Design and optimization of luminescent semiconductor nanocrystals for optoelectronic applications Erstellung und Optimierung von lumineszierenden Halbleiter-Nanokristallen für optoelektronische Anwendungen Der Technischen Fakultät der Friedrich-Alexander-Universität Erlangen-Nürnberg zur Erlangung des Doktorgrades Dr.-Ing. vorgelegt von Ievgen Levchuk aus Myhove (Ukraine) Als Dissertation genehmigt von der Technischen Fakultät der Friedrich-Alexander-Universität Erlangen-Nürnberg Tag der mündlichen Prüfung: 20 Juni 2017 Vorsitzender des Promotionsorgans: Prof. Dr. Reinhard Lerch Gutachter: Prof. Dr. Christoph J. Brabec Prof. Dr. Rainer Hock ii To my parents Volodymyr and Halyna Levchuk iii Acknowledgments First of all I would like to thank Prof. Dr. Christoph J. Brabec for the great opportunity to perform a Ph.D. in his group. It was a pleasure to work under his supervision, because his always the open-minded look to all of my research, perfect suggestions, new ideas, really great inspiration and guidance, encourage me to make a nice and very interesting scientific work. I highly appreciate that Prof. Brabec always has a time for my results discussion and other topics. My special thanks I would like to address to my group leader Priv.-Doz. Dr. Miroslaw Batentschuk who opened for me beautiful and colorful world of luminescent materials. He always was positive and ready to help me with any issue even beyond scientific life. Most importantly Dr. Batentschuk all the time support my ideas and giving me a lot of scientific freedom resulting in right choice of thesis direction. His role in making of this work is invaluable. I am very grateful to Dr. Andres Osvet for all of his help with optical characterization, generation and implementation of new ideas, fast correction and improvement of my manuscripts as well as for very nice environments in the group and always interesting daily discussion. This thesis would not be complete without successful collaboration. I would like to thank Patrick Herre for almost all TEM, HRTEM and SEM measurements, without which work in nanochemistry is almost impossible. I also would like to acknowledge Prof. Spiecker for his support in electron microscopy. I want to express my gratitude to Prof. Rik Tykwinski and Marco Gruber for their help in NMR spectroscopy and results evaluation. My next thanks I would like to address to Prof. Rainer Hock and his Ph.D. student Marco Brandl for performed XRD measurements. Dr. Christian Würth and Prof. Ute Resch-Genger thank you for the great collaboration and photoluminescence quantum yield measurements. I want to thank also Claudia Kolbeck and Prof. Hans-Peter Steinrück for XPS measurements. I would love to acknowledge i-MEET peoples for the warm atmosphere in the last 4 years. My special thanks going to my officemates Nicola. Cesar, Lili, Carina, Chaohong, Xie, Chao and always part of our screw Shreetu who brought to my scientific life not only great collaborations but also a lot of fun; they encourage me to be a better than I am. I also would like to thank Yi Hou and Gebhard J. Matt for many crazy ideas and fantastic realization thereof. Also, I am grateful to my former master and bachelor students, who not only contributed to my projects but also became my friends. Dr. Anastasiia Solodovnyk, thank you for more than 2 years of great collaboration and your colourful and bright LDS layers. Also my warmest thanks to Liudmyla for her great friendly support. iv Special thanks goes to Winfried Habel, Leonid Kuper, Uli Knerr, Claudia Koch and Corina Winkler for their unlimited support in administrative matters and my German language improvement. Especially, I want to thank warmly Prof. Maksym V. Kovalenko, who played a significant role in my decision to become a scientist and to join the group of Prof. Dr. Christoph J. Brabec. I am also thankful to my friend Anatolii Polovitsyn who convinced me to try the life of Ph.D. student and his support in the field of nanochemistry. Most importantly I would like to thank my parents Volodymyr and Halyna as well as my dear sisters Adriana and Natasha for unconditional support during my entire life regardless of the path on which I decided to go. Thousands of times I thank my girlfriend Nathalie for her patience, invaluable support and love. Last but not least, I would like to thank the members of the examination committee for their efforts and time and also to you, the reader, for considering my work v Summary Luminescent colloidal semiconductor nanocrystals have attracted prominent attention for the last three decades since their size-dependent optical properties were discovered. Numerous applications in fields of light conversion such as light-emitting diodes (LED), photovoltaics, medicine, lasers and TV displays were