Tobias Bertram

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

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.

The influence of Se pressure on the electronic properties of CuInSe2 grown under Cu-excess

Standard Cu-poor Cu(In,Ga)Se2 solar cell absorbers are usually prepared under high Se excess since the electronic properties of the absorbers are better if prepared under high Se pressure. However, in CuInSe2, grown under Cu-excess, it was found that solar cell properties improve with lowering the Se pressure, mostly because of reduced tunnel contribution to the recombination path. Lower Se pressure during Cu-rich growth leads to increased (112) texture of the absorber films, to better optical film quality, as seen by increased excitonic luminescence and to lower net doping levels, which explains the reduced tunnelling effect. These findings show an opposite trend from the one observed in Cu-poor Cu(In,Ga)Se2.

In-Se surface treatment of Cu-rich grown CuInSe 2

This work focuses on the chalcopyrite CuInSe2 as a model for the more complex but also more widely used thin-film material Cu(In,Ga)Se2. Both materials are characterized by a very broad existence region that allows Cu-poor as well as stoichiometric growth. Although Cu-poor solar cells are more studied and commercially available, Cu-rich CuInSe2 exhibits qualities that make it the superior material. But due to an inherently high doping and interface problems, it has not been possible to take advantage of these. On the other hand it has been shown in previous studies, that forming a Cu-poor surface layer on Cu-rich grown CuInSe2-absorbers can greatly improve the open-circuit voltage of these solar cells. Surface treatments will be discussed, that are comprised of an indium and selenium co-deposition stage with the goal to form the Cu-poor layer by copper migration. They were performed on a new Cu-rich material, which is characterized by a low Se environment during growth. Through this it was possible to reduce the doping level greatly, which results in reliably delivering devices with high currents. Making them excellent candidates for interface optimization, that mainly effects the open-circuit voltage. Thus it became possible to produce high efficiency Cu-rich devices. There is still room for improvement though, as the devices show absorption losses in a wavelength region in accordance with a remainder of InSe on top of the CIS surface. Optimization of the process is a straightforward approach to remove this layer and shows potential for even greater efficiencies. Still the striking point is, that the here presented solar cells, are already as efficient as the Cu-poor devices, that have been published by our group.

Metastable defect in CuInSe2 probed by modulated photo current experiments above 390 K

Modulated photocurrent experiments have been widely used to study defects in semiconductors. Previous studies have found a number of defects in CuInSe2, which is used as an absorber in solar cells. We apply a method of analysis, which has previously not been used for Cu(In,Ga)Se2 semiconductors and which allows the determination of defect concentrations in addition to defect energies. We found that at least one of the previously discovered defects shows a metastable behaviour, increasing in concentration, and can be related to the efficiency loss in corresponding solar cells.

Electrical Characterization of Defects in Cu-Rich Grown CuInSe

We study defects in CuInSe2 (CIS) grown under Cu-excess. Samples with different Cu/In and Se/metals flux ratios were characterized by thermal admittance spectroscopy, capacitance-voltage (C-V) measurements, and temperature-dependent current-voltage (IVT) measurements. All samples showed two different capacitance responses, which we attribute to defects with energies around 100 and 220 meV, plus the beginning of an additional step that we attribute to a freeze-out effect. By application of the Meyer-Neldel rule, the parameters of the two defects can be assigned to two different groups, both lying within the energy region of the so-called N1-defect that has been observed for Cu-poor absorbers.

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CuInSe2 films for photovoltaic applications grown under Cu-excess have been rarely investigated up to now. In general CuInSe2 solar cells use an overall Cu-poor absorber. In this work we argue that it is valuable to investigate Cu-rich solar cells, since all the basic material properties are better in Cu-rich absorbers. With less defects in the bulk and better transport properties it is somehow intriguing why devices with Cu-rich absorber perform less. We demonstrate that this can be attributed to the too high doping of these films. Such a high native doping leads to tunneling enhanced recombination and interface recombination, strongly affecting the devices performances. We demonstrate different attempts to overcome the problem of doping: at first a Cu-poor surface was grown on the Cu-rich absorbers which enables to decrease the doping in the space charge region, then to directly decrease the doping in the bulk, the influence of sodium content was investigated. Finally, here we show that different selenium activity during the absorber growth enables to decrease the doping of these films and to open thus a way to fully exploit the favorable properties of the Curich CuInSe2 films.