Effective electrochemical n-type doping of ZnO thin films for photovoltaic window applications
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Effective electrochemical n-type doping of ZnO thin films for photovoltaic window applications
B. Marí-Soucase1,*, P. Cembrero-Coca1, M. Mollar1, M. E. Calixto2 1
Física Aplicada-IDF, Universitat Politècnica de València, València, Spain.
2
Instituto de Física, Benemérita Universidad Autónoma de Puebla, Puebla, México.
ABSTRACT An effective n-type doping of ZnO using Cl was demonstrated in thin films electrochemically synthetized by adding different amounts of chlorine ions in the starting electrolyte. The ratio between chlorine and zinc cations was varied between 0 and 2 while the zinc concentration in the solution was kept constant. When the concentration of chloride in the bath increases an effective n-type doping of ZnO films takes place. n-type doping is evidenced by the rise of donors concentration, obtained from Mott-Schottky measurements, as well as from the blue shift observed in the optical gap owing to the Burstein-Moss effect. INTRODUCTION ZnO is an intrinsic, n-type semiconductor with a broad range of applications in optoelectronics. As a semiconducting oxide material ZnO has low resistivity and high transmittance down to the UV spectral range making it thus a well-suited material to be used as transparent electrode for photovoltaic solar cells and electrodes. However, some challenges have yet to be reached or improved for a broad usage of ZnO as TCO film, for example, the environmental stability of ZnO [1]. Although unintentionally doped ZnO is always n-type, due to the unavoidable presence of native defects that act as donors, a higher level of n-type doping can be attained by using group III metal elements such as Al, Ga, In, which substitute Zn. Substituting O by VII group elements, such as F or Cl, also results in an effective n-type doping [2]. Cu(In,Ga)Se2-based thin-film solar cells use ZnO as window layer. Best results are obtained by sputtering a ZnO bilayer made of an unintentionally doped ZnO layer followed by a n-type ZnO:Ga layer. However, ZnO thin films can also be prepared by wet routes such electrodeposition (ED). ED presents some advantages compared to other techniques because deposition of semiconductors is performed at low temperature, atmospheric pressure and over large areas. Moreover it allows good control of film thickness through the control of deposited charge. Depending on the electrolyte, ZnO thin films can be deposited under several
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morphologies ranging from nanostructured and discontinuous films to extremely flat, compact and smooth films [3]. For PV window applications films with moderate surface textures are desired because of their effective light trapping in a wide wavelength range [4]. This work reports on the synthesis and characterization of ZnO thin films prepared by ED from an organic electrolyte like DMSO to be used as optical window in PV devices. Effective ntype doping of ZnO was achieved by varying the chlorine ion concentration in the electrolyte. The electrodeposited layers were characterized by XRD to study their structural properties, SEM for mor
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