Formation of CU(IN,GA)SE2 from Solution Processed Sodium containing Metal Precursors

Doctoral thesis (2015)

In this thesis a new chemical deposition process for the fabrication of Cu(In,Ga)Se2 absorber layers applicable as absorber material in thin film solar cells is described. Within this work a power ... [more ▼]

In this thesis a new chemical deposition process for the fabrication of Cu(In,Ga)Se2 absorber layers applicable as absorber material in thin film solar cells is described. Within this work a power conversion efficiency of up to 13.3% has been achieved which is among the highest reported values for non-vacuum based processes. The main part of this thesis is divided into 4 chapters describing the synthesis of the base materials and the ink (i), the individual process steps coating and drying together with the metal film formation (ii) as well as the selenization of the metal layer to chalcopyrites (iii). The last chapter investigates the influence of dissolved sodium salts (iv). (i) Metal formates synthesized from their respective nitrates have been chosen as starting salts due to the high reduction potential of the anion and its clean decomposition. With the reasonably toxic solvents tetramethylguanidine and methanol an ink can be produced which is highly concentrated, chemically stable and cheap. (ii) The coating and drying steps have been identified as crucial production processes to deposit homogeneous layers. In this context the solvent mixture has been shown to be responsible for unwanted material agglomerations close to the substrate boundaries. In order to minimize this effect a combined coating-drying device has been developed. The formation of metallic layers has been studied in detail. It could be shown that the respective metal ions are getting reduced consecutively at elevated temperatures between 130 and 200°C. The morphology of the resulting layers is highly dependent on the process conditions and varies from thick and porous structures to incoherent particle agglomerations. In order to overcome this issue an ink additive has been identified to densify and homogenize the metallic layer. A mechanism is proposed based on the formation of an intermediate polynuclear metal organic structure. (iii) The metallic layers could be selenized with elemental selenium in a tube oven to form dense chalcopyrites. By simulating the selenium evaporation a strong dependency of the final photovoltaic cell properties on the process temperature of the substrate layer getting in contact with evaporated selenium could be identified. (iv) Several sodium salts conveniently added to the ink have been proven to be suitable as sodium source. Compared to diffusion from the glass substrate the immediate sodium availability resulted in significantly improved cell properties. Within the evaluated sodium salts large differences have been found for example regarding the layer morphology. [less ▲]

Solution-based processing of Cu(In,Ga)Se2 absorber layers for 11% efficiency solar cells via a metallic intermediate

in Progress in Photovoltaics (2014)

In this work, a low cost solution-based method for the deposition of uniform Cu-In-Ga layers compatible with roll-to-roll processing is described. As ink system we use metal carboxylates dissolved in a ... [more ▼]

In this work, a low cost solution-based method for the deposition of uniform Cu-In-Ga layers compatible with roll-to-roll processing is described. As ink system we use metal carboxylates dissolved in a mixture of a nitrogen containing base and an alcohol. This solution can be coated homogeneously under inert atmosphere using a doctor blade technique. With this method and appropriate precursor concentrations, crack-free metal layers with dry-film thicknesses of more than 700 nm can be deposited in one fast step. For the controlled film formation during the drying of the solvents a flow channel has been used to improve the evaporative mass transport and the convective gas flows of any unwanted organic species. Due to the absence of organic binders with high molecular weight, this step allows the formation of virtually pure metal layers. Elementary analyses of the dried thin films reveal less than 5 wt% of carbon residues at 200°C. In situ X-ray diffraction data of the drying step show the formation of Cu-In-Ga alloys. The subsequent processing of Cu(In,Ga)Se2 chalcopyrites with evaporated elemental selenium takes place in a separate tube oven under inert atmosphere. Photoelectric measurements of cells with CdS buffer and ZnO window layer reveal a short-circuit current of 29 mA/cm2, an open-circuit voltage of 533 mV, and a fill factor of 0.69 under standard conditions. Thus efficiencies of up to 11% on 0.5 cm2 area without antireflective coating have been achieved. [less ▲]