@misc{DasRichterTallaridaetal., author = {Das, Chittaranjan and Richter, Matthias and Tallarida, Massimo and Schmeißer, Dieter}, title = {Electronic properties of atomic layer deposited films, anatase and rutile TiO2 studied by resonant photoemission spectroscopy}, series = {Journal of Physics D: Applied Physics}, volume = {49}, journal = {Journal of Physics D: Applied Physics}, number = {27}, issn = {0022-3727}, doi = {10.1088/0022-3727/49/27/275304}, pages = {275304-1 -- 275304-17}, abstract = {The TiO2 films are prepared by atomic layer deposition (ALD) method using titanium isopropoxide precursors at 250 °C and analyzed using resonant photoemission spectroscopy (resPES). We report on the Ti2p and O1s core levels, on the valence band (VB) spectra and x-ray absorption spectroscopy (XAS) data, and on the resonant photoelectron spectroscopy (resPES) profiles at the O1s and the Ti3p absorption edges. We determine the elemental abundance, the position of the VB maxima, the partial density of states (PDOS) in the VB and in the conduction band (CB) and collect these data in a band scheme. In addition, we analyze the band-gap states as well as the intrinsic states due to polarons and charge-transfer excitations. These states are found to cause multiple Auger decay processes upon resonant excitation. We identify several of these processes and determine their relative contribution to the Auger signal quantitatively. As our resPES data allow a quantitative analysis of these defect states, we determine the relative abundance of the PDOS in the VB and in CB and also the charge neutrality level. The anatase and rutile polymorphs of TiO2 are analyzed in the same way as the TiO2 ALD layer. The electronic properties of the TiO2ALD layer are compared with the anatase and rutile polymorphs of TiO2. In our comparative study, we find that ALD has its own characteristic electronic structure that is distinct from that of anatase and rutile. However, many details of the electronic structure are comparable and we benefit from our spectroscopic data and our careful analysis to find these differences. These can be attributed to a stronger hybridization of the O2p and Ti3d4s states for the ALD films when compared to the anatase and rutile polymorphs.}, language = {en} } @incollection{SchmeisserKotCorreaetal., author = {Schmeißer, Dieter and Kot, Małgorzata and Corr{\^e}a, Silma Alberton and Das, Chittaranjan and Henkel, Karsten}, title = {Interface Potentials, Intrinsic Defects, and Passivation Mechanisms in Al₂O₃, HfO₂, and TiO₂ Ultrathin Films}, series = {Encyclopedia of Interfacial Chemistry: Surface Science and Electrochemistry, vol. 3.1}, booktitle = {Encyclopedia of Interfacial Chemistry: Surface Science and Electrochemistry, vol. 3.1}, editor = {Wandelt, Klaus}, publisher = {Elsevier}, address = {Oxford}, isbn = {978-0-12-809739-7}, doi = {10.1016/B978-0-12-409547-2.14119-8}, pages = {162 -- 171}, abstract = {We study the electronic structure of ultrathin Al₂O₃, HfO₂, and TiO₂ ALD films by resonant photoelectron spectroscopy. We identify intrinsic defects which are responsible for the active sites in interface reactions, for the incorporation of intrinsic charges, and for the formation of local dipole momenta. All of these features determine the surface potentials and the reactivity of the surface of the atomic layer deposition coated systems. We give examples of charges and dipoles in Al₂O₃, on a study of the surface potentials in HfO₂, and relate the intrinsic defects in TiO₂ to their electrochemical relevance.}, language = {en} } @misc{DasKotHellmannetal., author = {Das, Chittaranjan and Kot, Małgorzata and Hellmann, Tim and Wittich, Carolin and Mankel, Eric and Zimmermann, Iwan and Schmeißer, Dieter and Nazeeruddin, Mohammad Khaja and Jaegermann, Wolfram}, title = {Atomic Layer-Deposited Aluminum Oxide Hinders Iodide Migration and Stabilizes Perovskite Solar Cells}, series = {Cell Reports Physical Science}, volume = {1}, journal = {Cell Reports Physical Science}, number = {7}, issn = {2666-3864}, doi = {10.1016/j.xcrp.2020.100112}, pages = {18}, abstract = {Iodide migration causes degradation of the perovskite solar cells. Here,we observe the direct migration of iodide into the hole-transport layer in a device. We demonstrate that ultrathin room temperature atomic layer-deposited Al2O3 on the perovskite surface very effectively hinders the migration. The perovskite-Al2O3 interface enables charge transfer across the Al2O3 layer in the solar cells, without causing any drastic changes in the properties of the perovskite absorber. Furthermore, it helps to preserve the initial properties of the perovskite film during exposure to light and air under real operating conditions, and thus, improves the stability of the solar cells. The ultrathin Al2O3 layer deposited at room temperature significantly increases the lifetime of the perovskite solar cells, and we hope this may be a step toward the mass production of stable devices.