@article{ZuWolffRalaiarisoaetal.2019, author = {Zu, Fengshuo and Wolff, Christian Michael and Ralaiarisoa, Maryline and Amsalem, Patrick and Neher, Dieter and Koch, Norbert}, title = {Unraveling the Electronic Properties of Lead Halide Perovskites with Surface Photovoltage in Photoemission Studies}, series = {ACS applied materials \& interfaces}, volume = {11}, journal = {ACS applied materials \& interfaces}, number = {24}, publisher = {American Chemical Society}, address = {Washington}, issn = {1944-8244}, doi = {10.1021/acsami.9b05293}, pages = {21578 -- 21583}, year = {2019}, abstract = {The tremendous success of metal-halide perovskites, especially in the field of photovoltaics, has triggered a substantial number of studies in understanding their optoelectronic properties. However, consensus regarding the electronic properties of these perovskites is lacking due to a huge scatter in the reported key parameters, such as work function (Φ) and valence band maximum (VBM) values. Here, we demonstrate that the surface photovoltage (SPV) is a key phenomenon occurring at the perovskite surfaces that feature a non-negligible density of surface states, which is more the rule than an exception for most materials under study. With ultraviolet photoelectron spectroscopy (UPS) and Kelvin probe, we evidence that even minute UV photon fluxes (500 times lower than that used in typical UPS experiments) are sufficient to induce SPV and shift the perovskite Φ and VBM by several 100 meV compared to dark. By combining UV and visible light, we establish flat band conditions (i.e., compensate the surface-state-induced surface band bending) at the surface of four important perovskites, and find that all are p-type in the bulk, despite a pronounced n-type surface character in the dark. The present findings highlight that SPV effects must be considered in all surface studies to fully understand perovskites' photophysical properties.}, language = {en} } @article{ZuSchultzWolffetal.2020, author = {Zu, Fengshuo and Schultz, Thorsten and Wolff, Christian Michael and Shin, Dongguen and Frohloff, Lennart and Neher, Dieter and Amsalem, Patrick and Koch, Norbert}, title = {Position-locking of volatile reaction products by atmosphere and capping layers slows down photodecomposition of methylammonium lead triiodide perovskite}, series = {RSC Advances}, volume = {10}, journal = {RSC Advances}, number = {30}, publisher = {Royal Society of Chemistry}, address = {Cambridge}, issn = {2046-2069}, doi = {10.1039/d0ra03572f}, pages = {17534 -- 17542}, year = {2020}, abstract = {The remarkable progress of metal halide perovskites in photovoltaics has led to the power conversion efficiency approaching 26\%. However, practical applications of perovskite-based solar cells are challenged by the stability issues, of which the most critical one is photo-induced degradation. Bare CH3NH3PbI3 perovskite films are known to decompose rapidly, with methylammonium and iodine as volatile species and residual solid PbI2 and metallic Pb, under vacuum under white light illumination, on the timescale of minutes. We find, in agreement with previous work, that the degradation is non-uniform and proceeds predominantly from the surface, and that illumination under N-2 and ambient air (relative humidity 20\%) does not induce substantial degradation even after several hours. Yet, in all cases the release of iodine from the perovskite surface is directly identified by X-ray photoelectron spectroscopy. This goes in hand with a loss of organic cations and the formation of metallic Pb. When CH3NH3PbI3 films are covered with a few nm thick organic capping layer, either charge selective or non-selective, the rapid photodecomposition process under ultrahigh vacuum is reduced by more than one order of magnitude, and becomes similar in timescale to that under N-2 or air. We conclude that the light-induced decomposition reaction of CH3NH3PbI3, leading to volatile methylammonium and iodine, is largely reversible as long as these products are restrained from leaving the surface. This is readily achieved by ambient atmospheric pressure, as well as a thin organic capping layer even under ultrahigh vacuum. In addition to explaining the impact of gas pressure on the stability of this perovskite, our results indicate that covalently "locking" the position of perovskite components at the surface or an interface should enhance the overall photostability.