Tuning of Electrical and Optical Properties of Highly Conducting and Transparent Ta-Doped TiO₂Polycrystalline Films

Abstract

We present a detailed study on polycrystalline transparent conducting Ta-doped TiO2 films, obtained by room temperature pulsed laser deposition followed by an annealing treatment at 550°C in vacuum. The effect of Ta as a dopant element and of different synthesis conditions are explored in order to assess the relationship between material structure and functional properties, i.e. electrical conductivity and optical transparency. We show that for the doped samples it is possible to achieve low resistivity (of the order of 5×10-4 Ωcm) coupled with transmittance values exceeding 80% in the visible range, showing the potential of polycrystalline Ta:TiO2 for application as a transparent electrode in novel photovoltaic devices. The presence of trends in the structural (crystalline domain size, anatase cell parameters), electrical (resistivity, charge carrier density and mobility) and optical (transmittance, optical band gap, effective mass) properties as a function of the oxygen background pressures and laser fluence used during the deposition process and of the annealing atmosphere is discussed, and points towards a complex defect chemistry ruling the material behavior. The large mobility values obtained in this work for Ta:TiO2 polycrystalline films (up to 13 cm2V-1s-1) could represent a definitive advantage with respect to the more studied Nb-doped TiO2

Controlling the Electrical Properties of Undoped and Ta‐Doped TiO2 Polycrystalline Films via Ultra‐Fast‐Annealing Treatments

“…[ 12 ] The thickness of all the analyzed fi lms is 150 ± 5 nm, as measured by scanning electron microscopy (SEM). The resistivity of all the as-deposited thin fi lms is about 10 Ω cm, with no particular dependence on the presence of doping; for as-deposited fi lms it was not possible to discern the contributions of charge carrier concentration and charge mobility because of the highly scattered values obtained in Hall measurements.…”

“…[ 9 ] However, the discovery of donor doped TiO 2 as a promising new class of n-type TCO, [ 10 ] and the possibility of obtaining highly conducting and transparent polycrystalline Nb or Ta-doped TiO 2 (TaTO) thin fi lms on inexpensive glass substrates, open up new opportunities for a better suited energy level alignment at the device interfaces via an all TiO 2 -based confi guration (photoanode/selective layer and TCO). [11][12][13] Between the two mentioned donor doped compositions, Ta-doped TiO 2 is thought to have several advantages with respect to the more widely investigated Nb for transparent conductor applications, namely, higher mobility and dopant solubility. [ 14 ] Moreover, it has been suggested that donor-doped TiO 2 could be a superior material compared to undoped TiO 2 not only as a TCO but also as an electron selective layer (i.e., hole blocking layer) and photoanode.…”

“…[ 18 ] It is important to mention that depending on the adopted synthesis conditions (reducing vs oxidizing deposition/annealing atmosphere) it is possible to fi nely tune the mobile charge carrier density in TiO 2 -based fi lms as well as their functional (electrical/optical) properties. [ 12,15 ] Typically, in order to achieve the highest electrical conduction upon deposition, TiO 2 -based thin fi lms require an annealing process in a reducing atmosphere (vacuum or H 2 -based atmosphere, commonly at temperatures between 500 and 600 °C) so as to induce crystallization to a pure anatase phase and thus increase the carrier mobility, without compromising the charge carrier density. [ 12,13,19 ] In this context, wileyonlinelibrary.com it is noteworthy that annealing processes in oxidizing atmospheres (e.g., air) result in highly insulating fi lms.…”

“…[ 12,15 ] Typically, in order to achieve the highest electrical conduction upon deposition, TiO 2 -based thin fi lms require an annealing process in a reducing atmosphere (vacuum or H 2 -based atmosphere, commonly at temperatures between 500 and 600 °C) so as to induce crystallization to a pure anatase phase and thus increase the carrier mobility, without compromising the charge carrier density. [ 12,13,19 ] In this context, wileyonlinelibrary.com it is noteworthy that annealing processes in oxidizing atmospheres (e.g., air) result in highly insulating fi lms. [ 12 ] Although such a result is expected from defect chemistry considerations, the exact mechanisms underlying this process and involving different phenomena (crystallization, oxygen incorporation) are still debated especially in the case of donor-doped TiO 2 .…”

“…[ 12,13,19 ] In this context, wileyonlinelibrary.com it is noteworthy that annealing processes in oxidizing atmospheres (e.g., air) result in highly insulating fi lms. [ 12 ] Although such a result is expected from defect chemistry considerations, the exact mechanisms underlying this process and involving different phenomena (crystallization, oxygen incorporation) are still debated especially in the case of donor-doped TiO 2 . [20][21][22][23][24][25][26][27] In this contribution, we focus on how the post-deposition annealing treatments performed in different atmospheres and different heating/cooling rates affect the electrical properties of undoped as well as tantalum doped anatase TiO 2 thin fi lms.…”