Influence of Ga/(Ga + In) grading on deep-defect states of Cu(In,Ga)Se2solar cells

Autor: Vikto Fjällström, Romain Delamare, Denis Flandre, Raja Venkata Ratan Kotipalli, Bart Vermang, Marika Edoff
Rok vydání: 2015
Předmět:
Zdroj: physica status solidi (RRL) - Rapid Research Letters. 9:157-160
ISSN: 1862-6254
DOI: 10.1002/pssr.201510024
Popis: 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.
Databáze: OpenAIRE
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