High-Efficient ZnO/PVD-CdS/Cu(In,Ga)Se 2 Thin Film Solar Cells: Formation of the Buffer-Absorber Interface and Transport
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F14.25.1
High-Efficient ZnO/PVD-CdS/Cu(In,Ga)Se2 Thin Film Solar Cells: Formation of the Buffer-Absorber Interface and Transport Properties Marin Rusu∗, Thilo Glatzel†, Christian A. Kaufmann, Axel Neisser, Susanne Siebentritt, Sascha Sadewasser, Thomas Schedel-Niedrig and Martha Ch. Lux-Steiner Hahn-Meitner-Institut Berlin, Department of Solar Energy, Glienickerstr. 100, 14109 Berlin, Germany ABSTRACT For preparation of ZnO/CdS/Cu(In,Ga)Se2 solar cells, physical vapor deposition (PVD) was employed to deposit CdS buffer layers in ultrahigh vacuum on Se-decapped absorber surfaces, thus realizing an all ‘dry’ fabrication process of the device. An 14.1% total area and 14.5% active area efficient ZnO/CdS/Cu(In,Ga)Se2 solar cell under AM1.5 conditions was achieved after annealing the as-prepared solar cells in air. Kelvin probe force microscopy (KPFM) measurements were carried out in-situ to monitor the initial growth of the CdS buffer layer on the absorber, as well as its electronic properties, in particular, the work function. It was observed that the PVD-CdS growth is initially inhibited at the absorber grain boundaries. Quantum efficiency measurements allowed us to suppose that during the initial growth stage a passivation of the grain boundaries occurs. The latter explains the higher short-circuit currents of the cells with PVD-CdS compared to their references with CdS grown by chemical bath deposition (CBD). The beneficial effect of the annealing seems to originate from a formation of a region with higher band gap than that of the absorber bulk and inverted conductivity type at the absorber surface, close to the CdS/Cu(In,Ga)Se2 interface, leading to a dramatic change in the electronic transport properties and finally, to a significant enhancement of the open-circuit voltage. Annealing of the ZnO/PVD-CdS/Cu(In,Ga)Se2 solar cells provides formation of PVDCdS/Cu(In,Ga)Se2 interface with properties similar to that of reference samples with CBD-CdS. INTRODUCTION Thin film solar cells using Cu(In,Ga)Se2 (CIGSe) as absorber material achieved high efficiencies of 19.2% [1] when using a ‘wet’ chemical bath deposition (CBD) of the CdS buffer layer between the p-type absorber and the n-type ZnO window. However, in industrial production a ‘dry’ in-line vacuum deposition process like physical vapor deposition (PVD) is preferred. Namely the PVD technique was used for the CdS deposition at early stages of the development of the solar cells from CIGSe absorbers. Active area efficiencies of ∼ 12.4% were achieved on solar cells from CuInSe2 and Cu(In,Ga)Se2 thin films with ‘dry’ CdS window layers alloyed by ZnS [2, 3]. Despite of faster progress in solar cell’s efficiency with CBD-CdS, different research groups continued investigations of solar cells with the CdS buffer layers deposited by a dry process [4]. In this work high-efficient ZnO/CdS/CIGSe solar cells with PVD deposited buffer layer are achieved. We report on in-situ Kelvin probe force microscopy (KPFM) investigation of the PVD-CdS growth on the fresh Se-decapped CIGSe ab
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