High photo-conversion efficiency in double-graded Cu(In,Ga)(S,Se)2 thin film solar cells with two-step sulfurization post-treatment

Gee Yeong Kim, Jung Yup Yang, Trang Thi Thu Nguyen, Seokhyun Yoon, Junggyu Nam, Dongho Lee, Dongseop Kim, Minsu Kwon, Chan Wook Jeon, Yoon Koo Kim, Seung Yong Lee, Miyoung Kim, William Jo

Research output: Contribution to journalArticlepeer-review

12 Scopus citations

Abstract

Sulfur is extensively used to increase the bandgap of Cu(In,Ga)(S,Se)2(CIGSSe) solar cells and to improve the open circuit voltage (VOC) in order to optimize the characteristics of the devices. This study uses a sulfurization process to obtain a double-graded bandgap profile. Selenization was carried out on Cu(In,Ga) precursors, followed by one sulfurization process or two consecutive sulfurization processes on top of the CIGSe absorber layer surface. The optimum two-step sulfurization process provides an increase of VOCof 0.05 V and an improvement of conversion efficiency of 1.17%. The efficiency of the 30 × 30 cm2monolithic module, which has 64 CIGS cells connected in series (aperture area: 878.6 cm2), is 15.85%. The optical and electrical properties of the phase and the work function distribution were investigated using the depth profiles of the absorber layer as a function of the sulfurization conditions. The CIGSSe thin film formed by two-step sulfurization with a high sulfur concentration exhibits a single work function peak, better crystallinity, and higher conversion efficiency than those of the thin film formed by two-step sulfurization at low sulfur concentration. In terms of the Raman spectra depth profile, the phase areas for the CIGSSe thin film that underwent the optimized high sulfur concentration two-step-sulfurization appeared to have less of Cu2-xSe phase than that with low sulfur concentration. Consequently, surface and interface phase analysis is an essential consideration to improve cell efficiency.

Original languageEnglish
Pages (from-to)139-148
Number of pages10
JournalProgress in Photovoltaics: Research and Applications
Volume25
Issue number2
DOIs
StatePublished - 1 Feb 2017

Keywords

  • bandgap grading
  • CIGSSe solar cell
  • Kelvin probe force microscopy
  • Raman scattering spectroscopy

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