Abstract
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.
Highlights
Copper indium gallium diselenide-based technology provides the most efficient solar energy conversion among all thin-film photovoltaic devices
Different chemical and structural analyses were performed on these films (SIMS, nano-Auger electron spectroscopy (AES), scanning electron microscopy (SEM)-energydispersive X-ray spectroscopy (EDS), scanning transmission electron microscopy (STEM)-EDS, electron back scatter diffraction mapping (EBSD), X-ray diffraction (XRD), atom probe tomography (APT), spectrophotometry, photoluminescence (PL) and Raman spectroscopy) and the results are discussed here
The 23Na secondary ion mass spectrometry (SIMS) depth profiles in Fig. 1a confirm that Na is incorporated into the CIS films via the gas-phase[25], yet the absolute Na concentration for all samples is below the detection limit of EDS
Summary
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. 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. Besides controlling the Ga depth profile, chemical passivation by extrinsic doping with Na, K, Rb, and Cs seems essential to ensure the best optoelectronic properties in CIGS9,16–18 This is normally achieved by diffusion from soda-lime glass or postdeposition treatments (PDTs). Since the diffusion of In/Ga in Cu-poor CIGS is likely to occur via copper vacancies[21,27,28], a decrease of the concentration of copper vacancies is thought to decrease the chances for In and Ga to interdiffuse[28]
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