Tom Kollek

Porous and Shape‐Anisotropic Single Crystals of the Semiconductor Perovskite CH3NH3PbI3 from a Single‐Source Precursor

Significant progress in solar-cell research is currently made by the development of metal-organic perovskites (MOPs) owing to their superior properties, such as high absorption coefficients and effective transport of photogenerated charges. As for other semiconductors, it is expected that the properties of MOPs may be significantly improved by a defined nanostructure. However, their chemical sensitivity (e.g., towards hydrolysis) prohibits the application of methods already known for the synthesis of other nanomaterials. A new and general method for the synthesis of various (CH3NH3)PbI3 nanostructures from a novel single-source precursor is presented. Nanoporous MOP single crystals are obtained by a crystal-to-crystal transformation that is accompanied by spinodal demixing of the triethylene glycol containing precursor structure. Selective binding of a capping agent can be used to tune the particle shape of the MOP nanocrystals.

Highly Efficient Reproducible Perovskite Solar Cells Prepared by Low-Temperature Processing

In this work, we describe the role of the different layers in perovskite solar cells to achieve reproducible, ~16% efficient perovskite solar cells. We used a planar device architecture with PEDOT:PSS on the bottom, followed by the perovskite layer and an evaporated C60 layer before deposition of the top electrode. No high temperature annealing step is needed, which also allows processing on flexible plastic substrates. Only the optimization of all of these layers leads to highly efficient and reproducible results. In this work, we describe the effects of different processing conditions, especially the influence of the C60 top layer on the device performance.

Thiophene-Functionalized Hybrid Perovskite Microrods and their Application in Photodetector Devices for Investigating Charge Transport Through Interfaces in Particle-Based Materials

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 (tON = 190 ms, tOFF = 430 ms) compared to alkyl-functionalized particles (tON = 1055 ms, tOFF = 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.

Facet-controlled preparation of hybrid perovskite microcrystals in the gas phase and the remarkable effect on optoelectronic properties

Within only a few years, hybrid perovskites have become one of the most intriguing semiconductors in different light-harvesting and light-emitting applications. Their optimization for targeting technological implementation can only be achieved if one gathers knowledge of their fundamental material properties and how they are influenced by factors like composition, particle size, and shape. Not only is shaping hybrid perovskite particles difficult, but capping agents binding to crystal surfaces might influence the intrinsic properties as well. We present a new aerosol-assisted crystallization with a liquid single-source precursor for making shaped CH3NH3PbBr3 crystals with “naked facets”. The formation of microcrystals with either predominant (001) facets or the less favorable (011) facets is achieved. We were able to assemble the particles in a defined orientation on a substrate to investigate the facet influence on the optical properties. We find not only a pronounced influence on the lifetime of the photo-generated charge carriers, but also a shift in the photoluminescence energy, and, using confocal fluorescence spectroscopy, a facet-dependent local enhancement of the fluorescence features. Our study demonstrates that the particle shape is an important tool to modify the properties of hybrid perovskites for optoelectronic applications.