Elucidating the origins of high preferential crystal orientation in quasi‐2D perovskite solar cells

Incorporating large organic cations to form 2D and mixed 2D/3D structures significantly increases the stability of perovskite solar cells. However, due to their low electron mobility, aligning the organic sheets to ensure unimpeded charge transport is critical to rival the high performances of pure 3D systems. While additives such as methylammonium chloride (MACl) can enable this preferential orientation, so far, no complete description exists explaining how they influence the nucleation process to grow highly aligned crystals. Here, by investigating the initial stages of the crystallization, as well as partially and fully formed perovskites grown using MACl, the origins underlying this favorable alignment are inferred. This mechanism is studied by employing 3-fluorobenzylammonium in quasi-2D perovskite solar cells. Upon assisting the crystallization with MACl, films with a degree of preferential orientation of 94%, capable of withstanding moisture levels of 97% relative humidity for 10 h without significant changes in the crystal structure are achieved. Finally, by combining macroscopic, microscopic, and spectroscopic studies, the nucleation process leading to highly oriented perovskite films is elucidated. Understanding this mechanism will aid in the rational design of future additives to achieve more defect tolerant and stable perovskite optoelectronics

Restoration of soil quality using biochar and brown coal waste: A review

Soils in intensively farmed areas of the world are prone to degradation. Amendment of such soils with organic waste materials attempts to restore soil quality. Organic amendments are heterogeneous media, which are a source of soil organic matter (SOM) and maintain or restore chemical, physical, biological and ecological functionality. More specifically, an increase in SOM can influence the soil microclimate, microbial community structure, biomass turnover and mineralisation of nutrients. The search is on-going for locally sourced alternatives as many forms may be costly or geographically limiting. The present review focuses on a heterogeneous group of amendments i.e. biochar and brown coal waste (BCW). Both biochar (made from a variety of feedstocks at various temperatures) and BCW (mined extensively) are options that have worldwide applicability. These materials have very high C contents and soil stability, therefore can be used for long-term C sequestration to abate greenhouse gas emissions and as conditioners to improve soil quality. However, biochar is costly for large-scale applications and BCW may have inherently high moisture and pollutant contents. Future studies should focus on the long-term application of these amendments and determine the physicochemical properties of the soil, bioavailability of soil contaminants, diversity of soil communities and productivity of selected crops. Furthermore, the development of in situ technologies to lower production and processing costs of biochar and BCW would improve their economic feasibility for large-scale application

Role of Interfaces in Perovskite Devices

Perovskite solar cells are a class of swiftly evolving next-generation photovoltaic devices that have attracted a lot of interest due to their exceedingly improving performance, low processing costs and the excellent optoelectronic properties of the perovskite semiconductors. However, issues related to device processing reproducibility, anomalous hysteresis, scalability as well as factors, limiting the performance and stability of the devices, are hampering the advancement of this technology. The development of reproducible and optimal processing conditions and a detailed understanding of processes related to charge transport and recombination in the perovskite devices are required before considering large scale production and commercialization. Also, the proper choice of charge transport materials and the engineering of interfaces are found to be crucial for the overall performance and stability of perovskite devices. In this PhD thesis, the effect of different charge-transport layers and interfaces present in the device stack on the performance and stability of perovskite devices were studied. In addition, the role of processing additives, used to control the formation of the perovskite absorber material, was investigated. The main objective of the research was to prepare efficient, reproducible and stable so-called inverted (p-i-n) perovskite solar cells. In the framework of this thesis various electron transport materials were tested. Thin layers of low temperature processed titanium oxide or different polyphosphazene derivatives were used as an additional buffer layer in perovskite solar cells to improve the cathodic interface of the device. Solar cells with an additional buffer layer were found to exhibit improved photocurrent density-voltage characteristics with reduced hysteresis and improved overall device performance and stability. The morphology and crystallinity of the photoactive perovskite layer determine to a large extent the performance and stability of perovskite solar cells. Solvent engineering is an effective technique to design and control the formation of thin films of high-quality perovskite materials. In this thesis, acetylacetone was used as a solvent additive to modify the morphology and crystallinity of mixed halide perovskite films and to improve the device performance. Perovskite solar cells processed with acetylacetone show improved photovoltaic properties with good stability under continuous operation in ambient air. Thorough characterization of the perovskite devices was performed using various optical, electrical and microscopic techniques. Current density-voltage and external quantum efficiency measurements were applied to evaluate the photon utilization efficiency of the perovskite solar cells. Impedance spectroscopy was used to characterize the charge carrier dynamics in the bulk and at the interfaces of perovskite solar cells. To study the optical properties of perovskite films in combination with different charge transport interlayers, photoluminescence and photoluminescence decay spectroscopy were applied. Space-charge limited current and Mott-Schottky plot analysis were used to depict trap-states density and recombination channels in the perovskite devices. Atomic force microscopy and scanning electron microscopy were applied to characterize the topography of the sample devices. Furthermore, the long-term stability of the optimized perovskite solar cells was studied under continuous operation and ambient conditions. The experimental results suggest that the decay of the device performance under continuous operation is mainly related to changes in the photoactive perovskite film and ion migration. Overall, this dissertation reveals the importance of proper device design and processing. The findings suggest that to achieve highly efficient and stable perovskite solar cells, one needs to consider the combined effect of careful interface and compositional engineering and processing conditions of the perovskite devices.submitted by MSc. Bekele Hailegnaw TeklemariamIn Zusammenarbeit mit dem Linzer Institut für Organische Solarzellen (LIOS)Universität Linz, Dissertation, 2019OeBB(VLID)468403

