2022-09-30, Zimmermann, Eugen, Wong, Ka Kan, Seewald, Tobias, Kalb, Julian, Steffens, Jonathan, Hahn, Giso, Schmidt-Mende, Lukas

Atmospheric Pressure Spatial Atomic Layer Deposition (AP-SALD) is an upcoming deposition technique suitable for a variety of materials and combines the benefits of a regular atomic layer deposition with a significantly increased deposition rate at ambient conditions. In this work, amorphous and anatase TiO₂ layers are fabricated by AP-SALD via systematic variation of process conditions such as temperature, reactant (H₂O and O₃), and posttreatment. The formed layers are characterized for their structural and optoelectronic properties and utilized as a hole-blocking layer in hybrid perovskite solar cells. It is found that TiO₂ layers fabricated at elevated deposition temperatures possess strong anatase character but expose an unfavorable interface to the perovskite layer, promoting recombination and lowering the shunt resistance and open circuit voltage of the solar cells. Therefore, the interface is essential for efficient charge extraction, which can be significantly improved by controlling the process parameters.

2018, Ehrenreich, Philipp, Wong, Ka Kan, Zimmermann, Eugen, Dorman, James A., Wang, Wei, Fakharuddin, Azhar, Putnik, Martin, Kalb, Julian, Pfadler, Thomas, Schmidt-Mende, Lukas

ZnO is a widely used metal-oxide semiconductor for photovoltaic application. In solar cell heterostructures they not only serve as a charge selective contact, but also act as electron acceptor. Although ZnO offers a suitable interface for exciton dissociation, charge separation efficiencies have stayed rather poor and conceptual differences to organic acceptors are rarely investigated. In this work, we employ Sn doping to ZnO nanowires in order to understand the role of defect and surface states in the charge separation process. Upon doping we are able to modify the metal-oxide work function and we show its direct correlation with the charge separation efficiency. For this purpose, we use the polymer poly(3-hexylthiophene) as donor and the squaraine dye SQ2 as interlayer. Interestingly, neither mobilities nor defects are prime performance limiting factor, but rather the density of available states around the conduction band is of crucial importance for hybrid interfaces. This work highlights crucial aspects to improve the charge generation process of metal-oxide based solar cells and reveals new strategies to improve the power conversion efficiency of hybrid solar cells.

2017, Wang, Wei, Strössner, Frank, Zimmermann, Eugen, Schmidt-Mende, Lukas

Sb₂S₃ is a promising candidate for solar cell absorbers due to its high absorption coefficient, suitable band gap and earth-abundant constituents. Here we present the preparation of hybrid solar cells from an ink of colloidal Sb₂S₃ nanoparticles and P3HT. Colloidal Sb₂S₃ nanoparticles were prepared via hot injection method. Solar cells based on these nanoparticles achieves a power conversion efficiency of 1.5%, which is efficiency record for planar hybrid solar cells based on Sb₂S₃ nanoparticles. We investigated in detail the role of the capping agents on the performance of solar cells.

2015, Kollek, Tom, Gruber, Dominik, Gehring, Julia, Zimmermann, Eugen, Schmidt-Mende, Lukas, Polarz, Sebastian

Signifikante Fortschritte in der Solarzellforschung werden aktuell von der Substanzklasse der metallorganischen Perowskite (MOPs) dominiert. Ursächlich dafür sind herausragende Eigenschaften dieser Materialien, wie hohe Absorptionskoeffzienten und die Fähigkeit zur effektiven Trennung photogenerierter Ladungsträger. In Analogie zu etablierteren Halbleitern kann erwartet werden, dass die Materialeigenschaften auch der MOPs durch eine definierte Nanostrukturierung stark verbessert werden können. Ihre chemische Anfälligkeit (z. B. gegen Hydrolyse) erschwert aber die direkte Übertragung gängiger Verfahren zur Herstellung von Nanomaterialien. Wir berichten über einen originären Ansatz zur Synthese verschiedener (CH₃NH₃)PbI₃-Nanostrukturen unter Verwendung einer neuartigen Single-Source-Vorstufe. Nanoporöse MOP-Einkristalle können durch eine Kristall-Kristall-Umwandlung erhalten werden, die mit einer spinodalen Mikrophasenseparation der Triethylenglykol-haltigen Vorstufenverbindung einhergeht. Die Steuerung der Nanopartikelform gelingt durch Einsatz eines selektiv bindenden Kristallisationsinhibitors.

