Abstract:The incorporation of sodium from sodium fluoride in single-crystal CuInSe2 (CIS) is investigated to provide insight into the intra-granular aspects of sodium incorporation in CIS-based thin films. Sodium was incorporated by evaporating NaF onto two CIS crystals of varying compositions and defect structures followed by heating under vacuum. Diffusion profiles show a near-surface reaction before a deeper diffusion zone which follows a complementary error function, confirming Na diffusion into the crystals. Trans… Show more
“…A small Na incorporation at the surface and at the back is also observed for Se-only, i.e., in the absence of intentional Na sources, and is attributed to trace Na impurities in the Se employed 25 . The rapid Na incorporation in epitaxial CIS is consistent with the low activation energy of Na diffusion reported recently for single crystal CIS 38 .Fig. 1Compositional depth profiles of the CIS/GaAs samples subject to different treatments.…”
Section: Resultssupporting
confidence: 85%
“…7 reveals that some of such defects pre-existed even in the untreated film and it is likely that they incurred Na decoration/extension, during the PDT. The role of dislocations on the enhanced diffusion of In and Ga cannot be excluded here, but a recent study on single crystal CIS suggests that dislocations do not control the Na diffusion mechanism[38]. Presumably the enhancing effect of Na on In/Ga interdiffusion occurs also at the interior of CIGS grains in conventional polycrystalline films.…”
Copper indium gallium diselenide-based technology provides the most efficient solar energy conversion among all thin-film photovoltaic devices. This is possible due to engineered gallium depth gradients and alkali extrinsic doping. Sodium is well known to impede interdiffusion of indium and gallium in polycrystalline Cu(In,Ga)Se2 films, thus influencing the gallium depth distribution. Here, however, sodium is shown to have the opposite effect in monocrystalline gallium-free CuInSe2 grown on GaAs substrates. Gallium in-diffusion from the substrates is enhanced when sodium is incorporated into the film, leading to Cu(In,Ga)Se2 and Cu(In,Ga)3Se5 phase formation. These results show that sodium does not decrease per se indium and gallium interdiffusion. Instead, it is suggested that sodium promotes indium and gallium intragrain diffusion, while it hinders intergrain diffusion by segregating at grain boundaries. The deeper understanding of dopant-mediated atomic diffusion mechanisms should lead to more effective chemical and electrical passivation strategies, and more efficient solar cells.
“…A small Na incorporation at the surface and at the back is also observed for Se-only, i.e., in the absence of intentional Na sources, and is attributed to trace Na impurities in the Se employed 25 . The rapid Na incorporation in epitaxial CIS is consistent with the low activation energy of Na diffusion reported recently for single crystal CIS 38 .Fig. 1Compositional depth profiles of the CIS/GaAs samples subject to different treatments.…”
Section: Resultssupporting
confidence: 85%
“…7 reveals that some of such defects pre-existed even in the untreated film and it is likely that they incurred Na decoration/extension, during the PDT. The role of dislocations on the enhanced diffusion of In and Ga cannot be excluded here, but a recent study on single crystal CIS suggests that dislocations do not control the Na diffusion mechanism[38]. Presumably the enhancing effect of Na on In/Ga interdiffusion occurs also at the interior of CIGS grains in conventional polycrystalline films.…”
Copper indium gallium diselenide-based technology provides the most efficient solar energy conversion among all thin-film photovoltaic devices. This is possible due to engineered gallium depth gradients and alkali extrinsic doping. Sodium is well known to impede interdiffusion of indium and gallium in polycrystalline Cu(In,Ga)Se2 films, thus influencing the gallium depth distribution. Here, however, sodium is shown to have the opposite effect in monocrystalline gallium-free CuInSe2 grown on GaAs substrates. Gallium in-diffusion from the substrates is enhanced when sodium is incorporated into the film, leading to Cu(In,Ga)Se2 and Cu(In,Ga)3Se5 phase formation. These results show that sodium does not decrease per se indium and gallium interdiffusion. Instead, it is suggested that sodium promotes indium and gallium intragrain diffusion, while it hinders intergrain diffusion by segregating at grain boundaries. The deeper understanding of dopant-mediated atomic diffusion mechanisms should lead to more effective chemical and electrical passivation strategies, and more efficient solar cells.
“…[30] These findings were supported by first-principles calculations. [39] In addition, the presence of Na atoms in the grain interior (GI) indicates the high mobility of Na atoms (Na interstitial diffusion). It was shown by ab initio calculations that Cu atoms are the most mobile atoms in CISe, whereas In atoms play no role in the mass transport.…”
Adaptive kinetic Monte Carlo simulation (aKMC) is employed to study the dynamics and the diffusion of point defects in the CuInSe 2 lattice. The aKMC results show that lighter alkali atoms can diffuse into the CuInSe 2 grains, whereas the diffusion of heavier alkali atoms is limited to the Cu-poor region of the absorber. The key difference between the diffusion of lighter and heavier alkali elements is the energy barrier of the ion exchange between alkali interstitial atoms and Cu. For lighter alkali atoms like Na, the interstitial diffusion and the ion-exchange mechanism have comparable energy barriers. Therefore, Na interstitial atoms can diffuse into the grains and replace Cu atoms in the CuInSe 2 lattice. In contrast to Na, the ion-exchange mechanism occurs spontaneously for heavier alkali atoms like Rb and the further diffusion of these atoms depends on the availability of Cu vacancies. The outdiffusion of alkali substitutional atoms from the grains results in the formation of Cu vacancies which in turn increases the hole concentration in the absorber. In this respect, Na is more efficient than Rb due to the higher concentration of Na substitutional defects in the CuInSe 2 grains.
“…This has been known since the 1990s, when sodium was shown to dramatically improve the solar cell's open circuit voltage. The mechanisms by which sodium affects the composition [4,5], structure [6,7], and optoelectronic properties [8,9] of CIGS are still under debate. Recently, potassium [10,11] and the heavier alkali metals [12] have increased performance up to the staggering 22.9% record value established by Solar Frontier [13].…”
This study explores the rich chemistry of elemental selenium reduction to monoselenide anions. The simplest possible homogeneous electron transfer occurs with free electrons, which is only possible in plasmas; however, alkali metals in liquid ammonia can supply unbound electrons at much lower temperatures, allowing in situ analysis. Here, solvated electrons reduce elemental selenium to K 2 Se, a compound relevant for alkali metal doping of Cu(In,Ga)Se 2 solar cell material. It is proposed that the reaction follows pseudo first-order kinetics with an inner-sphere or outer-sphere oxidation semi reaction mechanism depending on the concentration of solvated electrons.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
