CdTe absorber layers grown under Cd-rich conditions by MOCVD: Impact on surface morphology and structure

The analyses of surface morphology and microstructural properties of CdTe thin film absorber layers, deposited by metal organic chemical vapour deposition (MOCVD) on CdS/CdZnS/FTO-coated glass substrates as a function of different growth conditions (non–saturated or Cd–saturated growth condition and post–growth CdCl₂ heat treatment) are presented. Scanning electron microscopy (SEM) images showed that the CdTe absorber surface morphology was relatively smoother with Cd-saturated growth compared to non-saturated conditions, while using a similar II/VI precursor partial pressure ratio. Microstructural characterisation using electron back scatter diffraction (EBSD) measurements indicate a strong influence of the II/VI ratios on the crystalline structure and degree of recrystallisation in Cd–saturated deposited CdTe absorber layers. CdTe absorber layers from a low II/VI ratio of 2 showed a strong Te(A1) Raman spectra band, characteristic of a Te rich composition, whilst a Cd–saturated composition was confirmed in the case of II/VI ratio of 4, as Te(A1) band was significantly suppressed. Cd–saturated grown CdTe:As (II/VI = 4), combined with a CdCl₂ activation process at 440 °C, yielded optimum, highly randomized textured absorber with large grains. Reduced [111] oriented grains and suppression of Te(A1) mode was found to lead to higher V_OC in devices. A high efficiency CdTe:As solar cell with a high V_OC of 825 mV was measured based on the optimum growth condition for the absorber layers.