Optimization of the post-deposition annealing process of high-mobility In2O3:H for photovoltaic applications
Introduction
Transparent conductive oxides (TCOs) are employed as contact layers thin film solar cells. They transmit incoming light to the absorber and conduct the generated charge carriers laterally to the metal contacts. Thus, the TCO should have a high optical transparency as well as a high electrical conductivity to ensure good solar cell performance. The free charge carriers, which are necessary for the electrical conductivity, also cause free carrier absorption in the near infrared (NIR) spectrum. In order to fulfill the two conflicting requirements mentioned above, it is necessary to reduce the free carrier density and increase the carrier mobility in the TCO at the same time. By accomplishing this, high optical transparency can be achieved while still maintaining a good electrical conductivity.
Increasing the transparency of the TCO increases the short-circuit current density (Jsc) in solar cells. Hydrogen-doped indium oxide In2O3:H (IOH) has been widely discussed as a suitable TCO for this purpose, because of its mobility higher than 100 cm2/Vs [1], [2], [3], [4]. Very low optical absorption can be achieved with this material at resistivities around 350 μΩcm. By introducing a small amount of water vapor into the sputtering chamber during the deposition, IOH films can be deposited in X-ray-amorphous state and crystallized in a post-deposition annealing step lasting usually 15–30 min at approx. 180 °C in vacuum, as has been shown in our previous work [5].
The development of amorphous In2O3:Sn (ITO) by adding water vapor or H2 to the sputtering gas has been shown in literature [6], [7]. The crystallization behavior of hydrogenated ITO is similar to IOH. Ando et al. show that the growth of crystallites in the ITO film is suppressed by the addition of water vapor during the sputtering process at room temperature [6]. After a post-deposition annealing step, the films crystallized to polycrystalline structures. Betz et al. also achieved suppression of crystal growth during ITO deposition at room temperature by adding H2 to the sputtering gas [7]. They showed that the amorphous ITO film reaches almost the same resistivity after a post-deposition annealing step as an ITO film which is deposited in polycrystalline phase at 200 °C substrate temperature. In our previous publication, we demonstrated that IOH has a lower absorption in the NIR spectrum than ITO due to its lower free carrier concentration and higher mobility [5], and thus has the possibility to reach a higher JSC in solar cells than ITO.
We synthesized IOH films using rf magnetron sputtering from a ceramic In2O3 target in Ar/O2/H2O gas mixtures. The crystallization of amorphous IOH films was analyzed using various annealing treatments in vacuum, air, and argon gas atmosphere.
In our previous work, we have shown that oven annealing in vacuum at 180 °C for periods longer than 10 min decreases the open-circuit voltage (VOC) of a-Si:H/c-Si heterojunction cells, because of a degradation of the passivating a-Si:H layer [5]. Improved VOC values can be achieved by annealing heterojunction cells in air at 200 °C for ≤ 10 min [5], [8]. Therefore, we investigated the effect of annealing in air on IOH films deposited at different process pressures.
Also for CIGS cells, a degradation of the solar cell parameters has been shown due to an annealing process for 2 h at 180 °C in vacuum [9]. In this paper, we performed very short-term flash lamp annealing (FLA) to crystallize amorphous IOH films within 2.7 ms in argon gas atmosphere at 5 × 104 Pa. The extremely short duration of FLA reduces thermal stress on heat-sensitive substrates like CIGS cells and may provide an approach to benefit from the high transitivity of an IOH-front-contact without degrading the cells during the annealing process. Finally, both annealing in air and FLA have the potential to reduce processing costs compared to the conventional vacuum annealing step.
Section snippets
Experimental details
The IOH films were deposited by rf (13.56 MHz) magnetron sputtering from a ceramic In2O3 target onto unheated substrates. Hydrogen doping was realized by connecting a reservoir filled with deionized water via manual needle- and blocking-valves to the sputtering chamber (Roth&Rau). Prior to the deposition process, the H2O partial pressure was set by adjusting the needle valve manually. Approximately 15 min were required to stabilize the H2O partial pressure. The sputtering system contains three
Results and discussion
In order to evaluate the influence of hydrogen doping on the ratio of the crystalline phase in the IOH films, XRD scans of undoped and hydrogen-doped In2O3 films were compared in our previous work [9]. Undoped In2O3 films grow polycrystalline when they were deposited by rf magnetron sputtering at room temperature. By introducing water vapor into the sputtering chamber during the deposition, OH molecules were incorporated, and the film grew amorphously. In a post-deposition annealing step, the
Conclusion
In conclusion, we systematically investigated the effect of the water vapor partial pressures pH2O on the electrical properties of IOH films prepared by magnetron sputtering from an In2O3 target in argon/oxygen atmosphere, subsequently annealed in vacuum and air at 180 °C. The highest mobilities of 130 Vs/cm2 were achieved between pH2O = 0.1 and 0.3 mPa. These polycrystalline films exhibit large grains (> 500 nm) confirmed by EBSD. The crystallization behavior in air atmosphere for IOH films deposited
Acknowledgments
The authors would like to thank Carola Klimm for performing the EBSD measurements.
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