Branched methoxydiphenylamine-substituted fluorene derivatives as hole transporting materials for high-performance perovskite solar cells

Small-molecule hole transporting materials based on methoxydiphenylamine-substituted fluorene fragments were synthesized and incorporated into a perovskite solar cell, which displayed a power conversion efficiency of up to 19.96%, one of the highest conversion efficiencies reported. The investigated hole transporting materials were synthesized in two steps from commercially available and relatively inexpensive starting reagents, resulting in up to fivefold cost reduction of the final product compared with spiro-OMeTAD. Electro-optical and thermoanalytical measurements such as UV/Vis, thin-film conductivity, hole mobility, DSC, TGA, ionization potential and current voltage scans of the full perovskite solar cells have been carried out to characterize the new materials.

Donor- π-donor type hole transporting materials: marked π-bridge effects on optoelectronic properties, solid-state structure, and perovskite solar cell efficiency

Peng Gao, Giulia Grancini, Paul Gratia, Mohammad Khaja Nazeeruddin, Sanghyun Paek, Kasparas Rakstys, Iwan Zimmermann

Donor-pi-bridge-donor type oligomers (D-pi-D) have been studied intensively as active materials for organic optoelectronic devices. In this study, we introduce three new D-pi-D type organic semiconductors incorporating thiophene or thienothiophene with two electron-rich TPA units, which can be easily synthesized from commercially available materials. A thorough comparison of their optoelectronic and structural properties was conducted, revealing the strong influence of the extent of longitudinal pi-bridge conjugation on both the solid structure of the organic semiconductive materials and their photovoltaic performance when applied as hole transporting materials (HTM) in perovskite solar cells. Single-crystal measurements and time-resolved photoluminescence (TRPL) studies indicate that these coplanar donor-pi-donor type HTMs could be promising alternatives to state-of-the-art spiro-OMeTAD, due to the multiple intermolecular short contacts as charge transporting channels and efficient charge extraction properties from the perovskite layer. The optimized devices with PEH-9 exhibited an impressive PCE of 16.9% under standard global AM 1.5 illumination with minimized hysteretic behaviour, which is comparable to that of devices using spiro-OMeTAD under similar conditions. Ambient stability after 400 h revealed that 93% of the energy conversion efficiency was retained for PEH-9, indicating that the devices had good long-term stability.

Royal Soc Chemistry2016

Surface-Enhanced NMR Spectroscopy

Dominik Józef Kubicki

Dynamic Nuclear Polarization (DNP) is currently one of the most efficient ways of enhancing sensitivity in solid-state NMR experiments. The DNP protocol consists in doping a sample with a small amount of para-magnetic species, typically nitroxide biradicals, and carrying out a magic-angle spinning (MAS) solid-state NMR experiment under microwave irradiation at around 100 K. Maximum theoretical enhancements that can be achieved this way reach 660, translating into the possibility of studying atomic-level composition of dilute surface species at picomolar concentrations that would be otherwise out of reach. Recent years wit-nessed very fast development of DNP instrumentation (high-power and high-frequency microwave sources, higher magnetic fields, faster sample spinning), polarizing agents (rigid dinitroxide biradicals) and new sam-ple preparation techniques (universally applicable and straightforward protocols, solutions for DNP of highly reactive organometallic species) which brought about sensitivity enhancements close to the theoret-ical maximum. The experimental part of this thesis reviews recent advances in the field of MAS DNP. We explores ways of improving currently existing polarizing agents by fine-tuning their performance (function-alization, deuteration). In particular, we have developed a new polarizing agent, TEKPol2, which performs better than current state-of-the-art TEKPol. We have subsequently explored factors that limit DNP en-hancements under typical experimental conditions. We describe how by simply mixing the DNP sample with dielectric particles (such as NaCl, KBr, sapphire) enhancements can be boosted by a factor of 2.5. We have rationalized this phenomenon in terms of improved microwave propagation through the sample using finite-element simulations. Further, we have applied conventional solid-state NMR to probe atomic level microstructure to determine key properties of hybrid lead halide perovskites materials. Methylammonium (MA)- and formamidinium (FA)-based lead halide perovskites attract much attention owing to their impressive performance as photo-voltaic materials, currently providing power conversion efficiencies over 22%. We have used 14N, 2H, 13C, 133Cs, 87Rb, 39K and 1H solid-state MAS NMR to explain in detail cation reorientation dynamics, phase com-position, and phase separation phenomena in Cs-, Rb-, K- guanidinium (GUA)-, MA-, and FA-containing pho-tovoltaic perovskites, both in bulk and on thin films prepared according to the usual device fabrication procedure. We have presented the first quantitative, cation-specific picture of cation reorientation in MA/FA and GUA/MA materials, which explains the exceptionally long charge carrier lifetimes observed in these materi-als. Our findings have provided a link between charge carrier lifetimes of ABX3-type lead halide perovskites and the structure of A-cation site. We also found that rubidium and potassium are not incorporated into the 3D lattice of materials previously described in the literature. Cesium and GUA, on the other hand, are easily incorporated. These advances were possible through the use of mechanochemistry as a straightfor-ward way of preparing large quantities of high quality materials for NMR studies.

