Nanoscale Surface Analysis Reveals Origins of Enhanced Interface Passivation in RbF Post Deposition Treated CIGSe Solar Cells

Abstract

Alkali post deposition treatments (PDTs) of Cu(In,Ga)Se 2 absorbers have boosted the power conversion efficiency (PCE) of the solar cell devices in the last years. A detailed model explaining how the PDTs impact the optoelectronic properties at the nanoscale is still lacking. Here, via various scanning probe techniques, X-Ray photo-electron spectroscopy and high resolution secondary ion mass spectroscopy it is shown that the RbF PDT treatments lead to a one to one exchange of Rb with Cu at the surface. This exchange takes place in the naturally occurring Cu-depleted CIGSe surface, known as the ordered vacancy compound. A detailed comparison between samples with different PDTs after various cleaning procedures furthermore highlights the necessity to perform all the measurements on NH 4 OH cleaning surfaces only. After NH 4 OH, no RbInSe 2 phase could be detected at the surface anymore and the surface bandgap, as measured with scanning tunneling spectroscopy is only 1.7 eV. The findings demonstrate that the existence of a RbInSe 2 phase is most likely not responsible for the recent improvements in power conversion efficiency for state of the art solar cells. The primary effect of the PDT treatment is a modification of the ordered vacancy compound, where Cu is exchanged with Rb.

Cesium Modulation in Cu(In, Ga)(S, Se)2 Solar Cells: Comprehensive Analysis on Interface, Surface, and Grain Boundary

“…These shifts in binding energies resemble the characteristics observed in the formation of K–In–Se or Rb–In–Se . Although recent research suggests that the improvements in PCE may not be solely attributed to the formation of the alkali–In-Se 2 phase but rather to the exchange of Cu with alkali metal in OVC, our data provides evidence of new bond formation, regardless of whether it is a stoichiometric alkali–In-Se 2 phase or not. This all contributes to widening of the surface bandgap.…”

“…The In 3d 5/2 in Cs-CIGSSe shows an increased intensity (denoted as In 3d 5/2 -1) at higher binding energy (BE); additionally, Se 3d, and S 2p display additional peaks at lower binding energies (denoted as Se 3d 3/2 -2, Se 3d 5/2 -2, S 2p 1/2 -2, S 2p 3/2 -2, Se 3p 1/2 -2, and Se 3p 3/2 -2, as shown in Figure S8). These shifts in binding energies resemble the characteristics observed in the formation of K–In–Se or Rb–In–Se . Although recent research suggests that the improvements in PCE may not be solely attributed to the formation of the alkali–In-Se 2 phase but rather to the exchange of Cu with alkali metal in OVC, our data provides evidence of new bond formation, regardless of whether it is a stoichiometric alkali–In-Se 2 phase or not.…”

“…The studies have reported the depletion of Cu and Ga near the front interface in alkali-PDT devices. 11 , 14 , 15 The ion exchange effect of heavier alkali metal ions on the CIGS-based solar cells with lighter alkali metal ions has also been observed. 13 , 14 Owing to the compatibility of atomic radius, lighter alkali metal ions tend to occupy the Cu sites, while heavier alkali metal ions tend to stay at the grain boundary.…”

“…Numerous research studies have delved into the modification of CIGS-based solar cells following alkali-PDT. The studies have reported the depletion of Cu and Ga near the front interface in alkali-PDT devices. ,, The ion exchange effect of heavier alkali metal ions on the CIGS-based solar cells with lighter alkali metal ions has also been observed. , Owing to the compatibility of atomic radius, lighter alkali metal ions tend to occupy the Cu sites, while heavier alkali metal ions tend to stay at the grain boundary. ,− In addition, the formation of high bandgap alkali–In-Se-related compounds may be considered as the passivation layer at the p–n junction. Nevertheless, the existence of the high bandgap alkali–In-Se-related compound following alkali-PDT strongly depends on the composition of the CIGS absorber or alkali incorporation degree, which is not universally observed on all alkali-PDT devices. − Therefore, the promising reasons for this efficiency boost are still the subject of controversy.…”

Grain boundaries are not the source of Urbach tails in Cu(In,Ga)Se2 absorbers

“…Over the past decade, researchers have also explored the incorporation of heavy alkali elements such as potassium (K), rubidium (Rb) and cesium (Cs) into the CIGSe absorbers [2,3,5]. This approach has yielded higher efficiencies due to surface passivation, reduction of bulk recombination, increase in the p-type doping level, and decrease in the Urbach energy [5,7,[9][10][11][12]. The primary effect of heavy alkali-fluoride PDT is the improvement of the V OC , thereby leading to an overall increase in the efficiency of CIGSe solar cells [5].…”