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Journal Article Ultrafast Photocarrier Dynamics at the p-n Junction in Cu(In,Ga)Se2 Solar Cell with Various Zn(O,S) Buffer Layers Measured by Optical Pump-Terahertz Probe Spectroscopy
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Authors
Woo-Jung Lee, Dae-Hyung Cho, Jae-Hyung Wi, Hye-Jung Yu, Won Seok Han, Jung Min Bae, Jaehun Park, Yong-Duck Chung
Issue Date
2018-01
Citation
ACS Applied Energy Materials, v.1, no.2, pp.522-530
ISSN
2574-0962
Publisher
American Chemical Society(ACS)
Language
English
Type
Journal Article
DOI
https://dx.doi.org/10.1021/acsaem.7b00127
Project Code
17PB5100, Development of Commercial Process Equipment for Light-weight Flexible CIGS Thin Film PV Module, Chung Yong-Duck
Abstract
We fabricated Cu(In,Ga)Se2 (CIGS) solar cells with three types of Zn(O,S) buffer layer produced by different techniques: S thermal cracking, chemical bath deposition (CBD), and reactive sputtering. The highest cell efficiency was obtained from the CIGS solar cell with the CBD-Zn(O,S) buffer layer. The solar cell performance is predominantly determined by the population of defect states distributed in the p-n junction and CIGS bulk layer. We utilized the optical pump-terahertz probe (OPTP) spectroscopy to measure the carrier lifetimes governed by the trapping time at defect states and found out short (?꼜) and long (?꼕) lifetimes related to the surface defects and Cu vacancy (VCu) defects, respectively. Both ?꼜 and ?꼕 increased after deposition of a Zn(O,S) layer, which is attributed to the surface curing and the decrease of VCu defect states with the substitution of Zn atoms for VCu, resulting in "ZnCu". To investigate band alignment at the p-n junction depending on the different buffer types, we measured the valence band offset (?봂V) along the depth direction using X-ray photoemission spectroscopy with sputtering process. Based on the ?봂V values, we suggested a band alignment schematically at the interface of CIGS/Zn(O,S). Taking the carrier lifetimes and barrier height (conduction band offset, ?봂C) in band alignment into account, we established an important correlation between cell efficiency and interfacial properties that the performance of the CIGS solar cell can be improved with a sufficiently long carrier lifetime of ?꼕 (approximately a nanosecond) and appropriate ?봂C (0.3-0.4 eV) at the p-n junction.
KSP Keywords
Buffer layer, CIGS solar cell, Cell Efficiency, Chemical bath deposition(CBD), Conduction band offset, Different techniques, Interfacial Properties, Optical pump-terahertz probe(OPTP), Optical pump-terahertz probe spectroscopy, P-N junction, Petri net(PN)