}, language = {en} } @article{ZuAmsalemEggeretal.2019, author = {Zu, Fengshuo and Amsalem, Patrick and Egger, David A. and Wang, Rongbin and Wolff, Christian Michael and Fang, Honghua and Loi, Maria Antonietta and Neher, Dieter and Kronik, Leeor and Duhm, Steffen and Koch, Norbert}, title = {Constructing the Electronic Structure of CH3NH3PbI3 and CH3NH3PbBr3 Perovskite Thin Films from Single-Crystal Band Structure Measurements}, series = {The journal of physical chemistry letters}, volume = {10}, journal = {The journal of physical chemistry letters}, number = {3}, publisher = {American Chemical Society}, address = {Washington}, issn = {1948-7185}, doi = {10.1021/acs.jpclett.8b03728}, pages = {601 -- 609}, year = {2019}, abstract = {Photovoltaic cells based on halide perovskites, possessing remarkably high power conversion efficiencies have been reported. To push the development of such devices further, a comprehensive and reliable understanding of their electronic properties is essential but presently not available. To provide a solid foundation for understanding the electronic properties of polycrystalline thin films, we employ single-crystal band structure data from angle-resolved photoemission measurements. For two prototypical perovskites (CH3NH3PbBr3 and CH3NH3PbI3), we reveal the band dispersion in two high-symmetry directions and identify the global valence band maxima. With these benchmark data, we construct "standard" photoemission spectra from polycrystalline thin film samples and resolve challenges discussed in the literature for determining the valence band onset with high reliability. Within the framework laid out here, the consistency of relating the energy level alignment in perovskite-based photovoltaic and optoelectronic devices with their functional parameters is substantially enhanced.}, language = {en} } @article{ZhangHosseiniGunderetal.2019, author = {Zhang, Shanshan and Hosseini, Seyed Mehrdad and Gunder, Rene and Petsiuk, Andrei and Caprioglio, Pietro and Wolff, Christian Michael and Shoaee, Safa and Meredith, Paul and Schorr, Susan and Unold, Thomas and Burn, Paul L. and Neher, Dieter and Stolterfoht, Martin}, title = {The Role of Bulk and Interface Recombination in High-Efficiency Low-Dimensional Perovskite Solar Cells}, series = {Advanced materials}, volume = {31}, journal = {Advanced materials}, number = {30}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {0935-9648}, doi = {10.1002/adma.201901090}, pages = {11}, year = {2019}, abstract = {2D Ruddlesden-Popper perovskite (RPP) solar cells have excellent environmental stability. However, the power conversion efficiency (PCE) of RPP cells remains inferior to 3D perovskite-based cells. Herein, 2D (CH3(CH2)(3)NH3)(2)(CH3NH3)(n-1)PbnI3n+1 perovskite cells with different numbers of [PbI6](4-) sheets (n = 2-4) are analyzed. Photoluminescence quantum yield (PLQY) measurements show that nonradiative open-circuit voltage (V-OC) losses outweigh radiative losses in materials with n > 2. The n = 3 and n = 4 films exhibit a higher PLQY than the standard 3D methylammonium lead iodide perovskite although this is accompanied by increased interfacial recombination at the top perovskite/C-60 interface. This tradeoff results in a similar PLQY in all devices, including the n = 2 system where the perovskite bulk dominates the recombination properties of the cell. In most cases the quasi-Fermi level splitting matches the device V-OC within 20 meV, which indicates minimal recombination losses at the metal contacts. The results show that poor charge transport rather than exciton dissociation is the primary reason for the reduction in fill factor of the RPP devices. Optimized n = 4 RPP solar cells had PCEs of 13\% with significant potential for further improvements.}, language = {en} } @article{ZeiskeSandbergZarrabietal.2022, author = {Zeiske, Stefan and Sandberg, Oskar J. and Zarrabi, Nasim and Wolff, Christian Michael and Raoufi, Meysam and Pe{\~n}a-Camargo, Francisco and Gutierrez-Partida, Emilio and Meredith, Paul and Stolterfoht, Martin and Armin, Ardalan}, title = {Static disorder in lead halide perovskites}, series = {The journal of physical chemistry letters}, volume = {13}, journal = {The journal of physical chemistry letters}, number = {31}, publisher = {American Chemical Society}, address = {Washington}, issn = {1948-7185}, doi = {10.1021/acs.jpclett.2c01652}, pages = {7280 -- 7285}, year = {2022}, abstract = {In crystalline and amorphous semiconductors, the temperature-dependent Urbach energy can be determined from the inverse slope of the logarithm of the absorption spectrum and reflects the static and dynamic energetic disorder. Using recent advances in the sensitivity of photocurrent spectroscopy methods, we elucidate the temperature-dependent Urbach energy in lead halide perovskites containing different numbers of cation components. We find Urbach energies at room temperature to be 13.0 +/- 1.0, 13.2 +/- 1.0, and 13.5 +/- 1.0 meV for single, double, and triple cation perovskite. Static, temperature-independent contributions to the Urbach energy are found to be as low as 5.1 ?+/- 0.5, 4.7 +/- 0.3, and 3.3 +/- 0.9 meV for the same systems. Our results suggest that, at a low temperature, the dominant static disorder in perovskites is derived from zero-point phonon energy rather than structural disorder. This is unusual for solution-processed semiconductors but broadens the potential application of perovskites further to quantum electronics and devices.