This paper aims at providing an updated overview of the main achievements in the development of solar cells based on Cu 2 ZnSn(S,Se) 4 (CZTS(Se)) Kesterite absorbers obtained by electrodeposition. Although undoubtedly challenging, the ultimate goal is to learn from the past works and build a solid framework for future advances in this field. What is the reason for the lower efficiency of electrodeposited CZTS(Se)-based devices (8%) compared to the world record efficiency achieved with a hydrazine-based solution approach (12.6%)? Can this gap be filled, or there are intrinsic limitations for this achievement? The review is divided into the three main electrodeposition approaches: sequential elemental layer, alloy co-deposition, and chalcogenide co-deposition. It is argued that considerable technical challenges must be overcome for the latter approach to be successfully applied.Plot of the record power conversion efficiencies of Kesterite sulfide-based solar cells obtained by electrodeposition (hollow dots), and world record efficiency of CZTS(Se)-based devices (full dots). The dashed line shows the 15% minimum efficiency threshold considered relevant for potential industrial application.

Secondary phases, such as ZnSe, occur in Cu2ZnSnSe4 and can be detrimental to the resulting solar cell performance. Therefore, it is important to have simple tools to detect them. We introduce subband gap defect excitation room temperature photoluminescence of ZnSe as a practical and non-destructive method to discern the ZnSe secondary phase in the solar cell absorber. The PL is excited by the green emission of an Ar ion laser and is detected in the energy range of 1.2–1.3 eV. A clear spatial correlation with the ZnSe Raman signal confirms this attribution.

Alkali metal doping is essential to achieve highly efficient energy conversion in Cu(In,Ga)Se 2 (CIGSe) solar cells. Doping is normally achieved through solid state reactions, but recent observations of gasphase alkali transport in the kesterite sulfide (Cu 2 ZnSnS 4 ) 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.Control of alkali doping is crucial for a range of technologically relevant chalcogenide materials, from photovoltaics (CdTe, Cu(In,Ga)Se 2 , Cu 2 ZnSn(S,Se) 4 ) 1-5 and thermoelectricity (Pb(S,Se,Te)) 6-9 potentially to superconductivity (KFeSe 2 ) 10,11 and quantum computing (Bi 2 Te 3 12 , MoS 2 and WSe 2 13 ). In the case of Cu(In,Ga)Se 2 (CIGSe) solar cell material, the current alkali metal doping procedures are overwhelmingly based on condensed state reactions. Two common approaches are taken. Either by indirect control of the diffusion from a sodium-containing substrate or back contact [14][15][16][17] , or by deliberate doping from the precursor surface through a post deposition treatment (PDT), e.g. by NaF or KF evaporation onto the surface of the absorber to form a tens of nanometer thick layer, followed by annealing [18][19][20] . Control of the sodium content in the former case is difficult as substrates are never identical 21 , and in the latter case at least one extra step is required to add the alkali metal. The subject has been extensively reviewed by Salomé et al. 22. CIGSe thin films are always grown in a controlled atmosphere containing a certain pressure of selenium. The semiconductor requires selenium for its formation and to prevent its decomposition, given that the reaction is ruled by a solid/gas-phase equilibrium 23,24 . The question arises, are any other gas-phase chemical species involved in the equilibrium? All the main binary compounds of CIGSe have low vapour pressures; however, usually CIGSe contains also a considerable amount of sodium incorporated in the film. Is the vapour pressure of sodium or its

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The EQE can be simulated from EQE R taking into account the filtering of photons through ZnSe and the collection losses of charge carriers in the absorber layer underneath ZnSe. To consider the collection losses it is necessary to compute the total volume of absorber layer underneath ZnSe and the fraction of it which is still subject to carrier collection.