Passivation of Deep-Level Defects by Cesium Fluoride Post-Deposition Treatment for Improved Device Performance of Cu(In,Ga)Se₂ Solar Cells

Abstract

Heavy-alkali post-deposition treatments (PDTs) utilizing Cs or Rb has become an indispensable step in producing high-performance Cu(In,Ga)Se 2 (CIGS) solar cells. However, full understanding of the mechanism behind the improvements of device performance by heavyalkali treatments, particularly in terms of potential modification of defect characteristics, has not been reached yet. Here, we present an extensive study on the effects of CsF-PDT on material properties of CIGS absorbers and the performance of the final solar devices. Incorporation of an optimized concentration of Cs into CIGS resulted in a significant improvement of the device efficiency from 15.9 to 18.4% mainly due to an increase in the open-circuit voltage by 50 mV. Strong segregation of Cs at the front and rear interfaces as well as along grain boundaries of CIGS was observed via high-resolution chemical analysis such as atomic probe tomography. The study of defect chemistry using photoluminescence and capacitance-based measurements revealed that both deep-level donor-like defects such as V Se and In Cu and deep-level acceptor-like defects such as V In or Cu In are passivated by CsF-PDT, which contribute to an increased hole concentration. Additionally, it was found that CsF-PDT induces a slight change in the energetics of V Cu , the most dominant point defect that is responsible for the p-type conductivity of CIGS.

Morphological–Electrical Property Relation in Cu(In,Ga)(S,Se)2 Solar Cells: Significance of Crystal Grain Growth and Band Grading by Potassium Treatment

“…Note that the K atoms incorporated in the CIGS film are expected to accumulate at the CdS/CIGS interface and CIGS grain boundaries, which passivate defects and provide doping characteristics to the CIGS absorber layer. [ 17,19,20 ] The BK‐CIGS device shows a decreasing hole concentration in comparison to the U‐CIGS and SK‐CIGS devices as shown in the C – V results (Figure 6a), which seems to be an opposite trend of increasing hole carrier density, often reported in the literature. [ 31,55 ] Although there might exist K involved defects, likely, defect passivation and p‐doping do not significantly occur in these K incorporated devices.…”

“…To improve the performance of CIGS solar cells fabricated through vacuum processing, alkali metal (AM) atoms are commonly incorporated on the CIGS surface through post-deposition treatment (PDT). [14][15][16][17][18][19][20][21][22][23][24][25] Such treatment has recently resulted in efficiencies as high as ≈23%. [15,16] While light AM atom such as sodium (Na) diffuses into grain interior and changes doping concentration, [17,19] heavier AM atoms like potassium (K), rubidium (Rb), and cesium (Cs) tend to form a segregated secondary phase (AM-In-Se) at the grain boundaries and CdS/ CIGS interface, [17,18] and this passivates defects.…”

“…[15,16] While light AM atom such as sodium (Na) diffuses into grain interior and changes doping concentration, [17,19] heavier AM atoms like potassium (K), rubidium (Rb), and cesium (Cs) tend to form a segregated secondary phase (AM-In-Se) at the grain boundaries and CdS/ CIGS interface, [17,18] and this passivates defects. [17][18][19][20][21] Besides, the AM-In-Se phase at the CdS/CIGS interface widens the bandgap and enhances charge selectivity. [22,23] Also, AM PDT reduces the CdS thickness by promoting Cd interdiffusion, and this reduces the current loss due to CdS parasitic absorption.…”

Temperature‐dependent current–voltage and admittance spectroscopy analysis on cesium‐treated Cu (In1 − x,Gax)Se2 solar cell before and after heat‐light soaking and subsequent heat‐soaking treatments

“…It has been suggested that Cs reduces deep donor‐like defects (V Se and In Cu ) as well as deep‐level acceptor‐like defects (V In or Cu In ) and increases p ‐type characterization. 65 This, in turn, increases N CV .…”

Multilayer Cascade Charge Transport Layer for High‐Performance Inverted Mesoscopic All‐Inorganic and Hybrid Wide‐Bandgap Perovskite Solar Cells

“…[ 41,42 ] It was reported that F - can drift between lattice defects due to its very small ionic radius compared with other reported elements, which theoretically has the potential to fill the interstice, to mediate the perovskite crystallization, and to passivate deep‐level defects in hybrid perovskite system. [ 43–45 ] Moreover, incorporating fluorine could also strengthen the hydrophobicity of the resulting materials. Therefore, fluorine was chosen to tailor the crystallization process of CsPbI 2 Br in this study.…”