Effect of Cu-deficient layer formation in Cu(In,Ga)Se2 solar-cell performance

Cu-deficient layer (CDL) on Cu(In,Ga)Se-2 (CIGS) promotes Cd diffusion from CdS buffer layer and forms a valence band offset (Delta E-V) between CDL and CIGS. We quantitively demonstrate the effects of CDL formation on the performance of CIGS solar cells through experiments and theoretical simulation. To investigate the effects of Cd diffusion and Delta E-V by CDL, theoretical analysis was carried out for a CIGS solar cell with a surface layer which simulated the CDL at CdS/CIGS interface. It was revealed that when electron concentration in n-type surface layer is higher than the absolute carrier concentration in CIGS absorber (N-D > vertical bar N-A,N- CIGS vertical bar), open-circuit voltage and fill factor are improved. Additionally, Delta E-V >= 0.15 eV leads to the highest open-circuit voltage by suppression of interfacial recombination. Transmission electron microscope energy dispersive X-ray spectrometry and scanning spreading resistance microscopy were employed for the same cross section of a CIGS solar cell fabricated by three-stage process. Despite CDL with Cu/(Ga + In) of 0.31 formed on the surface had high Cd contents of 3.4 at%, its carrier concentration of 4.8 x 10(10) cm(-3) was lower than that of 10(14) -10(16) cm(-3) in grain interior owing to insufficient activation of Cd atoms. These results indicate the effectiveness of Delta E-V formation by introducing CDL with low Cu/(Ga + In) of 0.31 to boost CIGS solar cell performance and difficulty in realizing N-D > vertical bar N-A,N- CIGS vertical bar by surface Cd doping.