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

Abstract

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 × 1016cm–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. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)

Addressing the impact of rear surface passivation mechanisms on ultra-thin Cu(In,Ga)Se2 solar cell performances using SCAPS 1-D model

“…The electric-field creation/modification is mainly attributed to the reformed position (relative) of the conduction-band edge with respect to the vacuum level (i.e. bandgap engineering) [11][12][13][14]. In principle, it is possible to implement effective E-fields by tailoring either the bandgap and/or the doping profiles within the absorber films [11,12].…”

Investigating the electronic properties of Al2O3/Cu(In,Ga)Se2 interface

“…These approaches involve novel methods to passivate the front and rear surface of the CIGS absorber by (i) implementing alkali post-deposition treatments at the front surface, 1-3 (ii) passivating the rear surface of the CIGS/Mo interface using ALD Al 2 O 3 films, 4,5 and (iii) implementing back surface field-effect passivation using gallium grading schemes within the CIGS absorber layer. 6 In our previous studies, 4,5 we reported that Al 2 O 3 rear-surface passivation of ultra-thin CIGS solar cells can significantly enhance the open-circuit voltage (V oc ) due to a reduced rear surface recombination velocity at the CIGS/Mo interface, ultimately leading to a notable enhancement in cell efficiency [i.e., by more than 3.5% (abs.)] compared with corresponding unpassivated reference a R. Kotipalli 8 reported that introducing ALD Al 2 O 3 passivation films on CIGS surfaces could reduce the effective surface recombination velocity (S eff ) to 14-44 cm/s.…”

Modeling and Investigation of Rear-Passivated Ultrathin CIGS Solar Cell

“…They are Shockley-Read-Hall (SRH), Auger, and radiative recombination. The total recombination rate is the sum of all these three recombination rates [7,11].…”

“…The ultrathin CIGS structures with Al 2 O 3 rear surface passivation layer were investigated and optimized by Jackson, Bart, Joel and our research group [1,8,9,10]. Kotipalli's group has reported that decreasing the deep-defect states can improve cell performance [11]. Several works reported different strategies that were considered to reduce the rear surface recombination by implementing a very thin oxide layer and using different contact materials on the rear side of the cells [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26].…”