Improved photovoltaic parameters of CdTe/PEDOT:PSS solar cells by CSS through their cooling in an O2 atmosphere

Abstract

This work presents the influence of O₂ during the cooling stage (CS) after the deposition of the CdTe absorber layer on the photovoltaic parameters of CdTe/PEDOT:PSS hybrid solar cells. The CdTe samples were processed by closed space sublimation (CSS) at a constant deposition pressure of 100 mTorr under 50 %Ar + 50 %O₂ atmosphere. After CdTe deposition, the CSS system remained at a constant pressure of 50 mTorr for 10 min of the CS under different Ar + O₂ atmospheres and in vacuum, in order to study the influence of the cooling atmosphere on the optical, morphological and structural properties of CdTe and on the photovoltaic parameters of the CdTe/PEDOT:PSS hybrid solar cells. In addition, different cooling times (from 10 to 90 min) were studied in an O₂-only atmosphere, when the cooling temperature decreased from 500 to 300 °C inside the CSS system, at the same pressure. SEM micrographs showed changes in the surface morphology of CdTe when cooled in O₂ atmosphere; while, EDS showed a slight modification in Te/Cd ratio for some samples with O₂ during CS. Carrying out a cooling process in O₂ atmosphere for 10 min allows improving the photovoltaic parameters J_sc, V_oc, FF, and η of the obtained hybrid solar cells. For cooling times longer than 30 min, the values of V_oc and J_sc started to decrease. The solar cells with cooling times of 10 and 30 min in an O₂ atmosphere show in the J vs. V curves an increase mainly in the V_oc, FF and shunt resistances associated with the mitigation of defects at the interfaces and inside the materials, which may be due to a passivation of the absorber material. Solar cells with an O₂ cooling atmosphere also show an essential increase in the EQE response.