@misc{DasKotHenkeletal., author = {Das, Chittaranjan and Kot, Małgorzata and Henkel, Karsten and Schmeißer, Dieter}, title = {Engineering of Sub-Nanometer SiOₓ Thickness in Si Photocathodes for Optimized Open Circuit Potential}, series = {ChemSusChem}, volume = {9}, journal = {ChemSusChem}, number = {17}, issn = {1864-5631}, doi = {10.1002/cssc.201600777}, pages = {2332 -- 2336}, abstract = {Silicon is one of the most promising materials to be used for tandem-cell water-splitting devices. However, the electrochemical instability of bare Si makes it difficult to be used for stable devices. Besides that, the photovoltage loss in Si, caused by several factors (e.g., metal oxide protection layer and/or SiO₂/Si or catalyst/Si interface), limits its use in these devices. In this work, we present that an optimized open circuit potential (OCP) of Si can be obtained by controlling the SiOₓ thickness in sub-nanometer range. It can be done by means of a simple and cost-effective way using the combination of a wet chemical etching and the low temperature atomic layer deposition (ALD) of TiO₂. We have found that a certain thickness of the native SiOₓ is necessary to prevent further oxidation of the Si photocathode during the ALD growth of TiO₂. Moreover, covering the Si photocathode with an ALD TiO₂ layer enhances its stability.}, language = {en} } @misc{HenkelKotSchmeisser, author = {Henkel, Karsten and Kot, Małgorzata and Schmeißer, Dieter}, title = {Localized defect states and charge trapping in atomic layer deposited-Al₂O₃ films}, series = {Journal of Vacuum Science and Technology, A : Vacuum, Surfaces, and Films}, volume = {35}, journal = {Journal of Vacuum Science and Technology, A : Vacuum, Surfaces, and Films}, number = {1}, issn = {0734-2101}, doi = {10.1116/1.4971991}, pages = {01B125-1 -- 01B125-18}, abstract = {In this study, the authors compared different Al₂O₃ films grown by atomic layer deposition (ALD) with the same aluminum precursor but on different substrates. The authors employed different process parameters such as thermal-ALD and plasma-enhanced-ALD using different substrate temperatures ranging from 280 °C down to room temperature. They characterized these films by resonant photoelectron spectroscopy and by electrical measurements. They established that generally the ALD-Al₂O₃ films show characteristic features of bulk Al₂O₃. For all films investigated, the authors found intrinsic defect states within the electronic band gap and identified excitonic, polaronic, and charge-transfer defect states. The authors gave an atomistic model to explain these intrinsic defects and found that their relative abundance is subject of the choice of ALD parameters and of the substrate used. They were able to relate the spectroscopic assigned in-gap defect states with the electronic charges as determined in our electrical measurements.}, language = {de} } @misc{KotDasWangetal., author = {Kot, Małgorzata and Das, Chittaranjan and Wang, Zhiping and Henkel, Karsten and Rouissi, Zied and Wojciechowski, Konrad and Snaith, Henry J. and Schmeißer, Dieter}, title = {Room-Temperature Atomic Layer Deposition of Al₂O₃: Impact on Efficiency, Stability and Surface Properties in Perovskite Solar Cells}, series = {ChemSusChem}, volume = {9}, journal = {ChemSusChem}, number = {24}, issn = {1864-5631}, doi = {10.1002/cssc.201601186}, pages = {3401 -- 3406}, abstract = {In this work, solar cells with a freshly made CH₃NH₃PbI₃ perovskite film showed a power conversion efficiency (PCE) of 15.4 \% whereas the one with 50 days aged perovskite film only 6.1 \%. However, when the aged perovskite was covered with a layer of Al₂O₃ deposited by atomic layer deposition (ALD) at room temperature (RT), the PCE value was clearly enhanced. X-ray photoelectron spectroscopy study showed that the ALD precursors are chemically active only at the perovskite surface and passivate it. Moreover, the RT-ALD-Al2O3-covered perovskite films showed enhanced ambient air stability.}, language = {en} } @misc{HenkelDasKotetal., author = {Henkel, Karsten and Das, Chittaranjan and Kot, Małgorzata and Schmeißer, Dieter and Naumann, Franziska and K{\"a}rkk{\"a}nen, Irina and Gargouri, Hassan}, title = {In-gap states in titanium dioxide and oxynitride atomic layer deposited films}, series = {Journal of Vacuum Science and Technology: A}, volume = {35}, journal = {Journal of Vacuum Science and Technology: A}, number = {1}, issn = {0734-2101}, doi = {10.1116/1.4972247}, pages = {01B135-1 -- 01B135-8}, abstract = {Valence band (VB) spectra of titanium dioxide (TiO2) and oxynitride (TiOxNy) films prepared by different atomic layer deposition (ALD) processes are compared and related to electrical characterization [current-voltage (JV) and capacitance-voltage (CV)] results. By increasing the nitrogen amount in the TiO2 film, band-gap narrowing is observed. The band-gap decrease is related to the contribution of the nitrogen density of states, which induces defects within the band-gap and thus reduces its optical band-gap. In-gap states are found in the VB spectra at 1 eV below the Fermi energy in all investigated ALD samples, i.e., in TiO2 as well as in TiOxNy films. An exponential correlation between leakage current density and in-gap state intensity is derived by the combination of JV measurements and VB spectra, whereas the in-gap states seem to have no influence on hysteresis and fixed oxide charges found in the CV data. It is argued that the in-gap states in TiO2 and TiOxNy have an excitonic or polaronic origin. Both, band-gap narrowing and in-gap state intensity can be tuned by the ALD process selection and the variation of its parameters.