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

in Scientific Reports (2017), 7

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 ... [more ▼]

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. [less ▲]

ELECTRODEPOSITION AND SELENIZATION OF METALLIC THIN FILMS FOR KESTERITE SOLAR CELLS APPLICATION

Doctoral thesis (2016)

Thin films of Kesterite Cu2ZnSnSe4 (CZTSe) are prepared via a low energy cost and high material efficiency process, to be potentially used as light absorbers in solar cell devices. The fabrication process ... [more ▼]

Thin films of Kesterite Cu2ZnSnSe4 (CZTSe) are prepared via a low energy cost and high material efficiency process, to be potentially used as light absorbers in solar cell devices. The fabrication process involves two main steps: (i) formation of a metallic stack of Cu/Sn/Zn by sequential electrodeposition of Cu, Sn and Zn onto glass/Mo substrates; (ii) reactive annealing at 550°C in presence of Se and SnSe powders to form Kesterite. This thesis mainly aims at understanding the mechanisms of metal alloying and selenization occurring during step (ii), and their effects on the microstructure of the final film, the presence of secondary phases and their distribution in the thin films synthesized. The second objective is to understand their effects on the solar cells parameters. The stoichiometry of the precursor layers Cu/Sn/Zn is deliberately chosen to be Cu-poor and Zn-rich (Cu/(Zn+Sn)<1 and Zn/Sn>1), as it allows to reach the best power conversion efficiencies. Under these conditions, Kesterite, SnSe2 and ZnSe are expected. However, a study of different compositions shows that the predominant phases present are only Kesterite and ZnSe. SnSe2 is not present because this phase is unstable under the conditions of selenization, which leads to a self-regulation of tin content via gas phase exchange of SnSe during the selenization. Analyses of the selenization of Cu/Sn/Zn layers at short times and lower temperatures allow to deconstruct the mechanism of Kesterite formation into sequential steps. Because of the diffusion of metals and the formation of alloys, a reorganization of metals is observed in the thin films. The layers are then composed of Sn, Cu-Sn and Cu-Zn phases mainly, which are found to be segregating at large scales of tens of micrometers. During selenium incorporation, a tin self-regulation process is established, in which tin is depleted during the first stages of selenization, and then tin is replenished. ZnSe segregates at the surface of the absorber layer as large islands of 10-20 micrometers. By analyzing a specific position of a sample after the different process steps, it is shown that the segregation of ZnSe at this large scale is originating in the segregation of metals during alloying. Because of the presence of ZnSe on the surface of the films, part of the photocurrent generated in the absorber layer is not collected, which decrases the short circuit current of the devices. In this sense, a linear decrease of short circuit current is observed when the ZnSe molar ratio is increasing, and confirmed by external quantum efficiency (EQE) measurements showing a decrease of current collected through the whole range of photon energies. An optimal molar ratio of ZnSe/(CZTSe + ZnSe)=0.2 is found. Below this value, the short circuit current decreases, probably due to the formation of other types of harmful secondary phases such as Cu2SnSe3 or Cu2Se. A strong decrease of open circuit voltage and fill factor of the solar cells is proved to be related to the formation of blisters in the thin films, which result in the creation of pinholes due to their fragility. Formation of these blisters is supposed to originate from hydrogen evolution under the Cu layer during the electrodeposition process. Finally, a study of an additional process of prealloying between the steps of electrodeposition and selenization is presented, which demonstrates the possibility to increase the open circuit voltage of the solar cells by varying the time of this alloying step. A best power conversion efficiency of 7.2% is achieved via this method, which is close to the highest value of 9.1% reported for an electrodeposition-based process of Kesterite synthesis. [less ▲]

Prediction of photovoltaic p-n device short circuit current by photoelectrochemical analysis of p-type CIGSe films

in Electrochemistry Communications (2014), 48

The quality control of individual semiconductor thin films during fabrication of multiple layers is important for industry and academia. The ultimate aim of this research is to predict the efficiency of p ... [more ▼]

The quality control of individual semiconductor thin films during fabrication of multiple layers is important for industry and academia. The ultimate aim of this research is to predict the efficiency of p-–n junction solar cells by photoelectrochemical analysis of the bare p-type semiconductor. A linear correlation between the photocurrent measured electrochemically on Cu(In,Ga)Se2 absorber layers through a Eu3+ electrolyte junction and short circuit current and efficiency of the corresponding solid state devices is found. However, the correlation is complicated by pronounced recombination at the semiconductor/electrolyte interface, while the solid state interface behaves more ideally. [less ▲]

Simplified formation process for Cu2ZnSnS4-based solar cells

in Thin Solid Films (2014), 573

Continuous wave solid phase laser annealing of single-pot electrodeposited CuInSe2 thin films: Effects of Cu/In stoichiometry

in Journal of Applied Physics (2013)