P-type CuInSe 2 thin films prepared by selenization of one-step electrodeposited precursors
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In this study, p-type CuInSe2 (CIS) films were prepared by selenization of one-step electrodeposited Cu-In-2Se (atomic ratio) precursors. To obtain high-quality, dense, and homogeneous CIS films for solar cell application, the effects of substrate temperatures during selenization and precursor compositions on the final microstructures were systematically investigated. The precursor layers evolved in very different ways under different selenization conditions. The final microstructures and phases of the films depended critically on the precursor compositions, selenization temperature, and the selenization thermal process history. Low melting temperature CuxSe phase, which tended to segregate at the film surface, can efficiently assist the CIS grain growth. Large hexagonal CuSe platelets were formed at a temperature as low as 170 C in Cu-rich precursor, which acted as an element-transport flux agent at higher temperature under high Se vapor and reacted with In-Se selenide to form CIS at temperatures above 500 C. Good crystalline quality chalcopyrite CIS film was obtained at a selenization temperature of 550 C.
I. INTRODUCTION
Copper indium diselenide (CuInSe2 or CIS) belongs to the I-III-VI2 family, which has received considerable attention in the past two decades because of its application in solar cells.1 Cu(InxGa1-x)Se2 solar cells with efficiencies of 19.9% have been fabricated over small areas using physical vapor co-evaporation technique.2 Although several different techniques have been used successfully to prepare CuInSe2 thin films, none of them has yet shown a highly reproducible and low-cost yield needed for large-scale commercial production of largearea devices. The production of CuInSe2 from nonvacuum and one-step electrodeposition (ED) is a promising method because low cost and good control of deposition of the absorber layer CIS could be achieved over a large area compared with the physical-vapor-deposition techniques.3 Several groups have investigated the electrodeposition of CIS materials.4–6 The problem that has hindered development of this technique was the poor crystalline quality and electronic properties obtained by different annealing treatments of the precursor layers such as vacuum annealing of the electrodeposited precursors.5–7 Selenium has a much higher vapor pressure
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0261 J. Mater. Res., Vol. 24, No. 7, Jul 2009
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than other elements, i.e., copper, indium, and gallium. Selenium would be lost through evaporation from the precursor layer if a high Se vapor pressure was not present during the high temperature annealing. We compared x-ray diffraction (XRD) and Raman results of both the vacuum-annealed films and the selenized films. Both the XRD and Raman spectra showed much stronger and narrower peaks for the selenized films than that of the vacuum-annealed films, indicating a much higher crystalline quality for the selenized layers. T
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