Secondary phase formation in (Ag,Cu)(In,Ga)Se2 thin films grown by three-stage co-evaporation

Abstract

Ag,Cu)(In,Ga)Se 2 thin films deposited by a three-stage co-evaporation process have shown potential as mid-and wide-bandgap absorber materials for solar cell devices. Further growth optimization requires a thorough understanding of secondary phases formed in addition to the primary chalcopyrite phase, during the group-I rich growth near the end of the second stage. In this study, we investigate how the composition ratio Ag/(Ag+Cu) controls the formation of the secondary phases containing Ag, Cu and Se. Group-I rich [composition ratio (Ag+Cu)/(In+Ga)>1] ACIGS films with varying silver content-Ag/(Ag+Cu)=0, 0.1 and 0.5-are grown until the end of the second stage of three-stage processes. Secondary phases are found to locate near film surfaces. However, they intermix with the chalcopyrite phase in the surface layer when Ag/(Ag+Cu)>0, while they form a separate layer if no Ag is alloyed. The crystal structures of the secondary phases shifts along the Cu 2 Se -Ag 2 Se pseudo-binary tie-line as Ag/(Ag+Cu) increases. Concurrently, the film surfaces become smoother with reduced facets and voids, the accumulation of sodium is suppressed, and both the degree of In and Ga gradient and the film emissivity are reduced, all of which affect the subsequent deposition process and final solar device performance.

Thin Ag Precursor Layer-Assisted Co-Evaporation Process for Low-Temperature Growth of Cu(In,Ga)Se2 Thin Film

“…Unlike Cu–Se binary system that starts to form through the liquid phase at 523 °C at eutectic point, the Ag–Se system can form a pure liquid phase at much lower temperature of 221 °C . Given the lower bond dissociation energy between Ag and Se ( E Ag–Se = 210 kJ/mol) than the energy between Cu and Se ( E Cu–Se = 255 kJ/mol), we believe that Ag atoms in the CIGS matrix can be preferentially precipitated to the grain boundaries after the point of stoichiometry and readily form the liquid Ag–Se phase under high Se flux as depicted in Figure h. The liquid Ag–Se phase at grain boundaries can provide mobile channels for atom diffusion and Ga can easily diffuse toward the film surface along the grain boundaries, which facilitates CIGS recrystallization process throughout the film.…”

“…The corresponding SIMS profile shows an abrupt Ga gradient near the film surface supporting the weak Ga diffusion to the film surface compared to the In diffusion (Figure f). When the process exceeds the point of stoichiometry, i.e., Cu/(Ga + In)> 1, an excess Cu would build upon the film surface and possibly forms the Cu–Se secondary phase mixed with the surface CuInSe 2 phase, which is supported by the increased surface Cu, In, and Se intensities in the SIMS profile (Figure i). It seems that the existence of the excess Cu within the CuInSe 2 phase somehow expedites the atomic movement in the surface layer because of the following reasons.…”

Ultrathin Cu(In,Ga)Se2 Solar Cells with Ag/AlOx Passivating Back Reflector

“…Lastly, the effect of etching was studied since we found stains in the SEM images presented in Figure 6. Furthermore, the addition of Ag could possibly lead to secondary phases [32]. While this tends to occur only at higher AAC and GGI ratios, it has to be noted that we start with an Ag layer, and thus a very high AAC.…”

High‐temperature fabrication of Ag(In,Ga)Se2 thin films for applications in solar cells

“…Photovoltaic applications of AIGS thin films as the light‐absorbing layer of solar cells have been reported in our previous work and that of other groups . Among the approaches for fabricating high efficiency CIGS solar cell, such as selenization of the CuInGa precursor , single stage co‐evaporation , and three‐stage co‐evaporation , the three‐stage method is regarded as the most effective approach . In our previous studies, we deposited AIGS films by a modified three‐stage method using molecular beam epitaxy, and we achieved an efficiency of 10.7%.…”