Growth of AZO on flexible substrate using ALD system as a transparent conducting oxide for solar cells
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Growth of AZO on flexible substrate using ALD system as a transparent conducting oxide for solar cells Tara Dhakal, Abhishek Nandur, Rachel Christian, Daniel Vanhart, Parag Vasekar, and Charles R. Westgate Center for Autonomous Solar Power (CASP), Binghamton University, Binghamton, NY 13902 ABSTRACT Aluminum doped zinc oxide (AZO) thin film is gaining momentum as a transparent conducting oxide because its optical and electrical properties are optimal for a photovoltaic device. We report growth of AZO thin film using an Atomic Layer Deposition (ALD) system, which is known to deposit layers on a substrate with atomic layer precision. The precursors used for aluminum and zinc were Trimethylaluminum (TMA) and Dimethylzinc (DMZ) respectively. Alternating cycles of TMA and DMZ were introduced to the heated deposition chamber such that a desired aluminum doping was obtained on the AZO films grown. The films were grown on flexible substrates like PET and also on single crystalline Si(100) and float-glass substrates. An optimal aluminum doping (3 at.%) on the AZO film of thickness 575 nm gave a sheet resistance (Rs) of 97 Ω/□ with corresponding resistivity of 5.6 x 10-3 Ω.cm. The value of the carrier concentration and hall mobility were 1.86 x 1020 cm-3 and 6.5 cm2V-1s-1 respectively. Optical measurement showed 90% transmission in most of the visible spectrum. X-ray diffraction spectra of the film showed all characteristic ZnO hexagonal lattice peaks. INTRODUCTION Aluminum doped zinc oxide (AZO) exhibits suitable electrical and optical properties to be used as a transparent conducting oxide (TCO) for solar cell applications [1]. The everincreasing cost of the tin-doped indium oxide (ITO) has prompted the search for an alternative TCO, which is cost effective and made from earth abundant materials. In this regard, AZO is considered as a possible replacement to ITO. Thin film and organic photovoltaic devices are already being fabricated using AZO as TCOs [2,3]. An area where AZO needs improvement as compared to ITO is its relatively weak stability against moisture. ITO is found more stable for use in oxidizing atmospheres (e.g., air and moisture) and high temperatures [4]. DC and RF magnetron sputtering have been often used to grow and synthesize AZO thin films as a front contact in thin film solar cells due to its relatively better transparent conductive oxide (TCO) properties [5,6]. In recent years, a technique called atomic layer deposition (ALD) is used to grow thin films with atomic layer precision [7]. In addition to the atomic layer precision, the ALD growth is very uniform and conformal. The conformal growth guarantees a smooth and continuous interface on the layer it is grown with. Since it is important to have a good interface between the window layer and the TCO layer, the ALD growth process has the potential to be a future technique for solar cell layers. This article presents the electrical and optical properties of the AZO films grown on three substrates; Si(100), float glass and flexible polyethylene terephthala