}, language = {en} } @inproceedings{ŁobazaKotSchmeisser, author = {Łobaza, Justyna and Kot, Małgorzata and Schmeißer, Dieter}, title = {Analysis of surface oxidation of TiON ALD films}, series = {Verhandlungen der Deutschen Physikalischen Gesellschaft}, booktitle = {Verhandlungen der Deutschen Physikalischen Gesellschaft}, publisher = {Deutsche Physikalische Gesellschaft}, address = {Bad Honnef}, issn = {0420-0195}, pages = {S. 202}, abstract = {Titanium oxynitride (TiON) films are interesting due to their remarkable optical and electronic properties which strongly depend on the O/N ratio. However, it is known that films containing Ti are prone to oxidation in contact with the air [1]. In this work, we study the thickness of a surface oxidation layer which is inherently formed on the atomic layer deposition (ALD) grown TiON/TiN films on Si substrate. We use an Ar+ ion bombardment source and X-ray photoelectron spectroscopy (XPS) for this analysis. We calibrate the sputter rate by using substrate signal intensity decay (here Si 2p) in the XPS spectra of the 5 nm thick TiON sample accordingly. This rate is assumed to be constant when films with a larger thickness are analyzed. We find that the surface oxidation layer is about 1 nm thick, independent on the detailed ALD parameters of the films. The TiN films found underneath are close to the stoichiometric values and have a residual O content below 5\%. Finally, we compare these data to our previous results collected with synchrotron-based radiation source [2,3]. References: [1] Sowinska et al., Applied Physics Letters 100, 233509 (2012). [2] M. Sowinska et al., Applied Surface Science 381, 42-47 (2016). [3] M. Sowińska et al., Journal of Vacuum Science and Tech- nology A, 01A12734 (2016).}, language = {en} } @inproceedings{KotWojciechowskiSnaithetal., author = {Kot, Małgorzata and Wojciechowski, Konrad and Snaith, Henry J. and Schmeißer, Dieter}, title = {Characterization of the perovskite solar cells containing atomic layer deposited Al2O3 buffer layer}, series = {Verhandlungen der Deutschen Physikalischen Gesellschaft e.V.}, booktitle = {Verhandlungen der Deutschen Physikalischen Gesellschaft e.V.}, publisher = {Deutsche Physikalische Gesellschaft}, address = {Bad Honnef}, issn = {0420-0195}, pages = {S. 147}, abstract = {Hybrid perovskites have potential to overcome performance limits of the current solar cell technologies and achieve low cost and high versatility. Nonetheless, they are prone to degradation in presence of moisture within a couple of hours or days. In this work, we use the atomic layer deposition (ALD) of Al2O3 on the CH3NH3PbI3 perovskite at room temperature in order to verify if this thin ALD layer may protect the perovskite film against moisture degradation and to check the impact of the Al2O3 on the solar to power conversion efficiency (PCE). Depth profiling X-ray photoelectron spectroscopy study shows that the ALD precursors are chemically active only at the perovskite surface and the film bulk is not affected. The perovskite film coated with Al2O3 layer has enhanced moisture stability. Solar cells with a fresh-made CH3NH3PbI3 perovskite film have shown PCE of 15.4\%, while the one with 50 days aged perovskite only 6.1\%. However, when the aged perovskite is covered with RT-ALD-Al2O3 the PCE value is clearly enhanced.[1] [1] M. Kot et al., Room temperature ALD impact on efficiency, stability and surface properties in perovskite solar cells, ChemSusChem,acctepted.