}, language = {en} } @article{WuerfelPerdigonToroKurpiersetal.2019, author = {W{\"u}rfel, Uli and Perdigon-Toro, Lorena and Kurpiers, Jona and Wolff, Christian Michael and Caprioglio, Pietro and Rech, Jeromy James and Zhu, Jingshuai and Zhan, Xiaowei and You, Wei and Shoaee, Safa and Neher, Dieter and Stolterfoht, Martin}, title = {Recombination between Photogenerated and Electrode-Induced Charges Dominates the Fill Factor Losses in Optimized Organic Solar Cells}, series = {The journal of physical chemistry letters}, volume = {10}, journal = {The journal of physical chemistry letters}, number = {12}, publisher = {American Chemical Society}, address = {Washington}, issn = {1948-7185}, doi = {10.1021/acs.jpclett.9b01175}, pages = {3473 -- 3480}, year = {2019}, abstract = {Charge extraction in organic solar cells (OSCs) is commonly believed to be limited by bimolecular recombination of photogenerated charges. However, the fill factor of OSCs is usually almost entirely governed by recombination processes that scale with the first order of the light intensity. This linear loss was often interpreted to be a consequence of geminate or trap-assisted recombination. Numerical simulations show that this linear dependence is a direct consequence of the large amount of excess dark charge near the contact. The first-order losses increase with decreasing mobility of minority carriers, and we discuss the impact of several material and device parameters on this loss mechanism. This work highlights that OSCs are especially vulnerable to injected charges as a result of their poor charge transport properties. This implies that dark charges need to be better accounted for when interpreting electro-optical measurements and charge collection based on simple figures of merit.}, language = {en} } @article{WolffZuPaulkeetal.2017, author = {Wolff, Christian Michael and Zu, Fengshuo and Paulke, Andreas and Perdigon-Toro, Lorena and Koch, Norbert and Neher, Dieter}, title = {Reduced Interface-Mediated Recombination for High Open-Circuit Voltages in CH3NH3PbI3 Solar Cells}, series = {Advanced materials}, volume = {29}, journal = {Advanced materials}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {0935-9648}, doi = {10.1002/adma.201700159}, pages = {8}, year = {2017}, abstract = {Perovskite solar cells with all-organic transport layers exhibit efficiencies rivaling their counterparts that employ inorganic transport layers, while avoiding high-temperature processing. Herein, it is investigated how the choice of the fullerene derivative employed in the electron-transporting layer of inverted perovskite cells affects the open-circuit voltage (V-OC). It is shown that nonradiative recombination mediated by the electron-transporting layer is the limiting factor for the V-OC in the cells. By inserting an ultrathin layer of an insulating polymer between the active CH3NH3PbI3 perovskite and the fullerene, an external radiative efficiency of up to 0.3\%, a V-OC as high as 1.16 V, and a power conversion efficiency of 19.4\% are realized. The results show that the reduction of nonradiative recombination due to charge-blocking at the perovskite/organic interface is more important than proper level alignment in the search for ideal selective contacts toward high V-OC and efficiency.}, language = {en} } @article{WolffKristenJennySchettleretal.2014, author = {Wolff, Christian Michael and Kristen-Jenny, Iris and Schettler, Georg and Plessen, Birgit and Meyer, Hanno and Dulski, Peter and Naumann, Rudolf and Brauer, Achim and Verschuren, Dirk and Haug, Gerald H.}, title = {Modern seasonality in Lake Challa (Kenya/Tanzania) and its sedimentary documentation in recent lake sediments}, series = {Limnology and oceanography}, volume = {59}, journal = {Limnology and oceanography}, number = {5}, publisher = {Wiley}, address = {Waco}, issn = {0024-3590}, doi = {10.4319/lo.2014.59.5.1621}, pages = {1621 -- 1636}, year = {2014}, abstract = {From November 2006 to January 2010, a sediment trap that was cleared monthly was deployed in Lake Challa, a deep stratified freshwater lake on the eastern slope of Mt. Kilimanjaro in southern Kenya. Geochemical data from sediment trap samples were compared with a broad range of limnological and meteorological parameters to characterize the effect of single parameters on productivity and sedimentation processes in the crater basin. During the southern hemisphere summer (November-March), when the water temperature is high and the lake is biologically productive (nondiatom algae), calcite predominated in the sediment trap samples. During the "long rain" season (March-May) a small amount of organic matter and lithogenic material caused by rainfall appeared. This was followed by the cool and windy months of the southern hemisphere winter (June-October) when diatoms were the main component, indicating a diatom bloom initiated by improvement of nutrient availability related to upwelling processes. The sediment trap data support the hypothesis that the light-dark lamination couplets, which are abundant in Lake Challa cores, reflect seasonal delivery to the sediments of diatom-rich particulates during the windy months and diatom-poor material during the wet season. However, interannual and spatial variability in upwelling and productivity patterns, as well as El Nino-Southern Oscillation (ENSO)-related rainfall and drought cycles, exert a strong influence on the magnitude and geochemical composition of particle export to the hypolimnion of Lake Challa.