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Influence of Ga/(Ga + In) grading on deep-defect states of Cu(In,Ga)Se2 solar cells

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Kotipalli, Raja Venkata Ratan [UCL] Vermang, Bart [Angström Solar Center, University of Uppsala, Sweden] Fjällström, Vikto [Angström Solar Center, University of Uppsala, Sweden] Edoff, Marika [Angström Solar Center, University of Uppsala, Sweden] Delamare, Romain [UCL] Flandre, Denis [UCL]

The benefits of gallium (Ga) grading on Cu(In,Ga)Se2 (CIGS) solar cell performance are demonstrated by comparing with ungraded CIGS cells. Using drive-level capacitance profiling (DLCP) and admittance spectroscopy (AS) analyses, we show the influence of Ga grading on the spatial variation of deep defects, free-carrier densities in the CIGS absorber, and their impact on the cell’s open-circuit voltage Voc. The parameter most constraining the cell’s Voc is found to be the deep-defect density close to the space charge region (SCR). In ungraded devices, high deep-defect concentrations (4.2 × 1016 cm–3) were observed near the SCR, offering a source for Shockley–Read–Hall recombination, reducing the cell’s Voc. In graded devices, the deep-defect densities near the SCR decreased by one order of magnitude (2.5 × 1015 cm–3) for back surface graded devices, and almost two orders of magnitude (8.6 × 1014 cm–3) for double surface graded devices, enhancing the cell’s Voc. In compositionally graded devices, the free-carrier density in the absorber’s bulk decreased in tandem with the ratio of gallium to gallium plus indium ratio GGI = Ga/(Ga + In), increasing the activation energy, hindering the ionization of the defect states at room temperature and enhancing their role as recombination centers within the energy band.

Document type Article de périodique (Journal article) – Article de recherche
Access type Accès restreint
Publication date 2015
Language Anglais
Journal information "Physica Status Solidi. Rapid Research Letters" - , p. 4 (2015)
Peer reviewed yes
Publisher Wiley - V C H Verlag GmbH & Co. KGaA ((Germany) Weinheim)
issn 1862-6254
e-issn 1862-6270
Publication status Publié
Affiliation UCL - SST/ICTM/ELEN - Pôle en ingénierie électrique
Keywords Gallium ; Deep levels ; Defects ; Admittance spectroscopy ; CuInGaSe2 ; Solar cells
Links
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Bibliographic reference Kotipalli, Raja Venkata Ratan ; Vermang, Bart ; Fjällström, Vikto ; Edoff, Marika ; Delamare, Romain ; et. al. Influence of Ga/(Ga + In) grading on deep-defect states of Cu(In,Ga)Se2 solar cells. In: Physica Status Solidi. Rapid Research Letters, , p. 4 (2015)
Permanent URL http://hdl.handle.net/2078.1/156770