How the selenium distribution in CdTe affects the carrier properties of CdSeTe/CdTe solar cells

Recently, the introduction of a CdSexTe1-x (CST) buffer in the CdTe absorber has increased the photon absorption at both short wavelengths in the visible range (by removal of the parasitic absorption of the CdS layer) and at the infrared range (by reduction of the absorber band gap). But, moreover, the efficiency improvement is also attributed to a longer minority carrier lifetime when Se is added. This paper presents CdSexTe1-x/CdTe devices fabricated by evaporation at low substrate temperature and with a reduced absorber thickness (2 µm). These cells achieve record current densities, exceeding 28 mA/cm2 for this absorber thickness with efficiencies close to 15 %. We have compared CdSexTe1-x/CdTe devices made with different approaches and we have found that the performance improvement is due to a lower amount of interstitial Te in the absorber, suggesting that the increase of the minority carrier lifetime is connected with a change of the stoichiometry due to the selenium diffusion.

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