}, language = {en} } @article{WolffHaugTimmermannetal.2011, author = {Wolff, Christian Michael and Haug, Gerald H. and Timmermann, Axel and Damste, Jaap S. Sinninghe and Brauer, Achim and Sigman, Daniel M. and Cane, Mark A. and Verschuren, Dirk}, title = {Reduced interannual rainfall variability in East Africa during the last Ice Age}, series = {Science}, volume = {333}, journal = {Science}, number = {6043}, publisher = {American Assoc. for the Advancement of Science}, address = {Washington}, issn = {0036-8075}, doi = {10.1126/science.1203724}, pages = {743 -- 747}, year = {2011}, abstract = {Interannual rainfall variations in equatorial East Africa are tightly linked to the El Nino Southern Oscillation (ENSO), with more rain and flooding during El Nino and droughts in La Nina years, both having severe impacts on human habitation and food security. Here we report evidence from an annually laminated lake sediment record from southeastern Kenya for interannual to centennial-scale changes in ENSO-related rainfall variability during the last three millennia and for reductions in both the mean rate and the variability of rainfall in East Africa during the Last Glacial period. Climate model simulations support forward extrapolation from these lake sediment data that future warming will intensify the interannual variability of East Africa's rainfall.}, language = {en} } @article{WolffFrischmannSchulzeetal.2018, author = {Wolff, Christian Michael and Frischmann, Peter D. and Schulze, Marcus and Bohn, Bernhard J. and Wein, Robin and Livadas, Panajotis and Carlson, Michael T. and J{\"a}ckel, Frank and Feldmann, Jochen and W{\"u}rthner, Frank and Stolarczyk, Jacek K.}, title = {All-in-one visible-light-driven water splitting by combining nanoparticulate and molecular co-catalysts on CdS nanorods}, series = {Nature Energy}, volume = {3}, journal = {Nature Energy}, number = {10}, publisher = {Nature Publ. Group}, address = {London}, issn = {2058-7546}, doi = {10.1038/s41560-018-0229-6}, pages = {862 -- 869}, year = {2018}, abstract = {Full water splitting into hydrogen and oxygen on semiconductor nanocrystals is a challenging task; overpotentials must be overcome for both half-reactions and different catalytic sites are needed to facilitate them. Additionally, efficient charge separation and prevention of back reactions are necessary. Here, we report simultaneous H-2 and O-2 evolution by CdS nanorods decorated with nanoparticulate reduction and molecular oxidation co-catalysts. The process proceeds entirely without sacrificial agents and relies on the nanorod morphology of CdS to spatially separate the reduction and oxidation sites. Hydrogen is generated on Pt nanoparticles grown at the nanorod tips, while Ru(tpy)(bpy)Cl-2-based oxidation catalysts are anchored through dithiocarbamate bonds onto the sides of the nanorod. O-2 generation from water was verified by O-18 isotope labelling experiments, and time-resolved spectroscopic results confirmed efficient charge separation and ultrafast electron and hole transfer to the reaction sites. The system demonstrates that combining nanoparticulate and molecular catalysts on anisotropic nanocrystals provides an effective pathway for visible-light-driven photocatalytic water splitting.}, language = {en} } @misc{WolffCaprioglioStolterfohtetal.2019, author = {Wolff, Christian Michael and Caprioglio, Pietro and Stolterfoht, Martin and Neher, Dieter}, title = {Nonradiative Recombination in Perovskite Solar Cells}, series = {Advanced materials}, volume = {31}, journal = {Advanced materials}, number = {52}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {0935-9648}, doi = {10.1002/adma.201902762}, pages = {20}, year = {2019}, abstract = {Perovskite solar cells combine high carrier mobilities with long carrier lifetimes and high radiative efficiencies. Despite this, full devices suffer from significant nonradiative recombination losses, limiting their V-OC to values well below the Shockley-Queisser limit. Here, recent advances in understanding nonradiative recombination in perovskite solar cells from picoseconds to steady state are presented, with an emphasis on the interfaces between the perovskite absorber and the charge transport layers. Quantification of the quasi-Fermi level splitting in perovskite films with and without attached transport layers allows to identify the origin of nonradiative recombination, and to explain the V-OC of operational devices. These measurements prove that in state-of-the-art solar cells, nonradiative recombination at the interfaces between the perovskite and the transport layers is more important than processes in the bulk or at grain boundaries. Optical pump-probe techniques give complementary access to the interfacial recombination pathways and provide quantitative information on transfer rates and recombination velocities. Promising optimization strategies are also highlighted, in particular in view of the role of energy level alignment and the importance of surface passivation. Recent record perovskite solar cells with low nonradiative losses are presented where interfacial recombination is effectively overcome-paving the way to the thermodynamic efficiency limit.