Cu(In,Ga)S 2 Phase Formation from Metallic Cu-In-Ga Precursor Stacks in Rapid Thermal Processes
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Cu(In,Ga)S Phase Formation from Metallic Cu-In-Ga Precursor Stacks in Rapid Thermal Processes
´ Axel Neisser , Jacobo Alvarez-Garc´ ıa , Lorenzo Calvo-Barrio , Reiner Klenk , Thomas W. Matthes , Ilka Luck , Martha Ch. Lux-Steiner , Alejandro P´erez-Rodr´ıguez , and J.R. Morante Hahn-Meitner-Institut Berlin GmbH, Glienicker Strasse 100, D - 14 109 Berlin, Germany Lab. Enginyeria i Materials Electr`onics (EME), Unitat Associada CNM-CSIC, Departament d’ Electr`onica, Universitat de Barcelona, C. Mart´ı i Franqu`es 1, 08028 Barcelona, Spain ABSTRACT This contribution compares the growth of Cu(Ga,In)S based thin film solar cell absorbers in rapid thermal systems using sulfur vapor S or H S/Ar as reactive atmosphere, focusing on Ga-related influences on film growth and phase formation. Cu-In alloying in the precursor is kinetically hindered by the presence of Cu-Ga phases. In sulfur vapor Ga-containing samples sulfurize via an intermediate CuIn S phase, thereby delaying the full sulfurization and recrystallization of the layer. In contrast, in H S/Ar fast Ga-In interdiffusion and no intermediate chalcogenide phases are observed. The inhomogeneous Ga depth distribution usually reported for sequentially prepared Cu(In,Ga)S films can be assigned to the segregation of CuGaS prior to CuInS . INTRODUCTION Conversion efficiencies above 11 % have been achieved with CuInS /CdS/ZnO thin film solar cells based on sequentially prepared absorber layers. However, this is still below the efficiency of current chalcopyrite champion devices, based on coevaporated Cu(In,Ga)Se absorber layers [1], which is mainly due to a moderate open circuit voltage . The incorporation of Gallium is a successful way to substantially increase . Values above 800 mV have been reported for Cu(In,Ga)S absorber layers [2], prepared by reactive annealing of Cu-In-Ga precursor stacks in sulfur vapor for 30 min. Recently, significant improvements of CuInS based solar cells have been reported for using a Rapid Thermal Processing (RTP) furnace for the precursor annealing step in sulfur vapor, which allowed for very fast heat up ramps and annealing times as short as 3 to 5 minutes [3]. By employing a similar RTP-furnace but using a H S/Ar mixture as the reactive atmosphere, we have shown that such short annealing times still lead to the Ga-induced open circuit voltage gain known from the conventional process [4]. However, reducing the annealing time of Cu-In-Ga precursors in sulfur vapor was not successful in a reproducible way. Especially the characteristic open circuit voltage increase was not observed with these devices. Therefore, this contribution studies the sulfurization process focusing on the influences of the reactive atmosphere on the reaction kinetics and phase formation of Cu(In,Ga)S thin films prepared from Cu-In-Ga precursor stacks. In order to investigate Ga-related differences of growth path and phase formation sequence Ga-containing as well as Ga-free precursors were sulfurized in sulfur vapor in the same annealing step.
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