Engineering ZnTe Thin Films by Thermal Control for Bandgap Tuning and Improved CdTe Solar Cell Performance

The study discusses a thorough experimental analysis on RF-sputtered ZnTe thin films prepared with different substrate temperatures in the range of room temperature up to 300 °C at a constant pressure of 5 mTorr and RF power of 30 W. Substrate temperature was established as a thermal design variable controlling the transition of the deposited thin film from Te-rich amorphous to polycrystalline ZnTe film. All films were maintained at a thickness of approximately 85-90 nm. Raman spectroscopy of the 300 °C film confirmed four characteristic vibrational peaks at 121, 142, 210, and 420 cm -1 , with a Zn/Te ratio of 45:55, close to the ideal stoichiometry. Increasing substrate temperature progressively improved crystallinity, enhanced p -type electrical conductivity with a bulk carrier concentration of 2.49 × 10 15 cm -3 , and tuned the direct optical bandgap from 3.04 eV (RT) to 3.39 eV (300 °C), accompanied by a reduction in Urbach energy to 0.58 eV and peak carrier mobility of 2.31 × 10 3 cm 2 V -1 s -1 . The film at 300 °C was determined to be the optimized thermal design point and was used as an interface layer in a CdTe solar cell architecture. Device characterization under illumination revealed that the ZnTe interlayer improved the V oc from 0.538 V to 0.546 V, fill factor from 30.47% to 33.20% and power conversion efficiency from 4.74% to 5.98% over an active area of 0.203 cm 2 . These results demonstrate that thermally engineered ZnTe is a viable undoped interface material for improving the performance of CdTe-based photovoltaics.