}, language = {en} } @misc{WolffCaprioglioStolterfohtetal.2019, author = {Wolff, Christian Michael and Caprioglio, Pietro and Stolterfoht, Martin and Neher, Dieter}, title = {Nonradiative recombination in perovskite solar cells}, series = {Postprints der Universit{\"a}t Potsdam Mathematisch-Naturwissenschaftliche Reihe}, journal = {Postprints der Universit{\"a}t Potsdam Mathematisch-Naturwissenschaftliche Reihe}, number = {772}, issn = {1866-8372}, doi = {10.25932/publishup-43762}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-437626}, pages = {20}, year = {2019}, abstract = {Perovskite solar cells combine high carrier mobilities with long carrier lifetimes and high radiative efficiencies. Despite this, full devices suffer from significant nonradiative recombination losses, limiting their VOC to values well below the Shockley-Queisser limit. Here, recent advances in understanding nonradiative recombination in perovskite solar cells from picoseconds to steady state are presented, with an emphasis on the interfaces between the perovskite absorber and the charge transport layers. Quantification of the quasi-Fermi level splitting in perovskite films with and without attached transport layers allows to identify the origin of nonradiative recombination, and to explain the VOC of operational devices. These measurements prove that in state-of-the-art solar cells, nonradiative recombination at the interfaces between the perovskite and the transport layers is more important than processes in the bulk or at grain boundaries. Optical pump-probe techniques give complementary access to the interfacial recombination pathways and provide quantitative information on transfer rates and recombination velocities. Promising optimization strategies are also highlighted, in particular in view of the role of energy level alignment and the importance of surface passivation. Recent record perovskite solar cells with low nonradiative losses are presented where interfacial recombination is effectively overcome—paving the way to the thermodynamic efficiency limit.}, language = {en} } @misc{WolffCanilRehermannetal.2020, author = {Wolff, Christian Michael and Canil, Laura and Rehermann, Carolin and Nguyen, Ngoc Linh and Zu, Fengshuo and Ralaiarisoa, Maryline and Caprioglio, Pietro and Fiedler, Lukas and Stolterfoht, Martin and Kogikoski, Junior, Sergio and Bald, Ilko and Koch, Norbert and Unger, Eva L. and Dittrich, Thomas and Abate, Antonio and Neher, Dieter}, title = {Correction to 'Perfluorinated self-assembled monolayers enhance the stability and efficiency of inverted perovskite solar cells' (2020, 14 (2), 1445-1456)}, series = {ACS nano}, volume = {14}, journal = {ACS nano}, number = {11}, publisher = {American Chemical Society}, address = {Washington, DC}, issn = {1936-0851}, doi = {10.1021/acsnano.0c08081}, pages = {16156 -- 16156}, year = {2020}, language = {en} } @phdthesis{Wolff2011, author = {Wolff, Christian Michael}, title = {East African monsoon variability since the last glacial}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-58079}, school = {Universit{\"a}t Potsdam}, year = {2011}, abstract = {The impact of global warming on human water resources is attracting increasing attention. No other region in this world is so strongly affected by changes in water supply than the tropics. Especially in Africa, the availability and access to water is more crucial to existence (basic livelihoods and economic growth) than anywhere else on Earth. In East Africa, rainfall is mainly influenced by the migration of the Inter-Tropical Convergence Zone (ITCZ) and by the El Ni{\~n}o Southern Oscillation (ENSO) with more rain and floods during El Ni{\~n}o and severe droughts during La Ni{\~n}a. The forecasting of East African rainfall in a warming world requires a better understanding of the response of ENSO-driven variability to mean climate. Unfortunately, existing meteorological data sets are too short or incomplete to establish a precise evaluation of future climate. From Lake Challa near Mount Kilimanjaro, we report records from a laminated lake sediment core spanning the last 25,000 years. Analyzing a monthly cleared sediment trap confirms the annual origin of the laminations and demonstrates that the varve-thicknesses are strongly linked to the duration and strength of the windy season. Given the modern control of seasonal ITCZ location on wind and rain in this region and the inverse relation between the two, thicker varves represent windier and thus drier years. El Ni{\~n}o (La Ni{\~n}a) events are associated with wetter (drier) conditions in east Africa and decreased (increased) surface wind speeds. Based on this fact, the thickness of the varves can be used as a tool to reconstruct a) annual rainfall b) wind season strength, and c) ENSO variability. Within this thesis, I found evidence for centennialscale changes in ENSO-related rainfall variability during the last three millennia, abrupt changes in variability during the Medieval Climate Anomaly and the Little Ice Age, and an overall reduction in East African rainfall and its variability during the Last Glacial period. Climate model simulations support forward extrapolation from these lake-sediment data, indicating that a future Indian Ocean warming will enhance East Africa's hydrological cycle and its interannual variability in rainfall. Furthermore, I compared geochemical analyses from the sediment trap samples with a broad range of limnological, meteorological, and geological parameters to characterize the impact of sedimentation processes from the in-situ rocks to the deposited sediments. As a result an excellent calibration for existing μXRF data from Lake Challa over the entire 25,000 year long profile was provided. The climate development during the last 25,000 years as reconstructed from the Lake Challa sediments is in good agreement with other studies and highlights the complex interactions between long-term orbital forcing, atmosphere, ocean and land surface conditions. My findings help to understand how abrupt climate changes occur and how these changes correlate with climate changes elsewhere on Earth.