}, language = {en} } @misc{DasKediaZuoetal., author = {Das, Chittaranjan and Kedia, Mayank and Zuo, Weiwei and Mortan, Claudiu and Kot, Małgorzata and Flege, Jan Ingo and Saliba, Michael}, title = {Band Bending at Hole Transporting Layer-Perovskite Interfaces in n-i-p and in p-i-n Architecture}, series = {Solar RRL}, volume = {6}, journal = {Solar RRL}, number = {9}, issn = {2367-198X}, doi = {10.1002/solr.202200348}, abstract = {Interfaces between hybrid perovskite absorber and its adjacent charge-transporting layers are of high importance for solar cells performance. Understanding their chemical and electronic properties is a key step in designing efficient and stable perovskite solar cells. In this work, the tapered cross-section photoemission spectroscopy (TCS-PES) method is used to study the methylammonium lead iodide (CH3NH3PbI3) (MAPI)-based solar cells in two configurations, that is, in an inverted p-i-n and in a classical n-i-p architecture. It is revealed in the results that the MAPI film deposited once on the n-type TiO2 and once on the p-type NiOx substrates is neither an intrinsic semiconductor nor adapts to the dopant nature of the substrate underneath, but it is heavily n-type doped on both substrates. In addition to that, the TCS-PES results identify that the band bending between the MAPI film and the hole transporting layer (HTL) layer depends on the perovskite solar cells architecture. In particular, a band bending on the HTL side in the n-i-p and at the MAPI in the p-i-n architecture is found. The flat band of NiOx at the NiOx/MAPI interface can be explained by the Fermi level pinning of the NiOx at the interface.}, language = {en} } @misc{KotDasKegelmannetal., author = {Kot, Małgorzata and Das, Chittaranjan and Kegelmann, Lukas and K{\"o}bler, Hans and Vorokhta, Mykhailo and Escudero, Carlos and Albrecht, Steve and Abate, Antonio and Flege, Jan Ingo}, title = {Application of atomic layer deposition and x-ray photoelectron spectroscopy in perovskite solar cells}, series = {Verhandlungen der DPG}, journal = {Verhandlungen der DPG}, publisher = {Deutsche Physikalische Gesellschaft}, address = {Bad Honnef}, issn = {0420-0195}, abstract = {In this work we have utilized near-ambient pressure and ultra-high vacuum X-ray photoelectron spectroscopy as well as atomic layer deposition to investigate perovskite solar cells (PSCs). We have demonstrated that ultrathin room temperature atomic layer-deposited aluminium oxide on the perovskite surface very effectively suppresses iodine migration[1] and improves the long term stability and efficiency of PSCs [2,3]. Furthermore, exposure to light proves more detrimental to the perovskite film than exposure to water vapor.[2] Absorbed photons create Frenkel defects in the perovskite crystal and their number strongly depends on the used illumination. The higher the photon flux, the higher the concentration of Frenkel defects, and thus the stronger the degradation of power conversion efficiency and the stronger the hysteresis in the J-V characteristics. [1] C. Das, M. Kot et al., Cell Reports Physical Science 2020, 1, 100112. [2] M. Kot et al., ChemSusChem 2020, 13, 5722. [3] M. Kot et al., ChemSusChem 2018, 11, 3640.}, language = {en} } @misc{KediaRaiPhirkeetal., author = {Kedia, Mayank and Rai, Monika and Phirke, Himanshu and Aranda, Clara A. and Das, Chittaranjan and Chirvony, Vladimir and Boehringer, Stephan and Kot, Małgorzata and Malekshahi Byranvand, Mahdi and Flege, Jan Ingo and Redinger, Alex and Saliba, Michael}, title = {Light Makes Right: Laser Polishing for Surface Modification of Perovskite Solar Cells}, series = {ACS Energy Letters}, volume = {8}, journal = {ACS Energy Letters}, issn = {2380-8195}, doi = {10.1021/acsenergylett.3c00469}, pages = {2603 -- 2610}, abstract = {Interface engineering is a common strategy for passivating surface defects to attain open circuit voltages (Voc) in perovskite solar cells (PSCs). In this work, we introduce the concept of polishing a perovskite thin-film surface using a nanosecond (ns) pulsed ultraviolet laser to reduce surface defects, such as dangling bonds, undesirable phases, and suboptimal stoichiometry. A careful control of laser energy and scanning speed improves the photophysical properties of the surface without compromising the thickness. Using laser polishing, a Voc of 1.21 V is achieved for planar PSCs with a triple cation composition, showing an improved perovskite/hole transport interface by mitigating surface recombination losses. We measure an efficiency boost from 18.0\% to 19.3\% with improved stability of up to 1000 h. The results open the door to a new class of surface modification using lasers for interface passivation in well-controllable, automated, scalable, and solvent-free surface treatments.