Heavy Alkali Treatment of Cu(In,Ga)Se₂Solar Cells: Surface versus Bulk Effects

Abstract

Chalcopyrite solar cells achieve efficiencies above 23%. The latest improvements are due to post‐deposition treatments (PDT) with heavy alkalis. This study provides a comprehensive description of the effect of PDT on the chemical and electronic structure of surface and bulk of Cu(In,Ga)Se2. Chemical changes at the surface appear similar, independent of absorber or alkali. However, the effect on the surface electronic structure differs with absorber or type of treatment, although the improvement of the solar cell efficiency is the same. Thus, changes at the surface cannot be the only effect of the PDT treatment. The main effect of PDT with heavy alkalis concerns bulk recombination. The reduction in bulk recombination goes along with a reduced density of electronic tail states. Improvements in open‐circuit voltage appear together with reduced band bending at grain boundaries. Heavy alkalis accumulate at grain boundaries and are not detected in the grains. This behavior is understood by the energetics of the formation of single‐phase Cu‐alkali compounds. Thus, the efficiency improvement with heavy alkali PDT can be attributed to reduced band bending at grain boundaries, which reduces tail states and nonradiative recombination and is caused by accumulation of heavy alkalis at grain boundaries.

The fox and the hound: in-depth and in-grain Na doping and Ga grading in Cu(In,Ga)Se2 solar cells

“…14 Most recently, based on the observation of similar open-circuit voltage improvements, instead of different surface conditions it was concluded that the improvements in the bulk absorber play the most important role. 17 Namely, heavy alkali metal atoms can diffuse along grain boundaries accumulating on them and also substitute the proximate Na atoms. 14,16 This leads to the passivation of charged defects at the grain boundaries which reduces the concentration of tail states inside the grains.…”

“…The result is reduced recombination and increased open-circuit voltage. 17 This contribution focuses on the inuence of Na and K on the interdiffusion of In and Ga. Generally, it is well known that grain boundaries in polycrystalline materials have a strong inuence on mass transport properties 18 and the same is true for Na along CIGS grain boundaries. 19 Therefore, it is important to understand if the hindering effect of Na and K on the interdiffusion of In and Ga in CIGS is related to transport at the grain boundaries or is intrinsically related to bulk CIGS properties.…”

Heavy Alkali Treatment of Post‐Sulfurized Cu(In,Ga)Se2 Layers: Effect on Absorber Properties and Solar Cell Performance

“…[8,[24][25][26][27][28][29] While the gain in J SC is clearly explained, there is still a very controversial discussion about the origin of the V OC (and FF) gain. [30,31] In most studies, a very thin, wide-gap surface phase is observed and commonly attributed to an Alk-In-Se (e.g., AlkInSe 2 ) compound. [32][33][34] It is argued that this layer may act as a surface passivation and thereby increases V OC .…”

“…However, it is not clear if this phase always forms and if/when it occurs as a closed layer or rather as distributed clusters. [30,35] On the contrary, a suppression of bulk recombination is discussed. While a reduced concentration of deep defects by the Alk-PDT appears unlikely, [30] it was suggested that a detrimental band-bending at GBs is reduced by the introduction of heavy alkalis (expressed as reduced band-tailing), which may agglomerate in GBs as AlkInSe 2 and reduce/passivate charged defects.…”

“…[30,35] On the contrary, a suppression of bulk recombination is discussed. While a reduced concentration of deep defects by the Alk-PDT appears unlikely, [30] it was suggested that a detrimental band-bending at GBs is reduced by the introduction of heavy alkalis (expressed as reduced band-tailing), which may agglomerate in GBs as AlkInSe 2 and reduce/passivate charged defects. [30,36,37] The Alk-PDT further affects the apparent doping density, but opposite trends (i.e., higher and lower doping) were observed.…”