}, language = {en} } @phdthesis{Wolff2020, author = {Wolff, Christian Michael}, title = {Identification and reduction of losses in perovskite solar cells}, doi = {10.25932/publishup-47930}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-479301}, school = {Universit{\"a}t Potsdam}, pages = {x, 158}, year = {2020}, abstract = {Perovskite solar cells have become one of the most studied systems in the quest for new, cheap and efficient solar cell materials. Within a decade device efficiencies have risen to >25\% in single-junction and >29\% in tandem devices on top of silicon. This rapid improvement was in many ways fortunate, as e. g. the energy levels of commonly used halide perovskites are compatible with already existing materials from other photovoltaic technologies such as dye-sensitized or organic solar cells. Despite this rapid success, fundamental working principles must be understood to allow concerted further improvements. This thesis focuses on a comprehensive understanding of recombination processes in functioning devices. First the impact the energy level alignment between the perovskite and the electron transport layer based on fullerenes is investigated. This controversial topic is comprehensively addressed and recombination is mitigated through reducing the energy difference between the perovskite conduction band minimum and the LUMO of the fullerene. Additionally, an insulating blocking layer is introduced, which is even more effective in reducing this recombination, without compromising carrier collection and thus efficiency. With the rapid efficiency development (certified efficiencies have broken through the 20\% ceiling) and thousands of researchers working on perovskite-based optoelectronic devices, reliable protocols on how to reach these efficiencies are lacking. Having established robust methods for >20\% devices, while keeping track of possible pitfalls, a detailed description of the fabrication of perovskite solar cells at the highest efficiency level (>20\%) is provided. The fabrication of low-temperature p-i-n structured devices is described, commenting on important factors such as practical experience, processing atmosphere \& temperature, material purity and solution age. Analogous to reliable fabrication methods, a method to identify recombination losses is needed to further improve efficiencies. Thus, absolute photoluminescence is identified as a direct way to quantify the Quasi-Fermi level splitting of the perovskite absorber (1.21eV) and interfacial recombination losses the transport layers impose, reducing the latter to ~1.1eV. Implementing very thin interlayers at both the p- and n-interface (PFN-P2 and LiF, respectively), these losses are suppressed, enabling a VOC of up to 1.17eV. Optimizing the device dimensions and the bandgap, 20\% devices with 1cm2 active area are demonstrated. Another important consideration is the solar cells' stability if subjected to field-relevant stressors during operation. In particular these are heat, light, bias or a combination thereof. Perovskite layers - especially those incorporating organic cations - have been shown to degrade if subjected to these stressors. Keeping in mind that several interlayers have been successfully used to mitigate recombination losses, a family of perfluorinated self-assembled monolayers (X-PFCn, where X denotes I/Br and n = 7-12) are introduced as interlayers at the n-interface. Indeed, they reduce interfacial recombination losses enabling device efficiencies up to 21.3\%. Even more importantly they improve the stability of the devices. The solar cells with IPFC10 are stable over 3000h stored in the ambient and withstand a harsh 250h of MPP at 85◦C without appreciable efficiency losses. To advance further and improve device efficiencies, a sound understanding of the photophysics of a device is imperative. Many experimental observations in recent years have however drawn an inconclusive picture, often suffering from technical of physical impediments, disguising e. g. capacitive discharge as recombination dynamics. To circumvent these obstacles, fully operational, highly efficient perovskites solar cells are investigated by a combination of multiple optical and optoelectronic probes, allowing to draw a conclusive picture of the recombination dynamics in operation. Supported by drift-diffusion simulations, the device recombination dynamics can be fully described by a combination of first-, second- and third-order