Passivation mechanism in CdTe solar cells: The hybrid role of Se

Abstract

In this Letter, we report on the role of Se incorporation in the increased efficiency recently measured in Se alloyed cadmium telluride (CdTe) absorbers. This is done by means of density functional theory calculations following an extensive exploration of all the possible diffusion paths of Se. We identify a unique two-step mechanism that accounts for bulk diffusion of chalcogenide interstitials in CdTe, explaining the Se diffusion measured in experiments. The interaction of the diffusing interstitial with the Cd vacancy and the Te antisite is further analyzed in order to understand the passivation of these two main non-radiative recombination centers. Taking into account the approach path of Se, we identify nine complexes that present different levels of passivation. The lowest formation energy is achieved for a ⟨100⟩ Te dimer with two Se in the first neighbor shell. This defect also presents the shallowest donor character defect state due to the presence of Se. This highlights the hybrid role of Se in the mechanism of increased efficiency: it first mediates the diffusion of chalcogenide toward the non-radiative recombination centers before it leads with Te to their optimal passivation. This comprehensive insight should allow further improvements in CdTe-based technologies.

Impact of metastable defect structures on carrier recombination in solar cells

“…hopping of the split-interstitial dimer). 25,32,33 We thus determine a diffusion barrier of 0.13 eV for Te i 0 using the HSE(34.5%)+SOC functional (Fig. S2 † ), similar to the literature value of 0.09 eV calculated using both GGA (PBE) and hybrid (HSE06) DFT.…”

“…9 An important factor in the emergence of the thermal-excitation capture pathway here, is the high mobility of the Te interstitial in CdTe, as demonstrated by the low diffusion barriers calculated in this and previous works. 25,32,33 The facile ionic diffusion of Te i is a simultaneous consequence of the soft, anharmonic PESs which cause the ready accessibility of distinct structural motifs (i.e. low-lying metastable structures), facilitating this complex recombination pathway.…”

Impact of metastable defect structures on carrier recombination in solar cells

“…hopping of the split-interstitial dimer). 26,33,34 We thus determine a diffusion barrier of 0.13 eV for Te 0 i using the HSE(34.5%) + SOC functional (Fig. S2 †), similar to the literature value of 0.09 eV calculated using both GGA (PBE) and hybrid (HSE06) DFT.…”

“…9 An important factor in the emergence of the thermal-excitation capture pathway here is the high mobility of the Te interstitial in CdTe, as demonstrated by the low diffusion barriers calculated in this and previous works. 26,33,34 The facile ionic diffusion of Te i is a simultaneous consequence of the soft, anharmonic PESs which cause the ready accessibility of distinct structural motifs ( i.e. low-lying metastable structures), facilitating this complex recombination pathway.…”

Microscopy Visualization of Carrier Transport in CdSeTe/CdTe Solar Cells

“…While the detailed injection-level determination requires rigorous modeling, to simplify the discussion, we assumed a high injection in this depletion region and did not consider the electric field effect. In region 2 (intermixture), however, when the carrier transport approached the materials with a higher Se concentration, the narrower band gap and the less nonradiative recombination centers , could yield a higher light signal. Hence, a higher luminescence is possible, even with carrier diffusion and the monotonously decreasing carrier concentration.…”