely-packed large grains,and smooth surface was fabricated in air from the aqueous solution using Cu TU₃Cl and In TU₃Cl₃ as precursors.?2?Fabrication of CISSe thin film soalr cells from the aqueous solutions.The influence of precursor raw material,the ratio of copper to indium,and selenization pressure on device performance and the short-term stability of the solar cells are explored.A champion CISSe solar cell with a power conversion efficiency of 12.3%has been obtained from the complex aqueous solution.The efficiency of the device is among the best performing non-vacuum processed CISSe solar cells and the open-circuit voltage and fill factor are comparable to CIS solar cells fabricated from vacuum method.?3?Optimization of absorber property by gallium alloying and potassium doping.Gallium alloying increased absorber band gap,enhanced device open-circuit voltage,and reduced open-circuit voltage deficit.Potassium doping significantly improved device open-circuit voltage.Our work provides a facile,green,and low-cost aqueous approach to fabricate high quality CIGS thin film absorber materials,which not only achieved the highest efficiency CIGS solar cells from aquous solution but also can be extended to fabricate other chalcogenide materials.