recombination and JV curves as well as luminescence efficiencies over multiple illumination intensities are well described within the model. On this basis steady state carrier densities, effective recombination constants, densities-of-states and effective masses are calculated, putting the devices at the brink of the radiative regime. Moreover, a comprehensive review of recombination in state-of-the-art devices is given, highlighting the importance of interfaces in nonradiative recombination. Different strategies to assess these are discussed, before emphasizing successful strategies to reduce interfacial recombination and pointing towards the necessary steps to further improve device efficiency and stability. Overall, the main findings represent an advancement in understanding loss mechanisms in highly efficient solar cells. Different reliable optoelectronic techniques are used and interfacial losses are found to be of grave importance for both efficiency and stability. Addressing the interfaces, several interlayers are introduced, which mitigate recombination losses and degradation.}, language = {en} } @misc{WangSmithSkroblinetal.2020, author = {Wang, Qiong and Smith, Joel A. and Skroblin, Dieter and Steele, Julian A. and Wolff, Christian Michael and Caprioglio, Pietro and Stolterfoht, Martin and K{\"o}bler, Hans and Turren-Cruz, Silver-Hamill and Li, Meng and Gollwitzer, Christian and Neher, Dieter and Abate, Antonio}, title = {Managing phase purities and crystal orientation for high-performance and photostable cesium lead halide perovskite solar cells}, series = {Postprints der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe}, journal = {Postprints der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe}, number = {9}, issn = {1866-8372}, doi = {10.25932/publishup-52537}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-525374}, pages = {11}, year = {2020}, abstract = {Inorganic perovskites with cesium (Cs+) as the cation have great potential as photovoltaic materials if their phase purity and stability can be addressed. Herein, a series of inorganic perovskites is studied, and it is found that the power conversion efficiency of solar cells with compositions CsPbI1.8Br1.2, CsPbI2.0Br1.0, and CsPbI2.2Br0.8 exhibits a high dependence on the initial annealing step that is found to significantly affect the crystallization and texture behavior of the final perovskite film. At its optimized annealing temperature, CsPbI1.8Br1.2 exhibits a pure orthorhombic phase and only one crystal orientation of the (110) plane. Consequently, this allows for the best efficiency of up to 14.6\% and the longest operational lifetime, T-S80, of approximate to 300 h, averaged of over six solar cells, during the maximum power point tracking measurement under continuous light illumination and nitrogen atmosphere. This work provides essential progress on the enhancement of photovoltaic performance and stability of CsPbI3 - xBrx perovskite solar cells.}, language = {en} } @article{WangSmithSkroblinetal.2020, author = {Wang, Qiong and Smith, Joel A. and Skroblin, Dieter and Steele, Julian A. and Wolff, Christian Michael and Caprioglio, Pietro and Stolterfoht, Martin and K{\"o}bler, Hans and Turren-Cruz, Silver-Hamill and Li, Meng and Gollwitzer, Christian and Neher, Dieter and Abate, Antonio}, title = {Managing phase purities and crystal orientation for high-performance and photostable cesium lead halide perovskite solar cells}, series = {Solar RRL}, volume = {4}, journal = {Solar RRL}, number = {9}, publisher = {WILEY-VCH}, address = {Weinheim}, pages = {9}, year = {2020}, abstract = {Inorganic perovskites with cesium (Cs+) as the cation have great potential as photovoltaic materials if their phase purity and stability can be addressed. Herein, a series of inorganic perovskites is studied, and it is found that the power conversion efficiency of solar cells with compositions CsPbI1.8Br1.2, CsPbI2.0Br1.0, and CsPbI2.2Br0.8 exhibits a high dependence on the initial annealing step that is found to significantly affect the crystallization and texture behavior of the final perovskite film. At its optimized annealing temperature, CsPbI1.8Br1.2 exhibits a pure orthorhombic phase and only one crystal orientation of the (110) plane. Consequently, this allows for the best efficiency of up to 14.6\% and the longest operational lifetime, T-S80, of approximate to 300 h, averaged of over six solar cells, during the maximum power point tracking measurement under continuous light illumination and nitrogen atmosphere. This work provides essential progress on the enhancement of photovoltaic performance and stability of CsPbI3 - xBrx perovskite solar cells.