@article{FohlmeisterPlessenDudashvilietal.2017,
  author    = {Fohlmeister, Jens Bernd and Plessen, Birgit and Dudashvili, Alexey Sergeevich and Tjallingii, Rik and Wolff, Christian Michael and Gafurov, Abror and Cheng, Hai},
  title     = {Winter precipitation changes during the Medieval Climate Anomaly and the Little Ice Age in arid Central Asia},
  series = {Quaternary science reviews : the international multidisciplinary research and review journal},
  volume    = {178},
  journal   = {Quaternary science reviews : the international multidisciplinary research and review journal},
  publisher = {Elsevier},
  address   = {Oxford},
  issn      = {0277-3791},
  doi       = {10.1016/j.quascirev.2017.10.026},
  pages     = {24 -- 36},
  year      = {2017},
  abstract  = {The strength of the North Atlantic Oscillation (NAO) is considered to be the main driver of climate changes over the European and western Asian continents throughout the last millennium. For example, the predominantly warm Medieval Climate Anomaly (MCA) and the following cold period of the Little Ice Age (LIA) over Europe have been associated with long-lasting phases with a positive and negative NAO index. Its climatic imprint is especially pronounced in European winter seasons. However, little is known about the influence of NAO with respect to its eastern extent over the Eurasian continent. Here we present speleothem records (PC, 8180 and Sr/Ca) from the southern rim of Fergana Basin (Central Asia) revealing annually resolved past climate variations during the last millennium. The age control of the stalagmite relies on radiocarbon dating as large amounts of detrital material inhibit accurate 230Th dating. Present-day calcification of the stalagmite is most effective during spring when the cave atmosphere and elevated water supply by snow melting and high amount of spring precipitation provide optimal conditions. Seasonal precipitation variations cause changes of the stable isotope and Sr/ Ca compositions. The simultaneous changes in these geochemical proxies, however, give also evidence for fractionation processes in the cave. By disentangling both processes, we demonstrate that the amount of winter precipitation during the MCA was generally higher than during the LIA, which is in line with climatic changes linked to the NAO index but opposite to the higher mountain records of Central Asia. Several events of strongly reduced winter precipitation are observed during the LIA in Central Asia. These dry winter events can be related to phases of a strong negative NAO index and all results reveal that winter precipitation over the central Eurasian continent is tightly linked to atmospheric NAO modes by the westerly wind systems. (C) 2017 Elsevier Ltd. All rights reserved.},
  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{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{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{FerTietjenJeltschetal.2017,
  author    = {Fer, Istem and Tietjen, Britta and Jeltsch, Florian and Wolff, Christian Michael},
  title     = {The influence of El Nino-Southern Oscillation regimes on eastern African vegetation and its future implications under the RCP8.5 warming scenario},
  series = {Biogeosciences},
  volume    = {14},
  journal   = {Biogeosciences},
  publisher = {Copernicus},
  address   = {G{\"o}ttingen},
  issn      = {1726-4170},
  doi       = {10.5194/bg-14-4355-2017},
  pages     = {4355 -- 4374},
  year      = {2017},
  abstract  = {The El Nino-Southern Oscillation (ENSO) is the main driver of the interannual variability in eastern African rainfall, with a significant impact on vegetation and agriculture and dire consequences for food and social security. In this study, we identify and quantify the ENSO contribution to the eastern African rainfall variability to forecast future eastern African vegetation response to rainfall variability related to a predicted intensified ENSO. To differentiate the vegetation variability due to ENSO, we removed the ENSO signal from the climate data using empirical orthogonal teleconnection (EOT) analysis. Then, we simulated the ecosystem carbon and water fluxes under the historical climate without components related to ENSO teleconnections. We found ENSO-driven patterns in vegetation response and confirmed that EOT analysis can successfully produce coupled tropical Pacific sea surface temperature-eastern African rainfall teleconnection from observed