CdS and Cd-Free Buffer Layers on Solution Phase Grown Cu 2 ZnSn(S x Se 1- x ) 4 :Band Alignments and Electronic Structur
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CdS and Cd-Free Buffer Layers on Solution Phase Grown Cu2ZnSn(SxSe1- x)4 :Band Alignments and Electronic Structure Determined with Femtosecond Ultraviolet Photoelectron Spectroscopy Richard Haight1, Aaron Barkhouse1, Wei Wang1, Yu Luo1 , Xiaoyan Shao1 , David B. Mitzi1, Homare Hiroi2, Hiroki Sugimoto2 1 2
IBM TJ Watson Research Center, PO Box 218, Yorktown Hts., NY 10598 Atsugi Research Center, Solar Frontier K.K., Atsugi, Kanagawa 243-0206, Japan
ABSTRACT The heterojunctions formed between solution phase grown Cu2ZnSn(SxSe1- x)4 (CZTS,Se) and a number of important buffer materials including CdS, ZnS, ZnO, and In2S3, were studied using femtosecond ultraviolet photoemission spectroscopy (fs-UPS) and photovoltage spectroscopy. With this approach we extract the magnitude and direction of the CZTS,Se band bending, locate the Fermi level within the band gaps of absorber and buffer and measure the absorber/buffer band offsets under flatband conditions. We will also discuss two-color pump/probe experiments in which the band bending in the buffer layer can be independently determined. Finally, studies of the bare CZTS,Se surface will be discussed including our observation of mid-gap Fermi level pinning and its relation to Voc limitations and bulk defects. INTRODUCTION A key parameter impacting the performance of heterojunction photovoltaic (PV) devices is the conduction band offset (cbo) between absorber and buffer. If the offset is negative (cliff), such that the conduction band of the n-type buffer layer is below that of the p-type absorber, interface recombination can significantly lower the open circuit voltage (Voc) thereby reducing the power efficiency of the device[1]. Conversely, if the cbo is positive (spike) but too large, the barrier to minority (electron) transport from the absorber to the buffer can reduce the short circuit current (Jsc) again lowering efficiency. In addition to the energetic alignment of the band edges, the interfacial chemical composition and the density of defects in both the absorber and buffer are important issues. This is particularly true for Cu2ZnSn(SxSe1-x)4 (CZTS,Se), an earth abundant material that is a potential replacement for Cu(In,Ga)(SxSe1-x)2 (CIGSSe). While achieving 20.4% [2] power efficiency in a full PV device, CIGSSe suffers from the relative rarity and high cost of two of its key constituent materials, In and Ga. To date, solution phase grown CZTS,Se PV devices have achieved a world record efficiency of 12.0% [3] with CdS buffer layers deposited via chemical bath deposition (cbd). While this is an important achievement, continued improvement is required for this material to be incorporated into an economically viable PV technology. It is therefore critically important to accurately measure the valence band offsets (vbo) and conduction band offsets (cbo) of CdS deposited on CZTS,Se, and to consider alternative non-Cd buffers in an attempt to both improve overall efficiency and reduce the environmental impact of Cd. In this paper we will describe femtosecond laser based u
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