EPFL2018

Compositional Characterization of Organo-Lead tri-Halide Perovskite Solar Cells

Paul Gratia

Perovskite photovoltaics is one of the fastest growing research fields in materials science. Apart from photovoltaics, a wide range of energy-related research fields have gained substan-tial new momentum owing to the fantastic optoelectronic properties of hybrid lead halide per-ovskites. Starting with solar cells showing a mere 3% power conversion efficiency in 2009, researchers have been able to increase this value to over 22% in 2017, comparable to the best monocrystalline silicon solar cells. These improvements are chiefly due to the compositional tuning and mixing of the ABX3 perovskite structure. However, this strategy increases not only the efficiency but also in the same time the complexity of the perovskite formulation. Differ-ent phases, from the nano- to macroscale, can potentially form. In order to rationalize the effi-ciency progress, one needs to understand and characterize the complex perovskite composi-tion and crystal structure in great detail. The main focus of this thesis lies in the in-depth compositional and phase analysis of perovskite thin films as well as full perovskite solar cells, trying to rationalize the consequences of A cation and X anion tuning and mixing. In the first part, it is shown that fabricating solar cells from a non-stoichiometric perovskite precursor composition, namely one with PbI2 excess, leads to increased grain size, enhanced cristallinity, reduced recombination as well as an improved TiO2/perovskite interface. As op-posed to lead iodide phases resulting from film degradation, which is hampering device per-formance, unreacted excess PbI2 phases originating from an excess of PbI2 in the precursor solution are very beneficial for the final solar cell efficiency, reproducibility and stability. The next scientific challenge adressed in this thesis is related to the nano- and microscale composition of the record-breaking mixed cation/mixed anion perovskite composition. Mac-roscale investigations have previously suggested that this formulation results in a single ho-mogenous phase. However, using nanoscale elemental and charge carrier distribution mapping as well as microscale structural and optical film analysis, it is here found that in high efficient solar cells partial phase segregation does take place at the micro- and nanoscale. Moreover, it is shown that mixed cation/anion formulations allow the formation of never re-ported 3D hexagonal lead halide perovskite polytypes, namely 4H and 6H. These polytypes are shown to play a key role during the crystallization process, which is also revealed for the first time here. Indeed, mixed cation/mixed anion perovskite films are fabricated via the crys-tallization sequence 2Hï 4Hï 6Hï 3R(3C). It is demonstrated that the complex crystalliza-tion via these defect-prone hexagonal intermediates can be by-passed by the incorporaton of low amounts of Cs+ cations into the structure. This can explain the improved stability as well as the increased power conversion efficiency and device reproducibility. It gives for the first time a rational explanation as to why the halide perovskite community rapidly adopted the incorporation of Cs+ in mixed perovskites. The last results presented in this thesis are related to the hole-transporting materials (HTM) used in perovskite solar cells. It is shown that carbazole-based small molecule HTMs are a cheap and efficient alternative to the much costlier commercial spiro-OMeTAD.

EPFL2018

Compositional Characterization of Organo-Lead tri-Halide Perovskite Solar Cells

Paul Gratia

EPFL2018

Surface-Enhanced NMR Spectroscopy

Dominik Józef Kubicki

EPFL2018