Effect of Na‐PDT and KF‐PDT on the photovoltaic performance of wide bandgap Cu (In,Ga)Se2 solar cells

Abstract

Cu (In,Ga)Se 2 solar cells exhibit higher efficiencies if an appropriate Ga gradient is introduced and if the absorber is doped with sodium (Na) plus a heavier alkali atom such as potassium. However, a Gallium gradient in the presence of Na is challenging because sodium impedes the interdiffusion of elements and influences the gradient. In this contribution, we show that the presence of sodium during growth with the combination of high Ga concentration creates a pronounced gradient that is detrimental for carrier collection. One solution is to avoid Na abundance during absorber growth but to add it to the grown absorber layer via a postdeposition treatment. We investigate the effect of different Na incorporation methods simultaneously with KF-PDT on wide band gap CIGSe absorbers. By preparation on alkali-free substrates and application of alkalis (NaF and KF) onto a grown CIGSe layer, we show by smoothening of the Gallium gradient a large improvement in the solar cell performance from 4% to 8%. Another way to optimize the gradient in the presence of Na is the modification of the three-stage method. This yields the best efficiency of 10% in our laboratory at an integral GGI of 0.8. By means of temperature-dependent JV measurements, we show that the additional postdeposition of KF induces a barrier for the diode current. We conclude that KF-PDT induces a new thin layer at the CIGSe surface that has a lower valence band edge relative to the CIGSe bulk and is responsible for the double-diode behavior. This barrier can also explain the V oc (T) saturation at low temperature.

Heat-light soaking treatments for high-performance CIGS solar cells on flexible substrates

“…Figure 4(a) shows the temperature-dependent V OC of a cell on SLG substrate, for different illumination intensities. Extrapolation of the data points to T=0 K can be interpreted as the transition energy of the dominant recombination channel [31][32][33] . In our case, the high-temperature data extrapolates to about 1.21 eV, very similarly both before and after treatment.…”

Extrinsic Doping of Ink‐Based Cu(In,Ga)(S,Se)2‐Absorbers for Photovoltaic Applications

“…PDTs have been used to incorporate both heavy (e.g., K, Rb, Cs), [ 127,197 ] and light (e.g., Na, Li), [ 127,197 ] dopant atoms into the CIGSSe absorber layer, and have been scaled for production of large‐area modules, illustrating their feasibility and industrial scalability. [ 198 ] Among the alkali halide salts, alkali‐fluorides are commonly employed owing to the high stability of the fluorine ion (F − ), and the compound's thermal decomposition facilitated by selenium, to volatile SeF 6 or SeF 2 which desorb from the CIGSSe surface thereby limiting halide contamination.…”

“…[ 116,118,206 ] These results cemented the importance of adding sodium to the absorber layer and spurred research on developing new incorporation strategies. [ 207 ] Over the years, several contradictory results relating to the effect of Na addition to the CIGSSe absorber appeared including i) grain size (increased, [ 204,208,209 ] decreased, [ 197,210–213 ] no relation, [ 28,214 ] ) ii) preferentially oriented growth (promoted (220/204) growth, [ 215 ] promoted (112) growth [ 216,217 ] ), iii) decreased concentration of traps or deep levels, [ 196,218 ] iv) GB passivation [ 28,104,214,219–221 ] ), and v) p‐type conductivity (increased, [ 28,115,222 ] decreased, [ 51,223,224 ] ). The differences in findings can be related to the vast differences in absorber composition and growth conditions.…”

“…Commonly used methods to incorporate dopants into the vacuum processed absorber layers include i) dopant diffusion from substrates, for example, SLG and Mo:Na substrates, [63,139,195] ii) pre-deposition, that is, deposition of dopant metals before absorber film deposition (e.g., Al 2 O 3 substrate with a Mo/NaF layer, [196] ) iii) in situ co-evaporation (gas phase doping, i.e., the dopant compound (e.g., NaF, KF) is evaporated during absorber deposition), and iv) PDTs (i.e., the deposited absorber is annealed with the dopant element/compound (e.g., alkali halides) and Se vapor, without breaking the vacuum (see Figure 6). [59,120,125,141] PDTs have been used to incorporate both heavy (e.g., K, Rb, Cs), [127,197] and light (e.g., Na, Li), [127,197] dopant atoms into the CIGSSe absorber layer, and have been scaled for production of large-area modules, illustrating their feasibility and industrial scalability. [198] Among the alkali halide salts, alkalifluorides are commonly employed owing to the high stability of the fluorine ion (F − ), and the compound's thermal decomposition facilitated by selenium, to volatile SeF 6 or SeF 2 which desorb from the CIGSSe surface thereby limiting halide contamination.…”

Chalcopyrite solar cells —state-of-the-art and options for improvement

“…Also the GGI grading, important to achieve high-efficiency absorbers, is influenced by the presence of Na [94,95], which however is different for polycrystalline and epitaxial films: it appears that Na supports the intragrain interdiffusion of In and Ga, whereas it hampers the intergrain diffusion. The latter effect was used, for example, by Zahedi-Azad et al [96] who used substrates with a diffusion barrier to reduce the Na concentration during the growth of wide bandgap (high Ga) absorbers to smoothen the Ga grading and increase the efficiency.…”