(Invited) Directing Reaction Paths to Electrodeposit Opportune Nano Pn-junctions from Cu-in-Se Compounds
Authors: Menezes Shalini 
Published: February 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 alternate paths that determine the film composition and electronic properties. Among the numerous contributions to this end is the pioneering voltammetric study of the mechanism of CuInSe2 formation by the Krishnan Rajeshwar group [1]. It provided a firm foundation for various researchers to unravel the rather complicated reaction mechanisms and ultimately electrodeposit stoichiometric CuInSe2 [2-8].
The complexity of the reaction mechanism for the ternary Cu-In-Se (CISe) 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. 1 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 [8]. 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.
The reactions leading to the formation of the ternary CISe compounds are thermodynamically driven, which enables the SSE of self-stabilized compositions from a single solution, at a single potential. 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. A brief post-anneal enhances crystallinity without altering the film composition [8-10]. This approach naturally creates highly-ordered, interlinked, space-filling CISe nanocrystalline films with homogeneous composition on large substrates, making it suitable for easy scale-up via roll-to-roll processing. Besides the processing advantages, the films exhibit extraordinary electro-optical attributes that can potentially maximize spectral absorption and reduce recombination loss. Under certain conditions, SSE can generate a mixture of p-CuInSe2 and n-CuIn3Se5 nanocrystals, Fig. 1R. Surface analytical microscopies and spectroscopies support the fortuitous formation of surprisingly ordered, sharp, abrupt, 3-dimensional, nanoscale p-CuInSe2/n-CuIn3Se5 bulk homojunctions (BHJs), as anticipated from Fig. 1L.
The specific CISe BHJ structure enables efficient separation and transport of free carriers, and essentially performs the same functions as planar pn junctions. The CISe nanocrystals are very different from colloidal nanocrystals, used in state-of-the-art BHJs; 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 the BHJ structure. When sandwiched between band-aligned contact electrode materials, the CISe BHJ film can transition into high performance practical 3G devices, Fig, 1L. The totality manifests a highly significant advance in semiconductor processing, providing a generally accessible, low-cost electrochemical method to fabricate high quality nanocrystalline pn BHJs 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.
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Figure 1