Interfacial barrier height modification of indium tin oxide/a-Si:H(p) via control of density of interstitial oxygen for silicon heterojunction solar cell application

An indium tin oxide (ITO) film with low carrier concentration (n), high mobility (μ) and high work function (Φ_ITO) is a beneficial material for the front electrode in heterojunction silicon (HJ) solar cells due to its low free-carrier absorption in the near-infraredwavelength and low Schottky barrier height at the ITO/emitter-layer front contact. This lowfree-carrier absorption as well as the low Schottky barrier height increase the open-circuit voltage (V_oc) and the short-circuit current density (J_sc), which in turn increases the overall cell efficiency (η). Hence, ITO films with lower n, higher μ and higher Φ_ITO were prepared by controlling the density of the interstitial oxygen [O_i] in the films and used as anti-reflection electrodes in HJ solar cells. With increasing [O_i] in the ITO, the preferential orientation of the (222) crystalline plane became more dominant. The Φ_ITO and μ increased from 4.87 eV and 38.9 cm² V^-1 s^-1 to 5.04 eV and 48.79 cm² V^-1 s^-1, respectively, whereas n decreased from 4.7 × 10²⁰ cm^-3 to 2.8 × 10²⁰ cm^-3. We attribute these changes to the chemisorbed oxygen into the ITO films, while the decrease of n is due to the ability of interstitial oxygen to capture electron, and the increase of μ is due to the reduction in free-carrier scattering. These ITO films were used to fabricate HJ solar cells. As [O_i] in the ITO film increased, the device performance improved and the best cell performance was obtained with V_oc of 714 mV, J_sc 34.79 mA/cm² and η of 17.82%. By computer simulation, we found that the higher Φ_ITO and μ but lower n were responsible for the enhanced cell performance. The cell performance, however, deteriorated due to poor film properties when [O _i] exceeded concentration limit from 3.2 × 10²⁰ cm^-3.