Directing Reaction Paths to Electrodeposit Cu-In-Se Films with Alternate Morphologies
Authors: Menezes Shalini 
Published: December 2020

Electrodeposition approach has been widely explored for CuInSe2 and its alloys as a low cost, scalable alternative to vacuum methods. Achieving device quality electrodeposited films of ternary compounds requires a profound knowledge of the intricate reaction mechanisms and the possible alternate paths that determine the film composition and electronic properties. Various studies of the Cu-In-Se (CISe) system have enabled unraveling the rather complicated reaction mechanisms and ultimately electrodeposit stoichiometric CuInSe2 [1-7].
The complexity of the reaction mechanism for the ternary system CuSe system arises mainly from the multiple oxidation states of its component elements. Single step electrodeposition (SSE) from a Cu+/In3+/Se4+ electrolyte is challenging due to many competing chemical (C) and electrochemical (E) reactions that invariably create multiple solid phases, as partly illustrated in the Fig. 1R schematic. Experimental investigations reveal the simultaneous reduction of Cu2+ and Se4+ ions (E1
) and chemical assimilation of In (C1
) to form CuSe(In) within the first minute [6,7]. The CuSe(In) further reduces to Cu2Se and Se2- (E2
). The Se2- ions may react with Se4+ ions to form Se0 (C2
) or with In3+ ions to form In2Se3 (C3
). Subsequent chemical reactions of In3+ ions with the Cu2Se, In2Se3 and Se2- reaction products form CuInSe2 (C4
) and further CuIn3Se5 (C5
) if sufficient flux of In3+ is available, Fig. 1R.
The reactions leading to the formation of the CISe compounds are thermodynamically driven, which enables SSE of self-stabilized stoichiometries from a single solution, at a single potential, ED. Manipulation of process parameters (temperature, electrolyte composition, pH, mass transport and deposition potential) allows controlling the kinetics of competing reactions for SSE of stoichiometric CuInSe2, CuIn3Se5, or a mixture of the two compounds, Fig. 1M. A brief post-anneal enhances crystallinity without altering the film composition [7, 8]. Surface analytical microscopies and spectroscopies support the fortuitous formation of surprisingly ordered, interlinked, space-filling CISe nanocrystals forming interconnected network of sharp, abrupt 3-dimensional p-CuInSe2/n-CuIn3Se5 nanocrystalline homojunctions (NHJs). This approach generates CISe NHJ films with homogeneous composition on large foil, facilitating easy scale-up via roll-to-roll processing.
Besides the processing advantages, the CISe NHJ films exhibit extraordinary electro-optical attributes that can potentially maximize spectral absorption and reduce recombination losses. The specific NHJ structure enables efficient separation and transport of free carriers, and essentially performs the same functions as standard planar pn junctions. The CISe nanocrystals are very different from colloidal nanocrystals, used in state-of-the-art bulk homojunction devices; they exhibit high doping densities, mixed conductivity and multiple bandgaps. These distinctive innate attributes of CISe films naturally create ordered morphology and facilitate interconnections between the nanocrystals to form optimum NHJ structures. The CISe pn NHJ layer can be directly inserted between 2 band-aligned contact electrodes to produce high performance practical 3G devices, Fig, 1L.
The direct deposition of CISe pn NHJ resolves problems with current approaches for device fabrication based on conventional planar pn junctions, organic/colloidal bulk heterojunctions or inorganic ordered heterojunctions. The totality manifests a highly significant advance in semiconductor processing, providing a generally accessible, low-cost electrochemical method to fabricate high quality pn NHJs in simple flexible thin-film form factor with inorganic material systems. The films can be directly used for harvesting solar energy on a large scale for photoelectrochemical or photovoltaic applications and other devices applications, including light emitting diodes.

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Figure 1. Schematics of (R) Reaction Mechanism (M) SSE of p-CuInSe2/n-CuIn3Se5 NHJ thin film and (L) CISe NHJ device

Figure 1