datasets. We further simulated eastern African vegetation response under future climate change as it is projected by climate models and under future climate change combined with a predicted increased ENSO intensity. Our EOT analysis highlights that climate simulations are still not good at capturing rainfall variability due to ENSO, and as we show here the future vegetation would be different from what is simulated under these climate model outputs lacking accurate ENSO contribution. We simulated considerable differences in eastern African vegetation growth under the influence of an intensified ENSO regime which will bring further environmental stress to a region with a reduced capacity to adapt effects of global climate change and food security.},
  language  = {en}
}
@article{FerTietjenJeltschetal.2017,
  author    = {Fer, Istem and Tietjen, Britta and Jeltsch, Florian and Wolff, Christian Michael},
  title     = {The influence of El Nino-Southern Oscillation regimes on eastern African vegetation and its future implications under the RCP8.5 warming scenario},
  series = {Biogeosciences},
  volume    = {14},
  journal   = {Biogeosciences},
  number    = {18},
  publisher = {Copernicus},
  address   = {Katlenburg-Lindau},
  issn      = {1726-4170},
  doi       = {10.5194/bg-14-4355-2017},
  pages     = {4355 -- 4374},
  year      = {2017},
  abstract  = {The El Nino-Southern Oscillation (ENSO) is the main driver of the interannual variability in eastern African rainfall, with a significant impact on vegetation and agriculture and dire consequences for food and social security. In this study, we identify and quantify the ENSO contribution to the eastern African rainfall variability to forecast future eastern African vegetation response to rainfall variability related to a predicted intensified ENSO. To differentiate the vegetation variability due to ENSO, we removed the ENSO signal from the climate data using empirical orthogonal teleconnection (EOT) analysis. Then, we simulated the ecosystem carbon and water fluxes under the historical climate without components related to ENSO teleconnections. We found ENSO-driven patterns in vegetation response and confirmed that EOT analysis can successfully produce coupled tropical Pacific sea surface temperature-eastern African rainfall teleconnection from observed datasets. We further simulated eastern African vegetation response under future climate change as it is projected by climate models and under future climate change combined with a predicted increased ENSO intensity. Our EOT analysis highlights that climate simulations are still not good at capturing rainfall variability due to ENSO, and as we show here the future vegetation would be different from what is simulated under these climate model outputs lacking accurate ENSO contribution. We simulated considerable differences in eastern African vegetation growth under the influence of an intensified ENSO regime which will bring further environmental stress to a region with a reduced capacity to adapt effects of global climate change and food security.},
  language  = {en}
}
@article{StolterfohtCaprioglioWolffetal.2019,
  author    = {Stolterfoht, Martin and Caprioglio, Pietro and Wolff, Christian Michael and Marquez, Jose A. and Nordmann, Joleik and Zhang, Shanshan and Rothhardt, Daniel and H{\"o}rmann, Ulrich and Amir, Yohai and Redinger, Alex and Kegelmann, Lukas and Zu, Fengshuo and Albrecht, Steve and Koch, Norbert and Kirchartz, Thomas and Saliba, Michael and Unold, Thomas and Neher, Dieter},
  title     = {The impact of energy alignment and interfacial recombination on the internal and external open-circuit voltage of perovskite solar cells},
  series = {Energy \& environmental science},
  volume    = {12},
  journal   = {Energy \& environmental science},
  number    = {9},
  publisher = {Royal Society of Chemistry},
  address   = {Cambridge},
  issn      = {1754-5692},
  doi       = {10.1039/c9ee02020a},
  pages     = {2778 -- 2788},
  year      = {2019},