Depolymerization of Cellulose in Water Catalyzed by Phenylboronic Acid Derivatives

Based on the knowledge that <i>o-</i>aminomethylphenylboronic acids reversibly bind to carbohydrates, relevant water-soluble derivatives of the former were prepared by appending hydrophilic tethers. In this way the phenylboronic acid derivatives were used to hydrolytically dissolve, i.e. depolymerize cellulose in water at nearly neutral pH values. Some of these hydrophilic tethers consisted of moieties that were surmised to be able to promote hydrolysis of the glycosidic bonds such as a carboxylic acid, phosphonic acid as weak Bronsted acids, or an imidazole functionality as a nucelophilic catalyst; water-soluble polyethylene glycol and polyethyenelimine appendages were also used. The results show that at around 120 °C efficient hydrolysis of cellulose to form water-soluble oligosaccharides could be attained in a period of 24 h. Importantly preimpregnation of a morpholine substituted <i>o-</i>aminomethylphenylboronic acid led to the very selective formation of glucose

Rain on Methylammonium Lead Iodide Based Perovskites: Possible Environmental Effects of Perovskite Solar Cells

The great promise of hybrid organic–inorganic lead halide perovskite (HOIP)-based solar cells is being challenged by its Pb content and its sensitivity to water. Here, the impact of rain on methylammonium lead iodide perovskite films was investigated by exposing such films to water of varying pH values, simulating exposure of the films to rain. The amount of Pb loss was determined using both gravimetric and inductively coupled plasma mass spectrometry measurements. Using our results, the extent of Pb loss to the environment, in the case of catastrophic module failure, was evaluated. Although very dependent on module siting, even total destruction of a large solar electrical power generating plant, based on HOIPs, while obviously highly undesirable, is estimated to be far from catastrophic for the environment

Optoelectronic Properties of Layered Perovskite Solar Cells

Herein, the optoelectronic properties of interfaceengineered perovskite 2D|3Dheterojunction structure solar cells are reported. The reciprocity theorem is applied to determine the maximum opencircuit voltage (Voc) the device can deliver under solar illumination. A Voc of 1.295V is found, analyzing the measured external quantum efficiency and assuming only radiative recombination. For comparison, the experimental opencircuit voltage found for the studied 2D|3D heterojunctions is 1.15V. The contribution of nonradiative recombination is explored by measuring the electroluminescence quantum yield. A quantum yield of 0.4% is found at current densities equivalent to 1sun illumination. This translates into a Voc loss of 140mV, which is in very good agreement with the experimental findings. In addition, the fundamental correlation between luminescence intensity and the chemical potential predicted by the generalized Planck law is confirmed for the photoluminescence measured at different light intensities when the device is operated under opencircuit conditions and for the electroluminescence when operated under a forward bias. The investigations in this study suggest that further efficiency improvements can be achieved by reducing the nonradiative recombination in the studied solar cell. At the same time, a highperformance near IR light emitting diode can be realized.(VLID)4843823Version of recor

Ion-driven nanograin formation in early-stage degradation of tri-cation perovskite films

The operational stability of organic–inorganic halide perovskite based solar cells is a challenge for widespread commercial adoption. The mobility of ionic species is a key contributor to perovskite instability since ion migration can lead to unfavourable changes in the crystal lattice and ultimately destabilisation of the perovskite phase. Here we study the nanoscale early-stage degradation of mixed-halide mixed-cation perovskite films under operation-like conditions using electrical scanning probe microscopy to investigate the formation of surface nanograin defects. We identify the nanograins as lead iodide and study their formation in ambient and inert environments with various optical, thermal, and electrical stress conditions in order to elucidate the different underlying degradation mechanisms. We find that the intrinsic instability is related to the polycrystalline morphology, where electrical bias stress leads to the build-up of charge at grain boundaries and lateral space charge gradients that destabilise the local perovskite lattice facilitating escape of the organic cation. This mechanism is accelerated by enhanced ionic mobility under optical excitation. Our findings highlight the importance of inhibiting the formation of local charge imbalance, either through compositions preventing ionic redistribution or local grain boundary passivation, in order to extend operational stability in perovskite photovoltaics