Gas-phase alkali doping of chalcogenide semiconductors


CALL: 2014

DOMAIN: MS - New Functional and Intelligent Materials and Surfaces


LAST NAME: Colombara



HOST INSTITUTION: University of Luxembourg

KEYWORDS: Chalcogenide solar cells; CISe; Photovoltaics; Extrinsic alkali doping; Gas-phase incorporation; Sodium selenide; Potassium selenide; Vapour pressure

START: 2015-06-01

END: 2017-05-31


Submitted Abstract

It has been recently discovered that alkali metals may be introduced during the synthesis of technologically important chalcogenide materials via a near ambient gas phase. The optoelectronic properties of chalcogenides, and thus their functionality, are heavily dependent on the alkali doping. The ultimate goal of this project proposal is to understand and control the gas-phase alkali doping of CuInSe2 chalcopyrite thin films. CuInSe2 is chosen because it is a well-established semiconductor for photovoltaic applications, but the gas-phase doping is also relevant to other important chalcogenide semiconductors such as PbSe, Bi2Te3, MoS2 and WSe2 with real or potential applications in thermoelectricity and quantum computing.Chalcogenide thin films are often grown in a two-step approach to allow an industrial production with lower capital expenditure. This consists of forming a metallic precursor and exposing it to a chalcogenizing reagent at high temperature. The current alkali doping procedures are mostly based on diffusion from a sodium-containing substrate, or by deliberate doping through post deposition treatment. Since substrates are seldom identical, control of the sodium content in the former case is difficult, while the second case requires at least one additional step.Preliminary findings indicate that alkali doping of Cu(In,Ga)Se2 can be achieved via the gas phase during the selenization of metal precursors by simply loading alkali chloride salts in the same reaction chamber. This novel doping strategy is of scientific relevance, because it represents a simplification over the current doping routines . However, in order for it to capitalize on these preliminary results, two fundamental research questions need to be answered. Firstly, what is the mechanism of alkali vapour transport? This proposal will test the hypothesis that the transport occurs via Na/K-Se molecular gas species. Secondly, what are the effects of the gas-phase alkali doping on the structural properties of CuInSe2 films grown from metal precursors and on the optoelectronic properties of the corresponding solar cells? This proposal will test the hypothesis that gas-phase alkali doping during annealing affects the properties of CuInSe2 films and this has an effect on the optoelectronic properties of the corresponding solar cells.The first research question will be investigated by synthesizing Na/K-Se compounds and characterizing their thermochemical behavior. Explicitly the gas-phase species will be identified using infra-red spectroscopy and mass spectrometry, while vapor pressure – temperature relationships will be established using a Knudsen effusion cell. The second research question will be investigated by growth and ex-situ analysis of a series of CuInSe2 films with different alkali-doping concentration in order to gain insights into the mechanism of alkali gas-phase doping on structural properties. The growth will be done by selenization of Cu-In metal precursors on Mo-coated, alkali-free substrates in a dedicated furnace furniture capable of independently controlling the pressure of the alkali gas species. The CISe films obtained will be completed into solar cell devices in order to ascertain the effects of gas-phase doping on device performance parameters. Deeper understanding of the electrical effects of alkali doping will be obtained by measuring carrier lengths, densities and profiles.If the vapor phase activity of alkali metal gas species is easily controllable, a new simpler methodology for doping of Cu(In,Ga)Se2 will have been created. Further, the doping approach is straightforward to apply to other technologically important chalcogenide materials which rely on alkali dopants for their functional properties.

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