2019-06, Kalb, Julian, Folger, Alena, Zimmermann, Eugen, Gerigk, Melanie, Trepka, Bastian, Scheu, Christina, Polarz, Sebastian, Schmidt-Mende, Lukas

Due to their interface properties, compounds of anatase membranes and hydrothermally grown rutile TiO₂ nanorods are valuable materials for (opto-) electronic applications. So far, dense nanorod arrays are typically grown on seeds such as polycrystalline rutile TiO₂, fluorine-doped tin oxide (FTO) or seed particles in dispersion and the anatase modification is added subsequently. Nanorods grown on existing anatase films usually suffer from poor adhesion. In this study, we demonstrate the fabrication of anatase films that act directly as seed layers for the hydrothermal growth. The presented compounds offer a strong adhesion between the two TiO₂ modifications and the substrate which resists even extensive sonication. So far, the density of nanorods is controlled with the HCl concentration, which affects also their size and shape. We control the density of nanorods with the average grain size of the anatase film without affecting their size and shape. This offers new scientific insights and applications of specific anatase/rutile compounds. The grain size of the anatase films is adjusted with the post-annealing temperature after film deposition. To satisfy the requirements of different applications, we provide suitable anatase seed layers with different deposition techniques such as sputter deposition, spray pyrolysis, and atmospheric spatial atomic layer deposition (SALD).

2017-02, Birkhold, Susanne T., Zimmermann, Eugen, Kollek, Tom, Wurmbrand, Daniel, Polarz, Sebastian, Schmidt-Mende, Lukas

Despite their outstanding photovoltaic performance, organic–inorganic perovskite solar cells still face severe stability issues and limitations in their device dimension. Further development of perovskite solar cells therefore requires a deeper understanding of loss mechanisms, in particular, concerning the origin and impact of trap states. Here, different surface properties of submicrometer sized CH3NH3PbI3 particles are studied as a model system by photoluminescence spectroscopy to investigate the impact of the perovskite crystal surface on photoexcited states. Comparison of single crystals with either isolating or electron-rich surface passivation indicates the presence of positively charged surface trap states that can be passivated in case of the latter. These surface trap states cause enhanced nonradiative recombination at room temperature, which is a loss mechanism for solar cell performance. In the orthorhombic phase, the origin of multiple emission peaks is identified as the recombination of free and bound excitons, whose population ratio critically depends on trap state properties. The dynamics of exciton trapping at 50 K are observed on a time-scale of tens of picoseconds by a simultaneous population decrease and increase of free and bound excitons, respectively. These results emphasize the potential of surface passivation to further improve the performance of perovskite solar cells.

2016-09-01, Ehrenreich, Philipp, Birkhold, Susanne T., Zimmermann, Eugen, Hu, Hao, Kim, Kwang-Dae, Weickert, Jonas, Pfadler, Thomas, Schmidt-Mende, Lukas

Polymer morphology and aggregation play an essential role for efficient charge carrier transport and charge separation in polymer-based electronic devices. It is a common method to apply the H-aggregate model to UV/Vis or photoluminescence spectra in order to analyze polymer aggregation. In this work we present strategies to obtain reliable and conclusive information on polymer aggregation and morphology based on the application of an H-aggregate analysis on UV/Vis and photoluminescence spectra. We demonstrate, with P3HT as model system, that thickness dependent reflection behavior can lead to misinterpretation of UV/Vis spectra within the H-aggregate model. Values for the exciton bandwidth can deviate by a factor of two for polymer thicknesses below 150 nm. In contrast, photoluminescence spectra are found to be a reliable basis for characterization of polymer aggregation due to their weaker dependence on the wavelength dependent refractive index of the polymer. We demonstrate this by studying the influence of surface characteristics on polymer aggregation for spin-coated thin-films that are commonly used in organic and hybrid solar cells.