developed. Despite the strong and rapid expansion of this field in the scope of material quality reflected by narrow size distribution and photoluminescence quantum yield, simplification of the production technology, fabrication in industrial scale and their cost reduction are still under development. Furthermore, reduction of toxicity and design of new Cd-free highly luminescent nanocrystalline materials is the last hurdle before commercial application. The primary goal of this thesis was to develop new cost-effective luminescent colloidal semiconductor materials as spectral convertor with tunable optical properties and application thereof. Being a chemist in and having taken my master’s degree in the field of bulk material growth (CdTe), during my Ph.D. study I delved into several new fields like synthesis and surface modification of colloidal nanocrystals, device processing and photophysics of the semiconductor materials . The thesis is subdivided into two introductory chapters (Chapter 1, Chapter 2), and 4 chapters on scientific results. Chapter 3 describes the large-scale and one-pot synthesis of highly luminescent core-shell ZnCdS:Mn/ZnS colloidal nanocrystals (NCs). The key task of this chapter was designing highly luminescent colloidal nanocrystals with zero reabsorption for down-conversion of UV-blue light to the visible region in Si solar cells. Before starting the successful study on highly luminescent zero-reabsorption ZnCdS:Mn/ZnS NCs, several NC systems such as La-doped ZnO, YVO4:Eu,Bi, Carbon Quantum Dots and highly luminescent PMMA-ZnO NCs were tested. Further study on the synthesis of ZnCdS:Mn/ZnS NC system and the optimization of all the reaction parameters affecting the photoluminescence properties lead to a record photoluminescence quantum yield (PLQY) of 70%. These highly fluorescent nanocrystals were efficiently employed as down shifting layers for the ultraviolet (UV) to yellow wavelength region to improve the efficiency of monocrystalline silicon (mc-Si) solar cells by nearly 0.5 percentage points. The resulting power conversion efficiency (PCE) of conventional solar modules with a 14.6 % energy yield, which are coated with the ZnCdS:Mn/ZnS light converter, will enable a cost reduction for the solar electricity production by 2.1 %. Chapter 4 devoted to the synthesis of hybrid organic-inorganic metal halide perovskite colloidal nanocrystals. In the first part of the chapter, a simple ligand-assisted re- precipitation approach allows to fabricate highly luminescent (PLQY 30-90%) and nearly vi monodisperse CH3NH3PbX3 (X=Br, I) colloidal nanoplatelets with tailored thicknesses. Broadly tunable emission wavelengths (450–730 nm) are achieved via the pronounced quantum size effect without anion–halide mixing. However, pure chemical and colloidal stability was a main motivation to find a more stable analog. Therefore, in the second part of this chapter, we employed the same approach to synthesize novel formamidinium (CH(NH3)2PbX3, X=Cl, Br, I) based perovskite nanocrystals, which is an analog to + methylammonium (CH3NH3 ) perovskite. The cubic and platelet-like nanocrystals with their emission in the range of 415-740 nm, full width at half maximum (FWHM) of 20-44 nm and lifetimes of 5−166 ns, enable band gap tuning by halide composition as well as by their thickness tailoring; they have a high photoluminescence quantum yield (up to 85%), colloidal and thermodynamic stability. Further optoelectronic measurements verify that the photodetector based on FAPbI3 nanocrystals paves the way for perovskite quantum dot photovoltaics. Despite of high photovoltaic performance of the bulk perovskites, a FAPbI3 nanocrystalline photodetector prototype has shown low photoresponse mainly due to the existence of insulating long-chain organic ligands on the NCs surface. Therefore, in Chapter 5 a new ligand-free and shape controlled synthesis of submicron perovskite CH3NH3PbX3 (X=Br, I) crystallites and their mixed-halide analogs was designed. This allowed fabricating stable perovskite inks for the large-scale printing. Photodetector devices bladed out of this ink have shown remarkably high photoresponse, indicating the possibility of precursor-free and large area perovskite solar cell module manufacturing. Chapter 6 is devoted to the study of significant importance of the chemical purity of the perovskite precursors and their impact on the semiconductor quality. The findings presented in this chapter show that certain amount of impurities formed during the CH3NH3I synthesis promote PbHPO3 nanoparticle formation in the perovskite precursor. These particles play the role of seeds for high quality large grain growth, which led finally to high efficiency of the solar cells. This study demonstrates that the reproducibility