}, language = {en} } @misc{DasRoyKediaetal., author = {Das, Chittaranjan and Roy, Rajarshi and Kedia, Mayank and Kot, Małgorzata and Zuo, Weiwei and F{\´e}lix, Roberto and Sobol, Tomasz and Flege, Jan Ingo and Saliba, Michael}, title = {Unraveling the Role of Perovskite in Buried Interface Passivation}, series = {ACS Applied Materials \& Interfaces}, volume = {15}, journal = {ACS Applied Materials \& Interfaces}, number = {48}, issn = {1944-8244}, doi = {10.1021/acsami.3c13085}, pages = {56500 -- 56510}, abstract = {Interfaces in perovskite solar cells play a crucial role in their overall performance, and therefore, detailed fundamental studies are needed for a better understanding. In the case of the classical n-i-p architecture, TiO2 is one of the most used electron-selective layers and can induce chemical reactions that influence the performance of the overall device stack. The interfacial properties at the TiO2/perovskite interface are often neglected, owing to the difficulty in accessing this interface. Here, we use X-rays of variable energies to study the interface of (compact and mesoporous) TiO2/perovskite in such a n-i-p architecture. The X-ray photoelectron spectroscopy and X-ray absorption spectroscopy methods show that the defect states present in the TiO2 layer are passivated by a chemical interaction of the perovskite precursor solution during the formation of the perovskite layer and form an organic layer at the interface. Such passivation of intrinsic defects in TiO2 removes charge recombination centers and shifts the bands upward. Therefore, interface defect passivation by oxidation of Ti3+ states, the organic cation layer, and an upward band bending at the TiO2/perovskite interface explain the origin of an improved electron extraction and hole-blocking nature of TiO2 in the n-i-p perovskite solar cells.}, language = {en} } @misc{DasZiaMortanetal., author = {Das, Chittaranjan and Zia, Waqas and Mortan, Claudiu and Hussain, Navid and Saliba, Michael and Flege, Jan Ingo and Kot, Małgorzata}, title = {Top-Down Approach to Study Chemical and Electronic Properties of Perovskite Solar Cells: Sputtered Depth Profiling Versus Tapered Cross-Sectional Photoelectron Spectroscopies}, series = {Solar RRL}, volume = {5}, journal = {Solar RRL}, number = {10}, issn = {2367-198X}, doi = {10.1002/solr.202100298}, abstract = {A study of the chemical and electronic properties of various layers across perovskite solar cell (PSC) stacks is challenging. Depth-profiling photoemission spectroscopy can be used to study the surface, interface, and bulk properties of different layers in PSCs, which influence the overall performance of these devices. Herein, sputter depth profiling (SDP) and tapered cross-sectional (TCS) photoelectron spectroscopies (PESs) are used to study highly efficient mixed halide PSCs. It is found that the most used SDP-PES technique degrades the organic and deforms the inorganic materials during sputtering of the PSCs while the TCS-PES method is less destructive and can determine the chemical and electronic properties of all layers precisely. The SDP-PES dissociates the chemical bonding in the spiro-MeOTAD and perovskite layer and reduces the TiO2, which causes the chemical analysis to be unreliable. The TCS-PES revealed a band bending only at the spiro-MeOTAD/perovskite interface of about 0.7 eV. Both the TCS and SDP-PES show that the perovskite layer is inhomogeneous and has a higher amount of bromine at the perovskite/TiO2 interface.}, language = {en} } @misc{KotDasBaranetal., author = {Kot, Małgorzata and Das, Chittaranjan and Baran, Derya and Saliba, Michael}, title = {Themed issue on electronic properties and characterisation of perovskites}, series = {Journal of Materials Chemistry C}, volume = {7}, journal = {Journal of Materials Chemistry C}, issn = {2050-7526}, doi = {10.1039/c9tc90085c}, pages = {5224 -- 5225}, language = {en} } @misc{KotKegelmannDasetal., author = {Kot, Małgorzata and Kegelmann, Lukas and Das, Chittaranjan and Kus, Peter and Tsud, Nataliya and Matol{\´i}nov{\´a}, Iva and Albrecht, Steve and Matolin, Vladimir and Schmeißer, Dieter}, title = {Room temperature atomic layer deposited Al₂O₃ improves perovskite solar cells efficiency over time}, series = {ChemSusChem}, volume = {11}, journal = {ChemSusChem}, number = {20}, issn = {1864-5631}, doi = {10.1002/cssc.201801434}, pages = {3640 -- 3648}, abstract = {Electrical characterisation of perovskite solar cells consisting of room-temperature atomic-layer-deposited aluminium oxide (RT-ALD-Al₂O₃) film on top of a methyl ammonium lead triiodide (CH₃NH₃PbI₃) absorber showed excellent stability of the power conversion efficiency (PCE) over along time. Under the same environmental conditions (for 355 d), the average PCE of solar cells without the ALD layer decreased from 13.6 to 9.6 \%, whereas that of solar cells containing 9 ALD cycles of depositing RT-ALD-Al₂O₃on top of CH₃NH₃PbI₃ increased from 9.4 to 10.8 \%. Spectromicroscopic investigations of the ALD/perovskite interface revealed that the maximum PCE with the ALD layer is obtained when the so-called perovskite cleaning process induced by ALD precursors is complete. The PCE enhancement over time is probably related to a self-healing process induced by the RT-ALD-Al₂O₃ film. This work may provide a new direction for further improving the long-term stability and performance of perovskite solar cells.}, language = {en} }