Patrick Herre

A comprehensive study on the mechanism behind formation and depletion of Cu2ZnSnS4 (CZTS) phases

High efficiency kesterite based solar cells have vigorously raised the research interests in this material. The challenge lies in understanding the formation and co-existence of more than 10 possible by-products during and after the synthesis of Cu2ZnSnS4 (CZTS) and their various different structural and electronic defects. The present contribution shows an in-depth study on the stages of formation and depletion of nanoparticulate CZTS. Employing a hot injection synthesis method, we give direct experimental evidence of the co-existence of cubic, tetragonal and defected CZTS structures and different by-products as a function of time and temperature. SEM, (HR)TEM, XRD, EDX, ICP-OES, Raman spectroscopy and UV-Vis-NIR spectroscopy have been used in order to better evaluate and interpret data for crystal structures and compositions. The obtained understanding on the formation of different phases suggests 250 °C as the most favourable synthesis temperature. Based on our study, general strategies can be developed for controlling the amount of formed phases, the by-products and the defects in kesterite and other similar multicomponent nanoparticles as well as in bulk systems.

Ligand-assisted thickness tailoring of highly luminescent colloidal CH3NH3PbX3 (X = Br and I) perovskite nanoplatelets

Quantum size-confined CH3NH3PbX3 (X = Br and I) perovskite nanoplatelets with remarkably high photoluminescence quantum yield (up to 90%) were synthesized by ligand-assisted re-precipitation. Thickness-tunability was realized by varying the oleylamine and oleic acid ligand ratio. This method allows tailoring the nanoplatelet thickness by adjusting the number of unit cell monolayers. Broadly tunable emission wavelengths (450-730 nm) are achieved via the pronounced quantum size effect without anion-halide mixing.

Industrially scalable and cost-effective Mn2+ doped ZnxCd1−xS/ZnS nanocrystals with 70% photoluminescence quantum yield, as efficient down-shifting materials in photovoltaics

We present colloidally stable and highly luminescent ZnxCd1−xS:Mn/ZnS core–shell nanocrystals (NCs) synthesized via a simple non-injection one-pot, two-step synthetic route, which can be easily upscaled. A systematic variation of the reaction component, parameters and thickness of the ZnS shell yielded doped nanocrystals with a very high photoluminescence quantum yield (Φpl) of 70%, which is the highest value yet reported for these Mn-doped sulfide-semiconductor NCs. These materials can be synthesized with high reproducibility in large quantities of the same high quality, i.e., the same Φpl using accordingly optimized reaction conditions. The application of these zero-reabsorption high quality NCs in the light conversion layers, deposited on top of a commercial monocrystalline silicon (mono-Si) solar cell, led to a significant enhancement of the external quantum efficiency (EQE) of this device in the ultraviolet spectral region between 300 and 400 nm up to ca. 12%. EQE enhancement is reflected by an increase in the power conversion efficiency (PCE) by nearly 0.5 percentage points and approached the theoretical limit (0.6%) expected from down-shifting for this Si solar cell. The resulting PCE may result in a BoM (bill of materials) cost reduction of app. 3% for mono-Si photovoltaic modules. Such small but distinct improvements are expected to pave the road for an industrial application of doped semiconductor NCs as cost-effective light converters for silicon photovoltaic (PV) and other optoelectronic applications.

A Novel Approach for Preparation and In Situ Tensile Testing of Silica Glass Membranes in the Transmission Electron Microscope

The mechanical behavior of glasses in the micro- and/or nanometer regime increasingly gains importance in nowadays modern technology. However, suitable small scale preparation and mechanical testing approaches for a reliable assessment of the mechanical properties of glasses still remain a big challenge. In the present work, a novel approach for site-specific preparation and quantitative in situ tensile testing of thin silica glass membranes in the transmission electron microscope is presented. Thereby, advanced focused ion beam techniques are used for the preparation of nanoscale dog bone shaped silica glass specimens suitable for in situ tensile testing. Small amounts of gallium are detected on the surface of the membranes resulting from redeposition effects during the focused ion beam preparation procedure. Possible structural changes of silica glass upon irradiation with electrons and gallium ions are investigated by controlled irradiation experiments, followed by a structural analysis using Raman spectroscopy. While moderate electron beam irradiation does not alter the structure of silica glass, ion beam irradiation results in minor densification of the silica glass membranes. In situ tensile testing of membranes under electron beam irradiation results in distinctive elongations without fracture confirming the phenomenon of superplasticity. In contrast, in situ tensile testing in the absence of the electron beam reveals an elastic/plastic deformation behavior, and finally leads to fracture of the membranes. The Young’s moduli of the glass membranes pulled at beam off conditions in the TEM are comparable with values known for bulk fused silica, while the tensile strength is in the range of values reported for silica glass fibers with comparable dimensions. The impact of electron beam irradiation on the mechanical properties of silica glass membranes is further discussed. The results of the present work open new avenues for dedicated preparation and nanomechanical characterization of silica glasses, and further contribute to a fundamental understanding of the mechanical behavior of such glasses when being scaled down to the nanometer regime.

Deciphering the role of impurities in methylammonium iodide and their impact on the performance of perovskite solar cells(Conference Presentation)

Solution based perovskite solar cell fabrication typically involves rather complex processing sequences to yield highest performance. While most studies concentrate on the exploration of processing conditions, we have investigated the purity levels of common perovskite precursor solutions and found a number of impurities which are most critically controlling the crystallization of perovskites. Moreover, we identified these impurities at different level of concentrations is all commercially available precursors. In detail, we present a detailed chemical study on the nature of the various impurities in CH3NH3I and explored their impact on the crystal formation. The detrimental role of the impurities is best demonstrated by comparing perovskite solar cell devices fabricated from impurity free precursors vs precursors containing different concentrations of impurities. Most interestingly, we revealed that a certain concentration of impurities is detrimental to facilitate the growth of large grained crystals. This study gives valuable insight into the rate determining steps of perovskite crystal growth and provides the basis for developing reliable and reproducible high performance recipes for Perovskite solar cell processing.

Organometallic perovskites for optoelectronic applications(Conference Presentation)

Organometallic halide perovskites CH3NH3BX3 (B= Pb, Sn, Ge; X = I, Br, Cl) have become one of the most promising semiconductors for solar cell applications, reaching power conversion efficiencies beyond 20%. Improving our ability to harness the full potential of organometal halide perovskites requires the development of more reliable synthesis routines of well defined, reproducible and defect free reference systems allowing to study the fundamental photo-physical processes. In this study we present size and band gap engineering for organo-lead perovskites crystallites with various shapes and sizes ranging from the 5 nm regime all the way to 1 cm. Colloidal nano-crystals, micro-crystlline particles as well as single crystals are demonstrated with excellent purity and control in shape and size are demonstrated. The structural, optical and photo-physical properties of these reference materials are investigated and analyzed as function of their size and shape.