AFRL-OSR-VA-TR-2015-0044 Review of Zinc Oxide Thin Films

The present review paper reports on the optimization attempts, so far, on two simultaneously occurring properties of zinc oxide (ZnO) thin films (electrical conductivity and optical transparency). These two properties are discussed in the context of zinc oxide material thin films as transparent conducting oxides (TCOs) for photovoltaic (PV) applications. They are very critical and so is their optimization for the realization of superior zinc oxide based TCOs for superior performance in photovoltaic devices. According to theory, the electrical resistivity (ρ)of the novel TCO material should be ~10Ωcm while the optical transparency should be over 80% in the visible radiation range as well as in the near infrared region [3, 4, 5, 99, 11]. This remarkable combination of conductivity and transparency is usually impossible in intrinsic stoichiometric TCOs; usually only partial transparency and fairly good conductivity may be obtained in the thin films. However, it is achieved by producing them with a non‐stoichiometric composition or by introducing appropriate dopants which decrease the resistivity while retaining a good transparency [3, 4]. Whereas high quality pure ZnO thin films have been fabricated before by a number of standard techniques [83‐86], however, they are limited in their properties and stability for PV applications and require further development. The stability temperature for ZnO is ~ 250 [78, 95], above which the film quality starts to degrade by thermal decomposition [78, 80, 95]. Intrinisic thin film resistivities as low as 10 ‐2 to 10 ‐3 Ωcm and transmission coefficients (T) as high as 80%‐90% have been achieved [3, 4, 76]. Past studies [15‐18, 20, 58, 79, 65, 99‐109, 128‐131, 66, 67, 137, 138, 142] have recommended extrinsic doping of ZnO with a range of elements to produce more stable ZnO TCOs with better properties (especially electrical, optical). These doping elements are categorized into two: n‐ type and p‐type conductivity dopants; the most commonly researched dopants include group III and VII elements for n‐type and group I or VI for p‐type conductivity. While impurity doping is