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

Abstract

Cu-deficient layer (CDL) on Cu(In,Ga)Se 2 (CIGS) promotes Cd diffusion from CdS buffer layer and forms a valence band offset (ΔE 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 ΔE 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 > |N A, CIGS |), open-circuit voltage and fill factor are improved. Additionally, ΔE 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 × 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 ΔE 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 > |N A, CIGS | by surface Cd doping.

Characterization of Interface Between Accurately Controlled Cu‐Deficient Layer and Cu(In,Ga)Se2 Absorber for Cu(In,Ga)Se2 Solar Cells

“…A positive effect is expected when a CDL is formed on the CIGS surface. It has been reported that the valence band maximum ( E V ) decreases in Cu‐deficient condition, which results in the formation of a valence band offset (Δ E V ) between CIGS and CDL, whereas Δ E V causes the suppression of interfacial recombination at the n‐type CdS buffer layer/CIGS absorber interface . However, an accurate control method for CDL formation has not yet been revealed, despite numerous studies on CIGS solar cells.…”

Accurate control and characterization of Cu depletion layer for highly efficient Cu(In,Ga)Se2 solar cells

“…This suggests that the regions of high resistance that formed on the surfaces in Figures indicate Cu depletion. So far, we have revealed that high resistance region partially exists on CIGS surface, indicating low Cu contents . We suggested that the difficulty of n ‐type conductivity by Cd doping into CDL on CIGS surface to enhance conversion efficiency in CIGS solar cells owing to insufficient activation of Cd atoms.…”

“…Location with a low resistance of 10 6 –10 7 Ω in CIGS films is uniformly distributed in Figure B, while the low resistance region randomly exists in Figure A,C. Grain boundaries in CIGS have low resistance owing to Na elements derived from SLG substrate, which passivate donor defects for In on Cu anti‐site, whereas resistance in the grain boundaries increases when CDL exists on the CIGS surface since diffusion of Cd elements, which act as donor, during CBD process for CdS buffer layer is promoted by the formation of CDL . Hence, inhomogeneous distribution of grain boundaries with low resistance in Figure A,C is due to the non‐uniform formation of CDL on the CIGS surface.…”

“…In Cu depletion whereas resistance in the grain boundaries increases when CDL exists on the CIGS surface since diffusion of Cd elements, which act as donor, during CBD process for CdS buffer layer is promoted by the formation of CDL. 33 Hence, inhomogeneous distribution of grain boundaries with low resistance in Figure 2A,C is due to the non-uniform formation of CDL on the CIGS surface.…”

“…So far, we have revealed that high resistance region partially exists on CIGS surface, indicating low Cu contents. 33 We suggested that the difficulty of n-type conductivity by Cd doping into CDL on CIGS surface to enhance conversion efficiency in CIGS solar cells owing to insufficient activation of Cd atoms. In Cu depletion whereas resistance in the grain boundaries increases when CDL exists on the CIGS surface since diffusion of Cd elements, which act as donor, during CBD process for CdS buffer layer is promoted by the formation of CDL.…”