}, language = {en} } @article{WangMosconiWolffetal.2019, author = {Wang, Qiong and Mosconi, Edoardo and Wolff, Christian Michael and Li, Junming and Neher, Dieter and De Angelis, Filippo and Suranna, Gian Paolo and Grisorio, Roberto and Abate, Antonio}, title = {Rationalizing the molecular design of hole-selective contacts to improve charge extraction in Perovskite solar cells}, series = {dvanced energy materials}, volume = {9}, journal = {dvanced energy materials}, number = {28}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {1614-6832}, doi = {10.1002/aenm.201900990}, pages = {9}, year = {2019}, abstract = {Two new hole selective materials (HSMs) based on dangling methylsulfanyl groups connected to the C-9 position of the fluorene core are synthesized and applied in perovskite solar cells. Being structurally similar to a half of Spiro-OMeTAD molecule, these HSMs (referred as FS and DFS) share similar redox potentials but are endowed with slightly higher hole mobility, due to the planarity and large extension of their structure. Competitive power conversion efficiency (up to 18.6\%) is achieved by using the new HSMs in suitable perovskite solar cells. Time-resolved photoluminescence decay measurements and electrochemical impedance spectroscopy show more efficient charge extraction at the HSM/perovskite interface with respect to Spiro-OMeTAD, which is reflected in higher photocurrents exhibited by DFS/FS-integrated perovskite solar cells. Density functional theory simulations reveal that the interactions of methylammonium with methylsulfanyl groups in DFS/FS strengthen their electrostatic attraction with the perovskite surface, providing an additional path for hole extraction compared to the sole presence of methoxy groups in Spiro-OMeTAD. Importantly, the low-cost synthesis of FS makes it significantly attractive for the future commercialization of perovskite solar cells.}, language = {en} } @article{StolterfohtWolffMarquezetal.2018, author = {Stolterfoht, Martin and Wolff, Christian Michael and Marquez, Jose A. and Zhang, Shanshan and Hages, Charles J. and Rothhardt, Daniel and Albrecht, Steve and Burn, Paul L. and Meredith, Paul and Unold, Thomas and Neher, Dieter}, title = {Visualization and suppression of interfacial recombination for high-efficiency large-area pin perovskite solar cells}, series = {Nature Energy}, volume = {3}, journal = {Nature Energy}, number = {10}, publisher = {Nature Publ. Group}, address = {London}, issn = {2058-7546}, doi = {10.1038/s41560-018-0219-8}, pages = {847 -- 854}, year = {2018}, abstract = {The performance of perovskite solar cells is predominantly limited by non-radiative recombination, either through trap-assisted recombination in the absorber layer or via minority carrier recombination at the perovskite/transport layer interfaces. Here, we use transient and absolute photoluminescence imaging to visualize all non-radiative recombination pathways in planar pintype perovskite solar cells with undoped organic charge transport layers. We find significant quasi-Fermi-level splitting losses (135 meV) in the perovskite bulk, whereas interfacial recombination results in an additional free energy loss of 80 meV at each individual interface, which limits the open-circuit voltage (V-oc) of the complete cell to similar to 1.12 V. Inserting ultrathin interlayers between the perovskite and transport layers leads to a substantial reduction of these interfacial losses at both the p and n contacts. Using this knowledge and approach, we demonstrate reproducible dopant-free 1 cm(2) perovskite solar cells surpassing 20\% efficiency (19.83\% certified) with stabilized power output, a high V-oc (1.17 V) and record fill factor (>81\%).}, language = {en} } @article{StolterfohtWolffAmiretal.2017, author = {Stolterfoht, Martin and Wolff, Christian Michael and Amir, Yohai and Paulke, Andreas and Perdigon-Toro, Lorena and Caprioglio, Pietro and Neher, Dieter}, title = {Approaching the fill factor Shockley-Queisser limit in stable, dopant-free triple cation perovskite solar cells}, series = {Energy \& Environmental Science}, volume = {10}, journal = {Energy \& Environmental Science}, publisher = {Royal Society of Chemistry}, address = {Cambridge}, issn = {1754-5692}, doi = {10.1039/c7ee00899f}, pages = {1530 -- 1539}, year = {2017}, abstract = {Perovskite solar cells now compete with their inorganic counterparts in terms of power conversion efficiency, not least because of their small open-circuit voltage (V-OC) losses. A key to surpass traditional thin-film solar cells is the fill factor (FF). Therefore, more insights into the physical mechanisms that define the bias dependence of the photocurrent are urgently required. In this work, we studied charge extraction and recombination in efficient triple cation perovskite solar cells with undoped organic electron/hole transport layers (ETL/HTL). Using integral time of flight we identify the transit time through the HTL as the key figure of merit for maximizing the fill factor (FF) and efficiency. Complementarily, intensity dependent photocurrent and V-OC measurements elucidate the role of the HTL on the bias dependence of non-radiative and transport-related loss channels. We show that charge transport losses can be completely avoided under certain conditions, yielding devices with FFs of up to 84\%. Optimized cells exhibit power conversion efficiencies of above 20\% for 6 mm(2) sized pixels and 18.9\% for a device area of 1 cm(2). These are record efficiencies for hybrid perovskite devices with dopant-free transport layers, highlighting the potential of this device technology to avoid charge-transport limitations and to approach the Shockley-Queisser limit.}, language = {en} }