  abstract  = {Charge transport layers (CTLs) are key components of diffusion controlled perovskite solar cells, however, they can induce additional non-radiative recombination pathways which limit the open circuit voltage (V-OC) of the cell. In order to realize the full thermodynamic potential of the perovskite absorber, both the electron and hole transport layer (ETL/HTL) need to be as selective as possible. By measuring the photoluminescence yield of perovskite/CTL heterojunctions, we quantify the non-radiative interfacial recombination currents in pin- and nip-type cells including high efficiency devices (21.4\%). Our study comprises a wide range of commonly used CTLs, including various hole-transporting polymers, spiro-OMeTAD, metal oxides and fullerenes. We find that all studied CTLs limit the V-OC by inducing an additional non-radiative recombination current that is in most cases substantially larger than the loss in the neat perovskite and that the least-selective interface sets the upper limit for the V-OC of the device. Importantly, the V-OC equals the internal quasi-Fermi level splitting (QFLS) in the absorber layer only in high efficiency cells, while in poor performing devices, the V-OC is substantially lower than the QFLS. Using ultraviolet photoelectron spectroscopy and differential charging capacitance experiments we show that this is due to an energy level mis-alignment at the p-interface. The findings are corroborated by rigorous device simulations which outline important considerations to maximize the V-OC. This work highlights that the challenge to suppress non-radiative recombination losses in perovskite cells on their way to the radiative limit lies in proper energy level alignment and in suppression of defect recombination at the interfaces.},
  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{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{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{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{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{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{CaprioglioStolterfohtWolffetal.2019,
  author    = {Caprioglio, Pietro and Stolterfoht, Martin and Wolff, Christian Michael and Unold, Thomas and Rech, Bernd and Albrecht, Steve and Neher, Dieter},
  title     = {On the relation between the open-circuit voltage and quasi-fermi level splitting in efficient perovskite solar cells},
  series = {advanced energy materials},
  volume    = {9},
  journal   = {advanced energy materials},
  number    = {33},
  publisher = {Wiley-VCH},
  address   = {Weinheim},
  issn      = {1614-6832},
  doi       = {10.1002/aenm.201901631},
  pages     = {10},
  year      = {2019},
  abstract  = {Today's perovskite solar cells (PSCs) are limited mainly by their open-circuit voltage (VOC) due to nonradiative recombination. Therefore, a comprehensive understanding of the relevant recombination pathways is needed. Here, intensity-dependent measurements of the quasi-Fermi level splitting (QFLS) and of the VOC on the very same devices, including pin-type PSCs with efficiencies above 20\%, are performed. It is found that the QFLS in the perovskite lies significantly below its radiative limit for all intensities but also that the VOC is generally lower than the QFLS, violating one main assumption of the Shockley-Queisser theory. This has far-reaching implications for the applicability of some well-established techniques, which use the VOC as a measure of the carrier densities in the absorber. By performing drift-diffusion simulations, the intensity dependence of the QFLS, the QFLS-VOC offset and the ideality factor are consistently explained by trap-assisted recombination and energetic misalignment at the interfaces. Additionally, it is found that the saturation of the VOC at high intensities is caused by insufficient contact selectivity while heating effects are of minor importance. It is concluded that the analysis of the VOC does not provide reliable conclusions of the recombination pathways and that the knowledge of the QFLS-VOC relation is of great importance.},
  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{KegelmannTockhornWolffetal.2019,
  author    = {Kegelmann, Lukas and Tockhorn, Philipp and Wolff, Christian Michael and M{\´a}rquez, Jos{\´e} A. and Caicedo D{\´a}vila, Sebasti{\´a}n and Korte, Lars and Unold, Thomas and Loevenich, Wilfried and Neher, Dieter and Rech, Bernd and Albrecht, Steve},
  title     = {Mixtures of Dopant-Free Spiro-OMeTAD and Water-Free PEDOT as a Passivating Hole Contact in Perovskite Solar Cells},
  series = {ACS applied materials \& interfaces},
  volume    = {11},
  journal   = {ACS applied materials \& interfaces},
  number    = {9},
  publisher = {American Chemical Society},
  address   = {Washington},
  issn      = {1944-8244},
  doi       = {10.1021/acsami.9b01332},
  pages     = {9172 -- 9181},
