Improved electrical transport in Al-doped zinc oxide by thermal treatment

Abstract

A postdeposition thermal treatment has been applied to sputtered Al-doped zinc oxide films and shown to strongly decrease the resistivity of the films. While high temperature annealing usually leads to deterioration of electrical transport properties, a silicon capping layer successfully prevented the degradation of carrier concentration during the annealing step. The effect of annealing time and temperature has been studied in detail. A mobility increase from values of around 40 cm2/Vs up to 67 cm2/Vs, resulting in a resistivity of 1.4×10−4 Ω cm has been obtained for annealing at temperatures of 650 °C. The high mobility increase is most likely obtained by reduced grain boundary scattering. Changes in carrier concentration in the films caused by the thermal treatment are the result of two competing processes. For short annealing procedures we observed an increase in carrier concentration that we attribute to hydrogen diffusing into the zinc oxide film from a silicon nitride barrier layer between the zinc oxide and the glass substrate and the silicon capping layer on top of the zinc oxide. Both are hydrogen-rich if deposited by plasma-enhanced chemical vapor deposition. For longer annealing times a decrease in carrier concentration can occur if a thin capping layer is used. This can be explained by the deteriorating effect of oxygen during thermal treatments which is well known from annealing of uncapped zinc oxide films. The reduction in carrier concentration can be prevented by the use of capping layers with thicknesses of 40 nm or more.

Highly transparent front electrodes with metal fingers for p-i-n thin-film silicon solar cells

“…Finally, sub-bandgap absorption in the spectral range below the fundamental absorption edge, i.e. from 380 nm to 500 nm, also decreases, as already reported in reference [9]. In accordance with the absorptance data of the glass/ZnO:Al superstrates, the external quantum efficiency (EQE ) of the cell deposited on the annealed superstrate surpasses that of the reference cell in the near-infrared (NIR) wavelength region; this EQE gain compensates for the slight optical loss below 380 nm, such that the photocurrent increases from 24.6 mA/cm 2 to 25.7 mA/cm 2 (by 4.5%).…”

“…Sputtered tin-doped indium oxide In 2 O 3 :SnO 2 (ITO) combined with a textured superstrate has also been proposed to fulfill this function [18], and more recently hydrogenated indium oxide In 2 O 3 :H (IOH) [19,20]. Other approaches have also been introduced to improve the FOM of ZnO:Al front electrodes and, with it, cell performance: for instance, two-step postdeposition annealing treatments, comprising a thermal treatment under a protective layer, result in a decrease in sub-bandgap and free-carrier absorption combined with a remarkable gain in carrier mobility [9,21,22].…”

“…between 300 nm and 1100 nm. As the optical and electrical properties of TCOs are interlinked via the charge-carrier density and mobility, a high transparency in the near infrared (NIR) range is generally associated with a high sheet resistance R sq and vice versa [9,10]. To compare the suitability of different TCOs to be used as electrode materials, a commonly accepted figure of merit (FOM) is defined as F OM = σ/α, where σ is the film conductivity in S cm −1 and α is its absorption coefficient in cm −1 at a given wavelength [11], the best TCO being the one with the highest conductivity and the lowest light absorption.…”

Transparent Electrodes for Efficient Optoelectronics

“…[147] The electron mobility of sputtered AZO has been successfully increased up to 40-67 cm 2 V −1 s −1 by incorporating a capping layer and applying a high-temperature (650°C) annealing step. [148] Considering the temperature restriction of several devices, low R sh values are therefore mainly achieved by increasing N e , at the cost of an inflated FCA. Although the FOM values for sputtered and chemical-bath-deposited (CBD) AZO are similar, deposition from solution presents some advantages: it is a simple and low-cost process, [149] and it is gentle, preventing physical damage to the substrates or active device layers where the material is deposited.…”

Hard x-ray photoelectron spectroscopy study of the buried Si/ZnO thin-film solar cell interface: Direct evidence for the formation of Si–O at the expense of Zn-O bonds

“…One major challenge of this solar cell concept is the implementation of a transparent conductive oxide (TCO) window layer, which would facilitate an easy electric contact and light trapping. [3][4][5] So far the proposed Al/a-Si:H(p þ )/polySi(p)/poly-Si(n þ )/ZnO:Al/glass device structure 3 yields lower efficiencies than the standard device without ZnO:Al. This suggests that either the Si(n þ )/ZnO:Al ("Si/ZnO") interface is not properly designed or/and that its properties deteriorate during subsequent annealing processes.…”