How to Optimize Iodine-Doped Cadmium Telluride for N-Type Solar Cells

Cadmium telluride (CdTe) solar cells face efficiency limitations due to material defects. Washington State University systematically characterized defects in iodine-doped CdTe and found self-compensation. This guide outlines how to optimize n-type iodine-doped CdTe single crystals through post-growth cadmium annealing, significantly reducing resistivity for enhanced solar cell performance.

1. Grow bulk ingots of iodine-doped Cadmium Telluride (CdTe:I) using the vertical gradient freezing (VGF) method in a modified vertical Bridgman (MVB) furnace. Ensure high-purity CdTe and CdI2 are batched, aiming for an iodine dopant concentration of approximately 1.8 × 1018 cm-3.
2. Prepare samples by axially slicing and cutting the grown crystals into 10 × 10 × 1-2 mm³ dimensions. Double-side polish these samples using alumina suspension with decreasing grit sizes (5, 1, and 0.05 µm), then etch them with a 1% bromine and methanol solution.
3. Perform post-growth cadmium annealing to activate carriers and remove compensation centers. Seal CdTe:I samples with 0.09 g of 7N purity Cd under vacuum in a high-purity fused quartz tube, then heat treat at 700 °C for 15 hours with a 5 °C/min heating/cooling ramp rate.
4. Characterize defects using Thermoelectric Effect Spectroscopy (TEES) or Thermally Stimulated Current (TSC). Deposit 50 nm of gold on both sides of the samples for ohmic contacts, validate contact quality, then cool the sample to ~20 K in a dark vacuum system.
5. Fill traps using a 910 nm sub-bandgap light-emitting diode for 1000 seconds, optionally applying a bias to assist carrier injection. Heat the sample with a 10 K temperature gradient and apply a 10 V voltage to amplify the current, measuring the de-trapping current at the defect ionization temperature to obtain the defect spectrum.
6. Measure electronic properties using Hall-effect measurements with a four-wire van der Pauw configuration. Attach soldered indium contacts to the sample corners and measure voltage and current in a 1.4 T magnetic field from 80 K to 235 K to determine resistivity, mobility, and free carrier density.
7. Analyze defect levels with Photoluminescence (PL) measurements by exciting samples with a 635 nm laser from 83 K to 298 K. Collect spectra at multiple points and perform spectral fitting using bi-Gaussian functions to identify and characterize defect levels. 8.  Conduct Density Functional Theory (DFT) calculations using the projector augmented wave method (PAW) and HSE06 hybrid functional. Model defects like substitutional iodine (ITe) and the ITe-Vcd complex in 64-atom supercells to calculate defect formation energies and transition energy levels, providing theoretical insights into experimental observations.