LetterElectrodeposition of p–i–n type CuInSe2 multilayers for photovoltaic applications
Introduction
Copper indium diselenide (CuInSe2 or CIS) has a direct bandgap of ∼1.05 eV, a high-absorption coefficient and a large minority carrier diffusion length, which are particularly suitable for photovoltaic applications. Various techniques such as molecular beam epitaxy (MBE), metalorganic chemical vapour deposition (MOCVD), physical vapour deposition (PVD), sputtering, e-beam thermal evaporation and electrodeposition have been reported for the preparation of CIS films with varying degrees of success. Among these methods, electrodeposition is an attractive method, which has successfully been used for the preparation of elemental, binary, intermetallic, ternary and quaternary compounds. At the same time, increasing emphasis on nanotechnology has been accompanied by a corresponding increase of interest in electrodeposition of semiconductors [1], metallic [2] and magnetic [3] nanostructures.
Important advantages of electrodeposition are low equipment cost, high deposition speed, negligible waste of chemicals, scalability and manufacturability of large area polycrystalline films. It is an isothermal process, mainly controlled by electrical parameters which can be adjusted to control film thickness, morphology and composition [4]. In addition, toxic gaseous precursors are not involved as in chemical gas phase methods.
In a separate paper [5], the electrodeposition of p+-, p-, i-, n- and n+-type CuInGaSe2 thin films and the initial performance of four-layer solar cells are reported. This work showed that the atomic percentage concentration of elements changed with cathodic potential producing different types of electrical conductivity for CIGS layers. This letter presents the electrodeposition of p-, i- and n-type CIS layers from a single electrolyte and material as well as preliminary device properties. X-ray diffraction (XRD), X-ray fluorescence (XRF), photoelectrochemical (PEC) cell and atomic force microscopy (AFM) were used to characterise the films, and current–voltage (I–V) technique was used to assess the electrical properties of four-layer n–n–i–p (glass/FTO/n-CdS/n-CIS/i-CIS/p-CIS/Au) solar cell structures.
Section snippets
Experimental details
Copper, indium and selenium were deposited simultaneously by electrodeposition using a standard three-electrode system. EG&G Princeton Applied Research potentiostat/galvanostat Model 362 was used for the deposition of thin films. Deposition was carried out potentiostatically in an aqueous solution containing 0.002 M CuSO4, 0.004 M In2(SO4)3 and 0.004 M H2SeO3 with Cu:In:Se ratio of 1:2:2 in the electrolyte. A saturated Ag/AgCl reference electrode, a graphite plate and fluorine-doped tin oxide
Phase characterisation (XRD)
Crystal structure of the as-deposited films was investigated by XRD. Figs. 1a–c show the XRD patterns of as-deposited films at 0.60, 0.75 and 1.00 V cathodic potentials with respect to Ag/AgCl reference electrode, drawn with the same intensity scale. Some peaks are due to the FTO substrate and they are marked with a star (*) sign. It is seen that the planes (1 1 2) and (2 0 4)/(2 2 0) of CuInSe2 of chalcopyrite phase are common in all three diffractograms. An additional CIS peak (1 1 6/3 1 2) also appears
Conclusions
In conclusion, polycrystalline p-, i- and n-type thin films of CuInSe2 layers have been deposited using one-step electrodeposition and from a single electrodeposition bath. Structural and electrical properties studied showed that the layers deposited at low cathodic potentials, below 0.60 V yield copper-rich CIS layers and the materials deposited at high cathodic potentials above 0.90 V yield indium-rich CIS layers. Both XRD and XRF results confirm this change in stoichiometry and the PEC
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