Electrodeposition and characterisation of CdTe films for solar cell applications

Thin film CdS/CdTe solar cells have been prepared by electrodeposition of CdTe on CdS coated conducting glass from an acidic electrolyte containing a high concentration of Cd2+ and a low concentration of TeO2. Deposition of a 2 mu m CdTe film from stirred solutions typically requires 3 h. High quality CdTe films have been grown much more rapidly using a channel flow cell: 2 mu m films were deposited in around 24 min. The CdTe\CdS thin film structures obtained in this way were characterised by photocurrent spectroscopy, electrolyte electroreflectance/absorbance spectroscopy (EER/A), XRD and AFM. CdS\CdTe films prepared by both methods were annealed at 415 degrees C to effect type conversion of the CdTe layer. As deposited CdTe is generally n-type and exhibits strong preferential [111] orientation. Type conversion is not necessarily accompanied by recrystallisation: most of the CdTe films deposited from stirred solution did not recrystallise. Recrystallisation did occur for films grown by pulsing the potential periodically from 50 mV to >350 mV versus Cd2+/Cd during deposition. Evidence for sulphur and tellurium diffusion leading to alloy formation during annealing was obtained from bandgap shifts detected by photocurrent spectroscopy and EER/A and from changes in lattice parameters measured by XRD. The composition of the annealed electrodeposited structures approached CdS0.95Te0.05\CdTe0.95S0.05 after 15 min. Test solar cells with AM 1.5 efficiencies approaching 6% were fabricated. Recrystallised samples gave higher solar cell efficiencies than non-recrystallised samples. (C) 2000 Elsevier Science Ltd. All rights reserved.