  year      = {2019},
  abstract  = {Doped spiro-OMeTAD at present is the most commonly used hole transport material (HTM) in n-i-p-type perovskite solar cells, enabling high efficiencies around 22\%. However, the required dopants were shown to induce nonradiative recombination of charge carriers and foster degradation of the solar cell. Here, in a novel approach, highly conductive and inexpensive water-free poly(3,4-ethylenedioxythiophene) (PEDOT) is used to replace these dopants. The resulting spiro-OMeTAD/PEDOT (SpiDOT) mixed films achieve higher lateral conductivities than layers of doped spiro-OMeTAD. Furthermore, combined transient and steady-state photoluminescence studies reveal a passivating effect of PEDOT, suppressing nonradiative recombination losses at the perovskite/HTM interface. This enables excellent quasi-Fermi level splitting values of up to 1.24 eV in perovskite/SpiDOT layer stacks and high open-circuit voltages (V-OC) up to 1.19 V in complete solar cells. Increasing the amount of dopant-free spiro-OMeTAD in SpiDOT layers is shown to enhance hole extraction and thereby improves the fill factor in solar cells. As a consequence, stabilized efficiencies up to 18.7\% are realized, exceeding cells with doped spiro-OMeTAD as a HTM in this study. Moreover, to the best of our knowledge, these results mark the lowest nonradiative recombination loss in the V-OC (140 mV with respect to the Shockley-Queisser limit) and highest efficiency reported so far for perovskite solar cells using PEDOT as a HTM.},
  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{LoveFeuersteinWolffetal.2017,
  author    = {Love, John A. and Feuerstein, Markus and Wolff, Christian Michael and Facchetti, Antonio and Neher, Dieter},
  title     = {Lead Halide Perovskites as Charge Generation Layers for Electron Mobility Measurement in Organic Semiconductors},
  series = {ACS applied materials \& interfaces},
  volume    = {9},
  journal   = {ACS applied materials \& interfaces},
  publisher = {American Chemical Society},
  address   = {Washington},
  issn      = {1944-8244},
  doi       = {10.1021/acsami.7b10361},
  pages     = {42011 -- 42019},
  year      = {2017},
  abstract  = {Hybrid lead halide perovskites are introduced as charge generation layers (CGLs) for the accurate determination of electron mobilities in thin organic semiconductors. Such hybrid perovskites have become a widely studied photovoltaic material in their own right, for their high efficiencies, ease of processing from solution, strong absorption, and efficient photogeneration of charge. Time-of-flight (ToF) measurements on bilayer samples consisting of the perovskite CGL and an organic semiconductor layer of different thickness are shown to be determined by the carrier motion through the organic material, consistent with the much higher charge carrier mobility in the perovskite. Together with the efficient photon-to-electron conversion in the perovskite, this high mobility imbalance enables electron-only mobility measurement on relatively thin application-relevant organic films, which would not be possible with traditional ToF measurements. This architecture enables electron-selective mobility measurements in single components as well as bulk-heterojunction films as demonstrated in the prototypical polymer/fullerene blends. To further demonstrate the potential of this approach, electron mobilities were measured as a function of electric field and temperature in an only 127 nm thick layer of a prototypical electron-transporting perylene diimide-based polymer, and found to be consistent with an exponential trap distribution of ca. 60 meV. Our study furthermore highlights the importance of high mobility charge transporting layers when designing perovskite solar cells.},
  language  = {en}
}
@article{KegelmannWolffAwinoetal.2017,
  author    = {Kegelmann, Lukas and Wolff, Christian Michael and Awino, Celline and Lang, Felix and Unger, Eva L. and Korte, Lars and Dittrich, Thomas and Neher, Dieter and Rech, Bernd and Albrecht, Steve},
  title     = {It Takes Two to Tango-Double-Layer Selective Contacts in Perovskite Solar Cells for Improved Device Performance and Reduced Hysteresis},
  series = {ACS applied materials \& interfaces},
  volume    = {9},
  journal   = {ACS applied materials \& interfaces},
  publisher = {American Chemical Society},
  address   = {Washington},
  issn      = {1944-8244},
  doi       = {10.1021/acsami.7b00900},
