Fingerprints Indicating Superior Properties of Internal Interfaces in Cu(In,Ga)Se₂ Thin‐Film Solar Cells

Abstract

Growth of Cu(In,Ga)Se2 (CIGS) absorbers under Cu‐poor conditions gives rise to incorporation of numerous defects into the bulk, whereas the same absorber grown under Cu‐rich conditions leads to a stoichiometric bulk with minimum defects. This suggests that CIGS absorbers grown under Cu‐rich conditions are more suitable for solar cell applications. However, the CIGS solar cell devices with record efficiencies have all been fabricated under Cu‐poor conditions, despite the expectations. Therefore, in the present work, both Cu‐poor and Cu‐rich CIGS cells are investigated, and the superior properties of the internal interfaces of the Cu‐poor CIGS cells, such as the p–n junction and grain boundaries, which always makes them the record‐efficiency devices, are shown. More precisely, by employing a correlative microscopy approach, the typical fingerprints for superior properties of internal interfaces necessary for maintaining a lower recombination activity in the cell is discovered. These are a Cu‐depleted and Cd‐enriched CIGS absorber surface, near the p–n junction, as well as a negative Cu factor (∆β) and high Na content (>1.5 at%) at the grain boundaries. Thus, this work provides key factors governing the device performance (efficiency), which can be considered in the design of next‐generation solar cells.

Revani diffusion model in Cu(In,Ga)Se2

“…2d), which is supported by the recent detection of sparse Cu-rich GBs even in nominally Cu-poor lms. 68 The combined effect is consistent with the general decrease of electrostatic potential uctuations and may explain why sodium doping happens to be essential in the rst place. In 2018, highly efficient single crystal devices were obtained for the rst time in the history of CIGS.…”

“…2d), which is supported by the recent detection of sparse Cu-rich GBs even in nominally Cu-poor films. 68…”

The Physics of Twin Boundary Termination in Cu(In, Ga)Se2 Absorbers

“…Such a configuration allows better separation and collection of electrons (minority carriers) and hence superior electrical properties of the TBs, which was also evidenced by EBIC. Our recent work [ 25 ] also showed that a strong variation in Cu content (whether in the grain or at the GBs) changes the trait of the GB. However, Cu‐poor GBs were always detected as electrically beneficial.…”

“…[24] Even more importantly, the termination of TBs was previously not experimentally correlated with their physical properties. Our very recent studies [18,25] have shown that GBs of multiple traits coexist in the same CIGS absorber and that these traits strongly correlate with the GB compositions determined by atom probe tomography (APT). More specifically, 58% of the GBs were found to be electrically beneficial, exhibiting Cu depletion and In enrichment characteristics, whereas a small percentage of GBs (15%) were found to be electrically detrimental, exhibiting Cu enrichment and In depletion characteristics.…”

Sodium Incorporation for Performance Improvement of Solution‐Processed Submicron CuIn(S,Se)2 Thin Film Solar Cells

“…[ 21 ] The back barrier height ( Φ B ) can be calculated as the difference between E a1 and E a2 , and drops from >0.3 eV for the reference to close to 0.2 eV for Pre‐ST CISSe (Table 2). [ 32 ] Na will also incorporate into the Mo back contact, which is beneficial for Mo(S,Se) 2 layer formation, resulting in the reduction of the Φ B . [ 15,32b ]…”