2018-08, Nawaz, Asmat, Wong, Ka Kan, Ebenhoch, Carola, Zimmermann, Eugen, Zheng, Zhaoke, Akram, Muhammad Nadeem, Kalb, Julian, Wang, Kaiying, Fakharuddin, Azhar, Schmidt-Mende, Lukas

High performance in perovskite solar cells is often achieved using compact metal oxide layers or their mesoporous analogues. One dimensional scaffold materials such as nanorods or nanotubes are also employed in order to improve charge collection, however, perovskite pore-filling in these nanostructures is rather low. Herein, we introduce a method to more efficiently fill the pores in two most common nanostructure architectures namely, TiO2 nanorods and nanotubes. The method employs recrystallization of perovskite films in a methylamine rich environment– the so called perovskite healing. Whereas the scanning electron microscopy imaging revealed an improved pore-filling and formation of large perovskite grains upon healing, the complementary photoluminescence and electrical characterizations revealed improved charge transfer in the healed films than their pristine rivals. We also report a notable improvement in photoconversion efficiency and a better stability under continuous light soaking in the healed perovskite films.

2017-01-11, Kollek, Tom, Wurmbrand, Daniel, Birkhold, Susanne T., Zimmermann, Eugen, Kalb, Julian, Schmidt-Mende, Lukas, Polarz, Sebastian

Particle-based semiconductor materials are promising constituents of future technologies. They are described by unique features resulting from the combination of discrete nanoparticle characteristics and the emergence of cooperative phenomena based on long-range interaction within their superstructure. (Nano)particles of outstanding quality with regards to size and shape can be prepared via colloidal synthesis using appropriate capping agents. The classical capping agents are electrically insulating, which impedes particle-particle electronic communication. Consequently, there exists a high demand for realizing charge transport through interfaces especially for semiconductors of relevance like hybrid perovskites (HYPEs), for example, CH₃NH₃PbI₃ (MAPI) as one of the most prominent representatives. Of particular interest are crystals in the micrometer range, as they possess synergistic advantages of single crystalline bulk properties, shape control as well as the possibility of being functionalized. Here we provide a synthetic strategy toward thiophene-functionalized single crystalline MAPI microrods originating from the single source precursor CH₃NH₃PbI₃TEG₂ (TEG = triethylene glycol). In the dark, the microrods show enhanced charge transport characteristics of holes over 2 orders of magnitude compared to microscale cuboids with insulating alkyl surface modifiers and nonfunctionalized random sized particles. In large-area prototype photodetector devices (2.21 cm²), the thiophene functionalization improves the response times because of the interparticle charge transport (t_ON = 190 ms, t_OFF = 430 ms) compared to alkyl-functionalized particles (t_ON = 1055 ms, t_OFF = 60 ms), at similar responsivities of 0.65 and 0.71 mA W^-1, respectively. Further, the surface functionalization and crystal grains on the micrometer scale improve the device stability. Therefore, this study provides clear evidence for the interplay and importance of crystal size, shape and surface modification of MAPI crystals, which is of major importance in every optoelectronic device.

2016-08-19, Zimmermann, Eugen, Wong, Ka Kan, Müller, Michael, Hu, Hao, Ehrenreich, Philipp, Kohlstädt, Markus, Würfel, Uli, Mastroianni, Simone, Pfadler, Thomas, Schmidt-Mende, Lukas

Lead halide perovskite solar cells have shown a tremendous rise in power conversion efficiency with reported record efficiencies of over 20% making this material very promising as a low cost alternative to conventional inorganic solar cells. However, due to a differently severe “hysteretic” behaviour during current density-voltage measurements, which strongly depends on scan rate, device and measurement history, preparation method, device architecture, etc., commonly used solar cell measurements do not give reliable or even reproducible results. For the aspect of commercialization and the possibility to compare results of different devices among different laboratories, it is necessary to establish a measurement protocol which gives reproducible results. Therefore, we compare device characteristics derived from standard current density-voltage measurements with stabilized values obtained from an adaptive tracking of the maximum power point and the open circuit voltage as well as characteristics extracted from time resolved current density-voltage measurements. Our results provide insight into the challenges of a correct determination of device performance and propose a measurement protocol for a reliable characterisation which is easy to implement and has been tested on varying perovskite solar cells fabricated in different laboratories.