  pages     = {17246 -- 17256},
  year      = {2017},
  abstract  = {Solar cells made from inorganic organic perovskites have gradually approached market requirements as their efficiency and stability have improved tremendously in recent years. Planar low-temperature processed perovskite solar cells are advantageous for possible large-scale production but are more prone to exhibiting photocurrent hysteresis, especially in the regular n-i-p structure. Here, a systematic characterization of different electron selective contacts with a variety of chemical and electrical properties in planar n-i-p devices processed below 180 degrees C is presented. The inorganic metal oxides TiO2 and SnO2, the organic fullerene derivatives C-60, PCBM, and ICMA, as well as double-layers with a metal oxide/PCBM structure are used as electron transport materials (ETMs). Perovskite layers deposited atop, the different ETMs with the herein applied fabrication method show a similar morphology according to scanning electron microscopy. Further, surface photovoltage spectroscopy measurements indicate comparable perovskite absorber qualities on all ETMs, except TiO2, which shows a more prominent influence of defect states. Transient photoluminescence studies together with current voltage scans over a broad range of scan speeds reveal faster charge extraction, less pronounced hysteresis effects, and higher efficiencies for devices with fullerene compared to those with metal oxide ETMs. Beyond this, only double-layer ETM structures substantially diminish hysteresis effects for all performed scan speeds and strongly enhance the power conversion efficiency up to a champion stabilized value of 18.0\%. The results indicate reduced recombination losses for a double-layer TiO2/PCBM contact design: First, a reduction of shunt paths through the fullerene to the ITO layer. Second, an improved hole blocking by the wide band gap metal oxide. Third, decreased transport losses due to an energetically more favorable contact, as implied by photoelectron spectroscopy measurements. The herein demonstrated improvements of multilayer selective contacts may serve as a general design guideline for perovskite solar cells.},
  language  = {en}
}
@article{StolterfohtGrischekCaprioglioetal.2020,
  author    = {Stolterfoht, Martin and Grischek, Max and Caprioglio, Pietro and Wolff, Christian Michael and Gutierrez-Partida, Emilio and Pe{\~n}a-Camargo, Francisco and Rothhardt, Daniel and Zhang, Shanshan and Raoufi, Meysam and Wolansky, Jakob and Abdi-Jalebi, Mojtaba and Stranks, Samuel D. and Albrecht, Steve and Kirchartz, Thomas and Neher, Dieter},
  title     = {How to quantify the efficiency potential of neat perovskite films},
  series = {Advanced Materials},
  volume    = {32},
  journal   = {Advanced Materials},
  number    = {17},
  publisher = {Wiley-VCH},
  address   = {Weinheim},
  issn      = {0935-9648},
  doi       = {10.1002/adma.202000080},
  pages     = {1 -- 10},
  year      = {2020},
  abstract  = {Perovskite photovoltaic (PV) cells have demonstrated power conversion efficiencies (PCE) that are close to those of monocrystalline silicon cells; however, in contrast to silicon PV, perovskites are not limited by Auger recombination under 1-sun illumination. Nevertheless, compared to GaAs and monocrystalline silicon PV, perovskite cells have significantly lower fill factors due to a combination of resistive and non-radiative recombination losses. This necessitates a deeper understanding of the underlying loss mechanisms and in particular the ideality factor of the cell. By measuring the intensity dependence of the external open-circuit voltage and the internal quasi-Fermi level splitting (QFLS), the transport resistance-free efficiency of the complete cell as well as the efficiency potential of any neat perovskite film with or without attached transport layers are quantified. Moreover, intensity-dependent QFLS measurements on different perovskite compositions allows for disentangling of the impact of the interfaces and the perovskite surface on the non-radiative fill factor and open-circuit voltage loss. It is found that potassium-passivated triple cation perovskite films stand out by their exceptionally high implied PCEs > 28\%, which could be achieved with ideal transport layers. Finally, strategies are presented to reduce both the ideality factor and transport losses to push the efficiency to the thermodynamic limit.},
  language  = {en}
}