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

Photoluminescence and solar cell studies of chalcopyrites - comparison of Cu-rich vs. Cu-poor and polycrystalline vs. epitaxial material

The quasi-Fermi level splitting (qFls) in a solar cell absorber limits the maximum achievable open circuit voltage of the final device. A calibrated photoluminescence set-up allows the determination of the qFls at room temperature under conditions equivalent to the illumination from the sun. In this work the qFls is used as an indicator for the quality of epitaxial and polycrystalline thin CuInSe2 films with different compositions. It is shown, that the epitaxial material exhibits improved optoelectronic quality noticeable in the higher qFls (50-100meV) when compared to the polycrystalline counterpart. Furthermore, a dependency on the Cu/In ratio is noticed: In Cu poor material (Cu/In1). The difference between absorbers grown under Cu-poor and Cu-rich conditions is found to be in the order of 150meV. Additionally, the compositional dependence of the Urbach energy and of the band gap energy has been evaluated from the PL spectra at room temperature. Slightly higher bandgaps and lower Urbach energies have been found for the Cu rich material compared to the Cu poor absorbers. The time dependent change of the qFls of bare CuInSe2 absorbers exposed to air is presented and shows for Cu-poor absorbers a pronounced and for Cu-rich material a slower degradation. It is shown, that a chemical etching in potassium cyanide refreshes the degraded material to an extent comparable to freshly grown samples, and that the deposition of a CdS buffer layer passivates the surface. Epitaxial Cu(In,Ga)Se2 is grown by means of metal organic vapour phase epitaxy and used to produce epitaxial solar cells. A maximum power conversion efficiency of 6.7% has been achieved. All solar cells suffer from dominant interface recombination processes which limit the device performance. The critical interface is between the absorber layer and the CdS buffer and related to the high gallium content

Tuning the gallium content of metal precursors for Cu(In, Ga)Se-2 thin film solar cells by electrodeposition from a deep eutectic solvent

Controlling the Ga incorporation of Cu-In-Ga metal precursors for Cu(In,Ga)Se2 (CIGS) solar cells is one of the main challenges for low cost electrodeposition processes, mainly due to the difficulty in electrodepositing metallic Ga from aqueous electrolytes. In this work we use the deep eutectic solvent (DES) Choline Chloride : Urea (ChCl : U - 1 : 2) to efficiently codeposit In-Ga on Cu and Mo electrodes. We control the Ga/(Ga+In) (Ga/III) ratio of the films via the mass fluxes. The electrochemical behavior of ChCl : U containing GaCl3 and InCl3 is studied by rotating disk electrode cyclic voltammetry (CV) on Mo and Cu electrodes. CV revealed on both Mo and Cu electrodes that the electrochemical behavior of the ChCl : U-GaCl3-InCl3 system is the superposition of the individual In and Ga electrochemistry. On a Cu electrode the morphology, crystal structure and element distribution of the deposits were a function of the Ga/III ratio. We demonstrate the precise control of Ga incorporation over a large composition range from 0.1 ≤ Ga/III ≤ 0.9 and proved that ED from DES is a straightforward, robust and efficient process. First solar cells based on Mo/Cu/In-Ga metal stacks achieved efficiencies as high as 7.9% with a Voc of 520 mV.status: publishe