}, language = {en} } @inproceedings{HenkelKotSchmeisser, author = {Henkel, Karsten and Kot, Małgorzata and Schmeißer, Dieter}, title = {Localized defect states and charge trapping in Al₂O₃ films prepared by atomic layer deposition}, series = {Verhandlungen der Deutschen Physikalischen Gesellschaft}, booktitle = {Verhandlungen der Deutschen Physikalischen Gesellschaft}, publisher = {Deutsche Physikalische Gesellschaft}, address = {Bad Honnef}, issn = {0420-0195}, pages = {S. 205}, abstract = {The evaluation of the electronic structure and intrinsic defect mechanisms in Al₂O₃ thin films is essential for their effective use in applications with desired functionality such as surface passivation schemes for solar cells [1]. We present a comparative study of different Al₂O₃ films grown by atomic layer deposition (ALD) [2]. The layers were deposited on different substrates using the same aluminum precursor (TMA, trimethylalumium) and employing different process parameters (thermal-ALD, plasma-enhanced-ALD, substrate temperature). These films were characterized by resonant photoelectron spectroscopy and by electrical measurements (capacitance-voltage). For all films investigated intrinsic defect states within the electronic band gap were observed including excitonic, polaronic, and charge-transfer defect states, where their relative abundance is subject of the choice of ALD parameters and of the used substrate. The spectroscopic assigned in-gap defect states are related with electronic charges as determined in the electrical measurements. [1] G. Dingemans and W.M.M. Kessels, J. Vac. Sci. Technol. A 30, 040802 (2012). [2] K. Henkel, M. Kot, D. Schmeißer, J. Vac. Sci. Technol. A 35, (2017), accepted.}, language = {en} } @misc{DasKotRouissietal., author = {Das, Chittaranjan and Kot, Małgorzata and Rouissi, Zied and Kędzierski, Kamil and Henkel, Karsten and Schmeißer, Dieter}, title = {Selective Deposition of an ultrathin Pt Layer on a Au-Nanoisland-Modified Si Photocathode for Hydrogen Generation}, series = {ACS Omega}, volume = {2}, journal = {ACS Omega}, number = {4}, issn = {2470-1343}, doi = {10.1021/acsomega.6b00374}, pages = {1360 -- 1366}, abstract = {Platinum, being the most efficient and stable catalyst, is used in photoelectrochemical (PEC) devices. However, a minimal amount of Pt with maximum catalytic activity is required to be used to minimize the cost of production. In this work, we use an environmentally friendly, ost-effective, and less Pt-consuming method to prepare PEC devices for the hydrogen evolution reaction (HER). The Pt monolayer catalyst is selectively deposited on a Au-nanoisland-supported boron-doped p-type Si (100) photocathode. The PEC device based on the Si photocathode with an ultralow loading of the Pt catalyst exhibits a comparable performance for the HER to that of devices with a thick Pt layer. In addition, we demonstrate that by using a thin TiO2 layer deposited by atomic layer deposition photo-oxidation of the Si photocathode can be blocked resulting in a stable PEC performance.}, language = {en} } @misc{KotHenkelDasetal., author = {Kot, Małgorzata and Henkel, Karsten and Das, Chittaranjan and Brizzi, Simone and K{\"a}rkk{\"a}nen, Irina and Schneidewind, Jessica and Naumann, Franziska and Gargouri, Hassan and Schmeißer, Dieter}, title = {Analysis of titanium species in titanium oxynitride films prepared by plasma enhanced atomic layer deposition}, series = {Surface and Coatings Technology}, volume = {324}, journal = {Surface and Coatings Technology}, issn = {0257-8972}, doi = {10.1016/j.surfcoat.2016.11.094}, pages = {586 -- 593}, abstract = {A comparative study of thin titanium oxynitride (TiOxNy) films prepared by plasma enhanced atomic layer deposition using tetrakis(dimethylamino)titanium (TDMAT) and N2 plasma as well as titanium(IV)isopropoxide and NH3 plasma is reported. The comparison is based on the combination of Ti2p core level and valence band spectroscopy and current-voltage measurements. The TDMAT/N2 process delivers generally higher fractions of TiN and TiON within the Ti2p spectra of the films and stronger photoemissions within the bandgap as resolved in detail by high energy resolution synchrotron-based spectroscopy. In particular, it is shown that higher TiN contributions and in-gap emission intensities correlate strongly with increased leakage currents within the films and might be modified by the process parameters and precursor selection.}, language = {en} } @misc{KotDasHenkeletal., author = {Kot, Małgorzata and Das, Chittaranjan and Henkel, Karsten and Wojciechowski, Konrad and Snaith, Henry J. and Schmeißer, Dieter}, title = {Room temperature atomic layer deposited Al₂O₃ on CH₃NH₃PbI₃ characterized by synchrotron-based X-ray photoelectron spectroscopy}, series = {Nuclear Instruments and Methods in Physics Research B}, volume = {411}, journal = {Nuclear Instruments and Methods in Physics Research B}, issn = {0168-583X}, doi = {10.1016/j.nimb.2017.01.082}, pages = {49 -- 52}, abstract = {An ultrathin Al₂O₃ film deposited on methylammonium lead triiodide (CH₃NH₃PbI₃) perovskite has the capability to suppress the carrier recombination process and improve the perovskite solar cells efficiency and stability. However, annealing at temperatures higher than 85°C degrades the CH₃NH₃PbI₃ perovskite film. The X-ray photoelectron spectroscopy study performed in this work indicates that it is possible to grow Al₂O₃ by atomic layer deposition on the perovskite at room temperature, however, besides pure Al₂O₃ some OH groups are found and the creation of lead and iodine oxides at the Al₂O₃/CH₃NH₃PbI₃ interface takes place.}, language = {en} }