Transparent conductive SnO2 thin films via resonant Ta doping

Transparent conductive oxide (TCO) thin films are highly desired as electrodes for modern flat-panel displays and solar cells. Alternative indium-free TCO materials are highly needed, because of the scarcity and the high price of indium. Based on the mechanism of resonant doping, Ta has been identified as an effective dopant for SnO2 to achieve highly conductive and transparent TCO. In this work, we fabricated a series of Ta-doped SnO2 thin films (Sn1−xTaxO2, x = 0.001, 0.01, 0.02, 0.03) with high conductivity and high optical transparency via a low-cost sol-gel spin coating method. The Sn0.98Ta0.02O2 film achieves the highest electrical conductivity of 855 S cm−1 with a carrier concentration of 2.3 × 1020 cm−3 and high mobility of 23 cm2 V−1 s−1. The films exhibit a very high optical transparency of 89.5% in the visible light region. High-resolution X-ray photoemission spectroscopy and optical spectroscopy were combined to gain insights into the electronic structure of the Sn1−xTaxO2 films. The optical bandgaps of the films are increased from 3.96 eV for the undoped SnO2 to 4.24 eV for the Sn0.98Ta0.02O2 film due to the occupation of the bottom of conduction band by free electrons, i.e., the Burstein-Moss effect. Interestingly, a bandgap shrinkage is also directly observed due to the bandgap renormalization arising from many-body interactions. The double guarantee of transparency and conductivity in Sn1−xTaxO2 films and the low-cost growth method provide a new platform for optoelectronic and solar cell applications.