ZnO and ZnO:Al layers obtained by atomic layer deposition for organic electronics

Grzegorz Łuka
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Thesis name in original language
Warstwy ZnO i ZnO:Al otrzymane metodą osadzania warstw atomowych do zastosowań w organicznej elektronice
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The ALD method was used to obtain semiconductor and conductive ZnO layers as well as ZnO: Al layers for organic electronics. Technological parameters were optimized in order to obtain layers ZnO semiconductors with a low concentration of free electrons (n ≤ 1017 cm-3) and deposited at low growth temperatures (Tgr ≤ 100 ° C). This made it possible to use these layers in the organic photovoltaic structure as pn junction element with p-type organic material - nickel phthalocyanine (NiPc). The obtained ITO / NiPc / ZnO / Al structure was characterized by better properties photovoltaics compared to the identical structure but without the ZnO layer, ie ITO / NiPc / Al. Moreover, the low growth temperature allowed the layer to be deposited ZnO on the NiPc layer, thus protecting it against contact with air. This significantly improved the time stability of such a structure compared to the structure without ZnO layers. Optimization of technological parameters allowing to obtain undoped ZnO layers having the lowest possible appropriate resistances while maintaining high optical transmission in the range visible. One of the main parameters was the growth temperature. Obtained values of ρ = 1.7 × 10-3 Ωcm and T ≈ 90% (at Tgr = 200 ° C) are among the best parameters reported in the literature for transparent, conductive and undoped ZnO layers obtained by various methods. The resulting conductive undoped ZnO layers were used as transparent electrode in OLED diode with Alq3 active layer (ZnO / CuI / Alq3 / PEGDE / Al). The obtained IV and LV characteristics, including high luminance ~ 3 × 103 cd / m2 and the luminous efficiency of 3 cd / A are comparable and even better compared to similar structures with an Alq3 layer, but with a ZnO: Al layer or ITO used as electrodes. To further improve conductivity, the ZnO layers were doped with aluminum. ZnO: Al layers having the lowest resistances were obtained at a temperature of 200 ° C equal to 8.2 × 10-4 Ωcm (for thicknesses of 200 nm) and 7.1 × 10-4 Ωcm (for thicknesses ≈1 µm) and high optical transmission (T ≈ 90%) in the visible range. The obtained layers are characterized by a homogeneous distribution of aluminum for Al content ≥ 2% at. Periodicity was observed at lower contents Al decomposition disappears after heating in nitrogen at 300 ° C. Based on the research techniques used (SEM, SIMS, XRD), no the presence of foreign Al phases was observed in the obtained ZnO: Al layers, even at high Al contents of 7.8% at. ρ values are among the lowest reported so far in the literature for ZnO: Al layers obtained by the ALD method. However, compared to the lowest reported resistance of ZnO: Al layers obtained by sputtering magnetron and the PLD method ( ρ = 2 × 10-4 Ωcm) they are higher, but the same row. ZnO: Al layers were obtained on a flexible polymer - PET substrate, for use as transparent electrodes in organic and flexible instruments electronic. The growth temperature was 110 ° C. Parameters obtained electrical components are comparable to those reported in the literature for ZnO: Al / PET layers obtained by magnetron sputtering. In the literature known to the author So far, there are no studies devoted to the characterization of the obtained ZnO: Al layers by the ALD method on a flexible substrate. The influence of native defects and unintentional admixtures was examined on the obtained electrical parameters of ZnO and ZnO: Al layers obtained by the ALD method. It was found that in the non-doped aluminum layers a fundamental role native defects play. Among them, the predominant influence on high conductivity most likely has interstitial zinc, not oxygen gaps. It means bigger stability of ZnO layers caused, among others, by greater resistance to possible diffusion oxygen from the air. It turned out that the presence of hydrogen does not play a significant role in increasing the conductivity ZnO layers obtained by the ALD method. In the case of undoped layers aluminum, a decrease in the concentration of n is observed with an increase in concentration hydrogen. Comparing the results of the cross-sectional tests of the ZnO and ZnO layers: Al with the profile of the hydrogen content in these layers it can be concluded that hydrogen it accumulates in greater amounts at the grain boundaries which may contribute to lower electron mobility in these layers. An additional argument this is due to the fact that both in the ZnO and ZnO layers: Al the increase in hydrogen concentration was always accompanied by a decrease in the mobility of µ. 

Institute of Physics, Polish Academy of Sciences, Warsaw, Poland
(Warsaw, Poland)
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